CN115421460A - Casting residual stress control optimization method based on computer numerical simulation and application - Google Patents
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Abstract
The invention belongs to the technical field of casting process optimization, and discloses a casting residual stress control optimization method based on computer numerical simulation and application thereof, wherein an original casting model is preprocessed and simulated and calculated by preliminarily analyzing a casting structure and a casting system; redesigning a pouring system, adopting a bottom pouring type pouring scheme, arranging a straight pouring way nest and a cross gate extension section for reducing flow speed and impact, and arranging a riser and a chill for reducing the volume of a shrinkage cavity; orthogonal test design is adopted to optimize the pouring temperature, the pouring time and the casting mold temperature, so that the optimal pouring temperature is 670 ℃, the pouring time is 5s and the casting mold temperature is 20 ℃. According to the invention, simulation tests show that the casting numerical simulation technology has unique advantages and considerable reliability in the aspects of simulating the casting process, exploring residual stress evolution, optimizing casting schemes and the like, and the large-scale application of the technology can make great contribution to the healthy development of the casting industry.
Description
Technical Field
The invention belongs to the technical field of casting process optimization, and particularly relates to a casting residual stress control optimization method based on computer numerical simulation and application thereof.
Background
At present, the casting technology refers to a production method in which liquid metal is poured into a mold cavity corresponding to the shape and size of a part, and is cooled and solidified to obtain a blank or a part. The casting technology has the advantages of low manufacturing cost, wide material source, strong applicability and the like, can be applied to the forming of complex workpieces, and is widely applied to the fields of machine manufacturing, automobile industry, aerospace, military industry and national defense and the like.
Modern casting can be divided into sand casting and special casting according to the casting mould materials, wherein the sand casting has convenient modeling, low cost and reusable molding sand, and is widely applied to medium and large castings and single batch manufacturing; the gravity casting technology is divided into gravity casting and pressure casting according to the casting technology, the gravity casting technology is the most widely used casting technology with the longest history, and has the advantages of simple technology, few pores, casting piece capable of being subjected to thermal processing post-treatment and the like; the pressure casting can form complex thin-wall workpieces, and the casting has smooth surface, excellent mechanical property and high production efficiency. With the development of material science and the improvement of process technology, more casting processes are continuously emerging, such as shell mold casting, investment casting, lost foam casting and the like, have different advantages, and are actively used in the high-precision manufacturing fields of aeroengine blades, motor casings, turbine blades and the like.
With the development of modern industry and the requirement of environmental protection, the application of aluminum alloy is more and more extensive, and the aluminum alloy becomes one of the most widely applied metal materials like the traditional steel materials. Compared with cast steel and cast iron, cast aluminum alloy has the most prominent characteristic of light weight, and is one of the lightest structural materials in common metals; the mature casting process can mold various aluminum alloys with complex shapes at a higher speed and a lower cost, and realize near net shape so as to reduce the metal processing amount; in addition, the alloy has the characteristics of high strength, excellent conductivity, high reflectivity, good corrosion resistance, excellent processability, attractive appearance and the like, and is widely applied to industrial products such as gas turbine blades, engine casings, automobile hubs, gearbox casings, ship parts and the like.
The cast aluminum alloy has the same alloy system and strengthening mechanism as the wrought aluminum alloy, but contains enough eutectic elements such as silicon, magnesium and the like to enable the alloy to have equivalent fluidity and be easy to fill and feed. Modern cast aluminum alloys can be divided into four families according to the alloying elements: the aluminum-silicon alloy has the advantages of optimal casting performance, balanced service performance and the widest application range.
The aluminum alloy melt has low melting point, small volumetric heat capacity and high heat conductivity, so that the temperature is rapidly reduced, the viscosity is increased, the mold filling capacity is reduced, and bubbles are difficult to remove and remain in a workpiece to form air holes in the flowing process; the aluminum alloy has active chemical property and strong affinity with gas, is easy to absorb gas and be oxidized, and the specific gravity of the oxide is similar to that of the aluminum liquid and is difficult to remove so as to form slag inclusion; the shrinkage is large in the solidification process of the aluminum alloy, the crystallization temperature range is wide, and the shrinkage porosity defect is easy to generate; the aluminum alloy has large linear shrinkage and large elastic modulus, and is easy to generate large residual stress and strain in a cooling project, thereby influencing the service performance and the service life.
The casting residual stress is present inside the casting without being released from the casting stress, which is the algebraic sum of the thermal stress, the phase transformation stress and the mechanical barrier stress. In the cooling process of the casting, the thickness of each part of the casting is different, and the cooling speed difference is generated, so that the non-uniformity of plastic deformation is generated, and the internal stress in different directions is generated after the casting is cooled to the room temperature, and the stress is called as thermal stress; in the crystallization process, because different parts have different temperatures, the eutectic crystallization and eutectoid transformation of the parts have different time and the tissue volume is different, so that the phase change stress opposite to that of a thermal stress method exists in the casting after the casting is cooled to room temperature; in the solid shrinkage process of the casting, the time for each part from plastic state to elastic state is different, and the core blocks the cooling shrinkage of the metal, so that strain with different degrees is caused in the casting, and mechanical blocking stress is generated.
The generation of casting residual stress can influence the service performance and precision of a casting, and is mainly embodied in the following aspects:
TABLE 1 influence of residual stress on casting Properties
The residual stress exists in a conjugated manner in the component in most cases, so that the distribution of the residual stress needs to be detected and analyzed to determine the influence of the residual stress on the component, and conventional detection methods can be divided into two main categories according to the principle:
(1) The mechanical measurement method is based on the principle that a part having residual stress is separated or cut from a member to release the stress, and the change in strain caused by the stress is measured to estimate the residual stress. The common methods comprise a hole rotating method, a ring core method, a layer stripping method, a strip cutting method and the like. The methods have high measurement accuracy and mature and reliable technology, but can damage and influence workpieces to a certain extent.
(2) A nondestructive testing method is a method for testing a stress field by applying various physical principles and chemical phenomena such as sound, light, magnetism, electricity and the like on the premise of not damaging the use performance of a tested object and by means of modern technical equipment. Common methods include magnetic powder detection, ray detection, ultrasonic detection, indentation strain detection and the like. These methods are characterized by a wider measuring range and more convenient operation, but relatively lower measuring accuracy and expensive equipment, compared with mechanical measurement, they do not adversely affect the workpiece.
The mechanical measurement method and the nondestructive detection method are difficult to carry out full-flow and all-around monitoring on the distribution of the residual stress of the casting, the evolution of the residual stress of the casting in the machining process is difficult to express, time and labor are wasted, the efficiency is not high, and the problems are solved by applying the numerical simulation technology in the casting field.
In production practice, the casting forming process is a series of physical and chemical change processes including heat transmission, momentum transmission, mass transmission and phase change, and relates to factors such as material physical property parameters, workpiece modeling, casting process parameters and the like, and quantitative statistical analysis is difficult to perform as a result. Operators carry out manual modeling and process design optimization by experience, and often leave excessive design allowance, which causes material waste; the manual operation efficiency is low, and the precision and the process stability are difficult to guarantee. Therefore, the traditional casting method cannot meet the modern industrialization requirement.
The casting numerical simulation technology is a computer simulation technology which is used for calculating and simulating the pouring, solidifying and cooling processes in the casting process by using numerical methods such as a finite element method, a finite difference method and the like and combining theories such as hydrodynamics, heat transfer science, metallogy and the like. The numerical simulation technology of the casting process at the present stage is mainly used for simulation of the coupling process of the alloy melt flow field and the temperature field at the mold filling stage, simulation of the temperature field, prediction of shrinkage porosity and shrinkage cavity, simulation of the stress field and simulation of the microstructure of a casting. The technology can simulate the casting process, predict the casting defects, optimize the casting process and provide guidance for casting production without actual tests.
The ProCAST system is a professional casting simulation CAE system based on a finite element method, can accurately simulate the flowing, solidifying and cooling processes of a metal casting process, calculate a temperature field, a stress field and a flow field of the whole casting forming process, combine casting technology science and related criteria to obtain defect positions, display residual stress strain, predict shrinkage cavity and microstructure change, and can calculate special technological measures appearing in the casting process. The ProCAST system has the advantages of modular design, accurate geometric description, CAD/CAE high integration, engineering interface, capability of independently completing heat-flow-stress complete coupling calculation and the like, and is widely applied to the special casting fields of conventional casting, semi-solid casting, centrifugal casting, inclined casting, precision casting, continuous casting and the like. The system casting simulation process comprises three parts:
(1) Pretreatment: the part mainly provides geometric information of castings and casting molds, performance parameter information of the castings and molding materials and casting process information for numerical simulation, and is mainly completed by an exogenous CAD system, a Mesh module and a Cast module.
(2) Intermediate calculation: the part provides a calculation model for a numerical technology according to a physical field involved in a casting process, and the casting quality is predicted according to the relation between the casting quality or defects and the physical field, and is mainly completed by a solver.
(3) And (3) post-treatment: the numerical calculation is visually output in the mode of images and curves, and a mathematical analysis tool is provided, and is mainly completed by a Viewer module.
In order to ensure the accuracy and reliability of the casting residual stress numerical simulation technology, chinese and foreign scholars make continuous efforts and make many progresses. Accurate modeling of materials is critical to simulation calculations. Baghani et al demonstrate that setting the sand mold as a rigid body results in a higher residual stress calculation result than the actual case, while using elasto-plastic mechanical curves results in more accurate results. The Motoyama research result also proves that the elastic-plastic behavior of the MOTOyama is more consistent with the change trend of the binding force of the sand mold on the casting through experiments, and the binding force is one of the key factors for generating the residual stress. The Cam-Clay sand model provided by Inoue further improves the consistency of the calculation result and the experiment result, and improves the modeling and calculation efficiency of the sand model. Metzege et al innovatively propose a surface cell equation that can replace the sand mold action with the normal force on the casting surface in the mold, chang and Dantzig improving the equation to enable it to be used on large rigid molds in applications.
Due to the phase change behavior, solid-liquid variation, and high strain rate sensitivity of cast metals at high temperatures, a fully elastic or elastoplastic behavior model may lead to inaccurate results, and modeling of cast metals is one of the difficulties in casting simulation calculations. The study of Palumbo shows that under the yield limit, the material has larger plastic deformation due to the action of viscous behavior, namely, the high-temperature creep phenomenon cannot be ignored, and if a numerical model only uses an elastic-plastic model, the error is greatly increased; on the contrary, the displacement result caused by stress release is calculated by using the behavior of the elastic-viscous plastic material. Thorborg proposes that to obtain accurate residual stress calculations, the numerical model should include strain rate sensitive equations. Motoyama introduces the temperature level at which the material loses resistance into an elastic-plastic constitutive equation to calculate the thermal stress release phenomenon of the material above the temperature, and obtains a result which is consistent with the experimental measurement.
In addition to the ongoing refinement of material models, numerical methods are also continually advancing. In general, the finite element method is more effective in stress field calculation, and the finite element method is not as efficient as the finite difference method or the finite volume method in temperature field calculation due to the lack of matrix symmetry. Si et al propose a three-dimensional temperature interpolation algorithm for data conversion between FDM and FEM models, introducing the temperature map computed by the FDM solver into the FEM environment to compute the stress and strain fields. Liu et al proposed a [ H ] - [ H | N ] - [ N | S ] rheological model for quasi-solid metals that could also perform fully coupled thermomechanical analysis. Fackeldey et al propose a numerical model based on an accurate microstructure model, which can not only predict temperature distribution and stress evolution, but also predict dendrite structure and eutectic fraction.
The casting processing technology has a long history and wide application, and has an irreplaceable status in the fields of mechanical manufacturing, automobile industry, aerospace military industry and the like. The aluminum alloy casting has low weight, high strength and beautiful appearance, and is more and more concerned by people. However, the traditional casting process is complicated, has a plurality of influencing factors, is difficult to effectively control and detect the casting defects, and hinders the development of the casting industry. The casting residual stress can cause the generation of undesirable phenomena such as fatigue fracture of the casting, shortened service life of the casting, deformation of the casting and the like, so that the control and detection of the casting residual stress are important links of casting production.
Through the above analysis, the problems and defects of the prior art are as follows:
(1) The mechanical measurement method can damage and influence a workpiece to a certain extent, the nondestructive detection method is relatively low in measurement accuracy and expensive in equipment, the distribution of the residual stress of the casting is difficult to monitor in a full-flow and all-around mode, the evolution of the residual stress of the casting in the machining process is difficult to show, time and labor are wasted, and the efficiency is not high.
(2) In production practice, quantitative statistical analysis is difficult to perform on the casting forming process result, and operators perform manual modeling and process design optimization by virtue of experience, so that excessive design allowance is always left, and material waste is caused; the manual operation efficiency is low, and the precision and the process stability are difficult to guarantee.
(3) Due to cast metal phase-change behavior at high temperatures, solid-liquid variation, and high strain rate sensitivity, a fully elastic or elastoplastic behavior model may lead to inaccurate results; the traditional casting process is complicated, has a plurality of influencing factors, is difficult to effectively control and detect the casting defects, and hinders the development of the casting industry.
Disclosure of Invention
Aiming at the problems in the prior art, the invention provides a casting residual stress control optimization method based on computer numerical simulation and application thereof.
The invention is realized in such a way that the evolution simulation method of the residual stress in the casting and processing process of the engineering component comprises the following steps:
establishing a related three-dimensional model by using CAD software, obtaining parameters of a temperature field, a stress field and a filling condition by using a finite element method, analyzing a simulation result, predicting possible casting defects, optimizing and improving a pouring process to eliminate the casting defects and obtain better residual stress expression of a casting;
performing preliminary analysis on a casting structure and a pouring system, preprocessing an original pouring model and performing simulation calculation; and (4) redesigning a pouring system, and optimizing the pouring temperature, the pouring time and the casting mold temperature by adopting an orthogonal test design.
Further, the evolution simulation method of the residual stress in the casting process of the engineering component further comprises the following steps:
establishing a related three-dimensional model by using CAD software, and importing the model into PROCAST MASH for grid division; setting material characteristics, pouring conditions and boundary conditions, and carrying out finite element simulation calculation to obtain a temperature field, a stress field and a filling condition; analyzing the size and distribution of residual stress after the casting is solidified according to the stress field of the casting, and determining a characteristic sensitive part of the engineering component;
analyzing the solidification time and sequence of the molten liquid according to the temperature field of the casting, and judging the generation of shrinkage porosity; analyzing the speed and the filling time of the molten liquid during pouring according to the filling condition graph, and observing the flowing process of the molten liquid; and regulating and controlling material parameters and process parameters according to the analysis result, and analyzing the level of residual stress in the casting process of the part.
Further, the evolution simulation method of the residual stress in the casting and machining process of the engineering component comprises the following steps:
step 1: simply designing an initial pouring scheme according to a casting drawing, and establishing a model 1 by using CAD software: 1, casting and gating system model;
step 2: importing the model into ProCAST software, preprocessing the model, setting simulation parameters according to a processing pouring process, material characteristics and a simulation requirement lamp, and performing simulation operation;
and step 3: analyzing the simulation operation result, and improving the design of a pouring system by combining casting technology;
and 4, step 4: carrying out grid independence verification by utilizing a grid test to obtain grid parameters;
and 5: re-dividing the finite element grids, and optimizing pouring parameters according to an orthogonal test design method to perform a simulation test to obtain an optimal scheme;
step 6: and (5) verifying the optimal scheme.
Further, the initial casting scheme in step 1 comprises:
the initial pouring scheme adopts a top pouring mode, molten metal is poured from the upper part of the casting mold and enters the cavity along the pouring cup and the main pouring gate, and the whole cavity is filled from bottom to top;
the casting material is aluminum alloy ZL114A, the liquidus of the alloy is 616 ℃, the solidus of the alloy is 556 ℃, and the density of the alloy is 2730Kg/m at room temperature and 20 DEG C 3 (ii) a The casting temperature is controlled within the range of 10-110 ℃ above the liquidus line; in casting practice, the pouring speed is controlled by controlling the pouring time, and the following empirical formula is obtained:
t＝AG n ；
wherein, for the coefficient of aluminum alloy A =2.4, N =0.387, G is 2.5 times of the casting quality, and the theoretical pouring time t =5.02s is obtained by bringing relevant data.
Further, the numerical simulation pretreatment in the step 2 comprises model pretreatment and model pretreatment, wherein the model pretreatment is to establish a model by using CAD software according to a casting drawing and reasonably modify and simplify a casting, the diameter of a transverse through hole and a vertical blind hole at the neck part of the casting is 9mm smaller than the diameter of a minimum casting hole of a sand mold by 20mm, and the two long holes are removed before the casting simulation through post-machining drilling; treating the sand mold as a part of a sand box and as an elastic-plastic mechanical model;
the model preprocessing leads the preprocessed model into a Visual-Mesh module of ProCAST software for grid division;
the grid is divided into a model and a virtual sand box is added, and the size of the model is 300 multiplied by 250mm; the surface grid type is triangular, the size is set to be 10mm, and the surface of the model and the surface of the virtual sand mold are subjected to grid division; and after the surface grids are qualified, converting the surface grids into body grids, wherein the types of the body grids are tetrahedral and six-node units.
The gravitational acceleration is taken to be 9.80m/s 2 (ii) a The casting mold is EN AC-42100AlSi7Mg0.3, the cooling speed is 10k/s, the initial temperature is the casting temperature of 700 ℃, the Sand mold is Resin Bonded Sand, the initial temperature is room temperature of 20 ℃, and the mechanical model is set as an elastic-plastic model.
Setting the interface heat exchange parameter of the sand mold and the metal as 500W/m 2 K, heat transfer coefficient of sand mold and outside air is 10W/m 2 K, cooling in an air cooling mode to room temperature; according to the alloy performance and the pouring conditions, the pouring time is set to be 10s, the pouring speed is 0.204kg/s, and the pouring temperature is set to be 700 ℃; and (3) in order to obtain a model stress field, constraining the sand mold, wherein the constraint surface is the outer surface except the gate, and the displacement is set to be 0.
Further, the analysis simulation result in step 3 includes:
simplifying the metal liquid into incompressible Newtonian fluid, and obtaining a motion differential equation of the viscous fluid according to Newton's second law:
where ρ is the fluid density, t is the fluid flow time, μ is the fluid kinematic viscosity, g x 、g y 、g z Is the gravity acceleration component of three coordinate axis directions,
the method is characterized in that the method is a Laplace operator, p is unit volume pressure of fluid, and u, v and w are velocity components of the fluid on X, Y and Z axes;
obtaining a fluid pressure formula according to a continuity equation derived from the conservation of mass equation in the volume element:
wherein D is the fluid divergence.
Calculating the coupling flow field and the temperature field of the fluid temperature distribution in the cavity, wherein an energy equation is as follows:
wherein rho and k are the density and thermal conductivity of the material,
is the slope of the enthalpy-temperature curve of the material, corresponding to the specific heat, and u is the fluid velocity.
The temperature of the workpiece is determined by a heat conduction equation in the solidification process, and the distribution is obtained by a heat balance equation:
wherein c is the specific heat of the material respectively.
During the cooling process, simplifying the material into an elastic-plastic model; before the material reaches the yield strength, the stress strain is in a linear relation, the stress keeps constant after reaching the yield strength, and finally the strain is the sum of the elastic strain and the plastic strain:
wherein the content of the first and second substances,
and
is the theoretical stress and strain of the material, E is the elastic modulus, sigma s Is the material yield strength, epsilon e And ε s Corresponding strain when the material is yielding and subsequent plastic strain;
calculating to obtain a temperature field, a flow field and a stress field of the material finite element unit, and reproducing the processes of filling, solidifying and cooling of the casting model through a finite element constitutive equation; and (3) performing mathematical simulation calculation, performing casting simulation to obtain the visual results of the temperature, the solid state rate, the shrinkage porosity, the effective stress, the flow field speed, the mold filling pressure, the mold filling time, the maximum stress and the direction stress of each point in the casting machining process, and comparing and analyzing the results by a mathematical method.
Further, the improved gating system design in step 3 comprises:
the describedThe pouring system is optimized by adopting a bottom pouring type pouring scheme, and a straight pouring gate pit and a cross gate extension section are arranged for reducing the flow speed and impact; a dead head and a chill are arranged for reducing the volume of a shrinkage cavity; the casting is placed upside down, the alloy liquid is stably filled from bottom to top, and the proportion of the area of each section of the pouring system is A Straight bar ：A Cross bar ：A Inner part ＝1：2：2。
Further, the shape of the straight pouring channel is designed into a straight cone with a large upper part and a small lower part, the radius of the section of the lower surface obtained by the section area of the straight pouring channel is 16mm, and the inclination is 2 degrees; and a conical pouring cup is arranged above the sprue, the diameter of the upper surface of the pouring cup is 55mm, the height of the pouring cup is 35mm, the diameter of the lower surface of the pouring cup is consistent with that of the upper surface of the sprue, and the pouring cup is integrally positioned below the upper surface of the sand box.
Further, the shape of the horizontal runner is a high trapezoid with a large upper part and a small lower part, and the length of the upper bottom of the horizontal runner is 30mm, the length of the lower bottom of the horizontal runner is 20mm and the height of the horizontal runner is 16mm are calculated according to the cross sectional area and the empirical rule; the thickness of the trapezoid is 1/2 of the wall thickness of a casting area where the ingate is positioned, and the ingate is arranged at the top of the horizontal pouring channel; arranging a direct-pouring channel pit at the joint; the optimized size of the casting pit is 2 times of the diameter of the straight pouring gate outlet and 2 times of the height of the horizontal pouring gate according to experience; the straight pouring gate pit is provided with an inward inclination of 5 degrees, and the upper surface of the straight pouring gate pit is flush with the horizontal pouring gate, so that molten metal firstly fills the straight pouring gate pit and then enters the horizontal pouring gate pit; the outer surface of the sleeve is provided with a chill, the riser adopts a cylindrical riser, the chill adopts cast iron outer chill, and a casting fillet with R =3mm is arranged at a corner.
Further, the grid test in step 4 utilizes a modification model and a grid repair tool of ProCAST to improve the grid quality, and includes:
firstly, in a Mesh module of ProCAST, checking the integrity of a model and the entity crossing condition, and eliminating a repeated surface by assembling an entity;
then, determining the size of each part of the surface mesh of the model and generating a triangular surface mesh, checking and repairing the quality of the surface mesh, and generating a tetrahedral mesh on the basis of the surface mesh;
finally, respectively carrying out pretreatment and simulation calculation on the grid size test scheme, and randomly selecting the calculation results of the residual stress of two points and the average residual stress of the whole casting as investigation indexes, wherein the difference between the calculation results of the surface grid size of 5mm and the calculation results of the surface grid size of 10mm is not large; the size of the casting grid is set to be 7mm, and the rest parts including the chill, a riser and a sand box are set to be 10mm.
Further, the orthogonal experimental design method in step 5 includes:
and determining the average residual stress as a test index for quantifying the test result. Factors influencing the test result comprise pouring temperature, pouring time and casting mold temperature, each factor is set to three levels, the level values are randomized, and a four-factor three-level orthogonal table L is selected 9 (3 4 ) Wherein, the casting technological parameters are preferably that the casting temperature is 670 ℃, the casting time is 5s, and the casting mold temperature is 20 ℃.
Another object of the present invention is to provide an evolution simulation system of residual stress during casting and machining of engineering components, which applies the method for simulating evolution of residual stress during casting and machining of engineering components, the evolution simulation system of residual stress during casting and machining of engineering components includes:
the casting defect condition prediction module is used for analyzing the original casting scheme structure and material physical property parameters by using a casting numerical simulation technology, simulating a casting processing process by using ProCAST and predicting the defect condition;
the casting process numerical simulation module is used for establishing a model 1 by using CAD software according to a casting drawing: 1, carrying out numerical simulation pretreatment on a model, and analyzing a pouring process, a solidification process and a casting cooling result;
the casting numerical simulation initial scheme design module is used for designing initial scheme process parameters and an initial gating system for carrying out numerical simulation in the casting process and analyzing the distribution of residual stress;
and the process parameter optimization design module is used for improving and optimizing the casting process parameters by combining the orthogonal experiment with the simulation result and summarizing the influence trend of the relevant parameters on the magnitude of the residual stress value.
Another object of the present invention is to provide a computer device, which comprises a memory and a processor, wherein the memory stores a computer program, and the computer program, when executed by the processor, causes the processor to execute the steps of the method for simulating evolution of residual stress during the casting process of engineering components.
Another object of the present invention is to provide a computer-readable storage medium, which stores a computer program, and when the computer program is executed by a processor, the computer program causes the processor to execute the steps of the method for simulating evolution of residual stress in the casting process of an engineering component.
The invention also aims to provide an information data processing terminal which is used for realizing the evolution simulation system of the residual stress in the casting process of the engineering component.
In combination with the technical solutions and the technical problems to be solved, please analyze the advantages and positive effects of the technical solutions to be protected in the present invention from the following aspects:
first, aiming at the technical problems existing in the prior art and the difficulty in solving the problems, the technical problems to be solved by the technical scheme of the present invention are closely combined with results, data and the like in the research and development process, and some creative technical effects are brought after the problems are solved. The specific description is as follows:
according to the method, casting pictures (two-dimensional and three-dimensional) are obtained according to the casting, and then necessary optimization is carried out on the casting according to the casting process requirement to obtain a process picture; designing an initial pouring scheme and pouring parameters on the basis of a process diagram according to casting technology; and (4) carrying out targeted optimization according to the result of the initial scheme, firstly optimizing the pouring scheme, and then optimizing the pouring process if the pouring scheme cannot meet the requirement.
The invention uses a casting numerical simulation technology to simulate the casting processing process, predicts the defect conditions of shrinkage porosity, shrinkage cavity, insufficient pouring and the like, analyzes the residual stress distribution, and combines the simulation result to carry out the improvement and optimization of the pouring scheme. The invention firstly analyzes the structure and material physical parameters of the original casting scheme, and carries out casting simulation by using ProCAST, and finds that the casting formed by using the original casting scheme has the defects of insufficient casting, shrinkage porosity and the like. Aiming at the problems, the invention redesigns the pouring system, optimizes the parameters of the pouring process through orthogonal experimental design, summarizes the influence trend of the relevant parameters on the magnitude of the residual stress value, and verifies the rationality of the optimization scheme through simulation tests.
The method comprises the steps of establishing a related three-dimensional model by using CAD software, and importing the model into PROCAST MASH for grid division; setting material characteristics, pouring conditions and boundary conditions, and carrying out finite element simulation calculation to obtain results such as a temperature field, a stress field, filling conditions and the like; then analyzing the size and distribution of residual stress after the casting is solidified according to the stress field of the casting, and determining a characteristic sensitive part of the engineering component; analyzing the solidification time and sequence of the molten liquid according to the temperature field of the casting, and judging the generation of shrinkage porosity; analyzing the speed and the filling time of the molten liquid pouring according to the filling condition graph, and observing the flowing process of the molten liquid; and regulating and controlling material parameters and process parameters according to the analysis result, and reducing the level of residual stress in the casting process of the part.
The method has the advantages that the evolution rule of the temperature field and the stress field of the typical metal engineering component under the condition of near-service working condition is known; preliminarily mastering a finite element simulation (CAE) analysis method based on a three-dimensional modeling (CAD) and a casting processing process; the evolution simulation of a temperature field and a stress field in the casting process is realized aiming at a certain typical metal engineering part; the characteristic sensitive part of the engineering component is determined by analyzing the simulation result of the stress field in the casting process, and the level of the residual stress in the casting process of the component is reduced by regulating and controlling material parameters and process parameters.
The invention carries out numerical simulation on the casting process of a certain project casting by using ProCAST, discovers the defects of the casting scheme, designs an optimized and improved scheme according to the simulation result, and verifies through the simulation experiment that the concrete results are as follows:
(1) Through carrying out preliminary analysis to casting structure and gating system, discover that the great easy residual stress concentration phenomenon that produces of each partial wall thickness difference of foundry goods, and the not good easy heat festival that produces of some position heat dissipation condition leads to shrinkage cavity defect, and the gating system is not conform to the aluminum alloy pouring characteristics and leads to the molten metal velocity of flow too big. The subsequent simulation experiment results basically accord with the analysis, and the accuracy of the numerical simulation is verified.
(2) The original casting model is preprocessed and simulated and calculated, and the result analysis shows that the metal liquid flow generates turbulence, jetting and splashing phenomena in the casting process and causes large impact on a cavity; in the solidification process, the workpiece is solidified from top to bottom in a reverse order, and the phenomenon of insufficient pouring caused by cold contraction appears above the workpiece; after cooling, the central part of the transverse sleeve of the workpiece has large-area shrinkage porosity, and the residual stress of the edge of the casting is larger.
(3) Aiming at the casting defects, the invention redesigns a pouring system, adopts a bottom pouring type pouring scheme, arranges a straight pouring way pit and a cross pouring way extension section to reduce the flow rate and impact, and arranges a riser and a chill to reduce the volume of a shrinkage cavity. The simulation experiment result shows that the improvement achieves obvious effect, the phenomenon of insufficient pouring disappears, the shrinkage cavity volume is greatly reduced, and the residual stress performance is also improved.
(4) In order to further reduce the residual stress level of the workpiece, the invention adopts orthogonal test design to optimize three main casting parameters of casting temperature, casting time and casting mold temperature, and obtains a better scheme: the casting temperature is 670 ℃, the casting time is 5 seconds, and the casting mold temperature is 20 ℃. The present invention also finds that the pouring time has the greatest effect on the residual stress results, the casting temperature is the second order, and the pouring temperature is the smallest.
Secondly, the technical solutions are taken as a whole or from the perspective of products, and the technical effects and advantages of the technical solutions to be protected by the present invention are specifically described as follows:
the invention can simply and quickly design the casting process under the condition of lacking theoretical knowledge reserves such as relevant casting process or material data and the like; the quality of the casting can be checked without actual tests, relevant defects are optimized, and all-part monitoring of all data of the casting in the machining process can be realized, which is difficult to realize in the traditional method (trial and error method). The invention can assist the casting process design in actual production, reduce the dependence on experience, reduce the design difficulty, reduce the trial-manufacture cost and improve the production efficiency.
According to the invention, simulation tests show that the casting numerical simulation technology has unique advantages and considerable reliability in the aspects of simulating the casting process, exploring residual stress evolution, optimizing casting schemes and the like, and the large-scale application of the technology can make great contribution to the healthy development of the casting industry.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings needed to be used in the embodiments of the present invention will be briefly described below, and it is obvious that the drawings described below are only some embodiments of the present invention, and it is obvious for those skilled in the art that other drawings can be obtained according to the drawings without creative efforts.
FIG. 1 is a flow chart of a method for simulating the evolution of residual stress during the casting process of an engineering component according to an embodiment of the present invention;
FIG. 2 is a casting practice and simulation flow chart provided by an embodiment of the present invention;
FIG. 3 is a block diagram of a casting provided by an embodiment of the present invention;
FIG. 4 is a design drawing of a gating system provided by an embodiment of the present invention;
FIG. 5 is a graph of the result of meshing provided by embodiments of the present invention;
FIG. 6 is a schematic diagram of physical parameters of a material provided by an embodiment of the present invention;
FIG. 7 is a graph of molten metal flow rates provided by an embodiment of the present invention;
FIG. 8 is a cloud view of a casting process provided by an embodiment of the present invention; t =0s for diagram (a), t =1.68s for diagram (b), t =7.78s for diagram (c), t =13.04s for diagram (d), t =17.21s for diagram (e), and t =17.89s for diagram (f);
FIG. 9 is a thermal budget profile provided by an embodiment of the present invention;
FIG. 10 is a cloud of solidification processes provided by an embodiment of the present invention; t =20.18s for graph (a), t =75.88s for graph (b), t =235.88s for graph (c), t =315.88s for graph (d), t =385.88s for graph (e), and t =525.88s for graph (f);
FIG. 11 is a graphical illustration of the cooling results provided by an embodiment of the present invention; the graph (a) is a solid fraction distribution cloud picture, the graph (b) is a shrinkage cavity distribution cloud picture, and the graph (c) is an effective stress distribution cloud picture;
FIG. 12 is a schematic diagram of the improvement of a gating system provided by an embodiment of the present invention;
FIG. 13 is a schematic view of a casting process according to an improvement provided by an embodiment of the present invention; 0s in panel (a), 1.43s in panel (b), 2.07s in panel (c), 3.91s in panel (d), 12.74s in panel (e), and 16.15s in panel (f);
FIG. 14 is a schematic illustration of a modified embodiment of the invention providing a coagulation process; 26.78s for panel (a), 136.66s for panel (b), 176.66s for panel (c), 256.66s for panel (d), 346.66s for panel (e), 496.66s for panel (f);
FIG. 15 is a cloud of clotting times provided by an embodiment of the present invention;
FIG. 16 is a schematic illustration of the cooling results of an improvement provided by an embodiment of the present invention; the graph (a) is a shrinkage cavity distribution cloud graph, and the graph (b) is a residual stress distribution graph;
FIG. 17 is a graph illustrating grid test results provided by an embodiment of the present invention;
FIG. 18 is a graph of the trend of various factors provided by an embodiment of the present invention;
FIG. 19 is a schematic diagram of an optimization result provided by an embodiment of the invention; the graph (a) is a shrinkage cavity distribution graph, and the graph (b) is a residual stress distribution graph.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail with reference to the following embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
Aiming at the problems in the prior art, the invention provides a casting residual stress control optimization method based on computer numerical simulation and application thereof, and the invention is described in detail below with reference to the accompanying drawings.
1. Illustrative embodiments are explained. This section is an explanatory embodiment expanding on the claims so as to fully understand how the present invention is embodied by those skilled in the art.
As shown in fig. 1, the method for simulating evolution of residual stress in a casting process of an engineering component according to an embodiment of the present invention includes the following steps:
s101: simply designing an initial pouring scheme according to a casting drawing, and establishing a model 1 by using CAD software: 1, casting and gating system model;
s102: importing the model into ProCAST software, preprocessing the model, setting simulation parameters according to a processing pouring process, material characteristics and a simulation requirement lamp, and performing simulation operation;
s103: analyzing the simulation operation result, and improving the design of a pouring system by combining casting technology;
s104: carrying out grid independence verification by utilizing a grid test to obtain grid parameters;
s105: re-dividing the finite element grids, and optimizing pouring parameters according to an orthogonal test design method to perform a simulation test to obtain an optimal scheme;
s106: and (5) verifying the optimal scheme.
Further, the initial pouring scheme in S101 includes:
the initial pouring scheme adopts a top pouring mode, molten metal is poured from the upper part of the casting mold and enters the cavity along the pouring cup and the main pouring gate, and the whole cavity is filled from bottom to top;
the casting material is aluminum alloy ZL114A, the liquidus of the alloy is 616 ℃, the solidus of the alloy is 556 ℃, and the density of the alloy is 2730Kg/m at room temperature and 20 DEG C 3 (ii) a The casting temperature is controlled within the range of 10-110 ℃ above the liquidus; in casting practice, the pouring speed is controlled by controlling the pouring time, and the following empirical formula is obtained:
t＝AG n ；
wherein, for the aluminum alloy coefficient A =2.4, N =0.387, G is 2.5 times of the casting quality, and the theoretical pouring time t =5.02s is obtained by bringing relevant data.
Further, the numerical simulation pretreatment in S102 includes model pretreatment and model pretreatment, the model pretreatment establishes a model using CAD software according to a casting drawing, and reasonably modifies and simplifies the casting, the diameter of the horizontal through hole and the vertical blind hole of the neck of the casting is 9mm smaller than the diameter of the minimum casting hole of the sand mold by 20mm, and the two long holes are removed before the casting simulation by post-machining drilling; treating the sand mold as a part of a sand box and as an elastic-plastic mechanical model;
the model preprocessing leads the preprocessed model into a Visual-Mesh module of ProCAST software for grid division;
dividing the grid into models and adding virtual sand boxes, wherein the size of each model is 300 multiplied by 250mm; the surface grid type is triangular, the size is set to be 10mm, and the surface of the model and the surface of the virtual sand mold are subjected to grid division; and after the surface grids are checked to be qualified, converting the surface grids into volume grids, wherein the types of the volume grids are tetrahedral and six-node units.
The gravitational acceleration is taken to be 9.80m/s 2 (ii) a The mold is EN AC-42100AlSi7Mg0.3, the cooling rate is 10k/s, the initial temperature is 700 ℃ and the Sand mold is Resin Bonded Sand, the initial temperature is 20 ℃ and the mechanical model is set as an elastic-plastic model.
Setting the interface heat exchange parameter of the sand mold and the metal as 500W/m 2 K, heat transfer coefficient of sand mold and outside air is 10W/m 2 K, cooling in an air cooling mode to room temperature; according to the alloy performance and the pouring conditions, the pouring time is set to be 10s, the pouring speed is 0.204kg/s, and the pouring temperature is set to be 700 ℃; and in order to obtain a model stress field, the sand mold is restrained, the restraining surface is the outer surface except the pouring gate, and the displacement is set to be 0.
Further, the analysis simulation result in S103 includes:
simplifying the metal liquid into incompressible Newtonian fluid, and obtaining a motion differential equation of the viscous fluid according to Newton's second law:
where ρ is the fluid density, t is the fluid flow time, μ is the fluid kinematic viscosity, g x 、g y 、g z Is the gravity acceleration component of three coordinate axis directions,
the method is characterized in that the method is a Laplace operator, p is unit volume pressure of fluid, and u, v and w are velocity components of the fluid on X, Y and Z axes;
obtaining a fluid pressure formula according to a continuity equation derived from the conservation of mass equation in the volume element:
wherein D is the fluid divergence.
Calculating the coupling flow field and the temperature field of the fluid temperature distribution in the cavity, wherein an energy equation is as follows:
wherein rho and k are the density and thermal conductivity of the material,
is the slope of the enthalpy-temperature curve of the material, corresponding to the specific heat, and u is the fluid velocity.
The temperature of the workpiece is determined by a heat conduction equation in the solidification process, and the distribution is obtained by a heat balance equation:
wherein c is the specific heat of the material respectively.
During the cooling process, the material is simplified into an elastic-plastic model; before the material reaches the yield strength, the stress strain is in a linear relation, the stress keeps constant after reaching the yield strength, and finally the strain is the sum of the elastic strain and the plastic strain:
wherein the content of the first and second substances,
and
is the theoretical stress and strain of the material, E is the elastic modulus, sigma s Is the material yield strength, epsilon e And epsilon s Corresponding strain when the material is yielding and subsequent plastic strain;
calculating to obtain a temperature field, a flow field and a stress field of the material finite element unit, and reproducing the processes of filling, solidifying and cooling of the casting model through a finite element constitutive equation; and (3) performing mathematical simulation calculation, performing casting simulation to obtain the visual results of the temperature, the solid state rate, the shrinkage porosity, the effective stress, the flow field speed, the mold filling pressure, the mold filling time, the maximum stress and the direction stress of each point in the casting machining process, and comparing and analyzing the results by a mathematical method.
Further, the improved gating system design in S103 includes:
the pouring system is optimized by adopting a bottom pouring type pouring scheme, and is provided with a straight pouring gate pit and a cross gate extension section for reducing flow speed and impact; a riser and a chill are arranged for reducing the volume of a shrinkage cavity; the casting is placed upside down, the alloy liquid is stably filled from bottom to top, and the proportion of the area of each section of the pouring system is A Straight bar ：A Cross bar ：A Inner part ＝1：2：2。
Further, the shape of the straight pouring channel is designed into a straight cone with a large upper part and a small lower part, the radius of the section of the lower surface obtained by the section area of the straight pouring channel is 16mm, and the inclination is 2 degrees; and a conical pouring cup is arranged above the sprue, the diameter of the upper surface of the pouring cup is 55mm, the height of the pouring cup is 35mm, the diameter of the lower surface of the pouring cup is consistent with that of the upper surface of the sprue, and the pouring cup is integrally positioned below the upper surface of the sand box.
Further, the shape of the horizontal runner is a high trapezoid with a large upper part and a small lower part, and the length of the upper bottom of the horizontal runner is 30mm, the length of the lower bottom of the horizontal runner is 20mm and the height of the horizontal runner is 16mm are calculated according to the cross sectional area and the empirical rule; the thickness of the trapezoid is 1/2 of the wall thickness of a casting area where the ingate is positioned, and the ingate is arranged at the top of the horizontal pouring channel; arranging a direct pouring gate pit at the joint; the optimized size of the casting pit is that the diameter is 2 times of the diameter of the outlet of the sprue and the height is 2 times of the height of the cross sprue according to experience; the straight pouring pit is provided with an inward inclination of 5 degrees, and the upper surface of the straight pouring pit is flush with the horizontal pouring gate so that molten metal firstly fills the straight pouring pit and then enters the horizontal pouring gate; the outer surface of the sleeve is provided with a chill, the riser adopts a cylindrical riser, the chill adopts cast iron outer chill, and a casting fillet with R =3mm is arranged at a corner.
Further, the mesh test in S104 utilizes a modified model and a mesh repair tool of ProCAST to improve the mesh quality, including:
firstly, in a Mesh module of ProCAST, checking the integrity of a model and the entity crossing condition, and eliminating a repeated surface by assembling an entity;
then, determining the size of each part of the surface mesh of the model and generating a triangular surface mesh, checking and repairing the quality of the surface mesh, and generating a tetrahedral mesh on the basis of the surface mesh;
finally, respectively carrying out pretreatment and simulation calculation on the grid size test scheme, and randomly selecting the calculation results of the residual stress of two points and the average residual stress of the whole casting as investigation indexes, wherein the difference between the calculation results of the surface grid sizes of 5mm and 10mm is not large; the casting grid size was set to 7mm, and the remainder, including the chill, riser and flask, was set to 10mm.
Further, the orthogonal experiment design method in S105 includes:
and determining the average residual stress as a test index for quantifying the test result. The factors influencing the test result comprise pouring temperature, pouring time and casting mold temperature, each factor is set with three levels, the level values are randomized, and a four-factor three-level orthogonal table L is selected 9 (3 4 ) Wherein, the casting technological parameters are preferably that the casting temperature is 670 ℃, the casting time is 5s, and the casting mold temperature is 20 ℃.
The evolution simulation system of the residual stress in the casting process of the engineering component provided by the embodiment of the invention comprises:
the casting defect condition prediction module is used for analyzing the original casting scheme structure and material physical property parameters by using a casting numerical simulation technology, simulating a casting processing process by using ProCAST and predicting the defect condition;
the casting process numerical simulation module is used for establishing a model 1 by using CAD software according to a casting drawing: 1, carrying out numerical simulation pretreatment on a model, and analyzing a pouring process, a solidification process and a casting cooling result;
the casting numerical simulation initial scheme design module is used for designing initial scheme process parameters and an initial gating system for carrying out numerical simulation in the casting process and analyzing the distribution of residual stress;
and the process parameter optimization design module is used for improving and optimizing the casting process parameters by combining the orthogonal experiment with the simulation result and summarizing the influence trend of the relevant parameters on the magnitude of the residual stress value.
2. Application examples. In order to prove the creativity and the technical value of the technical scheme of the invention, the part is the application example of the technical scheme of the claims on specific products or related technologies.
The invention can be applied to the casting process formulation and optimization of small and medium-sized castings such as connecting rod sleeves, engine crankshafts, hubs and the like so as to reduce defects and control residual stress expression.
3. Evidence of the relevant effects of the examples. The embodiment of the invention has some positive effects in the process of research and development or use, and indeed has great advantages compared with the prior art, and the following contents are described by combining data, charts and the like in the test process.
1. Casting numerical simulation theory and initial scheme design
The casting numerical simulation technology is based on the theories of hydrodynamics, engineering mechanics, heat transfer science and the like, takes a numerical method as a tool, takes a casting process as a research object, and aims at eliminating defects and controlling performance. The invention roughly analyzes the casting process of the workpiece and possible defects according to the casting theory knowledge and the original casting scheme, compares the casting process with results obtained by the subsequent casting numerical simulation calculation to verify the reliability of numerical simulation results, and explores the advantages and disadvantages of the two methods.
1.1 casting numerical simulation technical principle
The casting numerical simulation technology aims to accurately calculate the field changes of temperature field, flow field, concentration field, stress field and other stages in the casting forming process so as to obtain reasonable casting forming control parameters to eliminate casting defects and control the casting performance through structure simulation. The essence of the casting numerical simulation technology is that geometrically finite dispersion is carried out on a casting forming system, the change characteristics of a physical field in the casting process are analyzed through numerical calculation under the support of a physical model, and the casting quality is predicted by combining the formation criterion of related casting defects. Common numerical calculation methods include Finite Element Method (FEM), finite Difference Method (FDM), control volume method (VEM), and Boundary Element Method (BEM). In order to simulate the solidification process of a casting on the grain size, researchers have established a cellular automata model from macro to micro, the (CAFE) method, by combining the cellular automata method (CA) with the finite element method. Regardless of the numerical method, the numerical solution generally includes five steps:
(1) Univalent conditions such as geometric conditions, initial conditions, physical conditions and boundary conditions of a given research object;
(2) Discretizing the research area in time and space;
(3) Establishing numerical equations of nodes or units of the inner part and the boundary;
(4) Solving a linear algebraic equation by selecting a proper calculation method;
(5) And programming calculation, namely combining the calculation results of the nodes or the units into a field result.
1.2 application of numerical simulation technique in casting field
Taking gravity sand casting production practice as an example, a foundry carries out workpiece appearance and gating system design such as workpiece casting molds, sand molds, chill and the like according to customer requirements at the early stage, sets casting process parameters according to casting technology and workpiece performance requirements, and prepares raw materials such as gold materials, coatings, molding sand and the like. In the casting process, a metal raw material is smelted into molten alloy through the procedures of charging, melting, slagging off, modulating, refining, modifying and the like in a factory, a casting mold is formed through the procedures of sand mixing, molding, core making, drying, box assembling and the like, then the molten metal is poured into a preset pouring gate, the mold opening and sand cleaning are carried out after the molten metal is cooled to the room temperature, and finally the final product is obtained through the post-treatment procedures of machining, heat treatment, coating and the like. After the working procedure is finished, the microstructure, shape appearance, mechanical property and other aspects of the workpiece need to be detected and evaluated so as to judge whether the workpiece is qualified.
The casting simulation technology utilizes a CAD technology to carry out three-dimensional reconstruction on the appearance and the size of a workpiece, the mechanical, fluid, thermal and other parameters of raw materials carry out mathematical modeling on the raw materials, and simultaneously, the process parameters and the boundary conditions are quantitatively analyzed and reproduced by a mathematical method.
During filling, the metal liquid is generally simplified into incompressible Newtonian fluid, the fluid dynamics research is based on the law of conservation of momentum, and a differential equation of motion of the viscous fluid, namely a Navier-Stokes equation, can be obtained according to the second law of Newton:
where ρ is the fluid density, t is the fluid flow time, μ is the fluid kinematic viscosity, g x 、g y 、g z Is the gravity acceleration component of three coordinate axis directions,
for laplace, p is the pressure per unit volume of the fluid, and u, v, w are the velocity components of the fluid in the X, Y, Z axes.
The fluid pressure equation can be derived from the continuity equation derived from the conservation of mass equation in the volume element:
wherein D is the fluid divergence.
During mold filling, heat transmission not only comprises heat conduction between metal melt and metal in the cavity and the cavity wall, but also comprises heat input brought by flowing metal, so that the fluid temperature distribution in the cavity is calculated to be coupled with a flow field and a temperature field, and an energy equation is as follows:
wherein rho and k are the density and thermal conductivity of the material,
is made of woodThe slope of the enthalpy-temperature curve, corresponding to the specific heat, u is the fluid velocity.
Macroscopically, the temperature of the workpiece is determined by a heat conduction equation in the solidification process, and the distribution of the temperature can be obtained by a heat balance equation:
wherein c is the specific heat of the material respectively.
In the cooling process, the material can be simplified into an elastic-plastic model, namely the stress and the strain of the material are in a linear relation before the material reaches the yield strength, the stress keeps constant after the yield strength is reached, and the final strain is the sum of the elastic strain and the plastic strain:
wherein, the first and the second end of the pipe are connected with each other,
and
is the theoretical stress and strain of the material, E is the elastic modulus, sigma s Is the material yield strength, epsilon e And ε s The corresponding strain at material yield and the subsequent creation of plastic strain.
The temperature field, the flow field and the stress field of the material finite element unit can be calculated through the theorem and the formula, and then the processes of filling, solidifying and cooling the casting model are reproduced through a finite element constitutive equation. Through mathematical simulation calculation, the casting simulation can obtain visual results of temperature, solid state rate, shrinkage porosity, effective stress, flow field speed, mold filling pressure, mold filling time, maximum stress, direction stress and the like of each point at each moment in the casting processing process, and the results can be compared and analyzed through a mathematical method.
The casting practice and simulation flow chart is shown in fig. 2.
1.3 casting Structure and casting introduction
1.3.1 introduction to casting Structure
The total weight of the casting is 2.27KG, the external contour dimension is 160 multiplied by 160mm, the maximum wall thickness is 32mm, and the minimum wall thickness is 5mm. The casting as a whole can be divided into three parts according to the wall thickness: the upper part of the casting is provided with a vertical cylindrical sleeve, and the wall thickness is 6.5mm; casting rounded triangular prism neck with cross-sectional area of about 600mm 2 The upper part of the horizontal blind hole is provided with a horizontal through hole and a vertical blind hole which are intersected internally; the lower part of the casting comprises a cylindrical sleeve and a bottom plate, the wall thickness of the cylindrical sleeve is 12mm, the height of the bottom plate is 20mm, the upper part of the casting is provided with a short cylindrical sleeve which is vertically intersected with a transverse sleeve, and a stepped round hole is formed in the upper part of the casting. The wall thicknesses of three parts of a casting are greatly different, and a thermal joint is easily generated at the intersecting part, so that the phenomena of shrinkage porosity, shrinkage cavity and cold shrinkage are generated, and stress concentration is caused; the casting has more internal holes, needs more sand cores for modeling, has larger size change, and causes mechanical barrier stress and thermal stress due to the interaction of molten metal and the sand cores and different cooling time of different wall thicknesses during solidification and cooling, as shown in fig. 3.
1.3.2 initial recipe Process parameter design
The casting material is an aluminum alloy ZL114A (ZAlSi 7Mg 1A), which is a variety developed by increasing the content of alloy element magnesium on the basis of ZL101A and has excellent casting performance and higher strength and corrosion resistance. The alloy has a liquidus line of 616 deg.C, a solidus line of 556 deg.C, and a density of 2730Kg/m at room temperature (20 deg.C) 3 The main components and contents are shown in table 2. The casting sand mold material is ester-cured alkaline phenolic resin sand which has the advantages of high strength, good collapsibility, easy cleaning, high casting size precision and the like. The specific components are shown in table 3.
TABLE 2 ingredient content table of aluminum alloy ZL114A
TABLE 3 content of ester-cured alkaline phenolic resin sand component
For sand casting, the pouring temperature, pouring speed and mold temperature are the main process parameters, and are directly related to the quality of the casting. The casting temperature has a decisive influence on the mold filling capacity of the liquid metal, the pouring temperature is increased, the overheating heat of the alloy is increased, the fluidity of the molten metal is improved, and the mold filling capacity is improved. However, the casting temperature is too high, the gas absorption amount of molten metal is increased, the oxidation phenomenon is increased, pinholes, shrinkage cavities and inclusions are easily generated at thick walls, and the tendency of deformation and cracking is increased. The casting temperature is too low, the molten metal filling capability is not enough, and the defects of cold shut, flow lines, insufficient casting and the like are easy to occur. In order to obtain ideal casting performance, the casting temperature is controlled within the range of 10-110 ℃ above the liquidus, namely 626-726 ℃.
The pouring speed affects the casting quality primarily by affecting the temperature gradient of the mold filling and solidification. The higher the pouring speed, the better the molten metal filling capability. However, excessive casting speed can cause spraying and splashing phenomena, metal oxidation is increased, gas in a cavity cannot be discharged in time, and the defect of insufficient casting or cold shut can be formed. Lower pouring speed can reduce the temperature gradient of molten metal, reduce the generation of shrinkage cavity and reduce the level of thermal stress. However, the too low pouring speed can cause insufficient metal liquid filling capacity, and the cavity is heated and baked for a long time to cause the cavity to tilt and fall, thereby causing the defects of insufficient pouring, cold shut and slag inclusion. In casting practice, the pouring speed is usually controlled by controlling the pouring time, and the following empirical formula is obtained:
t＝AG n
wherein, for the coefficient A =2.4 of the aluminum alloy, N =0.387, G is 2.5 times of the casting quality, and the theoretical pouring time t =5.02s can be obtained by bringing relevant data.
The mold temperature determines the rate of temperature decrease during the initial stages of solidification and cooling. Higher die temperature can improve the fluidity of molten metal and prevent shrinkage porosity, but longer cooling temperature causes coarse grains and reduced mechanical properties. Lower mold temperatures increase cooling rates and refine the grains, but too rapid cooling may produce cold shut and under-pour, with resultant residual stress and crystallographic segregation.
1.3.3 initial gating System design
The original pouring system adopts a top pouring mode, namely, molten metal is poured from the upper part of a casting mold and enters a cavity along a pouring cup and a main pouring gate, and the whole cavity is filled from bottom to top. The pouring system is simple in design, easy to fill and convenient to mold and clear sand, but the impact force on a sand mold is large, molten metal is easy to splash and oxidize, and the defects of sand holes, air holes, slag inclusion and the like are caused. The pouring cup and the main pouring gate can store liquid metal besides the function of guiding molten metal, supply metal when a casting is cooled, prevent shrinkage porosity and play a role in collecting slag and exhausting gas, and are shown in figure 4.
2. Numerical simulation of casting process
The above results are all based on theoretical analysis of casting structure and pouring scheme, and have no vivid and accurate results, and are more difficult to quantify and provide improvement suggestions, so that numerical simulation of the casting is needed.
The casting numerical simulation process can be divided into three parts of pretreatment, calculation and post-treatment, and the model is subjected to necessary simplified modification treatment before the pretreatment so as to simplify the operation and improve the operation efficiency. The result can be simulated by putting the model for dividing the grid and setting the boundary condition and the thermophysical property parameter into a solver, and the defects of the casting can be found for subsequent scheme improvement by analyzing the pouring, solidifying and cooling processes in the result.
2.1 numerical simulation Pre-treatment
2.1.1 model pretreatment
Establishing 1 by using CAD software according to a casting drawing: 1, modeling, and reasonably modifying and simplifying castings:
(1) The diameter (9 mm) of the transverse through hole and the vertical blind hole of the neck of the casting is smaller than the diameter (20 mm) of the minimum casting hole of the sand mold, the length is long, the casting molding is difficult to perform, and the drilling can be performed through post machining, so that the two long holes are removed before the casting simulation.
(2) The sand molds and sand core materials are similar and the relevant parameters are similar, so the sand molds are not modeled separately, but are used as part of the sand box. The contact problem such as air gaps between the sand mold (sand core) and the molten metal is ignored, and the sand mold (sand core) is treated as an elastic-plastic mechanical model.
(3) The influence of factors such as machining allowance, casting shrinkage rate, drawing slope, casting fillet and the like on the external dimension and shape of the casting is small, and the effect on an accurate simulation result is not large, so that the factors are not considered.
2.1.2 model pretreatment
As shown in fig. 5, the preprocessed model is imported into the Visual-Mesh module of ProCAST software for Mesh partitioning. Firstly, adding a virtual sand box for a model, wherein the size of the virtual sand box is 300 multiplied by 250mm; the surface grid type is triangular, the size is set to be 10mm, the surface of the model and the surface of the virtual sand mold are subjected to grid division, and the total number is 15162; and after the surface grids are qualified, converting the surface grids into body grids, wherein the types of the body grids are tetrahedron and six-node units, and the total number of the body grids is 135383.
The reasonable setting of boundary conditions and material thermophysical parameters is the key to ensure the accuracy of the simulation result. Firstly, the pouring direction and the gravity direction of the two are ensured to be consistent, and the gravity acceleration is 9.80m/s 2 (ii) a The ENAC-42100AlSi7Mg0.3 is selected as the casting mould for endowing the model with materials, the component performance of the casting mould is similar to that of ZL114A, the cooling speed is 10k/s, the initial temperature is the casting temperature, namely 700 ℃, the mechanical model is an elastic-plastic model, and relevant characteristics are shown in figure 6. The Sand mold was Resin Bonded Sand (Resin Sand) and the initial temperature was room temperature (20 ℃ C.), and the mechanical model was set to an elastoplastic model.
Setting the interface heat exchange parameter of the sand mold and the metal as 500W/m 2 K, heat transfer coefficient of sand mold and outside airIs 10W/m 2 K, cooling by air cooling to room temperature. The casting time was set to 10s and the casting speed was 0.204kg/s, depending on the alloy properties and the casting conditions. The casting temperature was set at 700 ℃. In order to obtain a model stress field, the sand mold needs to be restrained, the restraining surface is the outer surface except the gate, and the displacement is set to be 0.
2.2 analysis of simulation results
2.2.1 pouring Process analysis
As can be seen from the cloud figure 8 of the flow field in the mold filling process, when t =0s, the pouring process starts, the molten metal enters the cavity through the pouring cup and the sprue, and the neck of the cavity serves as an ingate. When t =1.68s, the front end of the molten metal reaches the bottom of the cavity, the speed of the lower half part in the liquid flow exceeds 0.895m/s, and the highest speed in the pouring process occurs at the front end of the liquid flow: 1.492m/s. Research of John Campbell et al at Bigminghan university in UK shows that the filling critical speed of the molten aluminum alloy is about 0.5m/s, and the casting speed of the model is far higher than the critical speed. The internal pressure of the molten metal is greater than the surface pressure of the molten metal, the liquid metal breaks through a metal oxide film on the surface of the molten metal, and the oxide film is rolled into the liquid metal after being broken, so that oxidized slag inclusion is generated. t =1.68s, the molten metal first fills the periphery of the disc, and it can be seen from fig. 7 (a) that two oppositely directed molten metal streams join in the middle, causing convection, which may entrain oxides and sand on the surface of the molten metal stream into the stream, and at the same time entrap a large amount of air. t =13.04s, the metal bath fills the pan, fills both sides of the lower part of the casting, and prepares to fill the middle part of the casting in both directions. FIG. 7 (b) shows the flow direction at this time, and since the liquid level in the mold is higher than the liquid level in the ingate, the flow directions near the ingate are disordered and mutually collided. And when t =17.21s, the molten metal begins to fill the upper half part of the casting, and the molten metal gradually fills the upper cavity from bottom to top. However, as can be seen from fig. 7 (c), when the metal begins to enter the die cavity, there is a flow of metal which is at a much higher level and velocity than the surrounding liquid, and the jet phenomenon is obviously generated, which causes the air below the flow to be wrapped into the metal pool, and the air holes are generated. t =17.89s, the pouring process is completed, and the mold cavity is completely filled with the molten metal.
In the pouring process, as the cross section size of the ingate is larger than that of the sprue, the ingate is not completely filled with the metal liquid flow, the flow speed is overlarge, conditions are created for generating turbulence, and the point is proved by the fact that the cross section size of the metal liquid flow changes constantly. In addition, under the blocking action of the casting mold and the filled molten metal, the molten metal is lowered to a static state from a high speed in a short distance, and the impact force on the casting mold and the filled molten metal generated by the lowering of the molten metal is large, so that the molten metal can splash and wash the sand mold, and air holes and sand holes can be generated.
2.2.2 analysis of the solidification Process
As can be seen from the solidification process solid cloud 10, t =20.18s, the solidification process starts at the outer edge of the base pan at the ear, where the temperature drops rapidly below the solidus due to the earliest mold filling, away from the cast body and with a thinner wall thickness. t =75.88s, the upper part of the casting begins to solidify, the highest solid rate of the outer side of the upper part of the casting can be seen, the solidification is carried out completely, and the original positions of the ingate and the casting head are not completely solidified. However, the aluminum alloy has a large shrinkage rate, a large cold shrinkage phenomenon is formed at the position of the original ingate, and although the casting head has feeding capacity, the molten metal stored in the casting head is not enough to completely feed, so that a large cavity is formed. t =235.88s, the upper part of the casting is solidified, the casting is solidified from outside to inside on the casting base plate, and the solidification process of the lower part of the casting is slow, especially in the middle sleeve and below the middle sleeve. t =315.88s, the disc at the lower part of the casting is completely solidified, the middle of the upper part and the outer wall of the sleeve is not solidified, and the solidification progress is slow because the wall thickness of the inner part of the sleeve is large and is far away from the heat dissipation surface. t =385.88s, the exterior of the casting is substantially solidified, but there is still a partially unsolidified area in the center of the lower portion. t =525.88s, the casting is totally solidified, and the time is 505.7s.
From the above process, it can be seen that the casting does not follow the principle of sequential solidification, with the upper part of the casting solidifying first, followed by the bottom disc region, with the slowest sleeve region. As can be seen from the thermal-link distribution cloud picture 9, thermal links are formed at the central part of the lower part of the casting and the outer side of the upper part of the assembly, and the parts are solidified later than the surrounding parts, so that molten metal flows into holes formed by shrinkage of the solidified parts, and finally, insufficient molten metal is not fed, and shrinkage cavities with larger areas are formed. The difference between the solidification time of the periphery and the solidification time of the interior of the bottom disk is long, and the part which is firstly solidified is influenced by thermal stress and can accumulate large residual stress.
2.2.3 analysis of Cooling results of castings
From fig. 11 (a) and (b), the casting has a complete appearance for the most part, and no chill phenomenon occurs. But the upper part of the casting is not fully complemented to form a large cave. Because the front edge of a liquid-solid interface of the aluminum alloy is cooled to the crystallization temperature in the pasty solidification process to form a metal framework, the molten metal flows and is fed between dendrites under the action of solidification shrinkage, the solid phase rate is increased along with the solidification, a feeding channel is gradually narrowed until the feeding channel disappears, an isolated liquid phase area is formed at the last solidification part, and the cooling shrinkage of the molten metal can not be supplemented to form shrinkage porosity. The interior of the sleeve is solidified later, large-area shrinkage porosity is finally generated, partial shrinkage porosity extends to the outer surface, the maximum shrinkage porosity is 91.25%, the total volume is about 52.77%, and the volume fraction of defects is 4.7%.
As can be seen from FIG. 11 (c), the residual stress at the later solidified portions of the cast sleeve and the like is small, and below 53.7MPa, the residual stress at the early solidified portions of the cast upper outer side and bottom disc and the like are poor, and both of them are at the level of 85.9MPa and above. This is because in the process of solidification and cooling, the upper outer side and the bottom disc of the casting have large modulus and high cooling speed due to the thin wall thickness, and the shrinkage of the parts is larger than that of other parts at the beginning stage of cooling, so that the casting is subjected to tensile stress; when cooling is continued until the casting is in the elastic stage, the earlier solidification part shrinks completely, the later solidification part shrinks greatly, the mutual restriction of the earlier solidification part and the later solidification part causes the earlier solidification part to be subjected to larger compression stress, and the trend is more obvious at the places with larger wall thickness difference. In addition, the junction of the upper part of the casting and the neck of the casting and the junction of the neck of the casting and the lower part of the casting are right-angled corners and are not provided with casting fillets, so that the stress sectional area is suddenly changed, and larger residual stress is accumulated.
3. Process optimization
Through the experimental analysis, the original pouring scheme has important defects, and needs to be optimized from the aspects of improving the pouring scheme and optimizing process parameters. The improved pouring scheme is in accordance with the casting characteristics of the aluminum alloy, reduces the phenomena of impact, turbulence and the like, and ensures the complete appearance of the casting; optimization of process parameters should be aimed at reducing residual stress levels. Furthermore, since the results need to be analyzed quantitatively, it is necessary to eliminate the interference of grid quality on the simulation results.
3.1 improved pouring scheme design
The pouring position refers to the state and position of the casting in the mold during pouring, the pouring position is determined by focusing on controlling the solidification sequence of the casting, so that the casting solidified sequentially from bottom to top can eliminate casting, shrinkage porosity and air holes, a riser is conveniently arranged, and a compact casting is ensured to be obtained. According to the solidification time cloud chart, the casting under the original pouring scheme is solidified from top to bottom in reverse order, so that the upper molten metal is difficult to smoothly and effectively feed, and finally, the lower part of the casting is seriously shrunk and shrunk with holes. Therefore, in the improved pouring scheme, the casting is placed upside down, so that when the molten metal below is solidified, the molten metal above can flow into the hole to be fed, the molten metal in the final riser is finally solidified, and the shrinkage cavity is transferred into the upper riser and the pouring system. In addition, the size and the weight of the workpiece are medium, the reverse workpiece does not occupy too many working hours, and the difficulty in molding and unpacking is avoided. For aluminum alloy, because the aluminum alloy is easy to oxidize and roll up gas, the aluminum alloy is stably filled, so that the pouring scheme is improved to be a bottom pouring type, and the molten alloy is stably filled from bottom to top. The most important advantage of the method is that the liquid level is stable in the antigravity mold filling process, obvious turbulence and air entrainment can not occur, the air can be effectively exhausted, and the method is the most common pouring method in the prior cast aluminum alloy.
The relation of each cross section area of the pouring system of the improved pouring scheme conforms to the open pouring system standard, namely the cross section area of an inner pouring channel is maximum, the cross pouring channel is next, and a straight pouring channel is minimum. The adoption of the pouring system ensures that the flow velocity is small when the liquid metal enters the cavityThe filling is stable, the scouring force to the sand mold is small, and the aluminum alloy pouring requirement is met. The proportion of each section adopted by the scheme is A Straight bar ：A Horizontal bar ：A Inner part =1:2:2. the cross-sectional area of each component of the aluminum alloy gating system can be calculated, but is generally determined by an empirical method. It is first necessary to determine the dimensions of the flow-blocking area, i.e. the dimensions of the sprue cross-section. The weight of the casting is 2.313Kg, and A can be determined from Table 4 Straight bar ＝2cm 2 A can be deduced from the proportional relationship Inner part ＝4cm 2 ，A Horizontal bar ＝4cm 2 。
In order to ensure that molten metal contacts with the wall of the sprue in the falling process at the initial stage of a pouring stage so as to reduce an air gap and control the generation of liquid flow turbulence and entrainment, the sprue is designed into a straight cone shape with a large upper part and a small lower part, the radius of the section of the lower surface of the sprue is 16mm according to the section area of the sprue, the inclination is 2 degrees, and the height is slightly higher than the height of a riser so as to provide a sufficient mold filling pressure head and ensure that the profile and the edge angle of a casting are clear. The conical pouring cup is arranged above the sprue to facilitate pouring, the diameter of the upper surface of the pouring cup is 55mm, the height of the pouring cup is 35mm, the diameter of the lower surface of the pouring cup is consistent with that of the upper surface of the sprue, and the pouring cup is integrally positioned below the upper surface of the sand box. The cross gate is in a high trapezoid shape with a large upper part and a small lower part so as to improve the friction force on liquid flow to slow down the liquid flow speed, impurities are deposited at the bottom of the cross gate, the length of the upper bottom of the cross gate is 30mm, the length of the lower bottom of the cross gate is 20mm, the height of the cross gate is 16mm can be calculated through the cross sectional area and the empirical rule, and no fillet is arranged for simplifying the operation. The ingate is also configured as a trapezoid and is similar in size to the ingate. Meanwhile, in order to ensure that a hot spot is not generated near the inner runner opening, the thickness of the trapezoid is about 1/2 of the wall thickness of the casting area where the inner runner opening is located. The ingate is arranged at the top of the cross gate to prevent impurities from entering the ingate and not being left at the top of the cross gate when the cross gate is not filled in the pouring process.
Because the molten metal is constantly accelerated under the action of gravity in the sprue, the molten metal can constantly impact the joint of the cross gate to cause sand inclusion and turbulence, and a reduced flow section problem can be formed at the corner of the molten metal, so that the cross gate cannot be completely filled, is easy to wrap air and is transmitted into a cavity, and therefore, the sprue pit is necessary to be arranged at the joint. The optimum size of the casting nest is generally 2 times the diameter of the sprue outlet, i.e. 32mm in diameter, and 2 times the height of the runner, i.e. 32mm, according to general experience. The straight pouring pit is provided with an inward inclination of 5 degrees, the upper surface of the straight pouring pit is flush with the horizontal runner, and molten metal is filled in the straight pouring pit and then enters the horizontal runner, so that the effects of reducing impact and flow speed are achieved. Similarly, the tail end of the transverse pouring channel is provided with an extension section to reduce the speed of molten metal entering the cavity and prevent the molten metal from splashing to cause air entrainment and turbulence.
Because the casting base plate and the sleeve have larger shrinkage cavity and shrinkage porosity tendency and are far away from the ingate, and are difficult to be fed, a riser is required to be arranged for temporarily storing molten metal during pouring, and liquid metal is provided for supplementing holes during solidification and cooling of relevant parts. Because ZL114A aluminum alloy is solidified in a pasty state, has high heat conduction speed and short riser feeding distance, and is difficult to repair shrinkage porosity near the inner surface of the sleeve, cold iron needs to be arranged on the outer surface of the sleeve. The cold iron can increase the temperature gradient near the riser, and obviously improve the feeding distance of the riser; in addition, the chilling block can reduce chilling environment provided by the chilling block during pouring, refine crystal grains and improve mechanical properties. In an optimized scheme, the riser adopts a cylindrical riser to reduce the modulus, and the junction is appropriately contracted so as to facilitate later-stage cutting; the cold iron is cast iron external cold iron, so that the later separation is convenient, the manufacturing cost is low, and the heat conducting property is good. In order to reduce the stress concentration at the junction between the upper and lower sleeves and the neck, cast fillets with R =3mm were formed at the corners, and the final design result is shown in fig. 12.
TABLE 4 aluminum alloy sprue size table
3.2 improved protocol simulation results analysis
3.2.1 improved protocol casting Process analysis
As can be seen from the pouring process fig. 13, the pouring process starts from 0s and the molten metal starts to enter the sprue. And at 1.43s, the molten metal fills the straight pouring gate pit and then enters the cross gate. The molten metal is fully filled in the middle and lower parts of the sprue, the liquid flow speed and direction at the same height are approximately the same, the turbulence phenomenon is avoided, the deceleration effect of the sprue pit is obvious, the speed is reduced to 0.2m/s from about 1m/s of the inlet, and the outlet flow speed is stabilized at about 0.47m/s and is less than the critical speed. And 2.07s, the molten metal is filled in the transverse pouring channel, returns back and enters the cavity, and the inlet speed of the ingate is 0.238m/s, so that the molten metal has almost no impact on the cavity. And 3.91s, the lower cavity is filled with the molten metal and enters the neck. At 12.74s, the molten metal begins to fill the chassis, the molten metal height of each part of the chassis is almost consistent, and the phenomenon of mold filling asynchronism is avoided. And (5) when 16.15s, the upper riser is filled with the molten metal, and the pouring is finished. In the pouring process, the molten metal enters the cavity after sequentially passing through the pouring cup, the sprue pit, the cross gate extension section and the ingate, the speed and the direction of the molten metal flow in the same section are kept consistent, the filling speed is below 0.5m/s critical speed, the cavity is stably and slowly filled from bottom to top on the whole, and the phenomena of splashing, gas wrapping, turbulence and the like do not exist.
3.2.2 modified protocols coagulation Process analysis
At 26.78s available from fig. 14, the setting process starts from the outer edge of the bottom plate. And 136.66s, the outer edge of the casting begins to solidify, and the sleeve below the casting mould is basically solidified, so that the preset solidification sequence is met. At 176.66s, the solidification is completed below the mold, the casting neck is reduced in solid content from top to bottom, and the outer edge of the sleeve begins to solidify. And at 256.66s, the solidification is basically finished at other parts except the wall thickness parts such as the center of the bottom plate, the joint of the neck and the sleeve and the like. 346.66S, most of the casting is solidified, the casting is solidified at the latest at the top, and the solid content of a riser and the joint part of the riser and the casting is about 60 percent, which indicates that the riser still has feeding capacity. And 496.6s, the casting is solidified. As can be seen from the solidification time cloud chart 15, the whole solidification process basically follows the principle of sequential solidification from bottom to top, the feeder head is finally solidified and maintains feeding capacity, and finally the obtained casting is complete in appearance and clear in outline.
3.3.3 improved protocol Cooling results analysis
As can be seen from FIG. 16, after cooling, the outer surface had no significant pits and the inner part was completely formed. Through shrinkage porosityThe cloth cloud picture shows that no area with 100% shrinkage porosity exists, namely, no holes caused by shrinkage porosity exist in the whole casting, and the volume with the shrinkage porosity of more than 10% is 26.79cm 2 Compared with the original scheme, the method has the advantages that the shrinkage porosity is reduced by 50%, the maximum shrinkage porosity is distributed in the central position of the chassis, the using performance of a workpiece cannot be affected, and the workpiece can be machined and cut in the later stage. As can be seen from the residual stress distribution cloud chart, the residual stress at the central part of the sleeve is at a lower level, and the residual stress is larger due to early solidification below the edge and the neck of the chassis, but is reduced in value compared with the original scheme.
3.4 grid size test
The number and quality of the grids directly relate to the efficiency and precision of the simulation, the number of the grids determines the number of nodes and units, and the quality of the grids determines whether the operation result is converged. Improvement of grid quality can be achieved by modifying the model and the grid repair tool of ProCAST, but the number of grids is usually determined by the experience of the user, and it is difficult to ensure the accuracy of the result. In a Mesh module of ProCAST, firstly, the integrity of a model and the entity crossing condition need to be checked, and an entity is assembled to eliminate a repeated surface; then determining the size of each part of the surface mesh of the model, generating a triangular surface mesh, and checking and repairing the quality of the surface mesh; a tetrahedral volume mesh is then generated on the basis of the face mesh, and subsequent calculations will be performed based on the volume mesh and its nodes. From the above process, the size of the face mesh determines the number and quality of cells and nodes. The smaller the grid size is, the more the number of the elements and the nodes is, the more accurate the fitting result of the element method of the finite element is, and meanwhile, the calculation amount is greatly increased, so that more calculation and storage resources are used, and the efficiency is difficult to improve. The larger the grid size is, the smaller the number of cells and nodes is, and the calculation speed can be obviously improved, but the more precise parts are difficult to perform accurate discretization processing, the accuracy of technical results is difficult to guarantee, and especially, the quantitative analysis is adversely affected. Therefore, the balance calculation efficiency and the calculation precision are the premise of the model simulation technology and quantitative analysis, the traditional empirical method is difficult to give scientific and accurate explanation, and the experiment sets the following grid size gradient, takes the average stress of the casting obtained by improving the pouring scheme as an index, and determines the proper unit size and number for subsequent analysis. The face mesh size and the number of generated volume meshes are shown in table 5.
The above test schemes are subjected to pretreatment and simulation calculation, and the calculation results of the residual stress at two points and the average residual stress of the whole casting are arbitrarily selected as investigation indexes, so that fig. 17 can be obtained. As can be seen from the figure, as the cell size decreases, the calculation results of the residual stress have a tendency of decreasing first and then being stable, wherein the calculation results of the face grid sizes of 5mm and 10mm are not very different, indicating that the calculation results gradually converge. In order to improve the calculation efficiency and reduce the storage space occupied by result files while ensuring the accuracy of calculation results, the size of a casting grid is set to be 7mm, and the rest parts of a chill, a riser, a sand box and the like are set to be 10mm.
TABLE 5 comparison table of surface grid size and volume grid number
3.5 orthogonal Experimental design parameter optimization
Through improvement of a pouring system, defects such as casting surface holes disappear, shrinkage porosity and shrinkage cavity phenomena are greatly reduced, stress performance is improved, and influence of three main pouring process parameters on residual stress and the optimal parameter combination are explored through orthogonal experimental design in subsequent optimization. The orthogonal experimental design is a method for scientifically arranging and analyzing multi-factor tests by using an orthogonal table, and test points can be uniformly distributed by the method so as to have the same reliability as that of the comprehensive test. A better scheme is deduced through statistical analysis of a few experimental schemes, and the obtained better scheme is not included in the few schemes, so that more information besides experimental results can be obtained, such as the importance degree of the influence of the experimental results, the influence trend of various factors on the experimental results, and the like.
Firstly, the average residual stress is determined as a test index and used for quantifying the test result. Influence ofThe test results were based on casting temperature (a), casting time (B) and mold temperature (C), three levels were set for each factor, and the level values were randomized to avoid systematic errors due to artifacts, and the resulting factor level table is shown in table 6. Selecting a four-factor three-level orthogonal table L 9 (3 4 ) The blank column is placed in the second column and the experimental protocol can be designed according to the orthogonal table as shown in table 7.
TABLE 6 factor level comparison Table
Table 7 test protocol combination table
3.6 analysis of test results
The data obtained by simulating the experimental protocol using PROCAST software are shown in table 8.
Using an intuitive analysis method to obtain the following table, wherein Ki represents the sum of corresponding test results when the horizontal number of a row of people is i; ki = Ki/3, representing the arithmetic mean of the test results obtained for factors at level i on either column; r is called range, and is the difference between the maximum value and the minimum value of the experimental results on any row. The size of Ki and Ki indicates the superiority and inferiority of the test results at different levels of this factor. For the factor of A (casting temperature), k2< k1< k3, i.e. level 2, i.e. 670 ℃; for the factor of B (pouring time), k1< k3< k2, level 1 should be taken, i.e. 5s; for the C (mold temperature) factor, k1< k3< k3, should be taken to be level 1, i.e., 20 degrees Celsius. In summary, the preferred embodiment is A2B1C1, i.e. the casting process parameters are that the casting temperature is 670 ℃, the casting time is 5s, and the casting mold temperature is 20 ℃.
The difference of the range difference indicates that the level of each factor has different influence on the experimental result, and the larger the range difference is, the larger the change of the value of the column in the experimental range can cause the larger change of the value of the experimental index, that is, the larger the influence on the experimental result. The casting method is obtained by using S3, S2 and S1, the influence of the casting speed on the residual stress of the casting is the largest, the casting mold temperature is the second, and the influence of the casting temperature is the smallest. To more visually observe the results, a trend graph 18 can be made from the test results.
TABLE 8 test results
TABLE 9 visual analytic method data processing
3.7 optimization results
The results obtained by performing simulation tests according to the optimal scheme obtained by orthogonal test design are shown in fig. 19, the appearance is complete, and the outline is clear; the volume of shrinkage cavity is reduced to 19.77cm 3 Compared with the original scheme, the shrinkage porosity is reduced by 62.5 percent, the maximum shrinkage porosity is reduced to 55.18 percent, and the shrinkage porosity is distributed on the surface, so that the influence on the performance of a workpiece is small; the average residual stress is reduced to 63.18MPa, and the optimization effect is obvious.
4. Results
The invention carries out numerical simulation on the casting process of a certain project casting by using ProCAST, discovers the defects of the casting scheme, designs an optimized and improved scheme according to the simulation result, and verifies through the simulation experiment that the concrete results are as follows:
(1) Through carrying out preliminary analysis to casting structure and gating system, discover that the great easy residual stress concentration phenomenon that produces of each partial wall thickness difference of foundry goods, and the not good easy heat festival that produces of some position heat dissipation condition leads to shrinkage cavity defect, and the gating system is not conform to the aluminum alloy pouring characteristics and leads to the molten metal velocity of flow too big. The subsequent simulation experiment results basically accord with the analysis, and the accuracy of numerical simulation is verified.
(2) The original casting model is preprocessed and simulated and calculated, and the result analysis shows that the metal liquid flow generates turbulence, jetting and splashing phenomena in the casting process and causes large impact on a cavity; in the solidification process, the workpiece is solidified from top to bottom in reverse order, and the phenomenon of insufficient pouring caused by cold contraction occurs above the workpiece; after cooling, large-area shrinkage porosity appears in the central part of the transverse sleeve of the workpiece, and the residual stress of the edge of the casting is large.
(3) Aiming at the casting defects, the invention redesigns a pouring system, adopts a bottom pouring type pouring scheme, arranges a straight pouring gate pit and a cross pouring gate extension section to reduce the flow rate and impact, and arranges a riser and a chill to reduce the volume of a shrinkage cavity. Simulation experiment results show that the improvement achieves obvious effects, the phenomenon of insufficient watering disappears, the shrinkage cavity volume is greatly reduced, and the residual stress performance is also improved.
(4) In order to further reduce the residual stress level of the workpiece, the invention adopts orthogonal test design to optimize three main casting parameters of casting temperature, casting time and casting mold temperature, and obtains a better scheme: the casting temperature is 670 ℃, the casting time is 5 seconds, and the casting mold temperature is 20 ℃. The present invention also finds that the pouring time has the greatest effect on the residual stress results, the casting temperature is the second order, and the pouring temperature is the smallest.
It should be noted that the embodiments of the present invention can be realized by hardware, software, or a combination of software and hardware. The hardware portions may be implemented using dedicated logic; the software portions may be stored in a memory and executed by a suitable instruction execution system, such as a microprocessor or specially designed hardware. It will be appreciated by those skilled in the art that the apparatus and methods described above may be implemented using computer executable instructions and/or embodied in processor control code, for example such code provided on a carrier medium such as a diskette, CD-or DVD-ROM, a programmable memory such as read-only memory (firmware) or a data carrier such as an optical or electronic signal carrier. The apparatus and its modules of the present invention may be implemented by hardware circuits such as very large scale integrated circuits or gate arrays, semiconductors such as logic chips, transistors, or programmable hardware devices such as field programmable gate arrays, programmable logic devices, etc., or by software executed by various types of processors, or by a combination of hardware circuits and software, e.g., firmware.
The above description is only for the purpose of illustrating the present invention and the appended claims are not to be construed as limiting the scope of the invention, which is intended to cover all modifications, equivalents and improvements that are within the spirit and scope of the invention as defined by the appended claims.
Claims (10)
1. A method for simulating evolution of residual stress in the casting process of an engineering component is characterized by comprising the following steps:
establishing a related three-dimensional model by using CAD software, obtaining parameters of a temperature field, a stress field and a filling condition by using a finite element method, analyzing a simulation result, predicting possible casting defects, optimizing and improving a pouring process to eliminate the casting defects and obtain better residual stress expression of a casting;
performing preliminary analysis on a casting structure and a pouring system, and performing pretreatment and simulation calculation on an original pouring model; and (4) redesigning a pouring system, and determining the optimal pouring temperature, pouring time and casting mold temperature by adopting orthogonal test design.
2. The method for simulating evolution of residual stress during the casting process of an engineering component according to claim 1, wherein the method for simulating evolution of residual stress during the casting process of an engineering component further comprises:
establishing a related three-dimensional model by using CAD software, and importing the model into PROCAST MASH for grid division; setting material characteristics, pouring conditions and boundary conditions, and carrying out finite element simulation calculation to obtain a temperature field, a stress field and a filling condition; analyzing the size and distribution of residual stress after the casting is solidified according to the stress field of the casting, and determining a characteristic sensitive part of the engineering component;
analyzing the solidification time and sequence of the molten liquid according to the temperature field of the casting, and judging the generation of shrinkage porosity; analyzing the speed and the filling time of the molten liquid during pouring according to the filling condition graph, and observing the flowing process of the molten liquid; regulating and controlling material parameters and process parameters according to the analysis result, and analyzing the level of residual stress in the casting process of the part;
and the orthogonal test design is adopted to optimize the pouring temperature, the pouring time and the casting mold temperature.
3. The method for simulating evolution of residual stress during the casting process of engineering components according to claim 1, wherein the method for simulating evolution of residual stress during the casting process of engineering components comprises the following steps:
step 1: simply designing an initial pouring scheme according to a casting drawing, and establishing a model 1 by using CAD software: 1, casting and gating system model;
step 2: importing the model into ProCAST software, preprocessing the model, setting simulation parameters according to a processing pouring process, material characteristics and a simulation requirement lamp, and performing simulation operation;
and step 3: analyzing the simulation operation result, and improving the design of a pouring system by combining casting technology;
and 4, step 4: carrying out grid independence verification by utilizing a grid test to obtain grid parameters;
and 5: re-dividing the finite element grids, and optimizing pouring parameters according to an orthogonal test design method to perform a simulation test to obtain an optimal scheme;
step 6: and (5) verifying the optimal scheme.
4. The method for simulating evolution of residual stress during the casting process of engineering components according to claim 3, wherein the initial pouring scheme in step 1 comprises:
the initial pouring scheme adopts a top pouring mode, molten metal is poured from the upper part of the casting mold and enters the cavity along the pouring cup and the main pouring gate, and the whole cavity is filled from bottom to top;
the casting material is aluminum alloy ZL114A, the liquidus of the alloy is 616 ℃, the solidus of the alloy is 556 ℃, and the density of the alloy is 2730Kg/m at room temperature and 20 DEG C 3 (ii) a The casting temperature is controlled within the range of 10-110 ℃ above the liquidus line; in the practice of casting, in the casting,controlling the pouring speed by controlling the pouring time, and obtaining the following empirical formula:
t＝AG n ；
wherein, for the aluminum alloy coefficient A =2.4, N =0.387, G is 2.5 times of the casting quality, and the theoretical pouring time t =5.02s is obtained by bringing relevant data.
5. The evolution simulation method of the residual stress in the casting process of the engineering component according to claim 3, wherein the numerical simulation pretreatment in the step 2 comprises model pretreatment and model pretreatment, the model pretreatment is to establish a model by using CAD software according to a drawing of a casting, reasonable modification and simplification are performed on the casting, the diameter of a transverse through hole and a vertical blind hole at the neck part of the casting is 9mm smaller than the diameter of a minimum casting hole of a sand mold, and the two long holes are removed before the casting simulation through post-machining drilling; treating the sand mold as a part of a sand box and as an elastic-plastic mechanical model;
the model preprocessing leads the preprocessed model into a Visual-Mesh module of ProCAST software for grid division;
the grid is divided into a model and a virtual sand box is added, and the size of the model is 300 multiplied by 250mm; the surface grid type is triangular, the size is set to be 10mm, and the surface of the model and the surface of the virtual sand mold are subjected to grid division; after the surface grids are checked to be qualified, converting the surface grids into body grids, wherein the types of the body grids are tetrahedral and six-node units;
the gravitational acceleration is taken to be 9.80m/s 2 (ii) a Selecting ENAC-42100AlSi7Mg0.3 as a casting mold, cooling at the speed of 10k/s, setting the initial temperature to be the casting temperature of 700 ℃, adopting Resin Bonded Sand as a Sand mold, setting the initial temperature to be the room temperature of 20 ℃, and setting a mechanical model to be an elastic-plastic model;
setting the interface heat exchange parameter of the sand mold and the metal as 500W/m 2 K, heat transfer coefficient of sand mold and outside air is 10W/m 2 K, cooling in an air cooling mode to room temperature; according to the alloy performance and the pouring conditions, the pouring time is set to be 10s, the pouring speed is 0.204kg/s, and the pouring temperature is set to be 700 ℃; in order to obtain a model stress field, the sand mold is constrained, and the constraint surfaces are all except the pouring gateThe displacement is set to 0.
6. The method for simulating evolution of residual stress during casting and machining of engineering components according to claim 3, wherein the analyzing and simulating result in the step 3 comprises:
the metal liquid is simplified into incompressible Newtonian fluid, and a motion differential equation of the viscous fluid is obtained according to Newton's second law:
where ρ is the fluid density, t is the fluid flow time, μ is the fluid kinematic viscosity, g x 、g y 、g z Is the gravity acceleration component of three coordinate axis directions,
the method is characterized in that the method is a Laplace operator, p is the unit volume pressure of fluid, and u, ν and w are the velocity components of the fluid in X, Y and Z axes;
obtaining a fluid pressure formula according to a continuity equation derived from the conservation of mass equation in the volume element:
wherein D is the fluid divergence;
calculating the coupling flow field and the temperature field of the fluid temperature distribution in the cavity, wherein an energy equation is as follows:
wherein rho and k are the density and thermal conductivity of the material,
the slope of the enthalpy-temperature curve of the material is equivalent to specific heat, and u is the fluid velocity;
the temperature of the workpiece is determined by a heat conduction equation in the solidification process, and the distribution is obtained by a heat balance equation:
wherein c is the specific heat of the material respectively;
during the cooling process, the material is simplified into an elastic-plastic model; before the material reaches the yield strength, the stress strain is in a linear relation, the stress keeps constant after reaching the yield strength, and finally the strain is the sum of the elastic strain and the plastic strain:
wherein the content of the first and second substances,
and
is the theoretical stress and strain of the material, E is the elastic modulus, sigma s Is the material yield strength, ε e And epsilon s Corresponding strain when the material yields and subsequent generation of shaping strain;
calculating to obtain a temperature field, a flow field and a stress field of the material finite element unit, and reproducing the processes of filling, solidifying and cooling of the casting model through a finite element constitutive equation; and (3) performing mathematical simulation calculation, performing casting simulation to obtain the visual results of the temperature, the solid state rate, the shrinkage porosity, the effective stress, the flow field speed, the mold filling pressure, the mold filling time, the maximum stress and the direction stress of each point in the casting machining process, and comparing and analyzing the results by a mathematical method.
7. The method for simulating evolution of residual stress during the casting process of engineering components according to claim 3, wherein the improved gating system design in step 3 comprises:
the pouring system is optimized by adopting a bottom pouring type pouring scheme, and a straight pouring gate pit and a cross gate extending section are arranged for reducing flow speed and impact; a dead head and a chill are arranged for reducing the volume of a shrinkage cavity; the casting is placed upside down, the alloy liquid is stably filled from bottom to top, and the proportion of the area of each section of the pouring system is A Straight bar ：A Horizontal bar ：A Inner part ＝1：2：2。
8. The evolution simulation method of the residual stress during the casting process of the engineering component according to claim 7, wherein the shape of the sprue is designed to be a straight cone with a large top and a small bottom, the section radius of the lower surface obtained from the section area of the sprue is 16mm, and the inclination is 2 °; a conical pouring cup is arranged above the sprue, the diameter of the upper surface of the pouring cup is 55mm, the height of the pouring cup is 35mm, the diameter of the lower surface of the pouring cup is consistent with that of the upper surface of the sprue, and the pouring cup is integrally positioned below the upper surface of the sand box;
the shape of the horizontal runner is a high trapezoid with a large upper part and a small lower part, and the length of the upper bottom of the horizontal runner is 30mm, the length of the lower bottom of the horizontal runner is 20mm and the height of the horizontal runner is 16mm are calculated according to the cross sectional area and the empirical rule; the thickness of the trapezoid is 1/2 of the wall thickness of a casting area where the ingate is positioned, and the ingate is arranged at the top of the horizontal pouring channel; arranging a direct-pouring channel pit at the joint; the optimized size of the casting pit is 2 times of the diameter of the straight pouring gate outlet and 2 times of the height of the horizontal pouring gate according to experience; the straight pouring pit is provided with an inward inclination of 5 degrees, and the upper surface of the straight pouring pit is flush with the horizontal pouring gate so that molten metal firstly fills the straight pouring pit and then enters the horizontal pouring gate; the outer surface of the sleeve is provided with a chill, the riser adopts a cylindrical riser, the chill adopts cast iron outer chill, and a casting fillet with R =3mm is arranged at a corner.
9. The method for simulating evolution of residual stress during the casting process of engineering components according to claim 3, wherein the step 4 of grid testing utilizes a modified model and a grid repair tool of ProCAST to improve the quality of the grid, comprising:
firstly, in a Mesh module of ProCAST, checking the integrity of a model and the entity crossing condition, and eliminating a repeated surface by assembling an entity;
then, determining the size of each part of the surface mesh of the model, generating a triangular surface mesh, checking and repairing the quality of the surface mesh, and generating a tetrahedral mesh on the basis of the surface mesh;
finally, respectively carrying out pretreatment and simulation calculation on the grid size test scheme, and randomly selecting the calculation results of the residual stress of two points and the average residual stress of the whole casting as investigation indexes, wherein the difference between the calculation results of the surface grid sizes of 5mm and 10mm is not large; the size of the casting grid is set to be 7mm, and the rest parts including the chill, a riser and a sand box are set to be 10mm;
the orthogonal experiment design method in the step 5 comprises the following steps:
determining the average residual stress as a test index for quantifying the test result; the factors influencing the test result comprise pouring temperature, pouring time and casting mold temperature, each factor is set with three levels, the level values are randomized, and a four-factor three-level orthogonal table L is selected 9 (3 4 ) Wherein the casting process parameter is preferably the casting temperature of 670 ℃ and the casting is carried outThe time was 5s and the casting temperature was 20 ℃.
10. An evolution simulation system of residual stress during the casting process of an engineering component, which applies the evolution simulation method of residual stress during the casting process of an engineering component according to any one of claims 1 to 9, wherein the evolution simulation system of residual stress during the casting process of an engineering component comprises:
the casting defect condition prediction module is used for analyzing the original casting scheme structure and material physical property parameters by applying a casting numerical simulation technology, simulating a casting processing process by using ProCAST and predicting the defect condition;
the casting process numerical simulation module is used for establishing a model 1 according to a casting drawing by using CAD software: 1, carrying out numerical simulation pretreatment on a model, and analyzing a pouring process, a solidification process and a casting cooling result;
the casting numerical simulation initial scheme design module is used for designing initial scheme process parameters and an initial gating system for carrying out numerical simulation in the casting process and analyzing the distribution of residual stress;
and the process parameter optimization design module is used for improving and optimizing the casting process parameters by combining the orthogonal experiment with the simulation result and summarizing the influence trend of the relevant parameters on the magnitude of the residual stress value.
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