CN117259670A - Accurate forming method for gradient mixing printing magnesium alloy castings by freezing casting flame retardant - Google Patents

Accurate forming method for gradient mixing printing magnesium alloy castings by freezing casting flame retardant Download PDF

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Publication number
CN117259670A
CN117259670A CN202311251222.6A CN202311251222A CN117259670A CN 117259670 A CN117259670 A CN 117259670A CN 202311251222 A CN202311251222 A CN 202311251222A CN 117259670 A CN117259670 A CN 117259670A
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sand
flame retardant
casting
layer
printing
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杨浩秦
单忠德
戴宇峰
梁校
黄健
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Nanjing University of Aeronautics and Astronautics
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Nanjing University of Aeronautics and Astronautics
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Priority to CN202311251222.6A priority Critical patent/CN117259670A/en
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22C—FOUNDRY MOULDING
    • B22C9/00—Moulds or cores; Moulding processes
    • B22C9/02—Sand moulds or like moulds for shaped castings
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22C—FOUNDRY MOULDING
    • B22C1/00—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds
    • B22C1/02—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds characterised by additives for special purposes, e.g. indicators, breakdown additives
    • B22C1/04—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds characterised by additives for special purposes, e.g. indicators, breakdown additives for protection of the casting, e.g. against decarbonisation
    • B22C1/06—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds characterised by additives for special purposes, e.g. indicators, breakdown additives for protection of the casting, e.g. against decarbonisation for casting extremely oxidisable metals
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22C—FOUNDRY MOULDING
    • B22C1/00—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds
    • B22C1/16—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds characterised by the use of binding agents; Mixtures of binding agents
    • B22C1/18—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds characterised by the use of binding agents; Mixtures of binding agents of inorganic agents
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22C—FOUNDRY MOULDING
    • B22C9/00—Moulds or cores; Moulding processes
    • B22C9/12—Treating moulds or cores, e.g. drying, hardening
    • B22C9/126—Hardening by freezing
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D21/00—Casting non-ferrous metals or metallic compounds so far as their metallurgical properties are of importance for the casting procedure; Selection of compositions therefor
    • B22D21/02—Casting exceedingly oxidisable non-ferrous metals, e.g. in inert atmosphere
    • B22D21/04—Casting aluminium or magnesium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B33—ADDITIVE MANUFACTURING TECHNOLOGY
    • B33Y—ADDITIVE MANUFACTURING, i.e. MANUFACTURING OF THREE-DIMENSIONAL [3-D] OBJECTS BY ADDITIVE DEPOSITION, ADDITIVE AGGLOMERATION OR ADDITIVE LAYERING, e.g. BY 3-D PRINTING, STEREOLITHOGRAPHY OR SELECTIVE LASER SINTERING
    • B33Y10/00—Processes of additive manufacturing
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B33—ADDITIVE MANUFACTURING TECHNOLOGY
    • B33Y—ADDITIVE MANUFACTURING, i.e. MANUFACTURING OF THREE-DIMENSIONAL [3-D] OBJECTS BY ADDITIVE DEPOSITION, ADDITIVE AGGLOMERATION OR ADDITIVE LAYERING, e.g. BY 3-D PRINTING, STEREOLITHOGRAPHY OR SELECTIVE LASER SINTERING
    • B33Y40/00—Auxiliary operations or equipment, e.g. for material handling
    • B33Y40/20—Post-treatment, e.g. curing, coating or polishing
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B33—ADDITIVE MANUFACTURING TECHNOLOGY
    • B33Y—ADDITIVE MANUFACTURING, i.e. MANUFACTURING OF THREE-DIMENSIONAL [3-D] OBJECTS BY ADDITIVE DEPOSITION, ADDITIVE AGGLOMERATION OR ADDITIVE LAYERING, e.g. BY 3-D PRINTING, STEREOLITHOGRAPHY OR SELECTIVE LASER SINTERING
    • B33Y70/00—Materials specially adapted for additive manufacturing
    • B33Y70/10—Composites of different types of material, e.g. mixtures of ceramics and polymers or mixtures of metals and biomaterials

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  • Engineering & Computer Science (AREA)
  • Chemical & Material Sciences (AREA)
  • Mechanical Engineering (AREA)
  • Materials Engineering (AREA)
  • Manufacturing & Machinery (AREA)
  • Ceramic Engineering (AREA)
  • Civil Engineering (AREA)
  • Composite Materials (AREA)
  • Structural Engineering (AREA)
  • Mold Materials And Core Materials (AREA)

Abstract

The invention relates to a method for precisely forming a magnesium alloy casting by gradient mixing and printing of a freezing casting flame retardant. Firstly, proportioning fire retardant components, and then designing the middle and high height of a gradient sand mould according to the structural characteristics of a typical magnesium alloy casting to be pouredThe shape and the layer thickness of the flame retardant and the low flame retardant sand layer are combined with the casting structure to design a protective gas pipeline in the low flame retardant sand layer. And then spraying a water adhesive to the paved molding sand powder bed through a control system, and paving and printing layer by layer to finish the preparation of the gradient sand mold. Freezing the printed gradient sand mould, introducing SF (sulfur hexafluoride) through a reserved vent pipeline before casting 6 +CO 2 And (5) protecting gas, ventilating for a period of time, and then performing low-temperature casting to obtain the magnesium alloy casting. The invention can reduce the use of flame retardant, prevent the reaction explosion with water vapor, reduce the production cost, lead the freezing casting technology to play a role in the production of complex magnesium alloy castings, and meet the development requirement of green casting.

Description

Accurate forming method for gradient mixing printing magnesium alloy castings by freezing casting flame retardant
Technical Field
The invention relates to the field of additive manufacturing and forming of freezing casting sand molds, in particular to a precise forming method of a magnesium alloy casting by gradient mixing and printing of a freezing casting flame retardant.
Background
With the development of the aerospace industry, the requirement on the weight reduction of components is increasing. Magnesium alloy members are also increasingly used, and the structure of the members tends to be large and complicated. In order to meet the performance and size requirements of castings, sand casting is adopted for molding large-scale aerospace magnesium alloy castings.
When the magnesium alloy is cast by adopting the sand mold, the sand mold is easy to release a large amount of harmful gas with complex components in the casting process, and is easy to generate oxidation reaction with the magnesium alloy to cause combustion. The existing research at present cannot thoroughly solve the combustion problem in the magnesium alloy sand casting, and the flame retardation problem in the casting process becomes a key technical bottleneck and potential safety hazard for restricting the production of high-performance magnesium alloy castings.
Meanwhile, in the traditional casting industry, the wood mold/metal mold turnover molding has long manufacturing period, low dimensional precision and large pollution discharge, and is difficult to meet the quick response requirements of complex thin-wall and high-end castings of aerospace, national defense and military industry and the like. The frozen sand mould additive manufacturing technology aims at the problems of poor production flexibility, long manufacturing period, high resource waste and the like of the traditional sand mould casting method in small-batch product manufacturing, proposes to replace a hardening agent in molding sand by freezing water, and manufactures a product prototype in a mode of stacking materials layer by layer, and has high flexibility in the manufacturing process, short production period and high material utilization rate.
In the traditional casting process, the magnesium alloy sand mold is of a homogeneous material and structure, flame retardants with fixed component proportions are added into the sand mold to solve the combustion problem in the casting process of the magnesium alloy, but because of the process characteristics of layer-by-layer preparation of the frozen sand mold, the sand mold with the fixed flame retardant component proportions cannot meet different flame retardant requirements of each part of a casting, and meanwhile, the use amount of the flame retardants in the preparation process of the magnesium alloy sand mold is increased by adopting the sand mold with the fixed flame retardant component proportions in the manufacturing process, high-temperature magnesium alloy melt is easy to react and explode when meeting water, so that the production cost is increased, and the green production of the sand mold is not facilitated.
Disclosure of Invention
In order to solve the existing problems, the invention provides the accurate forming method for the gradient mixing printing magnesium alloy casting of the freezing casting flame retardant, which can solve the problem of combustion and oxidization in the casting process of the freezing sand mould magnesium alloy, obtain the qualified magnesium alloy casting and reduce the casting cost of sand casting.
According to the method for precisely forming the gradient mixed printing magnesium alloy casting of the frozen casting flame retardant, firstly, the flame retardant components are proportioned, then, the shape and the layer thickness of a high flame retardant and low flame retardant type sand layer in a gradient sand mold are designed according to the structural characteristics of the specific magnesium alloy casting, and meanwhile, a protective gas pipeline is designed in the low flame retardant type sand layer by combining a casting structure. And then filling two types of molding sand with the pre-added high and low flame retardant ratios into two identical sand paving devices, accurately paving the two types of molding sand with the high and low flame retardant ratios by combining a photoelectric sensor, and then spraying a water adhesive onto the paved molding sand under the control of a control system through a spray head, paving and printing layer by layer to finish the preparation of the gradient sand mold. Freezing the printed gradient sand mould, introducing SF (sulfur hexafluoride) through a reserved vent pipeline before casting 6 +CO 2 And (5) protecting gas, and pouring to obtain the casting.
In order to achieve the above purpose, the manufacturing method of the gradient mixed printing magnesium alloy of the freeze-cast flame retardant comprises the following steps:
step 1: the components of the flame retardant are designed, and the flame retardant is formed by mixing boric acid and magnesite sand according to a certain proportion. Boric acid and magnesite sand in the flame retardant are classified into high-flame-retardant molding sand and low-flame-retardant molding sand according to the different percentages of the boric acid and magnesite sand in the total mass. Wherein the facing sand is premixed with high flame retardant and the backing sand is premixed with low flame retardant;
step 2: designing a high flame retardant content layer, wherein the flame retardant content requirements of magnesium alloys with different sizes and structures on the layer are different, comprehensively considering the casting temperature, casting speed and thickness distribution of magnesium alloy casting, and determining the flame retardant content layer and the thickness of the layer by the pressure generated by shrinkage of the casting after subsequent solidification, wherein the thinner and the better the flame retardant content layer is under the premise of meeting the flame retardant requirement;
step 3: designing a layer with low flame retardant content, considering the supporting and reinforcing effects of the layer on the layer with high flame retardant, and designing a reserved protection gas pipeline in the layer for introducing SF before pouring 6 +CO 2 A shielding gas;
step 4: preparing molding sand with high and low flame retardant content by using a sand mixer, and pre-cooling the mixed molding sand to a temperature range of 0-minus 10 ℃ by using a liquid fluidized bed of dry ice or liquid nitrogen;
step 5: the two kinds of molding sand with high and low flame retardant contents which are mixed respectively are conveyed into two sand spreading mechanisms, a working table is moved downwards for a certain distance, a layer of molding sand with low flame retardant content is spread by using a sand spreading control device to serve as bottom sand, and after the bottom sand is spread, the sand spreading device returns to the original position;
step 6: the workbench moves down by one layer thickness, and the sand paving device lays molding sand with high and low flame retardant content according to the requirement under the control of the control system according to the section information of the sand mould of the current layer;
step 7: after the molding sand is paved, the sand paving device moves back to the initial position, and the control system controls the printing spray head to spray water adhesive according to the section information of the current layer of the sand mold as required to finish the printing of the current layer of the sand mold;
step 8: repeating the step 7 and the step 8, and returning the sand paving device to the sand feeding position and filling the sand from the sand mixing hopper whenever the sand storage amount in the sand paving device is insufficient to lay the sand with one layer thickness;
step 9: repeating the step 9, printing layer by layer until sand printing is finished by stacking layer by layer finally;
step 10: freezing the printed sand mould in a low-temperature chamber;
step 11: sand cleaning to obtain a sand mold to be poured;
step 12: SF is introduced into a preset pipeline half an hour before pouring 6 +CO 2 The volume ratio of the protective gas to the casting is 1:100, the flow rate is properly changed according to the size of the casting, and the flow rate is reduced during casting;
step 13: and pouring the magnesium alloy liquid into the gradient sand mould to obtain the casting.
Furthermore, the low flame retardant layer can continuously reduce the proportion of the flame retardant, reduce the cost of the sand mould and accord with the development direction of green casting.
Further, the mass ratio of borax to magnesite powder serving as a flame retardant is 1:2.5.
furthermore, when the sand mould is manufactured by adopting the micro-droplet spraying water adhesive, the borax with higher content can generate corrosion effect on the printing spray head, and the service life of the printing spray head is prolonged by additionally arranging a protective shell or attaching a protective film on the printing spray head.
Further, two kinds of sand mixed with high and low flame retardants are respectively sent into a sand paving device, and the sand paving device comprises two sand paving grooves and a photoelectric sensor, wherein the photoelectric sensor is used for positioning the sand paving position.
Furthermore, in order to reduce the severe reaction generated by the contact of the surface magnesium alloy liquid and the air, the upper part of the frozen sand mold is covered with a resin casting head to isolate most of the air from contacting with the surface and delay the reaction.
The additive amount ratio of the flame retardant in the high-flame-retardant molding sand and the low-flame-retardant molding sand is shown in table 1.
TABLE 1 Molding sand flame retardant addition amount
The beneficial effects of the invention are that
1. On the premise of ensuring the flame-retardant effect of the magnesium alloy sand mould, the traditional magnesium alloy sand mould and the flame retardant are designed into the gradient flame-retardant frozen sand mould, and the flexible sand laying device is manufactured through the frozen sand mould additive to carry out regional laying, so that the use of the flame retardant can be reduced, the influence of water explosion of the magnesium alloy is reduced, the production cost is reduced, and the development requirement of green casting is compounded.
2. The invention further solves the combustion problem in the casting process of the magnesium alloy casting by combining the integrated technology of hollowed-out design and manufacturing of additive manufacturing on the basis of the gradient flame-retardant sand mold and a method for introducing the shielding gas into the sand mold, thereby improving the quality of the finished product of the magnesium alloy casting.
Drawings
FIG. 1 is a flow chart of a method for precisely forming a gradient mixed printed magnesium alloy casting by freezing and casting a flame retardant;
FIG. 2 is a schematic diagram of a gradient sand mold of magnesium alloy;
FIG. 3 is a schematic diagram of a vent line;
fig. 4 is a schematic diagram of a gradient printing process of a frozen sand mold.
Description of the drawings: 1-a pressure regulating valve; 2-SF 6 A gas cylinder; 3-tracheal adapter; 4-trachea; 5-CO 2 A gas cylinder; 6-high flame retardant content layer molding sand; 7-a resin casting head; 8-a cavity; 9-a vent line; 10-low flame retardant content layer molding sand; 11-a droplet printing ejection head; 12-X axis sliding block; 13-Y axis sliding block; 14-connecting rod; 15-connecting rods; 16-sand feeding device; 17-a sanding tank; 18-a photosensor; 19-printing platform.
Detailed Description
The present invention is further illustrated in the following drawings and detailed description, which are to be understood as being merely illustrative of the invention and not limiting the scope of the invention. It should be noted that the words "front", "rear", "left", "right", "upper" and "lower" used in the following description refer to directions in the drawings, and the words "inner" and "outer" refer to directions toward or away from, respectively, the geometric center of a particular component.
The invention provides a precision forming method for a gradient mixing printing magnesium alloy casting of a freezing casting flame retardant, which is shown in figure 1.
The components of the flame retardant are designed, boric acid and magnesite sand are mixed according to the proportion shown in the table 1, and the mixed flame retardant is added into molding sand with 50-100 meshes, stirred and precooled for standby; the two kinds of molding sand having the high and low flame retardant contents mixed respectively are fed into two identical sand feeding devices 16 respectively. The sand discharging device 16 can send the molding sand into the sand paving groove 17, and the photoelectric sensor 18 of the sand paving groove 17 is used for controlling the sand paving groove 17 to open and close for paving sand after locating the sand paving position. Firstly, a layer of molding sand with low flame retardant content is paved as bottom sand by a sand paving device after the workbench 19 is moved downwards for a certain distance, and the sand paving device returns to the original position after the bottom sand is paved; the workbench 19 moves down by one layer thickness, the sand discharging device 16 sends the molding sand with high and low flame retardant content into the sand spreading groove 17 according to the requirement according to the section information of the sand mould of the current layer under the control of the control system, and the photoelectric sensor of the sand spreading groove 17 is positioned at the sand spreading position and then controls the sand spreading groove opening and closing 17 to spread the molding sand required at present to the appointed position; after the molding sand is paved, the sand paving device moves back to the initial position, and the control system controls the printing spray head 11 to spray water adhesive according to the section information of the current layer of the sand mold as required, so that the printing of the current layer of the sand mold is completed; the workbench 19 moves down one layer thick, the paving and printing processes are repeated, and the layers are paved and printed layer by layer to obtain a required sand mold; as shown in fig. 3, the casting cavity 8 obtained by printing is positioned at the middle part of the sand mold, the inner layer is high-flame-retardant molding sand 6, the outer layer is low-flame-retardant molding sand 10, and a ventilation pipeline 9 is arranged in the low-flame-retardant molding sand. The printed sand mold was frozen in a low temperature chamber to remove excess sand, as shown in fig. 2. SF is introduced into the preset pipeline 9 half an hour before pouring 6 +CO 2 The gas is protected, the gas is arranged in the gas cylinder 2 and the gas cylinder 5, the flow rate of the gas is controlled by the pressure regulating valve 1 above the gas cylinder, the gas is converged through the gas pipe adapter 3 and then is sent into the pipeline 9, and the flow rate of the gas is reduced by the pressure regulating valve 1 during pouring. Before casting, placing a resin casting head 7 at the upper part of the sand mould to reduce the contact between the surface of the casting and air, and casting magnesium alloy liquid into the gradient sand mould to obtain the magnesium alloy casting.
Example 1:
a method for precisely forming magnesium alloy castings by gradient mixing and printing of freezing casting flame retardants adopts a gradient printing mode to realize that facing sand is mixed with high flame retardants and back sand is mixed with low flame retardants, and the high flame retardants and the low flame retardants are changed in a gradient manner according to a certain proportion.
1) Sand mould preparation: boric acid and magnesite powder are mixed according to the mass ratio of 1:3, mixing to prepare a flame retardant, and mixing the flame retardant with the molding sand according to a proportion. Wherein boric acid in the facing sand accounts for 2.6wt.% of the total mass of the flame retardant and the molding sand, and magnesite powder accounts for 6.5wt.% of the total mass; boric acid in the back sand accounts for 1wt.% of the total mass of the flame retardant and the molding sand, and magnesite powder accounts for 2.5wt.% of the total mass.
2) Printing and forming: and conveying the mixed two types of molding sand with high and low flame retardant contents into two identical sand paving mechanisms, paving sand by using a photoelectric sensor to position the sand paving mechanism, and controlling the sand paving groove to open and close for shakeout after the sand paving mechanism reaches a designated position. And controlling the printing nozzle to spray the water adhesive to finish printing of the current layer, wherein the water content of the sand mold after printing accounts for 3wt.% of the total mass.
3) Pouring: the prepared frozen sand mould is placed at room temperature of 20 ℃ for casting, the casting temperature is 650 ℃, and SF is continuously introduced into the vent holes with low flame retardant for half an hour before casting 6 +CO 2 And a protective gas, wherein the volume ratio of the protective gas to the protective gas is 1:100. the diameter of the casting cylindrical bar is 4cm, the height is 10cm, the thickness of the high-flame-retardant sand layer is about 2cm, and the thickness of the low-flame-retardant sand layer is about 6cm. Covering a resin casting head at the contact position of the surface of the cylindrical bar at the upper part of the frozen sand mold and air, casting magnesium alloy liquid into the gradient sand mold, and obtaining the casting after the magnesium alloy liquid is in contact with water and does not explode.
Example 2:
a method for precisely forming magnesium alloy castings by gradient mixing and printing of freezing casting flame retardants adopts a gradient printing mode to realize that facing sand is mixed with high flame retardants and back sand is mixed with low flame retardants, and the high flame retardants and the low flame retardants are changed in a gradient manner according to a certain proportion.
1) Sand mould preparation: boric acid and magnesite powder are mixed according to the mass ratio of 1:3, mixing to prepare a flame retardant, and mixing the flame retardant with the molding sand according to a proportion. Wherein boric acid in the facing sand accounts for 2.8wt.% of the total mass of the flame retardant and the molding sand, and magnesite powder accounts for 7wt.% of the total mass; boric acid in the back sand accounts for 1wt.% of the total mass of the flame retardant and the molding sand, and magnesite powder accounts for 2.5wt.% of the total mass.
2) Printing and forming: and conveying the mixed two types of molding sand with high and low flame retardant contents into two identical sand paving mechanisms, paving sand by using a photoelectric sensor to position the sand paving mechanism, and controlling the sand paving groove to open and close for shakeout after the sand paving mechanism reaches a designated position. And controlling the printing nozzle to spray the water adhesive to finish printing of the current layer, wherein the water content of the sand mold after printing is 4wt.% of the total mass.
3) Pouring: the prepared frozen sand mould is placed at room temperature of 20 ℃ for casting, the casting temperature is 670 ℃, and SF is continuously introduced into a vent hole with low flame retardant for half an hour before casting 6 +CO 2 And a protective gas, wherein the volume ratio of the protective gas to the protective gas is 1:100. the diameter of the casting cylindrical bar is 4cm, the height is 10cm, the thickness of the high-flame-retardant sand layer is about 3cm, and the thickness of the low-flame-retardant sand layer is about 5cm. Covering a resin casting head at the contact position of the surface of the cylindrical bar at the upper part of the frozen sand mould and air,
and (3) pouring magnesium alloy liquid into the gradient sand mould, and obtaining the casting without explosion when the magnesium alloy liquid contacts water.
Example 3:
a method for precisely forming magnesium alloy castings by gradient mixing and printing of freezing casting flame retardants adopts a gradient printing mode to realize that facing sand is mixed with high flame retardants and back sand is mixed with low flame retardants, and the high flame retardants and the low flame retardants are changed in a gradient manner according to a certain proportion.
1) Sand mould preparation: boric acid and magnesite powder are mixed according to the mass ratio of 1:3, mixing to prepare a flame retardant, and mixing the flame retardant with the molding sand according to a proportion. Wherein boric acid in the facing sand accounts for 3wt.% of the total mass of the flame retardant and the molding sand, and magnesite powder accounts for 7.5wt.% of the total mass; boric acid in the back sand accounts for 1wt.% of the total mass of the flame retardant and the molding sand, and magnesite powder accounts for 2.5wt.% of the total mass.
2) Printing and forming: and conveying the mixed two types of molding sand with high and low flame retardant contents into two identical sand paving mechanisms, paving sand by using a photoelectric sensor to position the sand paving mechanism, and controlling the sand paving groove to open and close for shakeout after the sand paving mechanism reaches a designated position. And controlling the printing nozzle to spray the water adhesive to finish printing of the current layer, wherein the water content of the sand mold after printing accounts for 5wt.% of the total amount.
3) Pouring: the prepared frozen sand mould is placed at room temperature of 20 ℃ for casting, the casting temperature is 690 ℃, and SF is continuously introduced into a vent hole with low flame retardant for half an hour before casting 6 +CO 2 And a protective gas, wherein the volume ratio of the protective gas to the protective gas is 1:100. the diameter of the casting cylindrical bar is 4cm, the height is 10cm, the thickness of the high-flame-retardant sand layer is about 4cm, and the thickness of the low-flame-retardant sand layer is about 4cm. Covering a resin casting head at the contact position of the surface of the cylindrical bar at the upper part of the frozen sand mold and air, casting magnesium alloy liquid into the gradient sand mold, and obtaining the casting after the magnesium alloy liquid is in contact with water and does not explode.
The technical means disclosed by the scheme of the invention is not limited to the technical means disclosed by the embodiment, and also comprises the technical scheme formed by any combination of the technical features.

Claims (8)

1. The method for precisely forming the magnesium alloy casting by gradient mixing and printing of the freezing casting flame retardant is characterized by comprising the following steps of:
step 1: the components of the flame retardant are designed, and the flame retardant is formed by mixing boric acid and magnesite sand according to a certain proportion; boric acid and magnesite sand in the flame retardant are classified into high-flame-retardant molding sand and low-flame-retardant molding sand according to the different percentages of the boric acid and magnesite sand in the total mass; wherein the facing sand is premixed with high flame retardant and the backing sand is premixed with low flame retardant;
step 2: designing a high flame retardant content layer (6), wherein the magnesium alloy castings with different sizes and structures have different flame retardant content requirements on the layer, comprehensively considering the casting temperature, casting speed and wall thickness distribution of the magnesium alloy castings, and determining the flame retardant content layer and the layer thickness of the layer by the pressure generated by shrinkage of the solidified castings, wherein the thinner and the better the flame retardant content layer is under the premise of meeting the flame retardant requirements;
step 3: designing a layer (10) with low flame retardant content, considering the supporting and reinforcing effects of the layer on the layer with high flame retardant, designing a reserved protection gas pipeline (9) in the layer, and realizing by freezing and printing a hollow structure for introducing SF before low-temperature casting 6 (2)+CO 2 (5) A shielding gas;
step 4: preparing molding sand with high and low flame retardant content by using a sand mixer, and pre-cooling the mixed molding sand to-40 ℃ to-25 ℃ by using dry ice or liquid nitrogen;
step 5: and (3) conveying the two kinds of mixed molding sand with high and low flame retardant contents into two sand discharging devices (16) with the same structure respectively, and after the sand discharging devices (16) convey the molding sand with the required types into a sand spreading groove (17), controlling the sand spreading groove (17) to open and close to spread the required molding sand after a photoelectric sensor (18) beside the sand spreading groove (17) reads the required sand spreading position. A working table is moved downwards for a certain distance, a layer of molding sand with low flame retardant content is paved by using a sand paving device to serve as bottom sand, and after the bottom sand is paved, the sand paving device returns to the original position;
step 6: the workbench moves down by one layer thickness, and the sand paving device lays high-content and low-content molding sand according to the section information of the sand mould of the current layer under the control of the control system;
step 7: after the molding sand is paved, the sand paving device moves back to the initial position, and the control system controls the micro-droplet printing spray head (11) to spray water adhesive according to the section information of the current layer of the sand mold as required, so that the printing of the current layer of the sand mold is completed;
step 8: repeating the step 7 and the step 8, and returning the sand paving device to the sand feeding position and filling the sand from the sand mixing hopper whenever the sand storage amount in the sand paving device is insufficient to lay the sand with one layer thickness;
step 9: repeating the step 9, printing layer by layer until sand printing is finished by stacking layer by layer finally;
step 10: the printed sand mould is placed in a low-temperature chamber to be frozen for the second time;
step 11: sand cleaning to obtain a sand mold to be poured;
step 12: SF is introduced into a freezing printing preset pipeline half an hour before pouring 6 (2)+CO 2 (5) The volume ratio of the protective gas to the casting is 1:100, the flow rate is changed according to the size of the casting, and the flow rate is properly reduced during casting;
step 13: and pouring the magnesium alloy liquid into the gradient sand mould to obtain the casting.
2. The method for precisely forming the gradient mixed printing magnesium alloy castings by using the freeze-cast flame retardant according to claim 1, wherein the flame retardant in the step 1 consists of boric acid and magnesite powder, and the mass ratio of the boric acid to the magnesite powder is 1:2.5, borax content in the facing sand directly contacted with the casting surface is 2.6wt.% to 3wt.%, and boric acid content in the back sand is 1wt.%. The content of the flame retardant gradually decreases from the inside of the cavity to the outside.
3. The method for precisely forming the magnesium alloy castings by gradient mixing and printing of the freeze-cast flame retardant according to claim 1, wherein the larger the diameter of the cast castings in the step 2 is, the higher the casting temperature is, and the thickness of the high flame retardant content layer is increased.
4. The method for precisely forming the magnesium alloy castings by gradient mixing and printing of the freeze-cast flame retardant according to claim 1, wherein a protective gas pipeline is required to be designed in the low flame retardant content layer in the step 3, and the pipeline is introduced with SF before casting 6 +CO 2 The volume ratio of the protective gas to the air is 1:100, the flow rate of the shielding gas is about 1-10L/min.
5. The method for precisely forming the magnesium alloy castings by gradient mixing and printing of the flame retardant for the freeze casting according to claim 1, wherein the molding sand mixed in the step 5 is respectively conveyed into two sand discharging devices, the sand discharging devices are used for positioning the laying positions by photoelectric sensors after conveying different kinds of sand into the sand laying grooves, and the sand laying grooves are controlled to open and close to lay the flame retardant molding sand after being positioned to the accurate positions, so that gradient printing of the molding sand with high flame retardant and low flame retardant is realized.
6. The method for precisely forming the magnesium alloy castings by gradient mixing and printing of the freeze-cast flame retardant according to claim 1, wherein when the sand mold is manufactured by adopting the micro-droplet spraying water adhesive in the step 7, the borax with higher content in the sand mold can generate corrosion effect on the printing spray head, and the service life of the printing spray head is prolonged by additionally arranging a protective shell or attaching a protective film on the printing spray head.
7. The method for precisely forming the magnesium alloy castings by gradient mixing and printing of the freeze-cast flame retardant according to claim 1, wherein SF is obtained during the casting process in the step 12 6 +CO 2 The flow rate of the protective gas is reduced to 0.5L/min, and after all the steps are finished, the pressure regulating valve of the gas cylinder is closed to finish ventilation.
8. The method for precisely forming the magnesium alloy castings by gradient mixing and printing of the flame retardant for the freeze casting according to claim 1, wherein in the step 13, in order to reduce the severe reaction generated by the contact of the surface magnesium alloy liquid with the air, a resin casting head is covered on the upper part of the frozen sand mold, so that most of the contact of the air with the surface is isolated.
CN202311251222.6A 2023-09-26 2023-09-26 Accurate forming method for gradient mixing printing magnesium alloy castings by freezing casting flame retardant Pending CN117259670A (en)

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