Preparation method of surface layer shell for precise casting production of thick-wall titanium alloy

Abstract

The invention provides a preparation method of a surface layer shell for precise casting production of a thick-wall titanium alloy. The preparation method comprises the following steps: S1, preparing a surface layer binder, wherein the surface layer binder comprises modified silica sol, a stabilizer, a reinforcing agent, a wetting agent and an acid-base regulator; the modified silica sol is diluted sodium-stabilized alkaline colloidal silicon dioxide; s2, preparing surface layer refractory powder, wherein the surface layer refractory powder comprises yttrium oxide powder, zirconium oxide powder and calcium fluoride powder; s3, preparing a surface layer coating; adding the surface layer refractory powder in the step S2 into the surface material binder in the step S1; s4, preparation of a surface layer shell, specifically, a module for preparing the surface layer shell enters the surface layer coating in the S3 to be subjected to slurry hanging, sand spraying and drying, and the step is repeated 1-2 times; and S5, the surface layer shell in the step S4 is subjected to firing forming, and the surface layer shell for precise casting production of the thick-wall titanium alloy is obtained. The surface layer shell obtained through the preparation method can be used for producing titanium alloy castings with the wall thickness ranging from 25 mm to 50 mm.

Description

Technical Field

[0001] The present invention relates to the technical field of investment precision casting of titanium alloys, and more particularly to a method for preparing a surface layer shell for the production of thick-walled titanium alloy precision casting. Background Art

[0002] Titanium alloy is an excellent structural material with a series of excellent properties such as low density, high specific strength, corrosion resistance, and good biocompatibility, and is widely used in fields such as aerospace, ocean engineering, energy chemical engineering, and biomedicine.

[0003] Investment precision casting is an advanced manufacturing technology for titanium alloy parts, which can achieve near-net-shape or even net-shape integral forming of complex parts, and has the advantages of high dimensional accuracy, high production efficiency, high material utilization rate, and low production cost. However, titanium has extremely strong reactivity at high temperatures and can react with almost all refractory materials to varying degrees, which greatly affects the appearance quality and mechanical properties of titanium alloy precision parts. Especially for thick-walled castings, due to the scouring force of the large-volume high-temperature titanium liquid during vacuum pouring under the action of gravity on the surface layer shell is much greater than that of general structural titanium alloy castings, and the filling and solidification time in the mold cavity is longer, and the heating time of the surface layer shell is also much higher than that of general structural titanium alloy castings. Under the action of high scouring force and long-term contact with high-temperature titanium liquid, it is required that each component of the surface layer material has higher strength and chemical inertness. Therefore, the biggest difficulty in the precision casting of thick-walled titanium alloys lies in the preparation of the investment shell, and the most critical link lies in the preparation of the surface layer shell.

[0004] In the precision casting of titanium alloys, the oxide ceramic shell process is mostly used. The refractory material for the surface layer is refractory metal oxide, and the binder is refractory metal oxide colloid or metal organic compound. The key to the preparation of the shell for thick-walled titanium alloy precision castings lies in the selection of high-strength and high-inertness oxides for the surface layer and the preparation of the surface layer shell. At present, the mainstream surface layer shell materials use zirconia or yttria as refractory materials; yttrium sol and zirconium acetate are used as binders. However, yttrium sol is prone to hydration, is very sensitive to changes in pH value, is easy to gel, and has a short service life. It is mostly used for experiments in universities and research institutes and is rarely used in production; zirconium acetate has a wide application range in the surface layer shell of titanium alloy precision casting. It has high chemical stability and a small degree of interfacial reaction, but there are also deficiencies. It is easy to redissolve in a humid environment, which leads to serious redissolution of the surface layer shell when the wax mold of thick-walled castings is dewaxed in hot water vapor, restricting its application in thick-walled titanium alloy castings. Summary of the Invention

[0005] In view of this, the present invention aims to propose a preparation method for the surface layer shell used in the precision casting production of thick-walled titanium alloys, so as to solve the problems in the prior art that when using yttrium sol, the yttrium sol is prone to hydration, is very sensitive to changes in pH value, is easy to gel, and has a short service life; when using zirconium acetate, the zirconium acetate is prone to redissolution in a humid environment, which leads to serious redissolution of the surface layer shell when the wax mold of the thick-walled casting is dewaxed in hot steam.

[0006] To achieve the above object, the technical solution of the present invention is realized as follows:

[0007] A preparation method for the surface layer shell used in the precision casting production of thick-walled titanium alloys includes the following steps:

[0008] S1. Preparation of the surface layer binder: The surface layer binder includes modified silica sol, stabilizer, reinforcing agent, wetting agent, and acid-base regulator; the modified silica sol is diluted sodium-stabilized alkaline colloidal silica.

[0009] S2. Preparation of the surface layer refractory powder: The surface layer refractory powder includes yttrium oxide powder, zirconium oxide powder, and calcium fluoride powder; the mass ratio of the yttrium oxide powder, zirconium oxide powder, and calcium fluoride powder is (60~95):(5~40):(0.1~1).

[0010] S3. Preparation of the surface layer coating; Add the surface layer refractory powder in S2 to the surface layer binder in S1, with a mass ratio of 3.5~5.5:1, and stir for 8~24h.

[0011] S4. Preparation of the surface layer shell: Immerse the mold for preparing the surface layer shell into the surface layer coating in S3 for slurry hanging, sand sprinkling, and drying, and repeat this step 1~2 times.

[0012] S5. Fire the surface layer shell in S4 to form a shape, and obtain the surface layer shell used in the precision casting production of thick-walled titanium alloys.

[0013] The preparation method of the present invention is simple and feasible. The surface layer shell obtained by the preparation method can be used to produce titanium alloy castings with a wall thickness of 25mm~50mm. The castings have no defects in morphology, low roughness, and high dimensional accuracy. This preparation method can be widely promoted and used.

[0014] Further, in the modified silica sol, the pH is 9~10, the diameter of the SiO2 colloidal particles ≤6nm, and the total mass ratio of the SiO2 colloidal particles to the modified silica sol is 10~15%.

[0015] Further, the specific preparation steps of the modified silica sol are as follows: Using ordinary silica sol, deionized water as the solvent, diluting the silica content to 10% - 15%, adding a pH regulator to adjust the pH value to 9 - 10, and stirring for about 24 - 48 h. Among them, the pH regulator is NaOH solution and HCL solution.

[0016] Further, the stabilizer is Y2O3 powder, the reinforcing agent is carbon fiber, the wetting agent is n-octanol and JFC, and the acid-base regulator is NaOH solution and HCL solution.

[0017] Further, the diameter of the carbon fiber is 100 - 200 nm, and the length is 1 - 2 mm.

[0018] Further, the particle size of the Y2O3 powder is 9 - 20 nm.

[0019] Further, the mass ratio of the modified silica sol, Y2O3 powder, carbon fiber, n-octanol, and JFC is (87 - 93):(5 - 10):(1 - 3):(0.01 - 1):(0.03 - 0.5).

[0020] Further, in S2, the mesh numbers of the yttrium oxide powder and zirconium oxide powder are both 200 - 400 mesh, and the mesh number of the calcium fluoride powder is 1000 - 1500 mesh.

[0021] Further, in S4, the sandblasting process uses yttrium oxide or zirconium oxide sand with a mesh number of 60 - 120 mesh, where 60 mesh accounts for 60% - 65%, 80 mesh accounts for 20% - 25%, and 120 mesh accounts for 10% - 20%.

[0022] Further, in the drying process of S4, the environmental humidity used is ≥75%, the drying temperature is 23 - 27 °C, and the drying time is ≥12 h.

[0023] Compared with the prior art, the preparation method of the surface layer shell for thick-walled titanium alloy precision casting of the present invention has the following advantages:

[0024] 1) The modified silica sol of the present invention uses diluted sodium-stabilized alkaline colloidal silica. First, it can reduce the residual strength of the shell and improve the shell cleaning efficiency; second, it reduces the interfacial reaction between the silica sol and the titanium alloy liquid and improves the quality of thick-walled titanium alloy castings; then, it makes the surface layer shell resistant to high temperatures, and during the investment casting process of thick-walled titanium alloy, no defects will occur in the castings.

[0025] 2) The modified silica sol of the present invention has a synergistic effect with carbon fiber and Y2O3 powder. On the basis of ensuring the bonding strength of the surface shell, the quality of thick-walled castings is also improved. The addition of carbon fiber is beneficial to improving the wet strength of the surface shell, accelerating the drying speed of the surface layer, and improving the air permeability of the surface shell, further improving the cleaning efficiency of the casting;

[0026] 3) The present invention adopts modified silica sol, surface refractory powder, and multi-graded surface sand to prepare the surface shell. The first obvious effect is that the surface layer is naturally dried, and the surface layer is pressed by hand. It is obvious that the wet strength of the surface layer is quickly established and the adhesion strength is high. No casting defects are found in the titanium alloy castings cast by the shell with only one layer of surface layer; the second obvious effect is that the shell has good disintegration, and the efficiency of sand removal and shelling is nearly 2 times improved; the third obvious effect is that the surface roughness of the casting is reduced, and there are no surface defects such as pockmarks. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is an appearance diagram of a casting according to Example 2 of the present invention;

[0028] Figure 2 This is an appearance diagram of the casting of Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0029] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, specific embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0030] In the preparation of the surface shell for the production of thick-walled titanium alloy precision casting, silica sol is generally not used as the surface binder, because the roasting product of silica sol is SiO2, and there is a certain chemical reactivity between SiO2 and the titanium alloy liquid. In the casting process of high temperature and long-term contact, this reaction may be aggravated, resulting in defects such as pores on the surface or subcutaneous of the titanium alloy casting. Especially in thick-walled castings, due to the relatively slow cooling rate, the impact of this chemical reaction may be more significant. This chemical reaction also affects the dimensional accuracy and stability of the surface shell; the surface binder used in the present application is modified silica sol, and the modified silica sol is diluted by a certain proportion on the basis of conventional silica sol. The use of modified silica sol can, firstly, reduce the residual strength of the shell and improve the shell cleaning efficiency; secondly, reduce the interface reaction between silica sol and titanium alloy liquid, and improve the quality of thick-walled titanium alloy castings; finally, make the surface shell resistant to high temperature, and no defects will occur in the casting during the thick-walled titanium alloy investment casting process.

[0031] The present invention relates to a method for preparing a surface shell for thick-wall titanium alloy precision casting production, which specifically comprises the following steps:

[0032] S1. Preparation of surface layer adhesive: The surface layer adhesive includes modified silica sol, stabilizer, reinforcing agent, wetting agent and acid-base regulator.

[0033] Specifically, the modified silica sol is diluted sodium-stabilized alkaline colloidal silica, the stabilizer is Y2O3 powder, the reinforcing agent is carbon fiber, the wetting agent is n-octanol and JFC, and the acid-base regulator is NaOH solution and HCL solution. The modified silica sol, carbon fiber, and Y2O3 powder in the surface layer binder play a synergistic role. On the basis of ensuring the bonding strength of the surface layer shell mold, it also improves the quality of thick-walled castings. The addition of carbon fiber is beneficial to improving the wet strength of the surface layer shell mold, accelerating the drying speed of the surface layer, and improving the air permeability of the surface layer shell mold, further improving the shell cleaning efficiency of the casting.

[0034] Specifically, in the modified silica sol, the PH is 9-10, the diameter of the SiO2 colloidal particles is ≤6nm, and the total mass ratio of the SiO2 colloidal particles to the modified silica sol is 10-15%.

[0035] Specifically, the specific preparation steps of the modified silica sol are as follows: Using ordinary silica sol, deionized water as the solvent, diluting the silica dioxide content to 10%-15%, adding a PH value regulator to adjust the PH value to 9-10, and stirring for about 24-48h. Among them, the PH value regulator is NaOH solution and HCL solution.

[0036] Specifically, the concentrations of the above-mentioned NaOH solution and HCL solution are both 0.1-1mol / L.

[0037] Specifically, the diameter of the carbon fiber is 100-200nm and the length is 1-2mm. This setting can improve the strength and air permeability of the surface layer shell mold and improve the shell cleaning efficiency.

[0038] Specifically, the particle size of the Y2O3 powder is 9-20nm.

[0039] Specifically, the mass ratio of the modified silica sol, Y2O3 powder, carbon fiber, n-octanol, and JFC is (87-93):(5-10):(1-3):(0.01-1):(0.03-0.5).

[0040] Specifically, the preparation method of the surface layer binder is to add the modified silica sol to a container, and sequentially add Y2O3 powder, carbon fiber, n-octanol, JFC, and acid-base regulator under mechanical stirring. Among them, the stirring speed is 100-180r / min and the stirring time is 24-48h.

[0041] S2. Preparation of the surface layer refractory powder: The surface layer refractory powder includes yttrium oxide powder, zirconium oxide powder, and calcium fluoride powder; the mass ratio of the yttrium oxide powder, zirconium oxide powder, and calcium fluoride powder is (60-95):(5-40):(0.1-1); In this setting, adding calcium fluoride can reduce the roasting temperature of the shell mold.

[0042] Specifically, in S2, the mesh number of both the yttrium oxide powder and the zirconium oxide powder is 200 - 400 mesh, and the mesh number of the calcium fluoride powder is 1000 - 1500 mesh.

[0043] S3. Preparation of the surface coating: Add the surface refractory powder in S2 to the surface binder in S1, with a mass ratio of 3.5 - 5.5:1, and stir for 8 - 24 h.

[0044] S4. Preparation of the surface shell mold: Immerse the wax mold module for preparing the surface shell mold after cleaning into the surface coating in S3 for uniform slurry dipping and then control the slurry. When the surface slurry no longer flows, use a rain - type sand spraying machine to uniformly sprinkle the surface sand. The surface sand is one of yttrium oxide or zirconium oxide sand, with a mesh number of 60 - 120 mesh. The preferred grading is: 60 - mesh accounts for 60% - 65%, 80 - mesh accounts for 20% - 25%, and 120 - mesh accounts for 10% - 20%. After uniformly sprinkling the sand, dry it in an environment with a humidity ≥75% and a temperature of 25 ± 2°C for ≥12 h, and then repeat the operations of slurry dipping, sand spraying, and drying 1 - 2 times to obtain the surface shell mold.

[0045] Specifically, on the basis of the above - mentioned surface shell - making process, 4 - 7 layers of back layers are prepared using a special back - layer powder slurry and back - layer sand for titanium alloy precision casting, and the shell mold preparation is completed. The back - layer powder slurry, back - layer sand, and the corresponding usage methods are all prior arts and will not be described in detail here.

[0046] S5. Bake the surface shell mold in S4 to form a surface shell mold for thick - wall titanium alloy precision casting production.

[0047] Specifically, step S5 further includes:

[0048] S51: Dewaxing;

[0049] S52: Baking the shell mold;

[0050] S53: Washing and drying.

[0051] Specifically, in the dewaxing process of S51, steam dewaxing is adopted, aiming to reduce the technological links of wax recovery treatment and lower the production cost. The dewaxing process uses a dewaxing temperature of 103 - 170°C and a dewaxing time of 10 - 30 min.

[0052] Specifically, the specific process of baking the shell mold in S52 is to directly place the dewaxed surface shell mold in the furnace for baking. During baking, the temperature is gradually increased, and the surface shell mold is heated to 1050 - 1200°C and kept warm for 180 - 360 min.

[0053] Specifically, the specific process of water washing and drying in S53 is to perform running water washing on the surface layer shell cooled to room temperature to remove the floating ash on the surface. It is dried under the conditions of a drying temperature of 280-320 °C and a drying time of 100-140 min.

[0054] Next, in combination with the embodiments of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described.

[0055] Embodiment 1

[0056] The present invention relates to a preparation method for a surface layer shell used in the precision casting production of thick-walled titanium alloys, specifically including the following steps:

[0057] S1. Preparation of the surface layer binder: Add the diluted sodium-stabilized alkaline colloidal silica into a container, and successively add Y2O3 powder, carbon fiber, n-octanol, and JFC under mechanical stirring. Adjust the pH value with NaOH solution and HCL solution to make its pH 9, the stirring speed is 100 r / min, and the stirring time is 24 h. Among them, the mass ratio of the modified silica sol, Y2O3 powder, carbon fiber, n-octanol, and JFC is 90:7:2:0.07:0.03.

[0058] Specifically, the specific preparation steps of the diluted sodium-stabilized alkaline colloidal silica are as follows: Use ordinary silica sol with a SiO2 colloidal particle diameter of 3-5 mm for the ordinary sol, dilute the ordinary silica sol with deionized water to a silica content of 11%, and adjust it with NaOH solution and HCL solution to make the pH value of the diluted sodium-stabilized alkaline colloidal silica 9, and the stirring time is 24 h.

[0059] Specifically, the diameter of the carbon fiber is 100-200 nm, and the length is 1-2 mm.

[0060] Specifically, the particle size of the Y2O3 powder is 9-20 nm.

[0061] S2. Preparation of the surface layer refractory powder: Mix 70% yttrium oxide powder, 29% zirconia powder, and 1% calcium fluoride powder evenly; among them, the mesh numbers of the yttrium oxide powder and zirconia powder are both 200 mesh, and the mesh number of the calcium fluoride powder is 1000 mesh to obtain the refractory powder.

[0062] S3. Preparation of the surface layer coating; Add the surface layer refractory powder in S2 to the surface layer binder in S1, with a mass ratio of 3.5:1, and stir for 10 h to obtain the surface layer coating.

[0063] S4. Preparation of the surface layer shell: The wax mold module for preparing the surface layer shell after cleaning is immersed in the surface layer coating in S3 and evenly slurried, and then the slurry is controlled. When the surface layer slurry no longer flows, a rain-type sand spraying machine is used to evenly sprinkle the surface layer sand. The surface layer sand is yttrium oxide, with a mesh number of 60 - 120 meshes, and the preferred particle size distribution is: 60 meshes account for 60%, 80 meshes account for 20%, and 120 meshes account for 20%. After evenly spraying the sand, it is dried in an environment with a humidity ≥ 75% and a temperature of 23°C for 36 hours, and then the operations of hanging slurry, sand spraying, and drying are repeated once to obtain the surface layer shell.

[0064] Specifically, on the basis of the above surface layer shell-making process, 5 layers of back layers are prepared using a special back layer slurry and back layer sand for precision investment casting of titanium alloy, and the shell preparation is completed. The back layer slurry, back layer sand, and their corresponding usage methods are all prior arts and will not be described in detail here.

[0065] S5. The surface layer shell in S4 is subjected to dewaxing - shell baking - water washing and drying to obtain the surface layer shell for thick-walled titanium alloy precision casting production.

[0066] In the dewaxing process, steam dewaxing is adopted to reduce the technological process of wax recovery treatment and lower the production cost. The dewaxing process uses a dewaxing temperature of 110°C and a dewaxing time of 30 minutes.

[0067] The specific process of shell baking is to directly place the dewaxed surface layer shell in the furnace for baking. During baking, the temperature is gradually increased, and the surface layer shell is heated to 1100°C and kept warm for 360 minutes.

[0068] The specific process of water washing and drying is to wash the surface layer shell cooled to room temperature with running water to remove the surface floating ash. It is dried under the conditions of a drying temperature of 280°C and a drying time of 140 minutes.

[0069] Specifically, the surface layer shell in S5 is then cast and the shell is removed to obtain the titanium alloy casting blank.

[0070] The present invention also provides a surface layer shell for titanium alloy precision casting prepared by the above method, which is applicable to the production of titanium alloy castings with a wall thickness of 25 mm - 50 mm.

[0071] Example 2

[0072] The present invention relates to a method for preparing a surface layer shell for thick-walled titanium alloy precision casting production, which specifically includes the following steps:

[0073] S1. Preparation of the surface layer binder: Add the diluted sodium-stabilized alkaline colloidal silica into a container. Under mechanical stirring, sequentially add Y2O3 powder, carbon fiber, n-octanol, and JFC. Adjust the pH value with NaOH solution and HCl solution to make the pH 9. The stirring speed is 100 r / min, and the stirring time is 24 h. Among them, the mass ratio of the modified silica sol, Y2O3 powder, carbon fiber, n-octanol, and JFC is 89:8:2:0.06:0.04.

[0074] Specifically, the specific preparation steps of the diluted sodium-stabilized alkaline colloidal silica are as follows: Use ordinary silica sol with the SiO2 colloidal particle diameter of 3 - 5 mm. Dilute the ordinary silica sol with deionized water to a silica content of 13%. Adjust with NaOH solution and HCl solution to make the pH value of the diluted sodium-stabilized alkaline colloidal silica 9, and the stirring time is 36 h.

[0075] Specifically, the diameter of the carbon fiber is 100 - 200 nm, and the length is 1 - 2 mm.

[0076] Specifically, the particle size of the Y2O3 powder is 9 - 20 nm.

[0077] S2. Preparation of the surface layer refractory powder: Mix 80% yttrium oxide powder, 18.5% zirconia powder, and 0.5% calcium fluoride powder evenly. Among them, the mesh number of the yttrium oxide powder and the zirconia powder is 300 mesh, and the mesh number of the calcium fluoride powder is 1200 mesh to obtain the refractory powder.

[0078] S3. Preparation of the surface layer coating: Add the surface layer refractory powder in S2 to the surface layer binder in S1 with a mass ratio of 4.5:1, and stir for 16 h to obtain the surface layer coating.

[0079] S4. Preparation of the surface layer shell mold: Immerse the cleaned wax mold module for preparing the surface layer shell mold into the surface layer coating in S3, soak it evenly and then control the slurry. When the surface layer slurry no longer flows, use a rain-type sand spraying machine to evenly spray the surface layer sand. The surface layer sand is zirconia sand with a mesh number of 60 - 120 mesh. The preferred particle size distribution is: 62% of 60 mesh, 25% of 80 mesh, and 13% of 120 mesh. After evenly spraying the sand, dry it in an environment with a humidity ≥ 75% and a temperature of 27 °C for 12 h, and then repeat the operations of hanging slurry, sand spraying, and drying 2 times to obtain the surface layer shell mold.

[0080] Specifically, on the basis of the above surface layer shell mold manufacturing process, use the special back layer powder slurry and back layer sand for titanium alloy precision casting to prepare 6 back layers, and the shell mold preparation is completed. The back layer powder slurry, the back layer sand, and the corresponding usage methods are all prior arts and will not be described in detail here.

[0081] S5. Dewax the surface layer shell of S4, roast the shell, wash and dry it to form a surface layer shell for precision casting of thick-walled titanium alloy.

[0082] In the dewaxing process, steam dewaxing is adopted to reduce the technological links of wax recovery treatment and lower the production cost. The dewaxing temperature is 150 °C and the dewaxing time is 20 min.

[0083] The specific process of roasting the shell is to directly place the dewaxed surface layer shell in the furnace for roasting. During roasting, the temperature is gradually increased, and the surface layer shell is heated to 1150 °C and held for 200 min.

[0084] The specific process of washing and drying is to wash the surface layer shell cooled to room temperature with running water to remove the floating ash on the surface. It is dried under the conditions of a drying temperature of 300 °C and a drying time of 120 min.

[0085] Specifically, the surface layer shell in S5 is then cast and shelled to obtain a rough blank of titanium alloy casting.

[0086] The present invention also provides a surface layer shell for precision casting of titanium alloy prepared by the above method, which is suitable for producing titanium alloy castings with a wall thickness of 25 mm to 50 mm.

[0087] Example 3

[0088] The present invention relates to a preparation method of a surface layer shell for precision casting of thick-walled titanium alloy, which specifically includes the following steps:

[0089] S1. Preparation of surface layer binder: Add diluted sodium-stabilized alkaline colloidal silica into a container. Under mechanical stirring, add Y2O3 powder, carbon fiber, n-octanol, and JFC in sequence. Adjust the pH value with NaOH solution and HCL solution to make its pH 9. The stirring speed is 180 r / min and the stirring time is 24 h. Among them, the mass ratio of the modified silica sol, Y2O3 powder, carbon fiber, n-octanol, and JFC is 93:6:1:0.06:0.04.

[0090] Specifically, the specific preparation steps of the diluted sodium-stabilized alkaline colloidal silica are as follows: Use ordinary silica sol with the SiO2 colloidal particle diameter of 3 - 5 mm. Dilute the ordinary silica sol with deionized water to a silica content of 11%. Adjust it with NaOH solution and HCL solution to make the pH value of the diluted sodium-stabilized alkaline colloidal silica 9, and the stirring time is 24 h.

[0091] Specifically, the diameter of the carbon fiber is 100 - 200 nm and the length is 1 - 2 mm.

[0092] Specifically, the particle size of the Y2O3 powder is 9 - 20 nm;

[0093] S2. Preparation of the refractory powder for the surface layer: Mix 90% yttrium oxide powder, 9% zirconia powder, and 1% calcium fluoride powder evenly; among them, the mesh number of both the yttrium oxide powder and the zirconia powder is 400 mesh, and the mesh number of the calcium fluoride powder is 1500 mesh to obtain the refractory powder.

[0094] S3. Preparation of the surface layer coating: Add the refractory powder for the surface layer in S2 to the fabric binder in S1 at a mass ratio of 5.5:1 and stir for 24 h to obtain the surface layer coating.

[0095] S4. Preparation of the surface layer shell mold: Immerse the cleaned wax mold module for preparing the surface layer shell mold into the surface layer coating in S3, evenly dip the slurry, and then control the slurry. When the surface layer slurry no longer flows, use a rain - type sand spraying machine to evenly sprinkle the surface layer sand. The surface layer sand is yttrium oxide with a mesh number of 60 - 120 mesh, and the preferred grading is: 65% of 60 - mesh, 20% of 80 - mesh, and 15% of 120 - mesh. After evenly spraying the sand, dry it in an environment with a humidity ≥ 75% and a temperature of 27°C for 24 h, and then repeat the operations of hanging the slurry, sprinkling the sand, and drying 2 times to obtain the surface layer shell mold.

[0096] Specifically, on the basis of the above - mentioned surface layer shell - making process, 7 layers of back layers are prepared using a special back - layer powder slurry and back - layer sand for precision investment casting of titanium alloy, and the shell mold preparation is completed. The back - layer powder slurry, the back - layer sand, and their corresponding usage methods are all prior arts and will not be described in detail here.

[0097] S5. Dewax the surface layer shell mold in S4, roast the shell mold, wash it with water, and dry it to form a surface layer shell mold for precision casting of thick - wall titanium alloy.

[0098] In the dewaxing process, steam dewaxing is adopted to reduce the technological processes of wax recovery treatment and lower the production cost. The dewaxing process uses a dewaxing temperature of 160°C and a dewaxing time of 10 min.

[0099] The specific process of roasting the shell mold is to directly place the dewaxed surface layer shell mold in the furnace for roasting. When roasting, gradually increase the temperature, heat the surface layer shell mold to 1200°C, and keep it warm for 180 min.

[0100] The specific process of washing with water and drying is to wash the surface layer shell mold that has cooled to room temperature with running water to remove the surface floating ash. Dry it under the conditions of a drying temperature of 320°C and a drying time of 100 min.

[0101] Specifically, the surface layer shell mold in S5 is then cast and the shell is removed to obtain a rough blank of a titanium alloy casting.

[0102] The present invention also provides a precision casting surface layer shell of titanium alloy prepared by the above method, which is applicable to the production of titanium alloy castings with a wall thickness of 25 mm to 50 mm.

[0103] Comparative Example 1

[0104] In this comparative example, the casting was prepared by the method in the publication number CN 106825409 A.

[0105] Comparative Example 2

[0106] The difference between this Comparative Example 2 and Example 2 is that in the preparation of the surface layer binder, the mass fraction of silicon dioxide in the diluted sodium-stabilized alkaline colloidal silica is 5%.

[0107] The appearance diagram of the casting obtained by the preparation method of Example 2 is as Figure 1 shown, and the appearance diagram of the casting obtained by the method of Comparative Example 1 is as Figure 2 shown.

[0108] By Figure 1 and Figure 2 comparison, it can be known that the preparation method of the present invention improves the inertness of the surface layer shell, increases the dry adhesion strength of the surface layer, and thus improves the quality of the shell-cast titanium alloy casting.

[0109] The time used for separating the castings obtained by the preparation methods of Examples 1 to 3 and Comparative Examples 1 to 2 from the shell by mechanical vibration for shell cleaning, as well as the surface roughness of the obtained castings and the analysis results of the dimensional accuracy measured by the GB / T 6414 standard are shown in Table 1.

[0110] Table 1

[0111]

[0112] It can be obtained from Table 1 that the shell prepared by the preparation method of the present invention has good collapsibility. The sand cleaning and shell stripping efficiency of the surface layer shell and the casting is increased by nearly 2 to 3 times compared with that obtained by the conventional process. Moreover, for the casting obtained by casting with the surface layer shell of the present invention, the surface roughness is reduced and the dimensional accuracy is greatly improved compared with the prior art.

[0113] Although the present invention is disclosed as above, the present invention is not limited thereto. Any person skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the protection scope of the present invention should be subject to the scope defined by the claims.

Claims

1. A method for preparing a surface shell for thick-walled titanium alloy precision casting production, characterized in that: The steps include: S1. Preparation of surface layer adhesive: The surface layer adhesive includes modified silica sol, stabilizer, reinforcing agent, wetting agent, and acid-base regulator; the modified silica sol is diluted sodium-stabilized alkaline colloidal silica; S2. Preparation of surface refractory powder: the surface refractory powder includes yttrium oxide powder, zirconium oxide powder and calcium fluoride powder; the mass ratio of yttrium oxide powder, zirconium oxide powder and calcium fluoride powder is (60-95): (5-40): (0.1-1); S3, preparation of surface coating: add the surface refractory powder in S2 to the surface binder in S1, with a mass ratio of 3.5-5.5:1, and stir for 8-24 hours; S4, preparation of the surface shell: put the module for preparing the surface shell into the surface coating in S3 to coat, sand and dry, and repeat this step 1-2 times; S5. Sinter the surface shell of S4 to obtain a surface shell for thick-walled titanium alloy precision casting production.

2. The preparation method according to claim 1, characterized in that: In the modified silica sol, the pH is 9-10, the diameter of SiO2 particles is ≤6nm, and the total mass ratio of the SiO2 particles to the modified silica sol is 10-15%.

3. The preparation method according to claim 1, characterized in that: The specific preparation steps of the modified silica sol are: using ordinary silica sol, using deionized water as a solvent, diluting the silicon dioxide content to 10% to 15%, adding a pH regulator to adjust the pH value to 9 to 10, and stirring for about 24 to 48 hours, wherein the pH regulator is a NaOH solution and an HCL solution.

4. The preparation method according to claim 1, characterized in that: The stabilizer is Y2O3 powder, the reinforcing agent is carbon fiber, the wetting agent is n-octanol and JFC, and the acid-base regulator is NaOH solution and HCL solution.

5. The preparation method according to claim 4, characterized in that: The carbon fiber has a diameter of 100-200 nm and a length of 1-2 mm.

6. The preparation method according to claim 4, characterized in that: The particle size of the Y2O3 powder is 9-20nm.

7. The preparation method according to claim 4, characterized in that: The mass ratio of the modified silica sol, Y2O3 powder, carbon fiber, n-octanol and JFC is (87-93): (5-10): (1-3): (0.01-1): (0.03-0.5).

8. The preparation method according to claim 1, characterized in that: In S2, the mesh sizes of yttrium oxide powder and zirconium oxide powder are both 200-400 meshes, and the mesh size of calcium fluoride powder is 1000-1500 meshes.

9. The preparation method according to claim 1, characterized in that: In S4, the sand spreading process uses yttria or zirconium oxide sand with a mesh size of 60~120, of which 60 mesh accounts for 60%~65%, 80 mesh accounts for 20%~25%, and 120 mesh accounts for 10%~20%.

10. The preparation method according to claim 1, characterized in that: In the drying process of S4, the ambient humidity is ≥75%, the drying temperature is 23~27℃, and the drying time is ≥12h.

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