A method for preparing sand molds / cores for titanium alloy casting based on 3D printing

Abstract

This invention discloses a method for preparing sand molds / cores for titanium alloy casting based on 3D printing. The method employs a vacuum pressure impregnation process of an inorganic binder onto the 3D-printed sand mold / core. By adding processing aids, high-temperature sintering aids, and additives to the 3D-printed molding sand material, followed by low-temperature sand embedding and calcination, the 3D-printed sand mold / core is then subjected to vacuum pressure impregnation of the inorganic binder. Finally, the sand mold / core is dip-coated or spray-coated with an inert material before being embedded in sand and calcined at high temperature. This method enables rapid, moldless fabrication of complex integral sand molds / cores. The sand molds / cores prepared by this method exhibit excellent high-temperature strength and low gas evolution, do not crack, and have a stable inert coating. When applied to titanium alloy casting, this method offers advantages such as high production efficiency, low cost, and safety.

Description

Technical Field

[0001] This invention relates to the field of titanium alloy sand casting technology, and in particular to a method for preparing a sand mold / core for titanium alloy casting based on 3D printing. Background Technology

[0002] Titanium and titanium alloys are widely used due to their high specific strength, low density, low thermal conductivity, strong resistance to high-temperature oxidation, and excellent corrosion resistance, especially in the aerospace industry where they are crucial strategic metal materials. With the rapid development of various equipment manufacturing sectors, the requirements for the mechanical properties, dimensional accuracy, and reliability of key titanium alloy components are becoming increasingly stringent. In recent years, to meet the development needs of large, complex, thin-walled titanium alloy precision castings, processes such as titanium alloy sand casting have been continuously improved and developed. Compared to investment casting, titanium alloy sand casting has advantages in terms of rapid manufacturing, the ability to produce large castings, and cost. Currently, there is a demand in practical engineering for replacing some precision castings and for producing castings ranging from simple to complex. However, titanium alloy sand casting still has certain limitations. This is because molten titanium alloy has high chemical reactivity and can react with almost all refractory materials. The interaction between molten titanium and the mold can form a contamination layer on the casting surface, greatly affecting the surface quality and performance of the casting. Moreover, traditional sand mold preparation methods are difficult to quickly prepare complex sand molds / cores, especially complex and irregularly shaped sand molds / cores, and the dimensional accuracy of sand molds / cores cannot be guaranteed. These problems limit the development of titanium alloy sand mold casting based on 3D printed sand molds / cores.

[0003] With the rapid development of modern 3D printing technology, the direct fabrication of sand molds / cores using 3D printing technology has become widely used. Especially in recent years, 3D printing technology has been able to fabricate complex, integrated, thin-walled, and partially functional sand molds / cores, rapidly and effectively increasing the complexity of sand mold / core fabrication. Among these, the direct fabrication of sand molds / cores using Selective Laser Sintering (SLS) and Three-Dimensional Printing (3DP) technologies offers advantages such as fast response speed, short manufacturing cycle, high flexibility, good stability, integrated fabrication of sand molds and cores, and the ability to produce arbitrarily complex shapes. These advantages significantly promote the rapid prototyping and manufacturing of large and complex castings, demonstrating great potential in solving the production of some key castings in aerospace and automotive fields. Currently, sand molds / cores formed using SLS and 3DP methods are practically applied in the production of parts made of cast aluminum, cast steel, and cast iron, both domestically and internationally. However, molten titanium, due to its high chemical reactivity, readily undergoes interfacial reactions with conventional mold materials during casting, leading to a series of casting defects on the titanium casting surface, such as a thick oxide layer, sand adhesion, surface inclusions, and porosity. This limits the application of SLS and 3DP sand molds / cores in titanium alloy casting. Compared to traditional molding techniques, 3D printed sand molds / cores also suffer from low thermal strength, high gas generation, and cracking or even collapse during the pouring process due to the use of organic binders. Furthermore, the lack of a high-temperature inert coating on the surface of the sand molds / cores makes them unsuitable for pouring chemically reactive titanium alloys. Clearly, sand molds / cores used in titanium alloy casting should possess high thermal strength, a low coefficient of expansion, and high chemical inertness to prevent violent chemical reactions between the highly reactive molten titanium alloy and refractory oxides.

[0004] Clearly, the key to the development of this technology lies in how to replace the organic binder in 3D-printed sand molds / cores with inorganic binders through processing methods. Considering the technical characteristics of 3D-printed sand molds / cores, a vacuum pressure impregnation method is used to add processing aids, additives, and sintering aids to the molding sand material. This method forms an inorganic bonding layer of a certain thickness inside and outside the sand mold / core, which replaces the ablated organic binder after high-temperature calcination of the sand mold / core, thus achieving a coating with good thermal strength and a stable inert material coating. Therefore, researching the inorganic binder impregnation and preparation process for 3D-printed sand molds / cores for titanium alloy casting plays an important role in promoting the application of rapid prototyping technology in the field of titanium alloy casting. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing sand molds / cores for titanium alloy casting based on 3D printing.

[0006] The objective of this invention is achieved as follows:

[0007] A method for preparing sand molds / cores for titanium alloy casting based on 3D printing, characterized by the following process steps:

[0008] A. Preparation of molding sand materials suitable for 3D printing

[0009] Add 1-10 wt% of additives, 0.1-5 wt% of processing aids, and 0.1-5 wt% of sintering aids to the raw sand material. Add the additives, processing aids, and sintering aids to the raw sand in a set ratio, mix thoroughly, sieve, and package to obtain 3DP molding sand material suitable for titanium alloy casting; or use a hot coating process to coat the raw sand material with phenolic resin to obtain phenolic resin coated sand. Add 1-10 wt% of additives, 0.1-5 wt% of processing aids, and 0.1-5 wt% of sintering aids to the phenolic resin coated sand, mix thoroughly, sieve, and seal to obtain SLS forming molding sand material suitable for titanium alloy casting.

[0010] B. 3D Printing Sand Molds / Cores

[0011] The digital model to be printed is sliced, and then the molding sand material or SLS molding sand material used for 3DP is 3D printed using conventional methods to obtain the 3D printed sand mold / core.

[0012] C. Sand cleaning, SLS-formed sand mold / core surface spraying and low-temperature calcination with embedded sand

[0013] After the sand mold / core prepared by 3DP technology is formed, the uncured and connected supporting sand particles are removed. For the sand mold / core formed by SLS technology, the surface is first directly sprayed and fired to improve the surface strength of the sand mold / core. Then, the sand mold / core formed by SLS technology is placed in a forced-air drying oven and fired at a low temperature of 150-250℃ for 30-180 minutes. After the low temperature firing, the sand mold / core is cooled to 100℃-room temperature, and the supports used in the sand mold / core due to process design requirements are removed.

[0014] D. Vacuum pressure impregnation of inorganic binder into sand mold / core

[0015] Sand molds / cores prepared using 3DP technology, or SLS-formed sand molds / cores after low-temperature calcination, are placed in a vacuum pressure impregnation device for impregnation with inorganic binders. The vacuum environment is 0.1KPa-50KPa, the impregnation pressure is 0.1MPa-1MPa, the impregnation temperature is 10℃-100℃, the impregnation time is 5min-60min, and finally, they are naturally air-dried at room temperature for 12-48h.

[0016] E. Dipping or spraying inert material coating onto sand molds / cores

[0017] Then, an inert material coating is dipped or sprayed onto the surface of the sand mold cavity and the surface of the sand core to form an inert material coating.

[0018] F. High-temperature calcination process for embedding sand in sand molds / cores

[0019] After the inert material coating dries, the sand mold / core is then subjected to a stepped high-temperature calcination process at a temperature of 300-1250℃ and a holding time of 30-480 minutes. After cooling to 300℃-room temperature, the sand mold / core for titanium alloy casting is obtained.

[0020] In step A, the raw sand material is one of the following: alumina sand, zircon sand, corundum sand, bauxite, zircon sand and quartz sand particles, and the particle size range of the raw sand is 40 mesh to 400 mesh.

[0021] The additives are one or a combination of two of the following: calcium stearate powder, polyimide resin powder, calcium aluminate powder, barium aluminate powder, barium zirconium aluminate powder, yttrium oxide powder, and iron oxide powder, with a particle size range of 70 mesh to 800 mesh after grinding and sieving.

[0022] The processing aids are one or a combination of two of the following: sodium hydroxymethyl starch, polyvinyl alcohol (PVA) powder, polystyrene (PS) powder, and polyurethane (PU) powder. The particle size range after grinding and sieving is 70-800 mesh.

[0023] The sintering aids are one or a combination of two of the following: ZrO2, TiO2, CaO, MgO, Y2O3 and CeO2 powders, with a particle size range of 0.01-100µm after ball milling and sieving.

[0024] In step C, for sand molds / cores formed using 3DP technology, the unsintered and uncured supporting sand particles are blown clean with compressed air. After the sand molds / cores are cleaned, they are prepared for the impregnation of inorganic binders. For sand molds / cores formed using SLS technology, the surface is first directly sprayed with a gas torch, paying special attention to small structural parts to improve the surface strength of the sand molds / cores and prevent breakage or damage to the fine structural parts of the sand molds / cores during handling. Then, the sand molds / cores formed using SLS technology are placed in a forced-air oven for low-temperature buried sand baking. After the sand molds / cores cool to 100°C to room temperature, the supports used for the sand molds / cores are removed.

[0025] The embedded sand material is one of glass microspheres, abrasive sand and quartz sand, and the particle size range after sieving is 40 mesh to 800 mesh.

[0026] In step D, the inorganic binder is one or a combination of two of the following: silica sol, modified silica sol, water glass, aluminum dihydrogen phosphate solution, disodium hydrogen phosphate solution, magnesium dihydrogen phosphate solution, sodium hexametaphosphate solution, zirconium phosphate solution, and ethyl silicate hydrolysate.

[0027] In step E, the inert coating is one of yttrium oxide coating, zirconium oxide coating, and thorium oxide coating.

[0028] In step F, the embedded sand material is one or a combination of two of the following: corundum sand, bauxite, zircon sand, and alumina sand. After sieving, the particle size ranges from 40 mesh to 800 mesh. The high-temperature calcination of the embedded sand is a stepped heating process, divided into three stages: the first stage, heating from room temperature to T1 and holding for N1 minutes for degassing; the second stage, heating from T1 to T2 and holding for N2 minutes for ablating the resin; and the third stage, heating from T2 to T3 and holding for N3 minutes for sintering the 3D printed sand mold / core. Among these stages, T1 is 200-500℃, N1 is 30-180 min, T2 is 600-850℃, N2 is 60-180 min, and T3 is 900-1250℃, N3 is 90-480 min. After cooling to 300℃-room temperature, the sand mold / core for titanium alloy casting is obtained.

[0029] To address the problems of current 3D-printed sand molds / cores, which rely solely on organic binders, resulting in insufficient high-temperature strength, high gas generation, and the lack of a stable inert coating on the surface, thus limiting their use for casting chemically reactive titanium alloys, a vacuum pressure impregnation process using inorganic binders was adopted for 3D-printed sand molds / cores. Processing aids, high-temperature sintering aids, and additives were added to the 3D-printed molding sand material, achieving effective impregnation of the sand mold / core. After impregnation or spraying with an inert coating, the sand mold / core was buried and fired at high temperature, rapidly producing complex integral sand molds / cores without a mold. The prepared sand molds / cores exhibit excellent high-temperature strength and low gas generation, are crack-free, and have a stable inert coating. Their application in titanium alloy casting offers advantages such as high production efficiency, low cost, and safety. Attached Figure Description

[0030] Figure 1 This is a simplified process flow diagram of the present invention;

[0031] Figure 2 This is a schematic diagram of the high-temperature calcination curve of the sand mold / core in this invention. Detailed Implementation

[0032] The present invention will be further described below with reference to the embodiments and the accompanying drawings.

[0033] Example 1:

[0034] 1) The abrasive particles are sieved to obtain 100 / 200 mesh three-sieve sand. Solid thermoplastic phenolic resin is crushed into uniform phenolic resin powder with a particle size range of 140-400 mesh. Phenolic resin coated sand is prepared by thermal coating method, wherein: the amount of phenolic resin powder with a particle size range of 140-400 mesh is 1.9 wt% of the above three-sieve sand, the amount of silane coupling agent KH550 is 1 wt% of the phenolic resin powder, and a KH550 aqueous solution with a mass fraction of 10% is prepared. Hexamethylenetetramine is added at a mass fraction of 12% of the phenolic resin powder and a hexamethylenetetramine aqueous solution with a mass fraction of 50% is prepared. Phenolic resin coated sand is obtained by thermally processing phenolic resin powder, KH550 aqueous solution, hexamethylenetetramine aqueous solution and abrasive particles.

[0035] 2) Add calcium stearate powder and polyvinyl alcohol powder (PVA powder) with a particle size range of 140-400 mesh, at amounts of 5 wt% and 0.5 wt% of the original sand weight, respectively.

[0036] 3) Add nano-ZrO2 with a particle size range of 20-50nm as a sintering aid, at a dosage of 0.5wt% of the original sand weight;

[0037] 4) Mix the above materials evenly to obtain molding sand material suitable for selective laser sintering;

[0038] 5) Slice the digital model that needs to be printed into sand molds / cores, and then perform SLS forming on the above-mentioned molding sand material according to conventional methods to obtain SLS formed sand molds / cores;

[0039] 6) After SLS forming of the sand mold / core, remove the unsintered and uncured supporting sand particles and spray them directly onto the surface of the sand mold / core to improve the surface strength of the sand mold / core;

[0040] 7) The SLS-formed sand mold / core is embedded in glass microspheres and calcined at a low temperature of 170℃ for 180min. After the sand mold / core cools to room temperature, the support used for the sand mold / core due to process design requirements is removed.

[0041] 8) Place the sand mold / core after low-temperature buried sand firing in a vacuum pressure impregnation device for vacuum pressure impregnation of water glass with a modulus of 2.0. The vacuum degree is 0.8 kPa, the impregnation pressure is 0.1 MPa, the impregnation temperature is 20℃, and the impregnation time is 50 min. After impregnation is completed, take out the sand mold / core and let it dry naturally at room temperature for 15 h.

[0042] 9) Then, spray yttrium oxide coating onto the dried sand mold / core surface to form an inert material coating;

[0043] 10) After the inert material coating dries, the sand is buried at 300℃+90min, 500℃+90min, and 1050℃+200min for high-temperature calcination. The buried sand material is 100-800 mesh corundum powder. After cooling to room temperature, the sand mold / core for titanium alloy casting is obtained. The tensile strength of the obtained sand mold / core is 2.35MPa, the bending strength is 5.46MPa, and the surface of the sand mold / core is smooth and free of cracks.

[0044] Example 2:

[0045] 1) The bauxite particles were sieved to obtain 50 / 100 mesh three-sieve sand. Solid thermoplastic phenolic resin was crushed into uniform phenolic resin powder with a particle size range of 70-200 mesh. Phenolic resin coated sand was prepared by thermal coating method, wherein the amount of phenolic resin powder with a particle size range of 70-200 mesh was 2.5 wt% of the above three-sieve sand, the amount of silane coupling agent KH550 was 1 wt% of the phenolic resin powder, and a KH550 aqueous solution with a mass fraction of 10% was prepared. Hexamethylenetetramine was added with a mass fraction of 12% of the phenolic resin powder, and a hexamethylenetetramine aqueous solution with a mass fraction of 50% was prepared. Phenolic resin coated sand was obtained by thermal method with phenolic resin powder, KH550 aqueous solution, hexamethylenetetramine aqueous solution and bauxite.

[0046] 2) Add calcium stearate powder with a particle size range of 70-200 mesh and polystyrene (PS) powder, at amounts of 7 wt% and 2.5 wt% of the original sand weight, respectively;

[0047] 3) Add nano-Y2O3 with a particle size range of 50-100nm as a sintering aid, at a dosage of 1wt% of the original sand weight;

[0048] 4) Mix the above materials evenly to obtain molding sand material suitable for selective laser sintering;

[0049] 5) Slice the digital model that needs to be printed into sand molds / cores, and then perform SLS forming on the above-mentioned molding sand material according to conventional methods to obtain SLS formed sand molds / cores;

[0050] 6) After SLS forming of the sand mold / core, remove the unsintered and uncured supporting sand particles and spray them directly onto the surface of the sand mold / core to improve the surface strength of the sand mold / core;

[0051] 7) The SLS-formed sand mold / core is embedded in glass microspheres and calcined at a low temperature of 190℃ for 90 minutes. After the sand mold / core cools to room temperature, the support used for the sand mold / core due to process design requirements is removed.

[0052] 8) Place the sand mold / core after low-temperature buried sand firing in a vacuum pressure impregnation device for impregnation with aluminum dihydrogen phosphate solution. The vacuum degree is 45 kPa, the impregnation pressure is 0.1 MPa, the impregnation temperature is 50℃, and the impregnation time is 20 min. After impregnation is completed, take out the sand mold / core and let it dry naturally at room temperature for 18 h.

[0053] 9) Then, spray thorium oxide coating onto the dried sand mold / core surface to form an inert material coating;

[0054] 10) After the inert material coating dries, the sand is buried at 400℃+120min, 600℃+120min and 1100℃+240min for high-temperature calcination. The buried sand material is 200~400 mesh bauxite powder. After cooling to room temperature, the sand mold / core for titanium alloy casting is obtained. The tensile strength of the sand mold / core is 2.82MPa and the bending strength is 6.56MPa. The surface of the sand mold / core is smooth and free of cracks.

[0055] Example 3:

[0056] 1) The corundum sand particles are sieved to obtain 70 / 140 mesh three-sieve sand. Solid thermoplastic phenolic resin is crushed into uniform phenolic resin powder with a particle size range of 100-200 mesh. Phenolic resin coated sand is prepared by thermal coating method, wherein the amount of phenolic resin powder added accounts for 3.5 wt% of the above three-sieve sand, the amount of silane coupling agent KH550 added accounts for 1 wt% of the phenolic resin powder, and a KH550 aqueous solution with a mass fraction of 10% is prepared. Hexamethylenetetramine added accounts for 12% of the phenolic resin powder, and a hexamethylenetetramine aqueous solution with a mass fraction of 50% is prepared. Phenolic resin coated sand is obtained by thermal method with phenolic resin powder, KH550 aqueous solution, hexamethylenetetramine aqueous solution and corundum sand.

[0057] 2) Add calcium stearate powder with a particle size range of 100-200 mesh and polyurethane (PU) powder, at amounts of 5 wt% and 1 wt% of the original sand weight, respectively;

[0058] 3) Add CaO and MgO in a 1:1 weight ratio, with a particle size range of 10-50µm, as sintering aids, at a dosage of 4.5wt% of the original sand weight;

[0059] 4) Mix the above materials evenly to obtain a coated sand material suitable for selective laser sintering.

[0060] 5) Slice the digital model that needs to be printed into sand molds / cores, and then perform SLS forming on the above-mentioned molding sand material according to conventional methods to obtain SLS formed sand molds / cores;

[0061] 6) After SLS forming of the sand mold / core, remove the unsintered and uncured supporting sand particles and spray them directly onto the surface of the sand mold / core to improve the surface strength of the sand mold / core;

[0062] 7) The SLS-formed sand mold / core is embedded in the pearl sand and fired at a low temperature of 210℃ for 60 minutes. After the sand mold / core cools to room temperature, the support applied to the sand mold / core due to process design requirements is removed.

[0063] 8) Place the sand mold / core after low-temperature buried sand calcination in a vacuum pressure impregnation device for impregnation with a modified silica sol solution and a mixed solution of aluminum dihydrogen phosphate (mass fraction 1:1). The vacuum degree is 10 kPa, the impregnation pressure is 0.2 MPa, the impregnation temperature is 70℃, and the impregnation time is 70 min. After impregnation is completed, take out the sand mold / core and let it dry naturally at room temperature for 20 h.

[0064] 9) Then, spray zirconium oxide coating onto the dried sand mold / core surface to form an inert material coating;

[0065] 10) After the inert material coating dries, the sand is buried at 500℃+150min, 700℃+150min and 1150℃+300min for high-temperature calcination. The buried sand material is 200~600 mesh corundum powder. After cooling to room temperature, the sand mold / core for titanium alloy casting is obtained. The tensile strength of the sand mold / core is 2.59MPa and the bending strength is 5.75MPa. The surface of the sand mold / core is smooth and free of cracks.

[0066] Example 4:

[0067] 1) Zircon sand particles are sieved to obtain 50 / 100 mesh three-sieve sand. Solid thermoplastic phenolic resin is crushed into uniform phenolic resin powder with a particle size range of 100-200 mesh. Phenolic resin coated sand is prepared by thermal coating method, wherein the amount of phenolic resin powder added accounts for 3.5 wt% of the above three-sieve sand, the amount of silane coupling agent KH550 added accounts for 1 wt% of the phenolic resin powder, and a KH550 aqueous solution with a mass fraction of 10% is prepared. Hexamethylenetetramine added accounts for 12% of the phenolic resin powder, and a hexamethylenetetramine aqueous solution with a mass fraction of 50% is prepared. Phenolic resin coated sand is obtained by thermal method with phenolic resin powder, KH550 aqueous solution, hexamethylenetetramine aqueous solution and zircon sand.

[0068] 2) Add calcium stearate powder with a particle size range of 100-200 mesh and polystyrene (PS) powder, at amounts of 2 wt% and 0.5 wt% of the original sand weight, respectively;

[0069] 3) Add nano-CeO2 with a particle size range of 120-150nm as a sintering aid, at a dosage of 2.5wt% of the original sand weight;

[0070] 4) Mix the above materials evenly to obtain a coated sand material suitable for selective laser sintering.

[0071] 5) Slice the digital model that needs to be printed into sand molds / cores, and then perform SLS forming on the above-mentioned molding sand material according to conventional methods to obtain SLS formed sand molds / cores;

[0072] 6) After SLS forming of the sand mold / core, remove the unsintered and uncured supporting sand particles and spray them directly onto the surface of the sand mold / core to improve the surface strength of the sand mold / core;

[0073] 7) The SLS-formed sand mold / core is embedded in glass microspheres and calcined at a low temperature of 240℃ for 30 minutes. After the sand mold / core cools to room temperature, the support used for the sand mold / core due to process design requirements is removed.

[0074] 8) Place the sand mold / core after low-temperature buried sand calcination in a vacuum pressure impregnation device for impregnation with modified silica sol. The vacuum degree is 0.5 kPa, the impregnation pressure is 0.7 MPa, the impregnation temperature is 100℃, and the impregnation time is 120 min. After impregnation is completed, take out the sand mold / core and let it dry naturally at room temperature for 24 h.

[0075] 9) Then, spray yttrium oxide coating onto the dried sand mold / core surface to form an inert material coating;

[0076] 10) After the inert material coating dries, the sand is buried and calcined at 300℃+90min, 500℃+90min, and 1250℃+200min. The buried sand material is zircon sand powder with a particle size range of 100-300 mesh. After cooling to room temperature, the sand mold / core for titanium alloy casting is obtained. The tensile strength of the sand mold / core is 2.98MPa, the bending strength is 5.86MPa, and the surface of the sand mold / core is smooth and free of cracks.

[0077] Example 5:

[0078] 1) The abrasive particles are sieved to obtain 50 / 100 mesh three-sieve sand;

[0079] 2) Add calcium stearate powder with a particle size range of 70-200 mesh and sodium hydroxymethyl starch, at amounts of 3 wt% and 1.2 wt% of the original sand weight, respectively;

[0080] 3) Add nano-TiO2 with a particle size range of 50-100nm as a sintering aid, at a dosage of 1.5wt% of the original sand weight;

[0081] 4) Mix the above materials evenly to obtain molding sand material suitable for 3DP printing;

[0082] 5) Slice the digital model that needs to be printed into sand molds / cores, and then use the above-mentioned molding sand material to perform 3DP printing to obtain 3DP printed sand molds / cores.

[0083] 6) After 3DP printing forms the sand mold / core, it will be used to support the removal of sand particles;

[0084] 7) Place the 3DP printed sand mold / core in a vacuum pressure impregnation device for impregnation with aluminum dihydrogen phosphate solution. The vacuum degree is 1.5 kPa, the impregnation pressure is 0.1 MPa, the impregnation temperature is 40℃, and the impregnation time is 30 min. After impregnation is completed, take out the sand mold / core and let it dry naturally at room temperature for 15 h.

[0085] 8) Then, spray thorium oxide coating onto the dried sand mold / core surface to form an inert material coating;

[0086] 9) After the inert material coating dries, the sand is buried at 400℃+90min, 600℃+90min, and 1100℃+360min for high-temperature calcination. The buried sand material is 200~400 mesh spherical alumina powder. After cooling to room temperature, the sand mold / core for titanium alloy casting is obtained. The tensile strength of the sand mold / core is 3.46MPa, the bending strength is 6.23MPa, and the surface of the sand mold / core is smooth and free of cracks.

[0087] Example 6:

[0088] 1) The zircon sand particles were sieved to obtain 50 / 100 mesh three-sieve sand;

[0089] 2) Add calcium stearate powder with a particle size range of 70-200 mesh and sodium hydroxymethyl starch, at amounts of 5 wt% and 0.6 wt% of the original sand weight, respectively;

[0090] 3) Add nano-ZrO2 with a particle size range of 20-50nm as a sintering aid, at a dosage of 1.2wt% of the original sand weight;

[0091] 4) Mix the above materials evenly to obtain molding sand material suitable for 3DP printing;

[0092] 5) Slice the digital model that needs to be printed into sand molds / cores, and then use the above-mentioned molding sand material to perform 3DP printing to obtain 3DP printed sand molds / cores.

[0093] 6) After 3DP printing forms the sand mold / core, it will be used to support the removal of sand particles;

[0094] 7) Place the 3DP printed sand mold / core in a vacuum pressure impregnation device for impregnation with a modulus 18 water glass solution. The vacuum degree is 2.5 kPa, the impregnation pressure is 0.2 MPa, the impregnation temperature is 60℃, and the impregnation time is 30 min. After impregnation is completed, take out the sand mold / core and let it air dry at room temperature for 24 h.

[0095] 8) Then, spray yttrium oxide coating onto the dried sand mold / core surface to form an inert material coating;

[0096] 9) After the inert material coating dries, the sand is buried at 400℃+90min, 600℃+90min, and 1200℃+450min for high-temperature calcination. The buried sand material is 100-300 mesh zircon sand powder. After cooling to room temperature, the sand mold / core for titanium alloy casting is obtained. The tensile strength of the sand mold / core is 3.57MPa, the bending strength is 6.72MPa, and the surface of the sand mold / core is smooth and free of cracks.

[0097] Example 7:

[0098] 1) The quartz sand particles were sieved to obtain 70 / 140 mesh three-sieve sand.

[0099] 2) Add calcium stearate powder (particle size range 100-200 mesh), yttrium oxide powder, and polystyrene powder, at amounts of 3 wt%, 0.6 wt%, and 3.6 wt% of the original sand weight, respectively.

[0100] 3) Add nano-ZrO2 with a particle size range of 20-50nm as a sintering aid, at a dosage of 1.0wt% of the original sand weight;

[0101] 4) Mix the above materials evenly to obtain molding sand material suitable for 3DP printing;

[0102] 5) Slice the digital model that needs to be printed into sand molds / cores, and then use the above-mentioned molding sand material to perform 3DP printing to obtain 3DP printed sand molds / cores.

[0103] 6) After 3DP printing forms the sand mold / core, it will be used to support the removal of sand particles;

[0104] 7) Place the 3DP printed sand mold / core in a vacuum pressure impregnation device for impregnation with aluminum dihydrogen phosphate solution. The vacuum degree is 25 kPa, the impregnation pressure is 0.5 MPa, the impregnation temperature is 80℃, and the impregnation time is 45 min. After impregnation is completed, take out the sand mold / core and let it dry naturally at room temperature for 24 h.

[0105] 8) Then spray thorium oxide coating onto the dried sand mold / core surface to form an inert material coating.

[0106] 9) After the inert material coating dries, the sand is buried at 300℃+70min, 650℃+90min, and 1050℃+240min for high-temperature calcination. The buried sand material is 50-100 mesh bauxite powder. After cooling to room temperature, the sand mold / core for titanium alloy casting is obtained. The tensile strength of the sand mold / core is 4.27MPa, the bending strength is 7.53MPa, and the surface of the sand mold / core is smooth and free of cracks.

Claims

1. A method for preparing sand molds / cores for titanium alloy casting based on 3D printing, characterized in that: The process steps are as follows: A. Preparation of molding sand materials suitable for 3D printing The raw sand material is coated with phenolic resin using a thermal coating process to obtain phenolic resin coated sand. 1-10 wt% of additives, 0.1-5 wt% of processing aids, and 0.1-5 wt% of sintering aids are added to the obtained phenolic resin coated sand. The mixture is thoroughly mixed, sieved, and sealed and packaged to obtain SLS molding sand material suitable for titanium alloy casting. B. 3D Printing Sand Molds / Cores The digital model of the sand mold / core to be printed is sliced, and then the SLS molding sand material is 3D printed using conventional methods to obtain the 3D printed sand mold / core. C. Sand cleaning, SLS-formed sand mold / core surface spraying and low-temperature calcination with embedded sand The sand mold / core formed using SLS technology is first directly surface-sprayed to improve its surface strength. Then, the sand mold / core formed using SLS technology is placed in a forced-air drying oven and subjected to low-temperature sand-buried baking at 150-250℃ for 30-180 minutes. After low-temperature baking, the sand mold / core is cooled to 100℃-room temperature, and the supports used in the sand mold / core as required by the process design are removed. D. Vacuum pressure impregnation of inorganic binder into sand mold / core After low-temperature calcination, the SLS forming sand mold / core is placed in a vacuum pressure impregnation device for impregnation with inorganic binder. The vacuum environment is 0.1KPa-50KPa, the impregnation pressure is 0.1MPa-1MPa, the impregnation temperature is 10℃-100℃, the impregnation time is 5min-60min, and finally it is naturally air-dried at room temperature for 12-48h. E. Then, dip or spray an inert material coating onto the surface of the sand mold cavity and the surface of the sand core to form an inert material coating. F. High-temperature calcination process for embedding sand in sand molds / cores After the inert material coating dries, the sand mold / core is then subjected to a stepped high-temperature calcination with sand embedded in it. The calcination temperature is 300-1250℃, the holding time is 30-480min, and the core is cooled to 300℃-room temperature after being removed from the furnace to obtain the sand mold / core for titanium alloy casting. The additives are one or a combination of two of the following: calcium stearate powder, polyimide resin powder, calcium aluminate powder, barium aluminate powder, barium zirconium aluminate powder, yttrium oxide powder, and iron oxide powder, with a particle size range of 70 mesh to 800 mesh after grinding and sieving. The processing aids are one or a combination of two of the following: sodium hydroxymethyl starch, polyvinyl alcohol (PVA) powder, polystyrene (PS) powder, and polyurethane (PU) powder. The particle size range after grinding and sieving is 70-800 mesh. The sintering aids are one or a combination of two of the following powders: ZrO2, TiO2, CaO, MgO, Y2O3, and CeO2, with a particle size range of 0.01-100µm after ball milling and sieving.

2. The method for preparing sand molds / cores for titanium alloy casting based on 3D printing according to claim 1, characterized in that: In step A, the raw sand material is one of the following: alumina sand, zircon sand, corundum sand, bauxite, zircon sand and quartz sand particles, and the particle size range of the raw sand is 40 mesh to 400 mesh.

3. The method for preparing sand molds / cores for titanium alloy casting based on 3D printing according to claim 1, characterized in that: In step C, the SLS-formed sand mold / core is directly burned by a gas torch. Pay attention to small structural parts to improve the surface strength of the sand mold / core and prevent breakage or damage to the fine structural parts of the sand mold / core during the handling process.

4. The method for preparing sand molds / cores for titanium alloy casting based on 3D printing according to claim 1, characterized in that: In step C, the embedded sand material is one of glass microspheres, abrasive sand, and quartz sand, and the particle size range after sieving is 40 mesh to 800 mesh.

5. The method for preparing sand molds / cores for titanium alloy casting based on 3D printing according to claim 1, characterized in that: In step D, the inorganic binder is one or a combination of two of the following: silica sol, modified silica sol, water glass, aluminum dihydrogen phosphate solution, disodium hydrogen phosphate solution, magnesium dihydrogen phosphate solution, sodium hexametaphosphate solution, zirconium phosphate solution, and ethyl silicate hydrolysate.

6. The method for preparing sand molds / cores for titanium alloy casting based on 3D printing according to claim 1, characterized in that: In step E, the inert material coating is one of yttrium oxide coating, zirconium oxide coating, and thorium oxide coating.

7. The method for preparing sand molds / cores for titanium alloy casting based on 3D printing according to claim 1, characterized in that: In step F, the embedded sand material is one or a combination of two of the following: corundum sand, bauxite, zircon sand, and alumina sand. After sieving, the particle size ranges from 40 mesh to 800 mesh. The high-temperature calcination of the embedded sand is a stepped heating process, divided into three stages: the first stage, heating from room temperature to T1 and holding for N1 minutes for degassing; the second stage, heating from T1 to T2 and holding for N2 minutes for ablating the resin; and the third stage, heating from T2 to T3 and holding for N3 minutes for sintering the 3D printed sand mold / core. Among these stages, T1 is 200-500℃, N1 is 30-180 min, T2 is 600-850℃, N2 is 60-180 min, and T3 is 900-1250℃, N3 is 90-480 min. After cooling to 300℃-room temperature, the sand mold / core for titanium alloy casting is obtained.

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