Method for precision forming of magnesium alloy castings by freeze casting gradient hybrid printing of flame retardants

Abstract

The application is a kind of precise forming method for printing magnesium alloy castings by gradient mixing of flame retardant in freeze casting. First, the components of the flame retardant are proportioned, then the shape and layer thickness of the high flame retardant and low flame retardant sand layer in the gradient sand mold are designed according to the structure characteristics of the typical magnesium alloy castings to be poured, and at the same time, the protection gas pipeline is designed in the low flame retardant sand layer combined with the structure of the castings. Subsequently, the control system sprays water binder onto the laid sand powder bed, layer by layer, to complete the preparation of the gradient sand mold. The printed gradient sand mold is frozen, and before pouring, SF6+CO2 protection gas is introduced through the reserved vent pipe, and after a period of aeration, low-temperature pouring is carried out to obtain magnesium alloy castings. The application can reduce the use of flame retardant, prevent explosion reaction with water vapor, reduce production cost, make the freeze casting technology play a role in the production of complex magnesium alloy castings, and meet the development requirements of green casting.

Description

Technical Field

[0001] This invention relates to the field of cryogenic casting sand mold additive manufacturing, and in particular to a method for precise forming of magnesium alloy castings by cryogenic casting flame retardant gradient mixing printing. Background Technology

[0002] With the development of the aerospace industry, the requirements for lightweight components are becoming increasingly stringent. The application of magnesium alloy components is also becoming more widespread, and their structures are trending towards larger and more complex sizes. To meet the performance and dimensional requirements of castings, large aerospace magnesium alloy castings are mostly formed using sand casting.

[0003] When using sand casting to produce magnesium alloys, the sand mold easily releases a large amount of harmful gases with complex compositions during the pouring process. These gases can easily react with the magnesium alloy to cause combustion. Current research has not yet completely solved the combustion problem in magnesium alloy sand casting, and flame retardancy during the casting process has become a key technical bottleneck and safety hazard restricting the production of high-performance magnesium alloy castings.

[0004] Meanwhile, in the traditional casting industry, the production cycle of sand molds using wooden / metal molds is long, the dimensional accuracy is low, and the pollution emissions are high, making it difficult to meet the rapid response requirements of complex thin-walled, high-end castings in aerospace, defense, and other industries. Cryogenic sand casting additive manufacturing technology addresses the problems of poor production flexibility, long production cycles, and significant resource waste in small-batch production of traditional sand casting methods. It proposes using water freezing to replace the hardener in the molding sand, creating product prototypes by layering materials. This process offers high flexibility, short production cycles, and high material utilization.

[0005] In traditional casting processes, magnesium alloy sand molds are homogeneous materials and structures. The problem of combustion during the casting process is often solved by adding flame retardants with a fixed composition ratio to the molding sand. However, due to the process characteristics of layer-by-layer preparation of frozen sand molds, molding sand with a fixed composition ratio of flame retardant cannot meet the different flame retardant requirements of different parts of the casting. At the same time, using molding sand with a fixed composition ratio of flame retardant during the manufacturing process will increase the amount of flame retardant used in the preparation of magnesium alloy sand molds. High-temperature magnesium alloy melt is prone to reaction and explosion when it comes into contact with water, which increases production costs and is not conducive to achieving green production of sand molds. Summary of the Invention

[0006] To address the aforementioned problems, this invention provides a method for precise forming of magnesium alloy castings by gradient mixing of flame retardants in cryogenic casting. This method can solve the combustion and oxidation problem during the cryogenic sand casting process of magnesium alloys, resulting in qualified magnesium alloy castings while reducing the casting cost of sand casting.

[0007] This invention proposes a method for precise forming of magnesium alloy castings using gradient mixing of flame retardants in cryogenic casting. First, the flame retardant components are proportioned. Then, based on the specific structural characteristics of the magnesium alloy casting, the shape and thickness of the high- and low-flame-retardant sand layers in the gradient sand mold are designed. Simultaneously, protective gas channels are designed within the low-flame-retardant sand layer, taking into account the casting structure. Next, two types of molding sand with pre-added high and low flame retardant ratios are loaded into two identical sand-laying devices. Using photoelectric sensors, the two types of molding sand with varying flame retardant ratios are accurately laid. Subsequently, under the control of the control system, a water-based binder is sprayed onto the laid molding sand through a nozzle, layer by layer, to complete the preparation of the gradient sand mold. The printed gradient sand mold is frozen. Before casting, SF6 + CO2 protective gas is introduced through pre-reserved ventilation channels for casting, resulting in the final casting.

[0008] To achieve the above objectives, the method for manufacturing magnesium alloys by gradient mixing of cryogenic casting flame retardants according to the present invention is carried out according to the following steps:

[0009] Step 1: Design the composition of the flame retardant, which is a mixture of boric acid and magnesite sand in a certain proportion. Based on the different percentages of boric acid and magnesite sand in the total mass of the flame retardant, it is divided into high-flame-retardant molding sand and low-flame-retardant molding sand. The face sand is premixed with high-flame-retardant, and the back sand is premixed with low-flame-retardant.

[0010] Step 2: Design the high flame retardant content layer. Magnesium alloys of different sizes and structures have different requirements for the flame retardant content of this layer. Taking into account the pouring temperature and pouring speed of the magnesium alloy melt, the thickness distribution of the magnesium alloy casting, and the pressure generated by the shrinkage of the casting after solidification, the flame retardant content and thickness of this layer are determined. The thinner the better, while meeting the flame retardant requirements.

[0011] Step 3: Design the low flame retardant content layer, taking into account the support and reinforcement effect of this layer on the high flame retardant layer. At the same time, design a reserved protective gas pipeline in this layer for the introduction of SF6+CO2 protective gas before re-pouring.

[0012] Step 4: Use a sand mixer to prepare molding sand with high and low flame retardant content respectively, and pre-cool the mixed molding sand to a temperature range of 0 to -10℃ using a liquid fluidized bed of dry ice or liquid nitrogen;

[0013] Step 5: The two types of molding sand with high and low flame retardant content are respectively transported into two sand laying mechanisms. The workbench is lowered a certain distance and a layer of molding sand with low flame retardant content is laid as the base sand using the controlled sand laying device. After the base sand is laid, the sand laying device returns to its original position.

[0014] Step 6: The workbench moves down one layer thickness. Under the control of the control system, the sand laying device will lay molding sand with high and low flame retardant content as needed according to the sand mold cross-section information of the current layer.

[0015] Step 7: After the molding sand is laid, the sand laying device moves back to the initial position, and the control system controls the printing nozzle to spray water adhesive as needed according to the cross-sectional information of the current layer of the sand mold, so as to complete the printing of the current layer of sand mold;

[0016] Step 8: Repeat steps 7 and 8. Whenever the amount of sand stored in the sand spreading device is insufficient to lay a layer of molding sand, the sand spreading device returns to the sand loading position and fills the molding sand from the sand mixing hopper.

[0017] Step 9: Repeat step 9, printing layer by layer, until the sand mold printing is completed by stacking layers one by one;

[0018] Step 10: Place the printed sand mold in a low-temperature chamber to freeze;

[0019] Step 11: Clean the sand to obtain the sand mold to be poured;

[0020] Step 12: Half an hour before pouring, introduce SF6+CO2 protective gas into the preset pipeline. The volume ratio of the two is 1:100. The flow rate is adjusted according to the size of the casting. Reduce the flow rate during pouring.

[0021] Step 13: Pour molten magnesium alloy into a gradient sand mold to obtain a casting.

[0022] Furthermore, a low flame retardant layer can further reduce the proportion of flame retardants, thereby reducing sand mold costs and aligning with the development direction of green casting.

[0023] Furthermore, the mass ratio of the flame retardant component borax to magnesite powder is 1:2.5.

[0024] Furthermore, when using micro-droplet jet water binder to manufacture sand molds, the high content of borax can corrode the printhead. The lifespan of the printhead can be extended by adding a protective shell or applying a protective film to the printhead.

[0025] Furthermore, the two types of sand, which are mixed with high and low flame retardants, are fed into a sand-laying device, which includes two sand-laying troughs and a photoelectric sensor. The photoelectric sensor is used to locate the sand-laying position.

[0026] Furthermore, to reduce the violent reaction caused by the surface magnesium alloy liquid coming into contact with air, a resin gating riser is added to the top of the frozen sand mold to isolate most of the air from the surface and delay the reaction.

[0027] The proportions of flame retardant added in high flame retardant molding sand and low flame retardant molding sand are shown in Table 1.

[0028] Table 1. Flame retardant addition amount for molding sand

[0029]

[0030] Beneficial effects of the present invention

[0031] 1. Under the premise of ensuring the flame retardant effect of magnesium alloy sand mold, this invention designs the traditional magnesium alloy sand mold with flame retardant into a gradient flame retardant frozen sand mold, and lays it in sections by means of a flexible sand laying device for additive manufacturing of frozen sand mold. This can reduce the use of flame retardant, reduce the impact of magnesium alloy explosion when exposed to water, reduce production costs, and meet the development requirements of green casting.

[0032] 2. Based on gradient flame-retardant sand molds, this invention combines additive manufacturing hollow design and manufacturing integration technology with the method of introducing protective gas into the sand mold to further solve the combustion problem in the casting process of magnesium alloy castings, thereby improving the finished product quality of magnesium alloy castings. Attached Figure Description

[0033] Figure 1 Flowchart of a method for precise forming of magnesium alloy castings by gradient mixing of flame retardants in cryogenic casting;

[0034] Figure 2 A schematic diagram of a gradient sand mold for magnesium alloys;

[0035] Figure 3 This is a schematic diagram of the ventilation pipeline;

[0036] Figure 4 This is a schematic diagram of the gradient printing process for frozen sand molds.

[0037] Figure descriptions: 1-Pressure regulating valve; 2-SF6 gas cylinder; 3-Gas pipe adapter; 4-Gas pipe; 5-CO2 gas cylinder; 6-High flame retardant content layered molding sand; 7-Resin gating riser; 8-Mold cavity; 9-Ventilation pipeline; 10-Low flame retardant content layered molding sand; 11-Microdroplet printing nozzle; 12-X-axis slider; 13-Y-axis slider; 14-Connecting rod; 15-Connecting rod; 16-Sand feeding device; 17-Sand spreading trough; 18-Photoelectric sensor; 19-Printing platform. Detailed Implementation

[0038] The present invention will be further illustrated below with reference to the accompanying drawings and specific embodiments. It should be understood that the following specific embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. It should be noted that the terms "front," "rear," "left," "right," "up," and "down" used in the following description refer to directions in the accompanying drawings, and the terms "inner" and "outer" refer to directions toward or away from the geometric center of a specific component, respectively.

[0039] This invention provides a method for precise forming of magnesium alloy castings by gradient mixing of flame retardants in cryogenic casting, such as... Figure 1 As shown.

[0040] The composition of the flame retardant is designed, and boric acid and magnesite sand are mixed in the proportions shown in Table 1. The mixed flame retardant is added to 50-100 mesh molding sand and stirred, then pre-cooled for later use. Two types of molding sand with different flame retardant contents are separately fed into two identical sand feeding devices 16. The sand feeding device 16 can deliver the molding sand into the sand spreading trough 17. After the photoelectric sensor 18 of the sand spreading trough 17 locates the sand spreading position, it controls the opening and closing of the sand spreading trough 17 to spread sand. First, the workbench 19 is lowered a certain distance, and a layer of low flame retardant content molding sand is laid as the base sand using a sand-laying device. After the base sand is laid, the sand-laying device returns to its original position. The workbench 19 is then lowered by one layer thickness. Under the control of the control system, the sand-feeding device 16, based on the current layer's sand mold cross-sectional information, delivers high and low flame retardant content molding sand into the sand-laying trough 17 as needed. After the photoelectric sensor of the sand-laying trough 17 locates the sand-laying position, it controls the opening and closing of the sand-laying trough 17 to lay the required molding sand to the designated position. After the molding sand is laid, the sand-laying device moves back to its initial position, and the control system controls the printing nozzle 11 to spray water-based adhesive as needed based on the current layer's cross-sectional information, completing the printing of the current layer's sand mold. The workbench 19 is then lowered by one layer thickness, and the laying and printing process is repeated layer by layer to obtain the required sand mold. Figure 3 As shown, the printed casting cavity 8 is located in the middle of the sand mold. The inner layer is high-flame-retardant molding sand 6, and the outer layer is low-flame-retardant molding sand 10. A venting pipe 9 is provided in the low-flame-retardant molding sand. The printed sand mold is placed in a low-temperature chamber for freezing, and excess molding sand is removed. Figure 2 As shown. Half an hour before pouring, SF6+CO2 protective gas is introduced into the preset pipeline 9. The gas is contained in gas cylinders 2 and 5, and the gas flow rate is controlled by the pressure regulating valve 1 above the gas cylinders. The gas is combined through the gas pipe adapter 3 and then sent into pipeline 9. During pouring, the pressure regulating valve 1 is controlled to reduce the gas flow rate. Before pouring, a resin riser 7 is placed on the top of the sand mold to reduce the contact between the casting surface and air. Magnesium alloy liquid is poured into the gradient sand mold to obtain a magnesium alloy casting.

[0041] Example 1:

[0042] A method for precise forming of magnesium alloy castings by gradient mixing of flame retardants in cryogenic casting is disclosed. The method employs gradient printing to achieve mixing of high flame retardant with face sand and low flame retardant with back sand, with the high and low flame retardants varying in a certain ratio.

[0043] 1) Sand mold preparation: Boric acid and magnesite powder are mixed at a mass ratio of 1:3 to prepare a flame retardant. The flame retardant is then mixed with molding sand in the same proportion. In the face sand, boric acid accounts for 2.6 wt.% of the total mass of the flame retardant and molding sand, and magnesite powder accounts for 6.5 wt.% of the total mass. In the back sand, boric acid accounts for 1 wt.% of the total mass of the flame retardant and molding sand, and magnesite powder accounts for 2.5 wt.% of the total mass.

[0044] 2) Printing and Forming: Two types of molding sand, one with high and one with low flame retardant content, are conveyed into two identical sand-laying mechanisms. These mechanisms are positioned using photoelectric sensors, and once the sand reaches the designated position, the sand-laying troughs open and close to allow sand to fall. The printing nozzles are then controlled to spray water-based adhesive to complete the printing of the current layer. After printing, the moisture content of the sand mold is 3 wt.% of the total mass.

[0045] 3) Casting: The prepared frozen sand mold was placed at room temperature (20℃) for casting. The casting temperature was 650℃. Half an hour before casting, SF6 + CO2 protective gas was continuously introduced into the vent of the low flame retardant, with a volume ratio of 1:100. The diameter of the cast cylindrical rod was 4cm, the height was 10cm, the thickness of the high flame retardant sand layer was about 2cm, and the thickness of the low flame retardant sand layer was about 6cm. A resin riser was placed on the surface of the cylindrical rod at the top of the frozen sand mold where it was in contact with air. Molten magnesium alloy was poured into the gradient sand mold. No explosion occurred upon contact with water, resulting in the casting.

[0046] Example 2:

[0047] A method for precise forming of magnesium alloy castings by gradient mixing of flame retardants in cryogenic casting is disclosed. The method employs gradient printing to achieve mixing of high flame retardant with face sand and low flame retardant with back sand, with the high and low flame retardants varying in a certain ratio.

[0048] 1) Sand mold preparation: Boric acid and magnesite powder are mixed at a mass ratio of 1:3 to prepare a flame retardant. The flame retardant is then mixed with molding sand in the same proportion. In the face sand, boric acid accounts for 2.8 wt.% of the total mass of the flame retardant and molding sand, and magnesite powder accounts for 7 wt.% of the total mass. In the back sand, boric acid accounts for 1 wt.% of the total mass of the flame retardant and molding sand, and magnesite powder accounts for 2.5 wt.% of the total mass.

[0049] 2) Printing and Forming: Two types of molding sand, one with high and one with low flame retardant content, are conveyed into two identical sand-laying mechanisms. These mechanisms are positioned using photoelectric sensors, and once the sand reaches the designated position, the sand-laying troughs open and close to allow sand to fall. The printing nozzles are then controlled to spray water-based adhesive to complete the printing of the current layer. After printing, the moisture content of the sand mold is 4 wt.% of the total mass.

[0050] 3) Casting: The prepared frozen sand mold was placed at room temperature (20℃) for casting. The casting temperature was 670℃. Half an hour before casting, SF6 + CO2 protective gas was continuously introduced into the vent hole of the low flame retardant layer, with a volume ratio of 1:100. The diameter of the cast cylindrical rod was 4cm, the height was 10cm, the thickness of the high flame retardant sand layer was approximately 3cm, and the thickness of the low flame retardant sand layer was approximately 5cm. A resin gating riser was placed on the surface of the upper cylindrical rod of the frozen sand mold where it came into contact with air.

[0051] Magnesium alloy liquid was poured into a gradient sand mold, and no explosion occurred upon contact with water, resulting in a casting.

[0052] Example 3:

[0053] A method for precise forming of magnesium alloy castings by gradient mixing of flame retardants in cryogenic casting is disclosed. The method employs gradient printing to achieve mixing of high flame retardant with face sand and low flame retardant with back sand, with the high and low flame retardants varying in a certain ratio.

[0054] 1) Sand mold preparation: Boric acid and magnesite powder are mixed at a mass ratio of 1:3 to prepare a flame retardant. The flame retardant is then mixed with molding sand in the same proportion. In the face sand, boric acid accounts for 3 wt.% of the total mass of the flame retardant and molding sand, and magnesite powder accounts for 7.5 wt.% of the total mass. In the back sand, boric acid accounts for 1 wt.% of the total mass of the flame retardant and molding sand, and magnesite powder accounts for 2.5 wt.% of the total mass.

[0055] 2) Printing and Forming: Two types of molding sand, one with high and one with low flame retardant content, are conveyed into two identical sand-laying mechanisms. These mechanisms are positioned using photoelectric sensors, and once the sand reaches the designated position, the sand-laying troughs open and close to allow sand to fall. The printing nozzles are then controlled to spray water-based adhesive to complete the printing of the current layer. After printing, the sand mold's moisture content is 5 wt.% of the total volume.

[0056] 3) Casting: The prepared frozen sand mold was placed at room temperature (20℃) for casting. The casting temperature was 690℃. Half an hour before casting, SF6 + CO2 protective gas was continuously introduced into the vent of the low flame retardant, with a volume ratio of 1:100. The diameter of the cast cylindrical rod was 4cm, the height was 10cm, the thickness of the high flame retardant sand layer was approximately 4cm, and the thickness of the low flame retardant sand layer was approximately 4cm. A resin riser was placed on the surface of the cylindrical rod at the top of the frozen sand mold where it was in contact with air. Molten magnesium alloy was poured into the gradient sand mold. No explosion occurred upon contact with water, resulting in the casting.

[0057] The technical means disclosed in this invention are not limited to those disclosed in the above embodiments, but also include technical solutions composed of any combination of the above technical features.

Claims

1. A method for precise forming of magnesium alloy castings by gradient mixing of flame retardants in cryogenic casting, characterized in that, This method is performed in the following steps: Step 1: Design the composition of the flame retardant. The flame retardant is made by mixing boric acid and magnesite powder in a certain proportion. According to the different percentages of boric acid and magnesite powder in the total mass of the flame retardant, it is divided into high flame retardant content molding sand and low flame retardant content molding sand. Among them, the face sand is premixed with high flame retardant content molding sand, and the back sand is premixed with low flame retardant content molding sand. Step 2: Design the high flame retardant content layer (6). Magnesium alloy castings of different sizes and structures have different requirements for the flame retardant content of this layer. Taking into account the pouring temperature of the magnesium alloy melt, the pouring speed, the wall thickness distribution of the magnesium alloy casting, and the pressure generated by the shrinkage of the casting after solidification, determine the flame retardant content layer and the thickness of this layer. Step 3: Design the low flame retardant content layer (10). Consider the support and reinforcement effect of this layer on the high flame retardant content layer. At the same time, design a reserved protective gas pipeline (9) in this layer. It is realized by cryogenic printing of a hollow structure to introduce SF6 (2) + CO2 (5) protective gas before low temperature casting. Step 4: Use a sand mixer to prepare molding sand with high and low flame retardant content respectively, and pre-cool the mixed molding sand to -40℃~-25℃ using dry ice or liquid nitrogen; Step 5: The two types of molding sand with high and low flame retardant content are respectively transported into two identical sand feeding devices (16). After the sand feeding device (16) sends the required type of sand into the sand laying trough (17), the photoelectric sensor (18) next to the sand laying trough (17) reads the required sand laying position and controls the sand laying trough (17) to open and close to lay the required molding sand. The workbench is moved down a certain distance and the sand laying device is used to lay a layer of low flame retardant content molding sand as the bottom sand. After the bottom sand is laid, the sand laying device returns to the original position. Step 6: The workbench moves down one layer thickness. Under the control of the control system, the sand laying device will lay high and low flame retardant content molding sand as needed according to the sand mold cross-section information of the current layer. Step 7: After the molding sand is laid, the sand laying device moves back to the initial position, and the control system controls the micro-droplet printing nozzle (11) to spray water adhesive as needed according to the current layer cross-sectional information of the sand mold, and complete the printing of the current layer of sand mold; Step 8: Repeat step 7. Whenever the amount of sand stored in the sand spreading device is insufficient to lay a layer of molding sand, the sand spreading device returns to the sand loading position and fills the molding sand from the sand mixing hopper. Step 9: Print layer by layer until the sand mold printing is completed by stacking the layers one by one; Step 10: Place the printed sand mold in a low-temperature chamber for secondary freezing; Step 11: Clean the sand to obtain the sand mold to be poured; Step 12: Half an hour before casting, introduce SF6 (2) + CO2 (5) protective gas into the pre-set pipeline of the cryogenic printing. The volume ratio of the two is 1:

100. The flow rate varies according to the size of the casting. Reduce the flow rate appropriately during casting. Step 13: Pour molten magnesium alloy into a gradient sand mold to obtain a casting.

2. The method for precise forming of magnesium alloy castings by gradient mixing of flame retardants in cryogenic casting according to claim 1, characterized in that, In step 1, the flame retardant is composed of boric acid and magnesite powder, with a mass ratio of boric acid to magnesite powder of 1:2.

5. The borax content in the face sand that is in direct contact with the casting surface is 2.6wt.%-3wt.%, and the boric acid content in the back sand is 1wt.%. The flame retardant content decreases gradually from the inside of the mold cavity to the outside.

3. The method for precise forming of magnesium alloy castings by gradient mixing of flame retardants in cryogenic casting according to claim 1, characterized in that, In step 2, the larger the diameter of the casting and the higher the pouring temperature, the greater the thickness of the high flame retardant content layer.

4. The method for precise forming of magnesium alloy castings by gradient mixing of flame retardants in cryogenic casting according to claim 1, characterized in that, In step 3, a protective gas pipeline needs to be designed in the low flame retardant content layer. Before pouring, SF6 + CO2 protective gas is introduced into the pipeline, with a volume ratio of 1:100 and a flow rate of 1-10 L / min.

5. The method for precise forming of magnesium alloy castings by gradient mixing of flame retardants in cryogenic casting according to claim 1, characterized in that, In step 5, the mixed molding sand is fed into two sand feeding devices. The sand feeding devices send different types of sand into the sand laying trough, and the photoelectric sensor locates the laying position. After the accurate position is located, the sand laying trough is controlled to open and close to lay the flame retardant molding sand, so as to realize the gradient printing of molding sand with high and low flame retardant content.

6. The method for precise forming of magnesium alloy castings by gradient mixing of flame retardants in cryogenic casting according to claim 1, characterized in that, In step 7, when using micro-droplet jet water adhesive to manufacture sand molds, the high content of borax in the sand molds can corrode the printhead. The service life of the printhead can be extended by adding a protective shell or applying a protective film to the printhead.

7. The method for precise forming of magnesium alloy castings by gradient mixing of flame retardants in cryogenic casting according to claim 1, characterized in that, In step 12, during the pouring process, the flow rate of the SF6+CO2 protective gas is reduced to 0.5L / min, and the gas cylinder pressure regulating valve is closed to end the gas supply after all steps are completed.

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