A Low-Pressure Precision Casting Method for Magnesium Alloy Investment Casting Based on High-Temperature Sintered Shell

Abstract

This invention belongs to the field of non-ferrous metal casting technology and discloses a low-pressure process for precision casting of magnesium alloy using a high-temperature sintered shell, comprising the following steps: Step 1: Preparing an investment mold assembly; Step 2: Bonding graphite chills to the thick and hot sections of the investment mold assembly; Step 3: Applying slurry and sprinkling sand layer by layer onto the investment mold assembly after bonding the graphite chills; Step 4: Dewaxing and sintering the investment mold assembly after slurry application and sand sprinkling to obtain a ceramic shell; Step 5: Shaping the ceramic shell to form an upper sand mold and a lower sand mold; Step 6: Assembling the upper and lower sand molds to form a composite structure mold; Step 7: Melting the magnesium alloy; Step 8: Low-pressure casting of the magnesium alloy in the composite structure mold; Step 9: Inspecting the cast parts.

Description

Technical Field

[0001] This invention belongs to the field of non-ferrous metal casting technology, and relates to a low-pressure precision casting method for magnesium alloy investment casting based on a high-temperature sintered shell. Background Technology

[0002] Magnesium alloys possess low density, high specific strength, good thermal conductivity, damping properties, shock absorption properties, and electromagnetic shielding properties, and are easily recyclable. Driven by today's industrial demands for environmental protection, lightweighting, and multi-functionality, magnesium alloy castings are increasingly widely used in aerospace, shipbuilding, automotive, and 3C electronics industries, among others. Precision magnesium alloy castings, in particular, are gaining increasing proportions in the magnesium alloy casting market due to their excellent surface quality, weight, and dimensional accuracy.

[0003] In the new industrial era, castings exhibit a demand and development trend towards "precision, lightweight, near-net-shape castings, and zero-defect castings." Investment casting offers advantages in near-net-shape and precision forming, and its application in high-performance forming of magnesium alloys greatly aligns with the development direction of casting towards "net-shape, thin-walled, high-precision, high-performance, large-scale, complex, and integrated" designs. However, magnesium alloys are highly chemically reactive. During traditional gravity casting, the molten magnesium flows and fills the ceramic mold shell with an open riser, making it highly susceptible to oxidation and combustion. Furthermore, under gravity casting, the melt flow rate is difficult to control, resulting in unstable filling and a greater tendency for gas entrapment and slag inclusions. In addition, magnesium alloys have a relatively wide solidification temperature range and a large solidification shrinkage rate, limiting the feeding effect during gravity casting, filling, and solidification, leading to a relatively greater tendency for shrinkage porosity.

[0004] Low-pressure casting offers advantages such as pure melt filling, controllable filling speed, stable filling, filling under pressure, and solidification, effectively addressing many drawbacks of gravity casting. Introducing low-pressure technology into investment casting, combining the advantages of both processes, can produce high-quality magnesium alloy precision castings. However, low-pressure casting for magnesium alloy investment casting is still relatively rare in engineering applications, and the process system is not yet fully developed. Although some low-pressure casting methods exist for magnesium alloy investment casting, they all have their own drawbacks. For example, poor mold venting can cause air blockage and cold shuts in the casting. Furthermore, without mold preheating, the ceramic shell surface will strongly hinder the filling of the highly reactive magnesium alloy melt during pouring, leading to cold shuts and under-casting. If the mold is preheated at high temperature, it can easily cause arcing and combustion during pouring. Summary of the Invention

[0005] Purpose of the invention: To address the technical challenges of high-quality casting of magnesium alloys, this invention proposes a low-pressure precision casting method for magnesium alloys based on a high-temperature sintered shell. This method includes the development and application of precision casting chills, preparation of silica sol ceramic shells, composite structure modeling, and low-pressure filling and solidification. It solves the technical problems of insufficient surface finish, weight, and dimensional accuracy of magnesium alloy castings under sand casting, and the relatively large tendency of magnesium alloy castings to have metallurgical defects under investment casting.

[0006] The technical solution of this invention is:

[0007] A low-pressure process for investment casting of magnesium alloys based on a high-temperature sintered shell includes the following steps:

[0008] Step 1: Prepare the investment casting module;

[0009] Step 2: Attach graphite chills to the geometric hot spot and the actual hot spot positions of the investment casting module;

[0010] Step 3: Apply slurry and sprinkle sand layer by layer onto the investment casting mold after the graphite chills have been bonded;

[0011] Step 4: Dewaxing and sintering the casting mold assembly after slurry coating and sand application to obtain a ceramic shell;

[0012] Step 5: Shape the ceramic shell to form an upper sand mold and a lower sand mold;

[0013] Step Six: Assemble the upper and lower sand molds to form a composite structure mold;

[0014] Step 7: Smelt the magnesium alloy;

[0015] Step 8: Perform low-pressure casting of magnesium alloy in a composite structure mold;

[0016] Step 9: Inspect the cast parts.

[0017] Furthermore, in step one, the specific method for preparing the investment casting module is as follows:

[0018] Prepare a PS powder rapid prototyping investment mold with the exact same shape as the casting, and bond a wax material-made gating system, gate, and riser onto the PS powder rapid prototyping investment mold;

[0019] Venting wax rods are bonded to the riser of the investment mold, the thickest part of the investment mold, and the location away from the feeding channel of the gating system to form venting channels for the ceramic shell;

[0020] Three protective gas wax rods are attached to both ends and the middle position in the direction of the maximum outline dimension of the investment mold to serve as channels for the introduction of protective gas before pouring.

[0021] Two signal wire wax rods are bonded to the top sidewall of the largest module riser and the other module riser that is furthest away from it. The signal wire wax rods are used to form the top signal wire channel of the ceramic shell.

[0022] Furthermore, in step two, the graphite chill conforms to the bonding position, and a layer of low-temperature wax is bonded to the bonding surface of the graphite chill during bonding, and then the graphite chill is bonded to the casting mold assembly.

[0023] Furthermore, in step three, the slurry is applied and sand is sprinkled and dried layer by layer on the investment mold with graphite chills, for a total of eight layers, of which the eighth layer is a sealing layer with slurry applied but no sand is sprinkled.

[0024] The specific parameters for each layer of slurry coating and sanding drying are shown in the table below:

[0025]

[0026] Before each layer of slurry is applied, compressed air is used to clean the surface sand from the dried previous layer.

[0027] Furthermore, in step four, the mold assembly after the slurry coating and sand drying are placed in a steam dewaxing kettle to melt and remove the mold assembly, thus obtaining the shell.

[0028] The shell is fired twice. The first firing is after dewaxing, at a temperature of 750℃±10℃ and a holding time of 2 hours. The second firing is before shaping, at a temperature of 1150℃±10℃ and a holding time of 2 hours. Both firings are cooled in the furnace to form a ceramic shell.

[0029] An aluminum tube with an vent plug is inserted into the venting channel formed after the venting wax rod melts, and the opening of the aluminum tube is wrapped with aluminum foil; no aluminum tube is inserted into the protective gas inlet channel formed by the three protective gas wax rods and the top signal line channel formed by the signal line wax rod, but they are also wrapped with aluminum foil.

[0030] Furthermore, in step five, the ceramic shell is placed in a bottomless box with a height less than half the height of the ceramic shell, with the shell gate facing downwards and positioned in the center of the box. Then, phenolic resin sand is filled in to fix the ceramic shell, forming an upper sand box.

[0031] Select a bottomless box with the same length and width as the upper sand box and a height of 300mm, fill it with phenolic resin sand to prepare the lower sand box. A cylindrical passage with the same diameter as the shell gate needs to be made in the center of the lower sand box, running through the entire height.

[0032] After the upper and lower sand boxes are dried for 2 hours, the outer box is removed to form the upper and lower sand molds. The upper and lower sand molds are then placed at a temperature of 200℃±20℃ for 2 to 3 hours to warm up.

[0033] Furthermore, the specific process in step six is ​​as follows:

[0034] Use compressed air to blow away the loose sand on the casting base plate, and place a magnesium oxide ceramic filter with the same diameter as the water inlet at the center of the casting base plate.

[0035] The top signal line is embedded in the channel reserved for the top signal line in the upper sand mold, and the bottom signal line is placed on the lower sand mold; the top signal line and the bottom signal line are distinguished by different colors.

[0036] Align and stack the pouring base plate inlet, the central cylindrical passage of the lower sand mold, and the bottom shell gate of the upper sand mold in sequence, with the pouring base plate at the bottom, the lower sand mold in the middle, and the upper sand mold at the top. Place a cylindrical casing around the assembled sand mold, with the casing height at least 300mm higher than the assembled sand mold height. When the gap between the casing and the base plate exceeds 5mm, fill it with sand.

[0037] Tear off the aluminum foil in the protective gas inlet channel formed by the three protective wax rods and insert the aluminum tube;

[0038] Phenolic resin sand is continued to be filled into the casing until the sand level reaches 5cm above the top of the aluminum tube. Then, the aluminum foil wrapped around the top of the aluminum tube is removed, and clean coarse sand is filled in to form a composite structure mold.

[0039] Furthermore, in step seven, the casting material is ZM6 alloy, and the alloy is smelted using a resistance crucible furnace;

[0040] The cleaned crucible is heated by electricity for more than 1 hour until it turns dark red. At this time, the crucible temperature is 400℃~500℃. Then, magnesium ingots and recycled materials are added, and protective gas is introduced. When the alloy temperature rises to 770℃~780℃, the preheated magnesium-zirconium master alloy is added. The melt is refined using a rotary impeller gas refiner. The refining gas is argon, the argon flow rate is 15L / min~20L / min, and the refining time is 12min~15min.

[0041] Furthermore, in step eight, before low-pressure casting, protective gas is introduced into the reserved protective gas inlet channel. More precisely, this is achieved by connecting the protective gas cylinder to the reserved protective gas inlet channel via an aluminum pipe. The protective gas enters the ceramic mold cavity through the aluminum pipe and the protective gas inlet channel, filling the cavity. Then, it is extracted by a vacuum pump from the inlet of the casting base plate. The protective gas is continuously introduced and extracted, circulating and flushing the inside of the cavity, ensuring that the cavity remains filled with protective gas throughout the casting process. The protective gas contains 1%–2% SF6 gas, with the remainder being CO2 gas. The CO2 is dried, and the protective gas flow rate is not less than 20 L / min. A vacuum pump is used at the inlet of the casting base plate to extract the gas, creating a pathway within the mold cavity and strengthening the protective atmosphere. The circulation time is not less than 20 minutes.

[0042] Then, formal low-pressure molding was carried out, with a molding rate of 0.35 kPa / s to 0.6 kPa / s;

[0043] After low-pressure filling and solidification are completed, the pressure of the casting machine is released, the mold is unloaded, and after the mold cools, sand is removed by vibration and the shell is cleaned by high-pressure water. Then the casting is taken out.

[0044] Furthermore, the castings undergo visual inspection, X-ray inspection, fluorescence inspection, and dimensional scanning.

[0045] This invention has the following beneficial effects: It forms a low-pressure precision casting method for magnesium alloy investment casting based on a high-temperature sintered shell, which includes the development and application of investment casting chills, investment casting preparation and combination, preparation of silica sol ceramic shells, composite structure modeling, low-pressure filling and solidification, etc. It can realize the research and production of high-quality magnesium alloy investment castings in aerospace, shipbuilding, automotive and other fields, help achieve the strategic goal of high quality and lightweight industrial equipment in my country, has both military and civilian applications, has broad market prospects, and has significant technical, economic and social benefits. Detailed Implementation

[0046] This section describes embodiments of the present invention, used to explain and illustrate the technical solutions of the present invention.

[0047] Example 1

[0048] A low-pressure precision casting method for magnesium alloy investment casting based on high-temperature sintered shells combines the development and application of investment casting chills, the preparation of silica sol ceramic shells, composite structural modeling, and low-pressure filling and solidification. These functionally linked elements form an integrated low-pressure casting process for magnesium alloy investment casting that encompasses the entire process, including chill application, shell preparation, composite modeling, and low-pressure casting.

[0049] High-purity graphite is used as an embedded chill for investment casting solidification chilling, which avoids the reduction of chilling effect caused by oxidation of traditional chills due to high temperature baking of the shell, and shell cracking due to differences in expansion coefficients.

[0050] The mold riser is sealed, and an exhaust pipe is installed to connect all areas of the mold, forming a parallel exhaust passage for the mold and casting, thereby enhancing the exhaust effect.

[0051] A silica sol ceramic shell is used. The shell is first sintered at a low temperature of 750℃ for 2 hours, and then sintered at a high temperature of 1150℃ for 2 hours to fully sinter the phase transformation, eliminate the shell gas generation phenomenon when the magnesium alloy melt flows through the silica sol shell, and reduce the filling resistance of the magnesium alloy melt in the silica sol shell.

[0052] A resin sand molding process is employed, where the resin sand is used to fill and fix the venting pipes, thus securing the shell and forming the basic mold. Above the resin sand, the venting pipe section of the shell is covered with coarse sand, utilizing the gaps between the sand grains as venting channels within the outer mold to complete the composite structure molding.

[0053] The mold uses a mixture of SF6 and CO2 for gas protection during filling and solidification, with SF6 accounting for 1% to 2% and the remainder being CO2. The protective atmosphere is introduced through three evenly distributed venting pipes pre-installed in the mold, while a vacuum pump is used at the gating gate on the bottom plate to evacuate the gas, thus creating a pathway for the protective gas within the mold cavity and enhancing the protective atmosphere.

[0054] The shell and mold are not preheated, and room temperature pouring is used, which reduces the risk of human operation and process cost.

[0055] The low-pressure filling and solidification process, based on the characteristics of the molten mold shell, eliminates the shell formation and pressurization stage, reducing the risk of overpressure and fire escape.

[0056] Example 2

[0057] This implementation case is a low-pressure casting of a magnesium alloy transmission housing for aviation applications, produced through investment casting. The casting is a medium-sized "banana-shaped" cavity structure magnesium alloy casting with an outline dimension of approximately 850mm × 550mm × 200mm. The detailed casting process is shown below.

[0058] (1) Investment casting preparation and assembly:

[0059] A PS powder rapid prototyping investment mold with the exact same structure as the casting is used as the casting investment mold. The investment mold is assembled according to the gating system design, that is, the gating system, gates, risers, etc., made of wax material are adhered to the casting investment mold. In addition, to ensure the venting effect of the subsequent composite structure mold, a certain number of wax rods, approximately 20mm in diameter and 100mm in length, are adhered to the mold risers, the thickest parts of the casting investment mold, and positions far from the gating system's feeding channels after the investment mold is assembled. These wax rods will become venting channels for the entire mold cavity after dewaxing and shell preparation, venting the gas from the entire mold cavity during the alloy melt filling process. Simultaneously, three wax rods of the same size are adhered to both ends and the middle position along the maximum outline dimension direction of the entire mold (farthest distance). These three wax rods will serve as channels for introducing protective gas before pouring.

[0060] Two small wax rods, 5mm in diameter and 20mm in length, are attached to the top side of the largest module riser (if there are several largest risers on the module, one can be chosen) and the other module riser that is furthest from this riser. These will serve as channels for placing the top pin signal line during molding. At this point, the final investment casting module is formed.

[0061] (2) Adhesive chilling:

[0062] To accelerate the local cooling rate of the casting, according to the gating system design, high-purity graphite chills are adhered to the geometric hot spots (the thickest parts of the module) and the actual hot spots (the final solidification points of the casting). During adhesion, a layer of low-temperature wax is applied to the surface of the graphite chill, and then the chill is adhered to the corresponding position, ensuring that the graphite shape conforms to the adhesion location.

[0063] (3) Module coating, sanding and drying:

[0064] Each layer of slurry required for shell preparation was prepared separately. The slurry consisted of binder and refractory powder. The binder, powder, and flow rate requirements for each layer of slurry are shown in Table 1. The slurry flow rate was measured using a Chinese standard flow cup No. 4. After the slurry preparation was completed, the mold with the chills was coated with slurry, sand, and dried. The relevant parameters are shown in Table 1. Before each layer was applied, compressed air was used to clean the surface of the previous shell layer to prevent bridging. Material accumulation was avoided during coating. The next layer was applied after the previous layer was completely dry. A total of eight layers were applied, with the eighth layer being a sealing layer, which only had slurry applied and no sand applied. Finally, a shell was formed on the outside of the mold.

[0065] Table 1 Shell Coating Parameters

[0066]

[0067]

[0068] (4) Shell dewaxing and sintering:

[0069] The mold with the shell formed in (3) is placed in a steam dewaxing kettle, so that the mold, i.e., the casting rapid forming mold in (1) and the wax material such as gate, sprue, riser, wax rod, etc., which are adhering to the casting mold, melt and are removed from the mold shell. Then the shell is fired twice. The first firing is after dewaxing, at a firing temperature of (750±10)℃ and a holding time of 2h. The second firing is before molding, at a firing temperature of (1150±10)℃ and a holding time of 2h. Both firings are cooled with the furnace to form a ceramic shell.

[0070] The venting wax rod, protective gas inlet wax rod, and mold top signal line channel wax rod in (1) form channels for venting the mold cavity, introducing protective gas into the mold cavity before pouring, and placing the mold top signal line, respectively, after the mold shell is dewaxed and sintered. After the ceramic mold shell cools, the metal aluminum tube with the vent plug is connected to the reserved venting channel. The metal aluminum tube is about 18mm in diameter and about 100mm in length. Then the opening of the aluminum tube with the vent plug is wrapped with aluminum foil to prevent it from being blocked by resin sand during molding. Note that the three channels for introducing protective gas before pouring and the two channels for placing the mold top signal line reserved in (1) are not connected to the metal aluminum tube, but are also covered with aluminum foil.

[0071] (5) Design

[0072] Place the ceramic shell prepared in (4) into a bottomless box with appropriate length and width dimensions and a height less than half the height of the shell, with the shell gate facing down and positioned in the center of the box. Then, fill the box with phenolic resin sand to fix the ceramic shell, forming the upper sand box. The bottom surface of the shell gate is flush with the bottom surface of the upper sand box. When filling the box with phenolic resin sand, pay attention to filling and sealing the gaps in the cavity near the bottom of the shell and the gate. Fill the box with resin sand up to the height of the box. Then, select a bottomless box with the same length and width as the upper sand box and a height of about 300 mm and fill it with phenolic resin sand to prepare the lower sand box. A cylindrical passage with the same diameter as the shell gate needs to be made in the center of the lower sand box. After the upper and lower sand boxes are dried and cured for 2 hours, remove the box to form the upper and lower sand molds. Place the two sand molds at (200±20)℃ for 2-3 hours to reheat.

[0073] (6) Group

[0074] First, use compressed air to blow away the loose sand on the circular casting base plate. The casting base plate is a built-in device of the low-pressure equipment and has fixed specifications. Place a magnesium oxide ceramic filter plate with the same diameter as the water inlet at the center of the casting base plate.

[0075] Subsequently, the top signal wire is embedded in the pre-reserved top signal wire channel in the upper sand mold, and the bottom signal wire is placed on top of the lower sand mold. When placing the bottom signal wire, first, a small groove slightly larger than the diameter of the signal wire is dug in the upper sand mold, extending from the center channel of the lower sand mold to the outer edge of the lower sand mold. Then, the signal wire is placed in the groove, with the signal suspended at the center channel. The top and bottom signal wires should be distinguished by different colors.

[0076] Next, align and stack the inlet of the casting base plate, the center channel of the lower sand mold, and the bottom shell gate of the upper sand mold in sequence, with the casting base plate at the bottom, the lower sand mold in the middle, and the upper sand mold at the top. During the sand mold assembly process, the joints of the inlet, center channel, and bottom gate should be carefully checked. Since the diameters of the three are the same, the joints of the three should fit tightly, and the offset distance should not exceed 20mm. After stacking the three, place a circular sleeve on the casting base plate to completely fit the assembled upper and lower sand molds, and make sure it is more than 300mm higher than the height of the sand mold assembly. The bottom of the sleeve should be placed flat and close to the casting base plate. If the gap between the sleeve and the casting base plate exceeds 5mm, it should be filled with sand; otherwise, it is not allowed to be used. After placing the sleeve, connect metal aluminum pipes to the three channels reserved for protective gas in (1). The length of the aluminum pipes should be about 300mm higher than the height of the sleeve, and they are used to pass protective gas before casting.

[0077] Finally, phenolic resin sand is added to the mold box until it reaches approximately 5cm above the top of the exhaust aluminum pipe. The aluminum foil covering the top of the pipe is then removed, and the mold is filled with clean, coarse sand to form a composite structure. Care must be taken to protect the exhaust pipe during the resin sand filling process; it must not be broken. If a break occurs, the broken section must be cleaned, wrapped with refractory cotton, and then the resin sand is added again to continue the molding process.

[0078] (7) Magnesium alloy smelting:

[0079] The casting material is ZM6 alloy, and the alloy is smelted using a resistance crucible furnace. The cleaned crucible is heated electrically for more than 1 hour (until it turns dark red), approximately (400-500)℃. Then, magnesium ingots and recycled materials are added, and an appropriate amount of protective gas is introduced. When the alloy temperature rises to (770-780)℃, a preheated magnesium-zirconium master alloy is added. The melt is refined using a rotary impeller gas refiner, with argon as the refining gas. The argon flow rate is (15-20) L / min, and the refining time is (12-15) min.

[0080] (8) Low-pressure casting:

[0081] Before casting, a protective gas (SF6 + CO2) is introduced through the pre-reserved ventilation pipe in the mold, where SF6 accounts for 1% to 2% and the remainder is CO2. The CO2 must be dried, and the flow rate of the protective gas must reach at least 20 L / min. At the same time, a vacuum pump is used to evacuate the gas at the gating gate of the casting base plate to create a passage for the protective gas within the shell cavity, thereby strengthening the protective atmosphere. The circulation time should be no less than 20 minutes.

[0082] (1) The casting mold, gate, riser, and all wax rods and other wax materials are combined into a whole through the casting mold. After the shell is prepared, the casting mold and wax materials are completely melted and removed to form a hollow shell with all parts. During molding and assembly, the shell is buried in the bottomless box. The shell gate is flush with the bottom of the upper sand mold and is not sealed by sand. Then it is aligned and stacked with the central channel of the lower sand box and the water inlet of the pouring bottom plate. At this time, the water inlet of the pouring bottom plate is connected to the shell and the whole is connected. Moreover, during pouring, the magnesium alloy solution also flows from the water inlet through the central channel of the lower sand box and from the shell gate of the upper sand box to fill the entire shell cavity, and finally completes the filling.

[0083] To promote effective venting during mold filling and prevent entrapment and turbulent flow, the filling rate should not be too fast, generally not exceeding 0.6 kPa / s. However, an excessively slow filling rate will increase heat loss in the alloy melt during filling, reducing its filling capacity and leading to metallurgical defects such as cold shuts and porosity. Therefore, the filling rate should not be lower than 0.35 kPa / s. Furthermore, since the ceramic shell itself has crust-forming properties, the crystallization pressurization stage can be eliminated during casting. The optimized low-pressure casting process parameters for investment casting in this case are shown in Table 2. After low-pressure filling and solidification, the casting machine pressure is released, the mold is unloaded, and after cooling, sand is removed using vibration, and the shell is cleaned with high-pressure water before the casting is removed.

[0084] Table 2 Low-pressure casting process parameters

[0085]

[0086] (9) Casting inspection: The example castings are inspected by appearance inspection, X-ray inspection, fluorescence inspection, dimensional scanning and other inspections. The castings are well formed, have excellent surface quality, accurate weight and dimensions, excellent metallurgical quality, and high quality that meets the requirements of casting blanks.

Claims

1. A low-pressure process for investment casting of magnesium alloys based on a high-temperature sintered shell, characterized in that: The method includes the following steps: Step 1: Prepare the casting mold module, which has venting wax rods, protective wax rods and signal line wax rods attached to it; Step 2: Attach graphite chills to the geometric hot spot and the actual hot spot positions of the investment casting module; Step 3: Apply slurry and sprinkle sand layer by layer onto the investment casting mold after the graphite chills have been bonded; Step 4: Dewaxing and sintering the molded mold after coating and sanding the slurry to obtain a ceramic shell; inserting an aluminum tube with an vent plug into the venting channel formed after the venting wax rod melts, and wrapping the opening of the aluminum tube with aluminum foil; not inserting aluminum tubes into the protective gas inlet channel formed by the three protective gas wax rods and the top signal line channel formed by the signal line wax rod, but wrapping them with aluminum foil as well. Step 5: Shape the ceramic shell to form an upper sand mold and a lower sand mold; Step Six: Assemble the upper and lower sand molds to form a composite structure mold; the specific process is as follows: use compressed air to blow away the floating sand on the casting base plate, and place a magnesium oxide ceramic filter with the same diameter as the water inlet at the center of the casting base plate. The top signal line is embedded in the channel reserved for the top signal line in the upper sand mold, and the bottom signal line is placed on the lower sand mold; the top signal line and the bottom signal line are distinguished by different colors. Align and stack the pouring base plate inlet, the central cylindrical passage of the lower sand mold, and the bottom shell gate of the upper sand mold in sequence, with the pouring base plate at the bottom, the lower sand mold in the middle, and the upper sand mold at the top. Place a cylindrical casing around the assembled sand mold, with the casing height at least 300mm higher than the assembled sand mold height. If the gap between the casing and the base plate exceeds 5mm, fill it with sand. Tear off the aluminum foil in the protective gas inlet channel formed by the three protective wax rods and insert the aluminum tube; continue to fill the box with phenolic resin sand until the sand level is 5cm from the top of the aluminum tube, then remove the aluminum foil wrapped around the top of the aluminum tube and fill it with clean coarse sand to form a composite structure mold. Step 7: Smelt the magnesium alloy; Step 8: Perform low-pressure casting of magnesium alloy in the composite structure mold. Before low-pressure casting, introduce protective gas into the reserved protective gas inlet channel, and use a vacuum pump to evacuate the gas at the water inlet of the casting bottom plate to form a passage for the protective gas in the shell cavity. The circulation time shall not be less than 20 minutes, followed by formal low-pressure filling at a filling rate of 0.35 kPa / s to 0.6 kPa / s. Step 9: Inspect the cast parts.

2. The method according to claim 1, characterized in that: In step one, the specific preparation method of the investment casting module is as follows: Prepare a PS powder rapid prototyping investment mold with the exact same shape as the casting, and bond a wax material-made gating system, gate, and riser onto the PS powder rapid prototyping investment mold; Venting wax rods are bonded to the riser of the investment mold, the thickest part of the investment mold, and the location away from the feeding channel of the gating system to form venting channels for the ceramic shell; Three protective wax rods are attached to both ends and the middle position in the direction of the maximum outline dimension of the investment mold to serve as channels for the introduction of protective gas before pouring. Two signal wire wax rods are bonded to the top sidewall of the largest module riser and the other module riser that is furthest away from it. The signal wire wax rods are used to form the top signal wire channel of the ceramic shell.

3. The method according to claim 2, characterized in that: In step two, the graphite chill conforms to the bonding position. During bonding, a layer of low-temperature wax is applied to the bonding surface of the graphite chill, and then the graphite chill is bonded to the casting mold assembly.

4. The method according to claim 3, characterized in that: In step three, the slurry is applied and sand is sprinkled and dried layer by layer on the investment mold with graphite chills, for a total of eight layers. The eighth layer is a sealing layer, in which only slurry is applied and no sand is sprinkled. The specific parameters for each layer of slurry coating and sanding drying are shown in the table below: Before each layer of slurry is applied, compressed air is used to clean the surface sand from the dried previous layer.

5. The method according to claim 4, characterized in that: In step four, the mold assembly after the slurry coating and sand drying are placed in a steam dewaxing kettle to melt and remove the mold assembly, thus obtaining the shell. The shell is fired twice. The first firing is after dewaxing, at a temperature of 750℃±10℃ and a holding time of 2 hours. The second firing is before shaping, at a temperature of 1150℃±10℃ and a holding time of 2 hours. Both firings are cooled in the furnace to form a ceramic shell.

6. The method according to claim 5, characterized in that: In step five, the ceramic shell is placed in a bottomless box with a height less than half the height of the ceramic shell, with the shell gate facing down and positioned in the center of the box. Then, phenolic resin sand is filled in to fix the ceramic shell, forming an upper sand box. Select a bottomless box with the same length and width as the upper sand box and a height of 300mm, fill it with phenolic resin sand to prepare the lower sand box. A cylindrical passage with the same diameter as the shell gate needs to be made in the center of the lower sand box, running through the entire height. After the upper and lower sand boxes are dried for 2 hours, the outer box is removed to form the upper and lower sand molds. The upper and lower sand molds are then placed at a temperature of 200℃±20℃ for 2 to 3 hours to warm up.

7. The method according to claim 6, characterized in that: In step seven, the casting material is ZM6 alloy, and the alloy is smelted using a resistance crucible furnace. The cleaned crucible is heated by electricity for more than 1 hour until it turns dark red. At this time, the crucible temperature is 400℃~500℃. Then, magnesium ingots and recycled materials are added, and protective gas is introduced. When the alloy temperature rises to 770℃~780℃, the preheated magnesium-zirconium master alloy is added. The melt is refined using a rotary impeller gas refiner. The refining gas is argon, with an argon flow rate of 15 L / min~20 L / min and a refining time of 12 min~15 min.

8. The method according to claim 7, characterized in that: In step eight, SF6 gas accounts for 1% to 2% of the protective gas, and the remainder is CO2 gas. The CO2 is dried, and the flow rate of the protective gas is not less than 20 L / min. After low-pressure filling and solidification are completed, the pressure in the casting machine tank is released, the mold is unloaded, and after the mold cools, sand is removed by vibration and the shell is cleaned by high-pressure water. Then the casting is taken out.

9. The method according to claim 8, characterized in that: The castings undergo visual inspection, X-ray inspection, fluorescence inspection, and dimensional scanning.

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