A method for preparing molding sand for titanium alloy sand casting and selective laser sintering of composite sand mold / core

Abstract

The present invention discloses a method for preparing molding sand for titanium alloy sand casting and selective laser sintering of composite sand molds / cores. The method adopts a process idea of ​​preparing a molding sand material with an organic binder + an inorganic binder. By adding an inorganic binder, a processing aid, a sintering aid and an additive to the 3D-printed molding sand material, and calcining the sand at low and high temperatures, a sand mold / core suitable for titanium alloy casting is quickly prepared without a mold. The sand mold / core prepared by the method has the advantages of excellent high-temperature strength and low gas evolution, high dimensional accuracy, no cracking, stable inert coating, high production efficiency, low cost and safety, thereby realizing titanium alloy sand casting with complex structures.

Description

Technical Field

[0001] The invention relates to the technical field of titanium alloy sand casting, in particular to a method for preparing molding sand for titanium alloy sand casting and selective laser sintering of a composite sand mold / core thereof. Background Art

[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. They are particularly important in the aerospace industry and are important strategic metal materials. With the rapid development of various equipment manufacturing fields, the requirements for the mechanical properties, dimensional accuracy and reliability of key titanium alloy components are becoming increasingly stringent. In recent years, in order to meet the research and 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 with investment precision casting, titanium alloy sand casting has the advantages of fast manufacturing, the ability to produce large castings and cost advantages. At present, there is a demand in actual engineering to replace some precision castings and to prepare castings from simple castings to complex castings. However, titanium alloy sand casting still has certain limitations. This is because molten titanium alloy has high chemical activity and can react with almost all refractory materials. The interaction between molten titanium and sand molds / cores will form a contamination layer on the surface of the casting, which greatly affects the surface quality and performance of the casting. In addition, traditional sand mold / core preparation methods are difficult to quickly prepare complex sand molds, especially complex and special-shaped sand molds / cores, and the dimensional accuracy of the sand mold / core cannot be guaranteed. These problems limit the development of titanium alloy sand casting.

[0003] Therefore, the search for a rapid manufacturing process for titanium alloys with low manufacturing costs, short production cycles, and high dimensional accuracy is currently a focus of attention. With the rapid development of 3D printing technology, the direct preparation of sand molds and cores using 3D printing has gained widespread application. In recent years, 3D printing has enabled the production of complex, integrated, thin-walled, and partially functionalized sand molds and cores, rapidly and effectively increasing the complexity of sand mold and core preparation. Among these, the use of Selective Laser Sintering (SLS) technology for direct sand mold and core formation offers advantages such as fast response time, short manufacturing cycles, high flexibility, excellent stability, integrated sand mold and core manufacturing, and the ability to produce any complex shape. This technology significantly promotes the rapid trial production and manufacturing of large, complex castings, and shows great potential for solving the production challenges of critical castings in fields such as aerospace and automotive. Currently, sand molds and cores formed using SLS (Solid Laser Sintering) are being used in the production of parts made from materials such as cast aluminum, cast steel, and cast iron. However, due to its high chemical activity, molten titanium easily reacts with conventional casting materials during casting, leading to a series of casting defects such as thick oxide layers, sand sticking, surface inclusions, and pores on the titanium casting surface. This limits the application of SLS-coated sand molds and cores in titanium alloy casting. Compared to traditional molding techniques, SLS-formed sand molds and cores also suffer from lower thermal strength and high gas evolution due to the use of organic binders. This can lead to cracking and even collapse during the casting process. Furthermore, the lack of an inert coating on the surface of the sand molds and cores makes them unsuitable for casting chemically active titanium alloys. Clearly, sand molds and cores suitable for titanium alloy casting should possess high thermal strength, a low coefficient of expansion, and high chemical inertness to prevent the highly reactive molten titanium alloy from reacting violently with the refractory oxide. Therefore, research on the molding sand materials and sand mold / core preparation process suitable for SLS casting of titanium alloys plays an important role in promoting the application of SLS rapid prototyping technology in the field of titanium alloy casting. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for preparing molding sand for titanium alloy sand casting and selective laser sintering of composite sand mold / core.

[0005] The object of the present invention is achieved like this:

[0006] A method for preparing molding sand for titanium alloy sand casting and selective laser sintering of composite sand molds / cores is characterized by the following process steps:

[0007] A. Preparation of organic binder + inorganic binder molding sand suitable for SLS

[0008] The raw sand material is subjected to phenolic resin coating treatment to obtain phenolic resin coated sand, and an inorganic binder in an amount of 1-15 wt% by weight of the raw sand, an additive in an amount of 0.5-10 wt% by weight of the raw sand, a processing aid in an amount of 0.1-5 wt% by weight of the raw sand, and a sintering aid in an amount of 0.1-5 wt% by weight of the raw sand are added to the obtained phenolic resin coated sand; the mixture is thoroughly mixed, sieved, and sealed and packaged to obtain an organic binder + inorganic binder molding sand material suitable for SLS molding of titanium alloy casting;

[0009] B. SLS forming sand mold / core

[0010] The digital model of the sand mold / core to be printed is sliced, and then the above-mentioned organic binder + inorganic binder molding sand material is subjected to SLS forming according to the conventional method to obtain the SLS formed sand mold / core;

[0011] C. Sand cleaning and sand mold / core surface spraying

[0012] After SLS forming the sand mold / core with the organic binder + inorganic binder molding sand material, the unsintered and solidified support sand particles are removed, and the surface of the sand mold / core is directly sprayed to improve the surface strength of the sand mold / core;

[0013] D. Low temperature sintering and coating process

[0014] The SLS-formed sand mold / core is subjected to a low-temperature sand-embedded roasting process at 150-250°C for 20-180 minutes. After low-temperature roasting, the sand mold / core is cooled to 100°C-room temperature, and the supports applied to the sand mold / core due to process design requirements are removed. Then, the sand mold cavity surface and the sand core surface are dipped or sprayed with an inert material coating to form an inert material coating.

[0015] E. After the inert material coating is dried, the sand is finally baked at a stepped high temperature at a temperature of 300-1250°C for 30-480 minutes. The sand is then taken out of the furnace and cooled to 300°C-room temperature to obtain a sand mold / core for titanium alloy casting.

[0016] In step A, the raw sand material used for the coated sand is one of gem sand, zircon sand, corundum sand, bauxite and quartz sand granular materials, and the particle size of the raw sand material after sieving is in the range of 40 mesh to 400 mesh.

[0017] The inorganic binder powder is one of aluminum metaphosphate, aluminum dihydrogen phosphate, sodium hexametaphosphate, sodium tripolyphosphate, disodium hydrogen phosphate and ammonium dihydrogen phosphate powder, or a combination of two of them, and the particle size range after grinding and sieving is 70 mesh-800 mesh.

[0018] The additive is one of calcium stearate powder, calcium aluminate powder, barium aluminate powder, barium zirconium aluminate powder, polyimide resin powder, magnesium chloride, iron oxide and yttrium oxide powder, or a combination thereof, and the particle size after grinding and sieving is in the range of 70 mesh to 800 mesh.

[0019] The processing aid is one of sodium hydroxymethyl starch, polyvinyl alcohol (PVA) powder, polystyrene (PS) powder and polyurethane (PU) powder, or a combination of the two. The particle size after grinding and sieving is in the range of 70 meshes to 800 meshes.

[0020] The sintering aid is one of ZrO2, TiO2, CaO, MgO, Y2O3 and CeO2 powders, or a combination of two of them, and the particle size range after ball milling and screening is 0.01-100µm.

[0021] The combination of the inorganic binder powder, additives, processing aids and sintering aids is added to the prepared phenolic resin coated sand in a set ratio, fully mixed, sieved and packaged to obtain an organic binder + inorganic binder molding sand material suitable for SLS molding.

[0022] In step C, after the SLS sand mold / core is formed, the unsintered and solidified support sand particles are blown away with compressed air. After the sand mold / core is clean, it is directly sprayed with a gas blowtorch, paying special attention to the local fine structure parts, to improve the surface strength of the sand mold / core and prevent the fine structure parts of the sand mold and core from breaking or being damaged during the transportation process.

[0023] In step D, the burying sand material in the low-temperature roasting stage is one of glass beads, quartz sand and gem sand, with a particle size range of 40 mesh to 800 mesh; the inert material coating is one of yttria coating, zirconium oxide coating and thorium oxide coating.

[0024] In step E, the buried sand material in the high-temperature roasting stage is one of corundum sand, gemstone sand, bauxite and zircon sand, or a combination of two thereof, and the particle size range after screening is 40 mesh-800 mesh; the buried sand high-temperature roasting is a step-by-step temperature increase, and the roasting is divided into three stages: the first stage, from room temperature to T1, holding for N1 minute, for exhaust, the second stage, from T1 to T2, holding for N2 minutes, for ablating resin, and the third stage, from T2 to T3, holding for N3 minutes, for sintering SLS-formed sand mold / core; wherein T1 is 200-500°C, N1 is 30-180min, T2 is 600-850°C, N2 is 60-180min, T3 is 900-1250°C, and N3 is 90-480min; after being taken out of the furnace and cooled to 300°C-room temperature, a sand mold / core for titanium alloy casting is obtained.

[0025] To address the current problems with SLS-molded sand molds and cores, which lack high-temperature strength, generate significant gassing, and lack an inert coating on their surface, making them unsuitable for casting chemically active titanium alloys, the present invention employs a process approach to prepare a sand material with both an organic binder and an inorganic binder. By adding inorganic binders, processing aids, sintering aids, and additives to the 3D-printed sand material, and then subjecting it to low- and high-temperature sand burial and roasting, a moldless, rapid sand mold and core suitable for titanium alloy casting is produced. The sand molds and cores produced by this method exhibit excellent high-temperature strength and low gassing, high dimensional accuracy, no cracking, a stable inert coating, high production efficiency, low cost, and safety, thus enabling sand casting of complex titanium alloy structures. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a simplified process flow diagram of the present invention;

[0027] Figure 2 Schematic diagram of the high temperature calcination curve of the sand mold / core in the present invention. DETAILED DESCRIPTION

[0028] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0029] Example 1:

[0030] 1) Screening the jewel sand particles to obtain 100 / 200 mesh triple-sieve sand with a main content of ≥90%, crushing the solid thermoplastic phenolic resin into a uniform phenolic resin powder with a particle size range of 140 mesh to 400 mesh; preparing the phenolic resin coated sand by a thermal coating method, wherein: the phenolic resin powder is added in an amount of 1.9wt% of the triple-sieve sand, the silane coupling agent KH550 is added in an amount of 1wt% of the phenolic resin powder, and a KH550 aqueous solution with a mass fraction of 10% is prepared; and the hexamethylenetetramine aqueous solution is added in an amount of 12% of the phenolic resin powder to prepare a 50wt% hexamethylenetetramine aqueous solution, and the phenolic resin powder, the KH550 aqueous solution, the hexamethylenetetramine aqueous solution, and the jewel sand are thermally treated to obtain the phenolic resin coated sand;

[0031] 2) Grind and sieve the ammonium dihydrogen phosphate particles into a fine powder with a particle size range of 140-400 mesh, and add 10wt% of the original sand weight;

[0032] 3) Add polyimide resin powder and calcium aluminate powder with a particle size range of 140-400 mesh, 50% each, and the addition amount is 2wt% of the original sand weight;

[0033] 4) Add nano ZrO2 with a particle size range of 20-50nm as a sintering aid, the addition amount is 0.5wt% of the original sand weight;

[0034] 5) The above materials are mixed evenly to prepare a phenolic resin + ammonium dihydrogen phosphate dual binder molding sand material suitable for SLS for titanium alloy casting;

[0035] 6) Slice the digital model of the sand mold / core to be printed, and then perform selective laser sintering on the above-mentioned sand material prepared with phenolic resin + ammonium dihydrogen phosphate double binder according to the conventional method;

[0036] 7) After the phenolic resin + ammonium dihydrogen phosphate binder molding sand material SLS forms the sand mold / core, the unsintered and solidified support sand particles are removed, and the surface of the sand mold / core is directly sprayed to improve the surface strength of the sand mold / core;

[0037] 8) The SLS-formed sand mold / core is embedded in glass beads with a particle size range of 100-200 mesh, and then baked at 180°C for 30 minutes. After the sand mold / core cools to room temperature, the sand mold / core support required by the process design is removed, and then yttrium oxide coating is sprayed on the surface of the sand mold cavity and the sand core to form an inert material coating;

[0038] 9) After the inert material coating is dry, the sand mold / core is buried in bauxite with a particle size range of 100-200 mesh and calcined at 300°C+90 min, 650°C+90 min, and 1050°C+200 min. After being removed from the furnace and cooled to room temperature, the sand mold / core for titanium alloy casting is obtained. The tensile strength of the sand mold / core is 2.31 MPa, the bending strength is 5.36 MPa, and the surface of the sand mold / core is smooth and free of cracks.

[0039] Example 2:

[0040] 1) Screening the Baozhu sand particles to obtain 70 / 140 mesh triple-sieve sand with a main content of ≥90%; pulverizing the solid thermoplastic phenolic resin into a uniform phenolic resin powder with a particle size range of 140 mesh to 270 mesh; preparing phenolic resin coated sand by a thermal coating method, wherein the phenolic resin powder is added in an amount of 2.3wt% of the triple-sieve sand, the silane coupling agent KH550 is added in an amount of 1wt% of the phenolic resin powder, and a KH550 aqueous solution with a mass fraction of 10% is prepared, and the hexamethylenetetramine aqueous solution is added in an amount of 12wt% of the phenolic resin powder to prepare a 50wt% hexamethylenetetramine aqueous solution, and the phenolic resin powder, the KH550 aqueous solution, the hexamethylenetetramine aqueous solution and the Baozhu sand are thermally processed to obtain phenolic resin coated sand;

[0041] 2) Grind and sieve ammonium dihydrogen phosphate and aluminum metaphosphate in a weight ratio of 1:1 into a composite fine powder with a particle size range of 100-200 mesh, and add 12wt% of the original sand weight;

[0042] 3) Add calcium stearate and calcium aluminate powders in a weight ratio of 3:2 with a particle size range of 100-200 mesh, and the addition amount is 3wt% of the original sand weight; add polystyrene PS powder with a particle size range of 100-200 mesh as a processing aid, and the addition amount is 0.6wt% of the original sand weight;

[0043] 4) Add nano-TiO2 with a particle size range of 40-80nm as a sintering aid, the addition amount is 0.7wt% of the original sand weight;

[0044] 5) The above materials are mixed evenly to prepare a phenolic resin + ammonium dihydrogen phosphate, aluminum metaphosphate binder molding sand material suitable for SLS casting of titanium alloys;

[0045] 6) Slice the digital model of the sand mold / core to be printed, and then perform selective laser sintering on the above-mentioned sand material prepared with phenolic resin + ammonium dihydrogen phosphate and aluminum metaphosphate binder according to the conventional method;

[0046] 7) After the selected area is laser sintered to form the sand mold / core, the unsintered and solidified supporting sand particles are blown away with compressed air. After the sand mold / core is clean, it is directly sprayed with a gas blowtorch, paying special attention to the local fine structure parts to improve the surface strength of the sand mold / core and prevent the fine structure parts of the sand mold / core from breaking or being damaged during the transportation process;

[0047] 8) The SLS-formed sand mold / core is buried in quartz sand with a particle size range of 50-200 mesh and low-temperature calcined at 200°C for 60 minutes. After low-temperature calcination, the sand mold / core is cooled to 50°C, and zirconium oxide coating is dipped on the surface of the sand mold cavity and the sand core to form an inert material coating;

[0048] 9) After the inert material coating is dry, the sand mold / core is buried in corundum sand with a particle size range of 50 mesh to 200 mesh, and high-temperature roasting is carried out at 400°C for 120 minutes, 600°C for 120 minutes, and 950°C for 250 minutes. After being taken out of the furnace and cooled to room temperature, the sand mold / core for titanium alloy casting is obtained. The tensile strength of the sand mold / core is 3.11 MPa, the bending strength is 5.96 MPa, and the surface of the sand mold / core is smooth and free of cracks.

[0049] Example 3:

[0050] 1) Screening corundum sand particles to obtain 100 / 200 mesh triple-sieve sand with a main content of ≥90%, crushing solid thermoplastic phenolic resin into uniform phenolic resin powder with a particle size range of 100 mesh-200 mesh; preparing phenolic resin coated sand by a thermal coating method, wherein the phenolic resin powder is added in an amount of 3.0wt% of the triple-sieve sand, the silane coupling agent KH550 is added in an amount of 1wt% of the phenolic resin powder, and a KH550 aqueous solution with a mass fraction of 10% and a hexamethylenetetramine aqueous solution with a mass fraction of 12% of the phenolic resin powder are prepared, and the phenolic resin powder, the KH550 aqueous solution, the hexamethylenetetramine aqueous solution and the corundum sand are thermally processed to obtain phenolic resin coated sand;

[0051] 2) Grind and sieve the aluminum metaphosphate particles into a fine powder with a particle size range of 100-200 mesh, and add 9wt% of the original sand weight;

[0052] 3) Add calcium stearate, calcium aluminate and iron oxide powders with a weight ratio of 4:4:2 and a particle size range of 100-200 mesh, and the addition amount is 5wt% of the original sand weight; add urethane PU powder with a particle size range of 100-200 mesh, and the addition amount is 1wt% of the original sand weight;

[0053] 4) Add nano Y2O3 with a particle size range of 50-100nm as a sintering aid, the addition amount is 1wt% of the original sand weight;

[0054] 5) The above materials are mixed evenly to prepare a phenolic resin + aluminum metaphosphate dual binder molding sand material suitable for SLS for titanium alloy casting;

[0055] 6) Slice the digital model of the sand mold / core to be printed, and then perform selective laser sintering on the above-mentioned sand material prepared with phenolic resin + aluminum metaphosphate dual binder according to the conventional method;

[0056] 7) After the selected area is laser sintered to form the sand mold / core, the unsintered and solidified supporting sand particles are blown away with compressed air. After the sand mold / core is clean, it is directly sprayed with a gas blowtorch, paying special attention to the local fine structure parts to improve the surface strength of the sand mold / core and prevent the fine structure parts of the sand mold / core from breaking or being damaged during the transportation process;

[0057] 8) The SLS-formed sand mold / core is buried in 100-200 mesh gemstone sand and calcined at 210°C for 90 minutes. After low-temperature calcination, the sand mold / core is cooled to room temperature and thorium oxide coating is sprayed on the surface of the sand mold cavity and the sand core to form an inert material coating.

[0058] 9) After the inert material coating is dry, the sand mold / core is embedded in zircon sand with a particle size range of 100-200 mesh and calcined at 500°C for 150 min, 700°C for 150 min, and 1150°C for 300 min. After being removed from the furnace and cooled to room temperature, the sand mold / core for titanium alloy casting is obtained. The tensile strength of the sand mold / core is 3.61 MPa, the bending strength is 6.43 MPa, and the surface of the sand mold / core is smooth and free of cracks.

[0059] Example 4:

[0060] 1) sieving zircon sand particles to obtain 70 / 140 mesh triple-sieve sand, crushing solid thermoplastic phenolic resin into phenolic resin powder with a particle size range of 100 mesh-200 mesh; preparing phenolic resin coated sand by a thermal coating method, wherein the phenolic resin powder is added in an amount of 2.5wt% of the triple-sieve sand, the silane coupling agent KH550 is added in an amount of 1wt% of the phenolic resin powder, and a KH550 aqueous solution with a mass fraction of 10% is prepared, and the hexamethylenetetramine aqueous solution is added in an amount of 12% of the phenolic resin powder to prepare a 50wt% hexamethylenetetramine aqueous solution, and the phenolic resin powder, the KH550 aqueous solution, the hexamethylenetetramine aqueous solution and the zircon sand are thermally processed to obtain phenolic resin coated sand;

[0061] 2) Grind and sieve aluminum dihydrogen phosphate particles into a fine powder with a particle size range of 100-200 mesh, and add 7wt% of the original sand weight;

[0062] 3) Add calcium aluminate powder with a particle size range of 100-200 mesh, the addition amount is 3wt% of the original sand weight; add polyvinyl alcohol (PVA) powder with a particle size range of 100-200 mesh, the addition amount is 0.5wt% of the original sand weight;

[0063] 4) Add nano Y2O3 with a particle size range of 100-150nm as a sintering aid, the addition amount is 0.8wt% of the original sand weight;

[0064] 5) The above materials are mixed evenly to prepare a phenolic resin + aluminum dihydrogen phosphate dual binder molding sand material suitable for SLS for titanium alloy casting;

[0065] 6) Slice the digital model of the sand mold / core to be printed, and then perform selective laser sintering on the above-mentioned molding sand material prepared with a double binder of phenolic resin and aluminum dihydrogen phosphate according to conventional methods;

[0066] 7) After the selected area is laser sintered to form the sand mold / core, the unsintered and solidified supporting sand particles are blown away with compressed air. After the sand mold / core is clean, it is directly sprayed with a gas blowtorch, paying special attention to the local fine structure parts to improve the surface strength of the sand mold / core and prevent the fine structure parts of the sand mold / core from breaking or being damaged during the transportation process;

[0067] 8) The SLS-formed sand mold / core is buried in 100-200 mesh gemstone sand and calcined at 190°C for 120 minutes. After low-temperature calcination, the sand mold / core is cooled to 60°C and zirconium oxide coating is sprayed on the surface of the mold cavity and the sand core to form an inert material coating.

[0068] 9) After the inert material coating is dry, the sand mold / core is buried in bauxite with a particle size range of 100-200 mesh and calcined at 400°C+120 min, 650°C+120 min, and 1100°C+200 min. After being removed from the furnace and cooled to room temperature, the sand mold / core for titanium alloy casting is obtained. The tensile strength of the sand mold / core is 2.81 MPa, the bending strength is 5.73 MPa, and the surface of the sand mold / core is smooth and free of cracks.

[0069] Example 5:

[0070] 1) sieving zircon sand particles to obtain 70 / 140 mesh triple-sieve sand, crushing solid thermoplastic phenolic resin into uniform phenolic resin powder with a particle size range of 100 mesh to 200 mesh; preparing phenolic resin coated sand by a thermal coating method, wherein the phenolic resin powder is added in an amount of 2.7wt% of the triple-sieve sand, the silane coupling agent KH550 is added in an amount of 1wt% of the phenolic resin powder, and a KH550 aqueous solution with a mass fraction of 10% is prepared, and the hexamethylenetetramine aqueous solution with a mass fraction of 12% is prepared, and the phenolic resin powder, the KH550 aqueous solution, the hexamethylenetetramine aqueous solution and the zircon sand are thermally processed to obtain phenolic resin coated sand;

[0071] 2) Grind and sieve the sodium tripolyphosphate particles into a fine powder with a particle size range of 100-200 mesh, and add 9wt% of the original sand weight;

[0072] 3) Add yttrium oxide powder with a particle size range of 100-200 mesh, the addition amount is 0.9wt% of the original sand weight; add polyurethane PU powder with a particle size range of 100-200 mesh, the addition amount is 1wt% of the original sand weight;

[0073] 4) Add nano-TiO2 with a particle size range of 30-80nm as a sintering aid, the addition amount is 1wt% of the original sand weight;

[0074] 5) The above materials are mixed evenly to prepare a phenolic resin + sodium tripolyphosphate dual binder molding sand material suitable for SLS for titanium alloy casting;

[0075] 6) Slice the digital model of the sand mold / core to be printed, and then perform selective laser sintering on the above-mentioned molding sand material prepared with phenolic resin + sodium tripolyphosphate dual binder according to the conventional method;

[0076] 7) After the selected area is laser sintered to form the sand mold / core, the unsintered and solidified supporting sand particles are blown away with compressed air. After the sand mold / core is clean, it is directly sprayed with a gas blowtorch, paying special attention to the local fine structure parts to improve the surface strength of the sand mold / core and prevent the fine structure parts of the sand mold / core from breaking or being damaged during the transportation process;

[0077] 8) The SLS-formed sand mold / core is embedded in glass microspheres with a particle size range of 100-200 mesh and calcined at 200°C for 60 minutes. After low-temperature calcination, the sand mold / core is cooled to 50°C and yttrium oxide coating is dipped on the surface of the mold cavity and the sand core to form an inert material coating.

[0078] 9) After the inert material coating is dry, the sand mold / core is buried in precious sand with a particle size range of 100-200 mesh and calcined at 500°C for 150 minutes, 700°C for 150 minutes, and 1150°C for 300 minutes. After being removed from the furnace and cooled to room temperature, the sand mold / core for titanium alloy casting is obtained. The tensile strength of the sand mold / core is 3.31 MPa, the bending strength is 6.13 MPa, and the surface of the sand mold / core is smooth and free of cracks.

[0079] Example 6:

[0080] 1) Bauxite particles are screened to obtain 50 / 100 mesh triple-sieve sand, and solid thermoplastic phenolic resin is crushed into phenolic resin powder with a particle size range of 70 mesh to 200 mesh; phenolic resin coated sand is prepared by a thermal coating method, wherein the phenolic resin powder is added in an amount of 2.9wt% of the triple-sieve sand, the silane coupling agent KH550 is added in an amount of 1wt% of the phenolic resin powder, and a KH550 aqueous solution with a mass fraction of 10% is prepared, and a hexamethylenetetramine aqueous solution with a mass fraction of 12% of the phenolic resin powder is prepared, and the phenolic resin powder, the KH550 aqueous solution, the hexamethylenetetramine aqueous solution and the bauxite are thermally processed to obtain phenolic resin coated sand;

[0081] 2) Grind and sieve the disodium hydrogen phosphate particles into a fine powder with a particle size range of 70-200 mesh, and add 10wt% of the original sand weight;

[0082] 3) Add barium zirconium aluminate powder with a particle size range of 70-200 mesh, the addition amount is 4wt% of the original sand weight; add polyimide resin powder with a particle size range of 70-200 mesh, the addition amount is 1wt% of the original sand weight; add sodium hydroxymethyl starch powder with a particle size range of 100-200 mesh, the addition amount is 0.6wt% of the original sand weight;

[0083] 4) Add nano-CeO2 with a particle size range of 120-180nm as a sintering aid, and the addition amount is 0.5wt% of the original sand weight;

[0084] 5) The above materials are mixed evenly to prepare a phenolic resin + ammonium dihydrogen phosphate dual binder molding sand material suitable for SLS for titanium alloy casting;

[0085] 6) Slice the digital model of the sand mold / core to be printed, and then perform selective laser sintering on the above-mentioned sand material prepared with phenolic resin + ammonium dihydrogen phosphate double binder according to the conventional method;

[0086] 7) After the selected area is laser sintered to form the sand mold / core, the unsintered and solidified supporting sand particles are blown away with compressed air. After the sand mold / core is clean, it is directly sprayed with a gas blowtorch, paying special attention to the local fine structure parts to improve the surface strength of the sand mold / core and prevent the fine structure parts of the sand mold / core from breaking or being damaged during the transportation process;

[0087] 8) The SLS-formed sand mold / core is buried in quartz sand with a particle size range of 40-200 mesh and low-temperature calcined at 180°C for 90 minutes. After low-temperature calcination, the sand mold / core is cooled to room temperature and yttrium oxide coating is dipped on the surface of the sand mold cavity and the sand core to form an inert material coating;

[0088] 9) After the inert material coating is dried, the sand mold / core is buried in bauxite with a particle size range of 100-200 mesh, and high-temperature roasting is carried out at 400℃+150min, 650℃+150min, and 1050℃+300min. After being taken out of the furnace and cooled to room temperature-300℃, the sand mold / core for titanium alloy casting is obtained. The tensile strength of the sand mold / core is 3.89MPa, the bending strength is 6.27MPa, and the surface of the sand mold / core is smooth and free of cracks.

[0089] Example 7:

[0090] 1) quartz sand particles are screened to obtain 70 / 140 mesh triple-sieve sand, solid thermoplastic phenolic resin is crushed into uniform phenolic resin powder with a particle size range of 100 mesh to 200 mesh; phenolic resin coated sand is prepared by a thermal coating method, wherein the phenolic resin powder is added in an amount of 3.0 wt% of the triple-sieve sand, the silane coupling agent KH550 is added in an amount of 1 wt% of the phenolic resin powder, a KH550 aqueous solution with a mass fraction of 10%, and a hexamethylenetetramine aqueous solution with a mass fraction of 12% of the phenolic resin powder is prepared, and the phenolic resin powder, the KH550 aqueous solution, the hexamethylenetetramine aqueous solution, and the quartz sand are thermally processed to obtain phenolic resin coated sand;

[0091] 2) Grind and sieve the sodium hexametaphosphate particles into a fine powder with a particle size range of 100-200 mesh, and add 12wt% of the original sand weight;

[0092] 3) Add calcium stearate and barium zirconium aluminate powders with a particle size range of 100-200 mesh as additives, and the addition amount is 2wt% of the original sand weight; add polystyrene PS powder with a particle size range of 100-200 mesh as a processing aid, and the addition amount is 0.8wt% of the original sand weight;

[0093] 4) Add CaO and MgO with a particle size range of 10-50µm as sintering aids, mix thoroughly in a weight ratio of 1:1, and add 2wt% of the original sand weight;

[0094] 5) The above materials are mixed evenly to prepare a phenolic resin + aluminum dihydrogen phosphate dual binder molding sand material suitable for SLS for titanium alloy casting;

[0095] 6) Slice the digital model of the sand mold / core to be printed, and then perform selective laser sintering on the above-mentioned molding sand material prepared with a double binder of phenolic resin and aluminum dihydrogen phosphate according to conventional methods;

[0096] 7) After the selected area is laser sintered to form the sand mold / core, the unsintered and solidified supporting sand particles are blown away with compressed air. After the sand mold / core is clean, it is directly sprayed with a gas blowtorch, paying special attention to the local fine structure parts to improve the surface strength of the sand mold / core and prevent the fine structure parts of the sand mold / core from breaking or being damaged during the transportation process;

[0097] 8) The SLS-formed sand mold / core is embedded in glass microspheres with a particle size range of 70-200 mesh and calcined at 190°C for 120 minutes. After the low-temperature calcination, the sand mold / core is cooled to 40°C and a thorium oxide coating is applied to the surface of the mold cavity and the sand core to form an inert material coating.

[0098] 9) After the inert material coating is dry, the sand mold / core is buried in corundum sand with a particle size range of 70 mesh to 200 mesh, and is calcined at 500°C for 150 min, 700°C for 150 min, and 1050°C for 300 min. After being taken out of the furnace and cooled to room temperature -300°C, the sand mold / core for titanium alloy casting is obtained. The tensile strength of the sand mold / core is 4.24 MPa, the flexural strength is 6.57 MPa, and the surface of the sand mold / core is smooth and free of cracks.

Claims

1. A method for selective laser sintering of composite sand molds / cores for titanium alloy sand casting, characterized by: The preparation steps are as follows: A. Preparation of organic binder + inorganic binder molding sand suitable for SLS The raw sand material is subjected to phenolic resin coating treatment, and an inorganic binder in an amount of 1-15 wt% by weight of the raw sand, an additive in an amount of 0.5-10 wt% by weight of the raw sand, a processing aid in an amount of 0.1-5 wt% by weight of the raw sand, and a sintering aid in an amount of 0.1-5 wt% by weight of the raw sand are added to the obtained coated sand; the mixture is thoroughly mixed, sieved, and sealed and packaged to obtain an organic binder + inorganic binder molding sand material suitable for SLS molding of titanium alloy casting; Wherein: the inorganic binder is one of aluminum metaphosphate, aluminum dihydrogen phosphate, sodium hexametaphosphate, sodium tripolyphosphate, disodium hydrogen phosphate and ammonium dihydrogen phosphate powder, or a combination of two thereof, and the particle size range after grinding and sieving is 70 mesh to 800 mesh; B. Selective laser sintering sand mold / core The digital model of the sand mold / core to be printed is sliced, and then the above-mentioned sand material prepared with organic binder + inorganic binder is subjected to selective laser sintering according to the conventional method; C. Sand cleaning and sand mold / core surface spraying After SLS forming the sand mold / core with the organic binder + inorganic binder molding sand material, the unsintered and solidified support sand particles are removed and directly sprayed on the surface of the sand mold / core to improve the surface strength of the sand mold / core; Among them: After the sand mold / core is formed by selective laser sintering, the unsintered and solidified supporting sand particles are blown away with compressed air. After the sand mold / core is clean, it is directly sprayed with a gas blowtorch, paying attention to the local fine structure parts to improve the surface strength of the sand mold / core and prevent the fine structure parts of the sand mold / core from breaking or being damaged during the transportation process; D. Low temperature sintering and coating process The SLS formed sand mold / core is subjected to a low temperature sand embedding roasting at 150-250℃ for 20-180min. After low temperature roasting, the sand mold / core is cooled to 100℃-room temperature, and the supports applied to the sand mold / core due to process design requirements are removed. Then, the sand mold cavity surface and the sand core surface are dipped or sprayed with an inert material coating to form an inert material coating. E. After the inert material coating is dried, the sand is finally baked at high temperature in a stepped manner at a temperature of 300-1250°C for 30-480 minutes. The sand is then taken out of the furnace and cooled to room temperature -300°C to obtain a sand mold / core for titanium alloy casting.

2. The method for selective laser sintering of composite sand molds / cores of molding sand for titanium alloy sand casting according to claim 1, characterized in that: In step A, the raw sand material used for the coated sand is one of gem sand, zircon sand, corundum sand, bauxite and quartz sand granular materials, and the particle size of the raw sand ranges from 40 mesh to 400 mesh.

3. The method for selective laser sintering of composite sand molds / cores of molding sand for titanium alloy sand casting according to claim 1, characterized in that: The additive is one of calcium stearate powder, calcium aluminate powder, barium aluminate powder, barium zirconium aluminate powder, polyimide resin powder and iron oxide powder, or a combination thereof, and the particle size range after grinding and sieving is 70 meshes to 800 meshes.

4. The method for selective laser sintering of composite sand molds / cores of molding sand for titanium alloy sand casting according to claim 1, characterized in that: The processing aid is one of sodium hydroxymethyl starch, polyvinyl alcohol (PVA) powder, polystyrene (PS) powder and polyurethane (PU) powder, or a combination of the two. The particle size range after grinding and sieving is 70-800 meshes.

5. The method for selective laser sintering of composite sand molds / cores of molding sand for titanium alloy sand casting according to claim 1, characterized in that: The sintering aid is one of ZrO2, TiO2, CaO, MgO, Y2O3 and CeO2 powders, or a combination of two of them, and the particle size ranges from 0.01 to 100 μm after ball milling and sieving; The combination of the inorganic binder powder, additives, processing aids and sintering aids is added to the prepared phenolic resin coated sand in a set ratio, fully mixed, sieved and packaged to obtain an organic binder + inorganic binder molding sand material suitable for selective laser sintering.

6. The method for selective laser sintering of composite sand molds / cores of molding sand for titanium alloy sand casting according to claim 1, characterized in that: In step D, the burying sand material in the low-temperature roasting stage is one of glass beads, gem sand and quartz sand, and its particle size range is 40 mesh to 800 mesh; the inert material coating is one of yttria coating, zirconium oxide coating and thorium oxide coating.

7. The method for selective laser sintering of composite sand molds / cores of molding sand for titanium alloy sand casting according to claim 1, characterized in that: In step E, the buried sand material in the high-temperature roasting stage is one of corundum sand, gemstone sand, bauxite and zircon sand, or a combination of two thereof, and the particle size range after screening is 40 mesh-800 mesh; the buried sand high-temperature roasting is a step-by-step temperature increase, and the roasting is divided into three stages: the first stage, from room temperature to T1, holding for N1 minute, for exhaust, the second stage, from T1 to T2, holding for N2 minutes, for ablation of resin, and the third stage, from T2 to T3, holding for N3 minutes, for sintering SLS-formed sand mold / core; wherein T1 is 200-500°C, N1 is 30-180min, T2 is 600-850°C, N2 is 60-180min, T3 is 900-1250°C, and N3 is 90-480min; after being taken out of the furnace and cooled to 300°C-room temperature, a sand mold / core for titanium alloy casting is obtained.

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