CN115740493A - Finite element simulation method for selective electron beam melting temperature field and stress field - Google Patents

Abstract

The invention discloses a finite element simulation method of a melting temperature field and a stress field of an electron beam selection area, which develops a method for simulating the temperature field and the stress field of a forming process by a DFLUX subprogram for realizing a user-defined heat source, a UMATHT subprogram for realizing the switching of physical property parameters of materials from powder to entity, and a UEPACTIVATIONVOL subprogram for distinguishing powder and entity through an established heat-structure indirect coupling finite element model aiming at the melting process of a metal material electron beam selection area.

Description

A Finite Element Simulation Method of Electron Beam Selective Melting Temperature Field and Stress Field

technical field

The invention relates to the field of finite element simulation of metal additive manufacturing process, in particular to a finite element simulation method of electron beam selective melting temperature field and stress field.

Background technique

The rapid development of additive manufacturing technology in recent years is based on computer-aided design, and it is formed layer by layer according to the digital model file of the part, which provides an opportunity for the preparation of complex parts. After decades of exploration, additive manufacturing technologies suitable for various occasions have emerged. Taking metal powder bed additive manufacturing as an example, the representative processes include: selective laser sintering (Selective Laser Sintering, SLS), laser Selective Laser Melting (SLM) and Electron Beam Melting (Selective Electron Beam Melting, SEBM), etc. Due to its advantages such as high energy density, high energy absorption rate, fast scanning speed, low residual stress, and powder reusability, SEBM quickly stands out from many technologies and has become one of the most potential metal additive manufacturing technologies at present. However, there are many problems in the forming process, such as high instantaneous temperature, rapid temperature change, and difficult data collection, which make it difficult to study the dynamic temperature and stress distribution in the process through experiments, and the final precision and surface quality of the produced parts There is an inseparable relationship between performance and metal additive manufacturing process parameters including heat source temperature, heat source movement speed and material properties. In the case of high cost and low efficiency of experimental research, numerical simulation has become a more suitable and effective means to study the temperature distribution and stress evolution in the SEBM forming process. It can directly and effectively obtain the transient temperature distribution and stress during the forming process. The distribution provides guidance for revealing the inner mechanism of SEBM forming and process test research, and is of great significance in the monitoring and detection of temperature and stress evolution in the metal additive manufacturing process and the improvement and optimization of part quality.

However, simulating the above process with the help of finite element software mainly faces three technical difficulties: layer-by-layer powder spreading, heat source movement, and material physical parameter switching. The unit activation method, or life-death unit technology, is commonly used to spread the powder layer by layer; the heat source movement is usually realized by writing the user subroutine DFLUX; the two are not difficult to implement in the finite element software, so a large number of studies can find related reports . It is more difficult to switch the parameters of material properties. Physical parameters are one of the important and difficult to ignore factors in simulating the temperature and stress distribution in the metal additive manufacturing process, including thermal physical parameters such as thermal conductivity, specific heat capacity, density, etc. and mechanical parameters such as thermal expansion coefficient and yield strength , Poisson's ratio and modulus of elasticity, etc., generally change nonlinearly with the change of temperature. In addition, the physical parameters will also be affected by the state of the material. For example, the internal heat transfer mechanism, stiffness and strength of powder materials and solid materials are very different. These parameters will directly affect the calculation results. However, the finite element method currently used by researchers to simulate the SEBM forming process is limited to the physical parameters of solid materials that are constant or temperature-varying, and rarely considers the influence of powder, which will not accurately reflect the single-layer multi-pass and electron beam forming. The thermal effect of multi-layer and multi-pass also affects the prediction of residual stress in the later stage.

Contents of the invention

Aiming at the problems existing in the prior art, the present invention provides a finite element simulation method of electron beam selective melting temperature field and stress field, realizes the prediction of temperature field and stress field, and can reduce the cost of exploring its process parameters through trial and error. capital cost and time cost.

The present invention is achieved through the following technical solutions:

A finite element simulation method for electron beam selective melting temperature field and stress field, comprising the following steps:

Step 1. Establish a three-dimensional heat transfer calculation model for metal electron beam selective melting, establish a UMATHT subroutine associated with the three-dimensional heat transfer calculation model for switching powder to solid material thermophysical parameters, and DFLUX for custom heat sources subroutine;

Step 2. Establish a three-dimensional stress calculation model for metal electron beam selective melting, and establish a UEPACTIVATIONVOL subroutine associated with the three-dimensional stress calculation model for distinguishing mechanical parameters of powder and solid regions;

Step 3. When calculating the temperature field, the UMATHT subroutine obtains the thermophysical parameters of the material of each grid node in turn, obtains the temperature of each network node according to the thermophysical parameters, and obtains the temperature of the three-dimensional heat transfer calculation model according to the temperature of all network nodes distributed;

Step 4. When calculating the stress field, the temperature distribution is applied as a load to the stress calculation of the three-dimensional heat transfer calculation model. The UEPACTIVATIONVOL subroutine obtains the temperature of each element node in turn, and assigns the corresponding mechanical parameters according to the material state properties. According to The stress of the unit is obtained from the mechanical parameters, and the stress distribution of the entire 3D heat transfer calculation model is obtained according to the stress of each unit.

Preferably, the establishment method of the three-dimensional heat transfer calculation model in step 1 is as follows:

Create substrates and individual powder layers and assign material properties;

Assemble the substrate and each powder layer, set the initial heat transfer conditions, boundary conditions and set the solver;

The powder layer and the substrate are divided into meshes and the element types are set to obtain a three-dimensional heat transfer calculation model.

Preferably, the assembly method of the substrate and each powder layer is as follows:

The "Tie" contact is used to constrain the substrate and the powder layer, and between adjacent powder layers, and the powder layer is activated layer by layer through "Model Change" to simulate the powder spreading process.

Preferably, the initial, boundary conditions and solver types are as follows:

Electron beam selection is melted in the vacuum cavity, the initial temperature of the 3D heat transfer calculation model is the temperature in the cavity, only radiation heat dissipation exists on the top and side surfaces of the 3D heat transfer calculation model, and a constant temperature boundary condition is applied to the bottom surface of the substrate; the solver is set to "Heat Transfer".

Preferably, the method and unit type of the grid division are set as follows:

The powder layer and the top of the substrate adopt a dense mesh, the bottom of the substrate adopts a loose mesh, and a transition mesh is divided between the two; the unit type is set to DC3D8.

Preferably, the establishment method of described UMATHT subroutine is as follows:

Set MAT_ID as the state variable of the material, the node whose MAT_ID is 0 is assigned the powder material attribute, the node whose MAT_ID is 1 is assigned the solid material attribute, and the MAT_ID value of all powder areas in the initial state is 0; the UMATHT subroutine starts at the analysis step Read the MAT_ID value of each node and judge whether the node meets the solidification condition, update the MAT_ID value of the node that meets the condition and assign it to the corresponding thermal physical property parameters of the solid material.

Preferably, the establishment method of the stress calculation model in step 2 is as follows:

Create substrates and individual powder layers and assign material properties;

Assembling the substrate and individual powder layers, setting up boundary conditions and solvers;

The powder layer and the substrate are meshed separately, and the element type is set to obtain the stress calculation model.

Preferably, the boundary conditions and solver types are as follows:

Before electron beam loading, the substrate and the powder layer are freely placed in the vacuum chamber, the initial stress and strain are equal to 0, the degrees of freedom of all nodes in the bottom surface of the substrate along the forming direction are set to zero, and any two adjacent sides constrain their x or y direction translational degrees of freedom for ; the solver selects "Static General".

Preferably, the mesh division of the powder layer and the substrate is consistent with the temperature field calculation model, and the unit type is changed to C3D8R.

Preferably, the establishment method of the UEPACTIVATIONVOL subroutine is as follows:

Apply the temperature field as a load to the corresponding stress model, judge whether each grid node meets the solidification condition in turn according to the UEPACTIVATIONVOL subroutine, activate the solid elements that meet the solidification condition and assign corresponding solid material mechanical parameters, so as to calculate Its stress value, according to the stress of each element, gets the stress distribution of the whole model.

Compared with the prior art, the present invention has the following beneficial technical effects:

The invention provides a finite element simulation method for the temperature field and stress field of electron beam selective melting, through the establishment of a heat-structure indirect coupling finite element model for the electron beam selective melting process of metal materials, the development and realization of user-defined heat source DFLUX Subroutines, UMATHT subroutines for switching materials from powder to solid physical parameters, and UEPACTIVATIONVOL subroutines for distinguishing the difference in mechanical properties between powder and solid materials. The method of simulating the temperature field and stress field of the forming process is used to predict different structural parts The evolution law of temperature field and molten pool, as well as the distribution of dynamic stress and residual stress in the forming process, in order to determine the optimal forming parameters, lay the foundation for avoiding defects such as deformation, warping and cracking of formed parts in the actual processing process, and reduce the risk of passing experiments. Trial and error to explore the capital cost and time cost of its process parameters.

Description of drawings

Fig. 1 is the process of layer-by-layer powder coating of electron beam selective melting in the present invention.

Fig. 2 is the physical model and the double ellipsoidal heat source model of the electron beam selective melting process adopted in the present invention.

Fig. 3 is a schematic diagram of the deformation caused by different temperature gradients in the present invention.

Fig. 4 is a schematic diagram of the calculation model and local grid division used in simulating the electron beam selective melting process of pure tungsten in the present invention.

Fig. 5 is a flow chart of UMATHT program design in the present invention.

Fig. 6 is the temperature nephogram at different times obtained by using the calculation model in Fig. 4 for single-layer multi-pass forming of pure tungsten electron beam selective melting in the present invention. NT11 means temperature in K.

Fig. 7 is a flow chart of UEPACTIVATIONVOL program design in the present invention.

Fig. 8 is the single-layer multi-pass forming of pure tungsten electron beam selective melting according to the present invention along the centerline of the upper surface of the calculation model in Fig. 4 along the x direction (scanning line direction), along the y direction (perpendicular to the scanning line direction) centerline and along the z direction Direction (forming direction) stress component distribution of the centerline. S represents the Von Mises stress, S11, S22 and S33 represent the stress components along the x, y and z directions, in MPa.

Detailed ways

The present invention will be further described in detail below in conjunction with the accompanying drawings, which are explanations rather than limitations of the present invention.

Referring to Figures 1-7, a finite element simulation method for the temperature field and stress field of metal electron beam selective melting includes the following steps:

Step 1. Establish a three-dimensional heat transfer calculation model for metal electron beam selective melting, which specifically includes the following processes:

Create the substrate and each powder layer. The substrate gives the material properties of the solid metal, and the powder layer gives the material properties of the metal powder. The thermal physical parameters of the material include density, thermal conductivity, specific heat, etc. that vary with temperature.

Assemble the substrate and each powder layer, and constrain the substrate and powder layer, and between adjacent powder layers through "Tie" contact, so that the powder layer can be activated layer by layer through "Model Change" to simulate the powder spreading process, as shown in Figure 1 It is a schematic diagram of simulating the electron beam selective melting process;

Set the solver, initial conditions, and boundary conditions. The temperature field solver selects "Heat Transfer" and selects the type of transient heat transfer; electron beam selective melting is carried out in a vacuum cavity, the initial temperature of the heat transfer calculation model is the temperature in the cavity, and the top and side surfaces of the powder are only There is radiation heat dissipation, and a constant temperature boundary condition is applied to the bottom surface of the substrate;

The powder layer and the substrate are meshed and element set separately. The powder layer and the top of the substrate adopt a denser mesh to analyze the melting situation, and the rest adopt a loose mesh, and a transition mesh is divided between the powder layer and the substrate; the element type of the heat transfer calculation process is set to DC3D8;

Step 2, constructing the DFLUX subroutine and the UMATHT subroutine associated with the three-dimensional heat transfer calculation model;

The DFLUX subroutine is used to simulate the moving heat source of the electron beam selective melting process. The built-in functions FLUX(1) and FLUX(2) define the functional relationship between heat flow and time and space, and can realize custom heat source radius, power, speed, and scanning time. , scanning strategy, etc., see Figure 2, the heat source used is a double ellipsoid heat source model.

In addition, Figure 2 also shows the heat transfer model when the electron beam scans the forming layer. The model follows the three-dimensional unsteady heat transfer law internally, and the satisfied heat conduction equation is:

λ is the thermal conductivity (W/(m·K)), ρ is the density (kg/m ³ ), c _p is the specific heat (J/(kg·K)), all of which change with the change of temperature, and q represents the change from The heat flux absorbed by the mobile heat source, in J/m ³ . In order to simulate the temperature of the forming chamber, a constant temperature boundary condition is set on the base; in order to simulate the preheating of the powder layer, a predefined field whose temperature is the preheating temperature is defined when each powder layer is activated; the other surfaces of the forming layer except the bottom surface are set is the radiation cooling boundary condition.

In addition, the temperature range of the SEBM forming process is large, and the metal powder undergoes the transition from powder state to liquid state and solid state under the action of electron beams. The thermal physical parameters of materials vary greatly under different temperatures and states. Therefore, in the calculation, it is necessary to consider not only the change of the thermal physical properties of the material with the temperature, but also the change of the thermal physical properties of the material with the state. The thermophysical parameters that change with temperature can be realized in the finite element software, and the thermophysical parameters that change with the material state must be realized by the UMATHT subroutine based on the secondary development of the finite element software ABAQUS. For built-in functions such as heat, you only need to assign each material parameter to the corresponding function and call the function. Referring to Figure 5, the establishment method of the UMATHT subroutine is as follows:

Define MAT_ID as the state variable of the material, the grid node whose MAT_ID is 0 is assigned the powder material attribute, the node whose MAT_ID is 1 is assigned the solid material attribute, the MAT_ID value of all powder regions in the initial state is 0, and when the temperature field is calculated, The UMATHT subroutine reads the MAT_ID value of each grid node and judges whether the grid node meets the solidification condition (T≤T _m &dT/dt<0), updates the MAT_ID value of the node that meets the condition and assigns it the corresponding Solid state material properties. Once endowed with solid material properties, when the temperature continues to drop to room temperature, the thermophysical parameters of the material will only change according to the solid material and will not change back to the powder state parameters, so as to realize the one-way transformation of the material from powder state to liquid state and then to solid state . According to the material state attributes, the corresponding thermophysical parameters are assigned, and then the temperature of the grid node is obtained, and the temperature distribution of the entire 3D heat transfer model is obtained according to the temperature of each grid node.

Step 3. Establish a stress calculation model for metal electron beam selective melting, specifically including the following process:

Create the substrate and each powder layer, the substrate endows the material properties of the solid metal, the powder layer endows the material properties of the metal powder, and the mechanical parameters of the material include Poisson's ratio, Young's modulus, thermal expansion coefficient and yield stress that vary with temperature;

The connection method between the substrate and the powder layer and between adjacent powder layers remains unchanged, and the "Tie" connection is still used;

Sets the solver for stress calculations and the initial and boundary conditions. Set the stress solver to "StaticGeneral"; since the substrate and the powder layer are freely placed in the vacuum chamber before electron beam loading, the initial stress and strain are considered to be equal to 0, and the degrees of freedom of all nodes in the bottom surface of the substrate along the forming direction are set to zero , any two adjacent sides constrain their translational degrees of freedom in the x or y direction;

Mesh the stress calculation model and set the element type. The mesh division is consistent with the temperature calculation process, and the element type is changed to C3D8R;

Step 4, construct the UEPACTIVATIONVOL subroutine associated with the stress calculation model;

Specifically, before the powder reaches the melting point, its Young's modulus, thermal expansion coefficient and yield stress are different from solid tungsten. Although it also changes with temperature, its influence is almost negligible compared with solid tungsten. From preheating When the temperature is about to solidify, the mechanical properties are almost zero, and the influence on deformation can be ignored. Therefore, the mechanical parameters of tungsten powder can be regarded as zero values to participate in the calculation, but the strength and stiffness of solid tungsten are very large, and the calculation process cannot be ignored. Therefore, the design The UEPACTIVATIONVOL subroutine shown in Figure 7 is used to distinguish the mechanical parameters of the material on the melted area and the unmelted area on the powder layer.

In the initial stage, the entire forming layer is not activated. The UEPACTIVATIONVOL subroutine judges in turn whether the temperature at the node is lower than the melting point of the material and whether the temperature change rate is less than zero (that is, whether the solidification condition is satisfied). If a certain element node meets the above conditions, the unit is fully activated, and is given solid material properties. Similarly, the units that do not meet the solidification conditions are not activated and are in an unconstrained free state. The corresponding mechanical parameters are assigned according to the material state properties, so as to calculate the stress value, and the stress distribution of the whole model is obtained according to the stress of each unit.

Referring to Fig. 1, the first technical difficulty in simulating the powder bed electron beam selective melting process is the problem of layer-by-layer powder spreading. Fig. 1 shows the basic idea of the present invention to solve this problem. A calculation model including the substrate and powder layer is established in advance, and Set up the rig and constraints. Before forming, all powder layers are not activated; after forming, each layer of powder layer is activated layer by layer according to the set powder spreading time. During the powder spreading interval, the electron beam scans the powder layer in a selected area and undergoes proper cooling. Solidification, alternating cycles, forms the entire part.

Refer to Figure 3, which shows the deformation of the forming layer affected by the temperature during the electron beam selective melting process. The electron beam reaches the surface of the sample, and the powder layer absorbs heat and the temperature rises suddenly and expands. Due to the continuity of the material, the bottom material must be opposite to the top. When the yield strength of the material is reached, plastic deformation occurs at the top, and the hot top layer and even the entire specimen will bend away from the electron beam; after the electron beam leaves, the top begins to cool and shrink, and is also affected by the bottom material. , the top is pulled, causing the specimen to bend toward the electron beam.

Figure 4 shows the calculation model for electron beam selective melting of pure tungsten in this example. The model includes a forming layer composed of a pure tungsten substrate and a single layer of pure tungsten powder. The size of the substrate is 6.0mm×6.0mm×2.0 mm, the size of the powder bed is 6.0mm×6.0mm×0.05mm, and the frame of the powder bed is the electron beam scanning area, and its size is 4.2mm×4.2mm×0.05mm. The scanning line pitch is 0.05 mm, so the number of scanning passes on the shaping layer is 21. Considering the two aspects of calculation accuracy and efficiency comprehensively, the model is meshed with density transition. The unit size of the powder layer is set to 0.15mm×0.15mm×0.025mm, the grid size of the solid tungsten substrate is set to 0.9mm×0.15mm×0.4mm, and the final grid model contains 15200 units. Sequential coupling is used to solve the temperature field and stress of the forming process. When calculating the temperature field, all grids use DC3D8 grid type. When calculating the stress field, all grids use C3D8 eight-node hexahedral grid type. Two process nets The grid division remains unchanged.

Figure 6 shows the temperature distribution at different moments in the forming process calculated by the model in Figure 4, (a) is the temperature cloud map when scanning to the midpoint of the first track; (b) is the temperature when scanning to the center point of the forming layer Cloud image; (c) is the temperature cloud image when scanning to the midpoint of the last track, due to the preheating of the front scanning channel, the temperature peak gradually increases as the scanning continues; (d) is the electron beam cooling to a certain level after leaving The temperature cloud map of the moment; (e) is the temperature cloud map of the entire forming layer cooled to the preheating temperature.

Figure 8 shows the distribution curves of residual stress components along the centerline of the x-direction (scanning line direction), along the centerline of the y-direction (perpendicular to the scanning line direction) and along the centerline of the z-direction (forming direction) of the upper surface of the model in Figure 4. The stress component along the x-direction path is almost equal to the stress component peak value along the y-direction path.

The present invention has the following beneficial technical effects:

First, "Model Change" is used to activate the powder layer layer by layer, and at the same time, the radiation heat dissipation of the activated part and the preheating of the powder bed are simulated by applying a predefined field, which restores the metal electron beam selective melting process more realistically.

Secondly, the thermophysical parameters and mechanical parameters of the material directly affect the calculation results, and the accurate material parameters have an important impact on the simulation results. In the present invention, by developing the user subroutine UMATHT subroutine and UEPACTIVATIONVOL subroutine, the influence of material parameters under different temperatures and states on temperature and stress is considered in the calculation of temperature field and stress field, and the accuracy of simulation is improved.

In addition, in the process of electron beam selective melting, the influence of forming parameters on the results is very important. By controlling the heat source parameters through the DFLUX subroutine, it is convenient to explore the influence of different process parameters on the thermomechanical coupling behavior of metal electron beam selective melting. For example, research on electron beam radius, scanning rate, heat source power, scanning distance, absorption rate, interlayer phase angle, scanning line length, overlapping rate, interlayer cooling time, etc.

Finally, compared with the experiment, the present invention can not only obtain the residual stress and deformation after forming, but also extract the temperature change, melting situation and stress distribution at each moment during the forming process, which provides a great advantage for the study of its thermal-mechanical behavior. convenience.

The above content is only to illustrate the technical ideas of the present invention, and cannot limit the protection scope of the present invention. Any changes made on the basis of the technical solutions according to the technical ideas proposed in the present invention shall fall within the scope of the claims of the present invention. within the scope of protection.

Claims (10)

1. A finite element simulation method for a melting temperature field and a stress field of an electron beam selection area is characterized by comprising the following steps:

step 1, establishing a three-dimensional heat transfer calculation model for selective melting of metal electron beams, establishing a UMATHT subprogram which is associated with the three-dimensional heat transfer calculation model and is used for realizing the switching of thermal physical parameters from powder to solid materials, and establishing a DFLUX subprogram which is used for self-defining a heat source;

step 2, establishing a three-dimensional stress calculation model for selective melting of the metal electron beam, and establishing a UEPACTIVATIONVOL subprogram which is associated with the three-dimensional stress calculation model and is used for distinguishing powder and material mechanical parameters of a solid area;

step 3, when calculating a temperature field, sequentially acquiring the thermophysical property parameters of each grid node material by the UMATHT subprogram, acquiring the temperature of each network node according to the thermophysical property parameters, and acquiring the temperature distribution of the three-dimensional heat transfer calculation model according to the temperatures of all the network nodes;

and 4, when stress field calculation is carried out, applying temperature distribution as load to stress calculation of the three-dimensional heat transfer calculation model, sequentially acquiring the temperature of each unit node by the UEPACTIVATIONVOL subprogram, assigning corresponding mechanical parameters according to the material state attribute, obtaining the stress of the unit according to the mechanical parameters, and obtaining the stress distribution of the whole three-dimensional heat transfer calculation model according to the stress of each unit.

2. A finite element simulation method of electron beam selective melting temperature field and stress field according to claim 1, wherein the three-dimensional heat transfer calculation model in step 1 is established by the following method:

creating a substrate and respective powder layers and imparting material properties;

assembling the substrate and each powder layer, setting heat transfer initial conditions, boundary conditions and setting a solver;

and respectively carrying out grid division on the powder layer and the substrate and setting unit types to obtain a three-dimensional heat transfer calculation model.

3. The finite element simulation method of electron beam selective melting temperature field and stress field of claim 2, the method for assembling the substrate and each powder layer is characterized by comprising the following steps:

the substrate and the powder layers and the adjacent powder layers are restrained by Tie contact, and the powder layers are activated layer by layer through Model Change to simulate the powder paving process.

4. A finite element simulation method of electron beam selective melting temperature and stress fields according to claim 2, wherein the initial and boundary conditions and solver types are as follows:

the electron beam selection area is melted in the vacuum cavity, the initial temperature of the three-dimensional heat transfer calculation model is the temperature in the cavity, only radiation heat dissipation exists on the top surface and the side surface of the three-dimensional heat transfer calculation model, and constant temperature boundary conditions are applied to the bottom surface of the substrate; the solver was set to "Heat Transfer".

5. A finite element simulation method of electron beam selective melting temperature field and stress field according to claim 2, wherein the gridding method and element types are set as follows:

dense grids are adopted at the top of the powder layer and the top of the substrate, loose grids are adopted at the bottom of the substrate, and transition grids are divided between the dense grids and the loose grids; the cell type is set to DC3D8.

6. A finite element modeling method for selective electron beam melting temperature and stress fields as claimed in claim 1, wherein the umat subroutine is established as follows:

setting MAT _ ID as a state variable of a material, wherein a node with MAT _ ID of 0 is endowed with a powder material attribute, a node with MAT _ ID of 1 is endowed with a solid material attribute, and MAT _ ID values of all powder areas are 0 in an initial state; and the UMATHT subroutine reads the MAT _ ID value of each node when the analysis step is started, judges whether the node meets the solidification condition or not, updates the MAT _ ID value of the node meeting the condition and gives the MAT _ ID value to a corresponding solid material thermophysical property parameter.

7. A finite element simulation method of electron beam selective melting temperature field and stress field according to claim 1, wherein the stress calculation model in step 2 is established by the following method:

creating a substrate and respective powder layers and imparting material properties;

assembling the substrate and each powder layer, and setting a boundary condition and a solver;

and respectively carrying out grid division on the powder layer and the substrate, and setting unit types to obtain a stress calculation model.

8. A finite element simulation method of melting temperature field and stress field of electron beam selection area established according to claim 7, wherein the boundary conditions and solver type are as follows:

the substrate and the powder layer are freely placed in a vacuum cavity before the electron beam is loaded, the initial stress and the strain are both equal to 0, the degree of freedom of all nodes in the bottom surface of the substrate along the forming direction is set to be zero, and the translational degree of freedom of any two adjacent sides in the x or y direction is restricted; the solver selects "Static General".

9. A finite element simulation method of electron beam selective melting temperature field and stress field established according to claim 7, characterized in that the grid division of the powder layer and the substrate is kept consistent with the temperature field calculation model, and the cell type is changed to C3D8R.

10. A finite element simulation method of selective electron beam melting temperature and stress fields according to claim 1, wherein the UEPACTIVATIONVOL subroutine is established as follows:

and applying a temperature field as a load to a corresponding stress model, sequentially judging whether each grid node meets the solidification condition or not according to a UEPACTIVATIONVOL subprogram, activating the entity units meeting the solidification condition, applying corresponding mechanical parameters to the solid materials, calculating the stress values of the entity units, and obtaining the stress distribution of the whole model according to the stress of each unit.

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