A shell layer slurry for investment casting and a vacuum impregnation process method

Abstract

The present application relates to the technical field of titanium alloy investment casting process, in particular to a kind of shell layer slurry for investment casting and vacuum impregnation process method, comprising: 15~25% silicon yttrium composite sol and 75~85% zirconium dioxide powder;Wherein, the molar ratio of Y:Si in the silicon yttrium composite sol is 1:(1.2~1.8).The present application uses the low-cost surface layer slurry formulation of "zirconium dioxide powder+silicon yttrium composite sol", replaces traditional high-cost yttrium oxide material, reduces the cost of surface layer refractory material by about 40%, and forms Y2SiO5 high-temperature barrier layer using silicon yttrium composite sol, ensures the excellent chemical compatibility of shell and titanium liquid, significantly improves the economy of material, guarantees the surface quality of casting, avoids defects such as titanium inclusion and under-casting caused by interface reaction.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of titanium alloy investment casting process, in particular to a shell layer slurry for investment casting and a vacuum impregnation process. BACKGROUND

[0002] Titanium alloy has become the core manufacturing material for complex thin-walled components such as aerospace blades and medical implants due to its high strength, low density, excellent biocompatibility, corrosion resistance, and good high-temperature stability. However, during the process of titanium alloy investment precision casting, the following difficulties are faced in the forming quality of complex thin-walled components: 1. High incidence of titanium and slag inclusion at narrow structures and concave corners; 2. Poor filling of 0.7mm and below thin-walled, sharp corners, with a failure rate of up to 70%. The core cause of such defects lies in the limitations of the existing shell surface layer preparation process: 1. Low bubble removal efficiency, which easily gets stuck in narrow gaps and concave corners and cannot be efficiently removed; 2. Insufficient slurry penetration, poor adhesion on the wax film surface, or even "gas retention" forming a new gas film. This leads to the appearance of defects such as "ant holes" on the shell surface layer, which is a direct cause of titanium liquid flow mark type filling defects and can also cause high-temperature interface reactions, further leading to quality problems such as titanium and slag inclusion and underfilling (such as Figure 1 ).

[0003] In the exploration of improving the quality of shell surface layer, the vacuum impregnation process has been verified in the practice of investment casting of other metal materials such as stainless steel, which has shown its technical potential in strengthening the filling and penetration of slurry. However, for titanium alloy investment casting, this process still has key technical gaps: on the one hand, there is a lack of quantitative research on the parameter mechanism, and the negative pressure value and pumping time are mostly set by experience, lacking quantitative rules of "parameter-slurry rheology-coating quality", while empirical parameters often lead to problems such as slurry rheology decay, uneven coating thickness, and local sand exposure in industrial production, which cannot be stably solved in batch production; on the other hand, the mainstream formula of the shell surface layer material for existing titanium alloy investment precision casting uses high-purity yttria as the surface layer refractory powder, but the price of yttria powder is as high as 420 yuan / kg, which increases the manufacturing cost by 40%, further hindering the industrial application of the vacuum impregnation process in the field of titanium alloy casting.

[0004] Yttrium rare earth ceramic shell titanium alloy investment casting method, including yttrium rare earth refractory material production, shell surface coating, shell reinforcement and other process. Yttrium rare earth refractory material production is to mix yttrium rare earth oxide and stabilizer, calcine in high temperature kiln, temperature control at 1850℃, shell surface coating is to mix yttrium rare earth refractory material and zirconium acetate solvent, ingredient ratio is 3.4:1. The prepared surface coating, then immerse the prepared wax mold into the surface coating, working environment temperature is 25℃, relative humidity is 70%, surface coating temperature is 20℃, surface layer viscosity control is 22sec, yttrium rare earth oxide composition is Y2O3+ReO75-90%, the stabilizer selects ZrO2 or CaO, ZrO2 is 6.5-20%, CaO is 3.5-10%. But the surface coating of the scheme is mainly composed of Y2O3, the proportion of ZrO2 is low, the cost is high, still belongs to the prior art, there are problems of low casting surface quality and high production cost.

[0005] Therefore, there is an urgent need for a new shell layer slurry for investment casting and a vacuum dipping process method to solve the problems of low casting surface quality and high production cost in the prior art. SUMMARY

[0006] Therefore, there is an urgent need for a new shell layer slurry for investment casting and a vacuum dipping process method to solve the problems of low casting surface quality and high production cost in the prior art.

[0007] Based on the prior art background of titanium alloy investment casting, the present application combines low-cost slurry with precise negative pressure process, taking into account economy and reliability, develops a low-cost surface layer formula, ensures compatibility with high-temperature titanium liquid, and realizes the performance-price ratio upgrade of surface layer material; Precise vacuum dipping process, clear negative pressure parameter mechanism on slurry rheology and coating quality, respectively provide precise process guidance for short process efficient production and long period stable production.

[0008] The application breaks through the conventional use of expensive yttrium oxide for the face layer of titanium alloy investment casting, for the first time proposes and verifies a low-cost face layer slurry formula mainly composed of zirconium dioxide powder and silicon-yttrium composite sol as a binder, directly reduces the material cost by replacing the traditional expensive yttrium oxide with zirconium dioxide powder which is only 35% of the price of yttrium oxide as the face layer refractory powder, and uses the silicon-yttrium composite sol as a binder, which can form a stable ternary mixture ZrO2-SiO2-Y2O3 with zirconium dioxide powder at high temperature and generate a high-temperature barrier layer Y2SiO5 with excellent chemical inertness, ensuring the compatibility of the face layer of the mold shell with high-temperature titanium liquid and preventing interfacial reaction, and through the gradient grading (7:3) of 200# and 400# zirconium dioxide powder, the coarse particles construct the skeleton and the fine particles fill the gaps, effectively maintaining the rheological properties of the slurry, ensuring good coating and permeability. The application clearly proposes a "low negative pressure and short time" suitable for small batch efficient production and a "high negative pressure and long time" suitable for large batch stable production precise process window, and quantifies the influence of different parameters on the slurry quality, apparent porosity and batch stability, such as the yield rate is increased to 96.3% and the batch fluctuation is only 1.8%, through systematic research, the quantification rule of "parameter-slurry rheology-coating quality" is established, the application bottleneck of vacuum dipping slurry in the field of titanium alloy is solved, the forming difficulty of complex thin-walled parts in titanium alloy precision casting is overcome, and reliable and replicable process guidance is provided for industrial application.

[0009] The technical scheme of the application is as follows:

[0010] An object of the application is to disclose a mold shell layer slurry for investment casting, comprising: 15-25% of a silicon-yttrium composite sol and 75-85% of zirconium dioxide powder; wherein the molar ratio of Y:Si in the silicon-yttrium composite sol is 1:(1.2-1.8).

[0011] Further, the zirconium dioxide powder is composed of 200# zirconium dioxide powder and 400# zirconium dioxide powder in a mass ratio of 7:3.

[0012] Further, the powder-liquid mass ratio of the slurry is (3.5-6.5):1, and the flow cup viscosity at room temperature is 18-25 seconds.

[0013] Further, 10-15 ml of defoaming agent is added to every 5 kg of the silicon-yttrium composite sol.

[0014] Further, the silicon-yttrium composite sol as a binder can form a ternary mixture containing Y2SiO5 phase with the zirconium dioxide powder at high temperature to provide a chemical inert barrier for high-temperature titanium liquid.

[0015] Another object of the application is to disclose a vacuum dipping slurry process method for investment casting, using any of the above-mentioned mold shell layer slurries for investment casting, specifically comprising the following steps:

[0016] S1: immerse the wax tree hanging module in the shell layer slurry for investment casting;

[0017] S2: seal the container containing the module and the slurry as a whole and vacuumize to achieve a preset negative pressure in the container;

[0018] S3: select a pressure stabilization time t according to the negative pressure, and release the air after the time t to complete the slurry immersion;

[0019] S4: after completing the vacuum slurry immersion and releasing the air, take out the module and rotate for a predetermined time;

[0020] S5: perform the sand spraying, drying and subsequent back layer preparation process.

[0021] Further, in step S2, the container is provided with a transparent sealing cover for observing the bubble discharge state of the slurry during vacuumization.

[0022] Further, in step S2, after observing that the slurry presents a state of dense bubbles rising and rolling into large bubbles, continue to vacuumize for 5 minutes during vacuumization.

[0023] Further, in step S3, the preset negative pressure is-0.04~-0.06MPa, and the maintenance time is 3 minutes, which is suitable for small batch efficient production.

[0024] Further, in step S3, the preset negative pressure is-0.08~-0.1MPa, and the maintenance time is greater than 5 minutes, which is suitable for batch continuous production to improve batch stability.

[0025] Compared with the prior art, the shell layer slurry for investment casting and the vacuum slurry immersion process method of the present application has the following advantages:

[0026] 1. The present application uses a low-cost surface layer slurry formula of "zirconium dioxide powder + silicon yttrium composite sol" to replace the traditional high-cost yttrium oxide material, reducing the cost of surface layer refractory material by about 40%, and using the silicon yttrium composite sol to form a Y2SiO5 high temperature barrier layer to ensure the excellent chemical compatibility of the shell and the titanium liquid, significantly improving the material economy while ensuring the surface quality of the casting and avoiding defects such as titanium inclusion and under-casting caused by interface reaction.

[0027] 2. The present application realizes efficient production by precise synergistic optimization of vacuum impregnation process parameters, low negative pressure and short time, improves the coating quality by 15.8%; realizes stable production by high negative pressure and long time, the apparent porosity is ≤27.3%, the coating quality fluctuation is <3%, effectively eliminates the trapped bubbles in the fine and narrow structure and the concave corner, solves the problems of "gas retention" and insufficient penetration of the slurry, makes the shell surface layer dense and smooth, and fundamentally reduces the source defects such as pinholes and ant holes that cause poor filling.

[0028] 3. The present application significantly improves the casting forming quality and production stability by the synergistic effect of low-cost slurry formula and precise vacuum impregnation process, greatly reduces the defect rate of slender thin-walled castings from more than 70% of the traditional process, and in the implementation case, the good product rate is stable to reach 96.3%, the batch fluctuation is only 1.8%, effectively solves the filling problem of 0.7mm and below thin-walled, sharp corners, greatly reduces the scrap rate and repair cost, and is especially suitable for high-quality, batch industrial production demand of complex thin-walled titanium alloy components such as aerospace and medical implants. BRIEF DESCRIPTION OF DRAWINGS

[0029] The accompanying drawings, which form a part of the present application, are used to provide a further understanding of the present application, and the illustrative embodiments of the present application and their description serve the purpose of explaining the present application. The accompanying drawings do not constitute an inappropriate limitation on the present application. In the drawings:

[0030] Figure 1 is a slurry bubble, slurry adhesion or accumulation, shell surface layer pinhole, pouring titanium bead slag and casting defect diagram, wherein, Figure 1 a is a slurry bubble defect diagram, Figure 1 b is a slurry adhesion or accumulation diagram, Figure 1 c is a shell surface layer pinhole diagram, Figure 1 d is a front view of pouring titanium bead slag and casting, Figure 1 e is a rear view of pouring titanium bead slag and casting;

[0031] Figure 2 is a slurry quality column chart of different impregnation processes;

[0032] Figure 3 is a shell surface layer surface morphology of different impregnation processes, wherein, Figure 3 a is a 1# no negative pressure impregnation, pore is dense, there are pinholes and even "ant hole" shell diagram, Figure 3 b is a 2# low negative pressure and short time impregnation, dense and smooth shell diagram, Figure 3 c is a 3# low negative pressure and long time impregnation, dense but transparent and sand exposed shell diagram; Figure 3 d is a 4# high negative pressure and short time impregnation, dense and smooth shell diagram, Figure 3 e is a 5# high negative pressure and long time impregnation, dense and smooth shell diagram;

[0033] Figure 4 is the variation curve of the shell bending strength and the apparent porosity under different impregnation processes;

[0034] Figure 5 is the blank figure after the low negative pressure short-time impregnation process of the aviation sleeve;

[0035] Figure 6 is the figure of serious titanium and slag clamping at the concave corner of the comparative example 1;

[0036] Figure 7 is the figure of the blank after the high negative pressure impregnation process of the batch ball truck series gear connecting piece, without titanium clamping, and the tooth tip is completely and sharply shaped;

[0037] Figure 8 is the figure of titanium clamping and insufficient tooth tip shaping of the comparative example 2. DETAILED DESCRIPTION

[0038] In order to make the technical means and purposes of the present application easy to understand and achieve, the embodiments of the present application are described in detail below in combination with specific figures.

[0039] It should be noted that all the terms for indicating direction and position in the present application, such as "up", "down", "left", "right", "front", "back", "vertical", "horizontal", "inner", "outer", "top", "low", "transverse", "longitudinal", "center", etc., are only used to explain the relative position relationship, connection condition, etc. between the components in a certain state, and are only for the convenience of describing the present application, and are not required to construct and operate the present application in a specific orientation, and therefore cannot be understood as a limitation on the present application. In addition, the description of "first", "second", etc. in the present application is only for the purpose of description, and cannot be understood as indicating or implying the relative importance of the indicated technical features or implicitly indicating the number of the indicated technical features.

[0040] In the description of the present application, unless otherwise explicitly specified and limited, the terms "mounting", "connecting", "connecting" should be understood broadly, for example, it can be fixedly connected, or detachably connected, or integrally connected; it can be mechanically connected; it can be directly connected, or indirectly connected through an intermediate medium; it can be the communication inside two elements. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0041] In the description of the present specification, the description referring to the terms "one embodiment", "some embodiments", "exemplary embodiment", "example", "specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In the present specification, the exemplary description of the above terms does not necessarily mean the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0042] As shown in Table 1, the existing titanium alloy investment precision casting original surface layer slurry component formula is as follows:

[0043] Table 1: Existing titanium alloy investment precision casting original surface layer slurry component formula

[0044]

[0045] As Figures 1-8 shown, one object of the present application is to disclose a shell layer slurry for investment casting, comprising: 15-25% of silicon-yttrium composite sol and 75-85% of zirconium dioxide powder (ZrO2); wherein the molar ratio of Y:Si in the silicon-yttrium composite sol is 1:(1.2-1.8), as shown in Table 2:

[0046] Table 2: Titanium alloy investment precision casting shell surface layer slurry component formula of the present application

[0047]

[0048] The zirconium dioxide powder ZrO2, which is much cheaper than yttrium oxide Y2O3, is used as the main refractory powder accounting for 75-85% to replace the traditional expensive yttrium oxide, thereby directly reducing the manufacturing cost of the shell surface layer from the raw material level. The yttrium and silicon composite sol is used as the binder. In the high-temperature baking and pouring process, Y2O3 and SiO2 in the components can react with the zirconium dioxide powder ZrO2 to form a stable ternary mixture, and preferentially generate yttrium silicate Y2SiO5 phase with excellent chemical inertness. The phase can effectively block the chemical reaction between the high-temperature titanium liquid and the shell material, prevent interface pollution defects such as "titanium inclusion", and the like. The yttrium and silicon composite sol not only provides Y and Si elements, but also has good self-bonding property, which can effectively bond the zirconium dioxide powder particles to form a stable slurry suspension system. The specific Y:Si molar ratio (1:1.2-1.8) is the key to form the effective Y2SiO5 phase and ensure the stability of the bonding network, so as to ensure that the slurry has appropriate viscosity and rheological property, and is easy to be uniformly coated.

[0049] The setting reduces the material cost of the shell surface layer by about 40% by replacing high-purity yttrium oxide with zirconium dioxide powder on a large scale, greatly improving the economy of titanium alloy investment casting. The yttrium element introduced by the yttrium and silicon composite sol can still form a key Y2SiO5 barrier layer at high temperature, ensuring the chemical compatibility of the shell surface layer and the active titanium liquid, avoiding serious interface reaction and casting pollution caused by the use of ordinary refractory materials, improving performance, and the self-bonding property of the yttrium and silicon composite sol is good. Compared with the traditional formula, additional adhesion aids can be reduced or even eliminated, the formula composition is simplified, and the raw material types and process complexity are reduced.

[0050] Specifically, the zirconium dioxide powder is composed of 200# zirconium dioxide powder and 400# zirconium dioxide powder in a mass ratio of 7:3.

[0051] The 200# zirconium dioxide powder has a relatively coarse particle size, and plays a "skeleton" role in the slurry, providing basic support and anti-deformation ability for the entire coating, and helping to control the thickness and shape of the final shell. The 400# zirconium dioxide powder has a relatively fine particle size, and can effectively fill the gaps between the skeleton formed by the 200# coarse particles, reduce the porosity inside the coating, and make the slurry system more dense. Through reasonable grading of coarse and fine particles, the bulk density of the powder can be improved, and in the case of achieving the same powder-to-liquid ratio, the amount of binder required is relatively less, thereby helping to regulate the viscosity and flowability of the slurry, so that it can fully wet the wax mold without being too thin to cause serious flow, ensuring that the slurry can uniformly cover the surface of the wax mold during coating, forming a dense, continuous, and defect-free coating.

[0052] This setting significantly reduces the apparent porosity of the coating, forms a more dense surface layer, can more effectively block the penetration of titanium liquid and high-temperature interfacial reaction, reduces defects such as "ant holes" and "pits", and improves the mechanical strength and bending strength of the shell surface layer, making it less likely to break during dewaxing, handling, and pouring, and having good suspensibility and leveling property.

[0053] Specifically, the powder-to-liquid mass ratio of the slurry is (3.5~6.5):1, and the flow cup viscosity at room temperature is 18~25 seconds.

[0054] The range of 3.5~6.5:1 ensures that the slurry has enough refractory powder to build an effective coating, while retaining an appropriate amount of liquid binder to ensure flowability and adhesion, and a flow cup viscosity of 18~25 seconds can ensure effective penetration into the small grooves and corners of the wax mold in a vacuum environment, so that a certain amount of adhesion is maintained during the dripping process after slurry immersion, forming a uniform coating.

[0055] This setting helps to obtain a surface layer coating with moderate thickness and uniform density, ensures the quality of the final coating, provides clear, quantitative, and easy-to-execute quality control standards, greatly improves production efficiency and batch stability, and enhances the robustness of the process.

[0056] Specifically, 10~15ml of defoaming agent is added to every 5kg of silicon-yttrium composite sol.

[0057] During the process of mixing the silicon-yttrium composite sol with zirconium dioxide powder by high-speed stirring to prepare the slurry, a large amount of air can be entrained by mechanical stirring, which is prone to generate foam. The addition of a defoaming agent can quickly destroy the stability of the foam, prevent the generation and accumulation of foam, and make the slurry as much as possible to remove the internal entrained air bubbles before use, reduce the surface tension of the slurry, help the slurry to better wet the wax mold, and reduce the retention of air bubbles on the surface of the wax mold.

[0058] The setting can effectively reduce bubbles from the source, thereby significantly reducing the occurrence rate of serious internal defects such as 'clamping titanium' and 'poor filling', shortening the time required to achieve ideal exhaust effect, and improving production efficiency.

[0059] Specifically, the silicon-yttrium composite sol serves as a binder and can form a ternary mixture containing Y2SiO5 phase with zirconia powder at high temperature to provide a chemical inert barrier for high-temperature titanium liquid.

[0060] The silicon-yttrium composite sol is a carrier of yttrium Y element necessary for forming the key protective phase Y2SiO5 at high temperature. Without Y2O3 provided by the sol, Y2SiO5 cannot be effectively generated from zirconia powder ZrO2 and ordinary silicon sol alone. In the process of baking the mold shell and subsequent pouring of high-temperature titanium liquid, SiO2 and Y2O3 in the silicon-yttrium composite sol react with refractory powder zirconia powder ZrO2 at high temperature to generate Y2SiO5 phase in situ. This phase has extremely high chemical stability and thermodynamic stability, and the overall chemical inertness of the ZrO2-SiO2-Y2O3 ternary mixture or reaction layer is much higher than that of single ZrO2 or ZrO2-SiO2 system, which can more effectively resist the erosion of active titanium liquid. The generated Y2SiO5 phase forms a dense 'inert barrier' at the contact interface between the mold shell and the molten titanium liquid, physically isolating the titanium liquid from direct contact with the mold shell material, thereby inhibiting element diffusion and chemical reaction.

[0061] The setting can effectively prevent the interface reaction between the titanium liquid and the mold shell, avoid the generation of fatal defects such as 'clamping titanium' caused by the contamination of the titanium liquid by the mold shell material, ensure the purity and mechanical properties of the casting, and achieve a technical breakthrough in chemical stability close to that of a yttria full-shell with low-cost materials.

[0062] Another object of the present application is to disclose a vacuum dipping slurry process for investment casting, comprising the following steps:

[0063] S1: immerse the mold group with wax trees in any of the above face layer slurries;

[0064] Ensure that all surfaces of the wax tree mold group, including complex cavities, narrow channels, and recessed corners, are completely wetted and covered with the face layer slurry.

[0065] S2: seal the container containing the mold group and the slurry as a whole and vacuumize it to achieve a preset negative pressure in the container;

[0066] By reducing the ambient pressure, the bubbles trapped on the surface of the wax mold, inside the slurry, and in the gaps of the mold structure can rapidly expand, merge, and float out due to the pressure difference between the inside and outside, especially the small bubbles and 'dead angle' bubbles that are difficult to remove by traditional normal-pressure dipping.

[0067] S3: According to the negative pressure selection of stable voltage time t, the time t is reached, and the gas is released, and the impregnation is completed;

[0068] According to the different negative pressure, different stable voltage time t is selected to achieve different process goals, such as high efficiency production or high stability production.

[0069] S4: After completing the vacuum impregnation and releasing the gas, the mold group is taken out and rotated for a predetermined time;

[0070] Through rotation, the slurry attached to the surface of the mold group is uniformly distributed by the action of centrifugal force and gravity, avoiding local over-thickness. The dripping process can remove excess slurry, control the thickness of the final surface layer within a reasonable range, and prevent cracking or increase cost due to over-thickness.

[0071] S5: Perform sanding, drying and subsequent back layer preparation process.

[0072] Sanding makes the surface layer have a rough surface, and drying is to solidify the coating and remove moisture.

[0073] This setting significantly improves the quality of the surface layer, eliminates the "air retention" defect, effectively avoids the "pitting" and "ant hole" surface defects caused by air bubble retention, thereby reducing the risk of causing serious internal defects such as "titanium inclusion" and "under casting" from the source. Through effective exhaust and precise control, the defect rate of thin-walled parts is significantly reduced from more than 70% to 96.3%, greatly improving production efficiency and economic benefits.

[0074] Specifically, in step S2, the container is provided with a transparent sealing cover for observing the bubble discharge state of the slurry during vacuum pumping.

[0075] The transparent sealing cover enables the operator to directly observe the real-time state of the slurry and mold group inside the container while the vacuum pump is continuously running and the container is in a negative pressure sealed state, providing an intuitive basis for determining the stable voltage time t. It can also observe whether there are abnormal phenomena such as excessive boiling, splashing of the slurry, displacement of the mold group, etc., so as to take timely measures.

[0076] This setting can confirm that the exhaust process has been fully started by observing the bubble discharge state of the slurry, thereby ensuring that the stable voltage time t is performed under effective exhaust, avoiding parameter failure caused by vacuum pumping efficiency fluctuations, and making the process more precise and reliable.

[0077] Specifically, in step S2, during vacuum pumping, after observing that the slurry presents a state of dense bubbles rising and rolling into large bubbles, continue to pump negative pressure for 5 minutes.

[0078] The state of "slurry presents dense bubbles upwelling and rolling into large bubbles" is taken as a clear visual signal that the exhaust process has been fully started and reached an efficient stage, indicating that most of the bubbles on the surface of the wax mold and inside the slurry have begun to be violently and concentratedly exhausted. After this state appears, the "5 minutes of continuous negative pressure" is calculated to ensure that the 5 minutes of effective negative pressure time is fully effective for the violent exhaust and slurry densification process, rather than including the low-efficiency stage of the initial exhaust and pressure establishment.

[0079] This setting can make the most of the advantages of high negative pressure -0.08~-0.1MPa, deeply remove micro-bubbles, and maximize the penetration and densification of the slurry, so as to obtain a high-quality surface layer with a porosity of ≤27.3%, effectively compensating for the slight differences in equipment and environment.

[0080] Specifically, in step S3, the preset negative pressure is -0.04~-0.06MPa, and the maintenance time is 3 minutes, which is suitable for small-batch efficient production.

[0081] The 3-minute processing time greatly shortens the cycle of single dipping, can significantly improve the production capacity per unit time compared with long-time vacuum treatment, and is particularly suitable for production modes with small order quantity but fast delivery requirement. The lower negative pressure requirement means that the load of the vacuum pump is smaller and the air exhaust demand is lower, thereby reducing energy consumption. At the same time, the shorter running time also reduces the running wear of the equipment and prolongs the service life of the equipment. For example Figure 3 b, at this negative pressure, most of the bubbles can be effectively removed, the quality of the dipped slurry is improved by 15.8%, the coating quality is significantly improved, and defects such as pinholes and ant holes are effectively reduced. If the maintenance time is greater than 5 minutes, as shown in Figure 3 c, it will cause particle sedimentation and colloid stratification, and the quality of the dipped slurry will decrease by 9.6%.

[0082] Specifically, in step S3, the preset negative pressure is -0.08~-0.1MPa, and the maintenance time is greater than 5 minutes, which is suitable for batch continuous production to improve batch stability.

[0083] The negative pressure of -0.08~-0.1MPa, which is equivalent to an absolute pressure of about 0.02~0.00MPa close to complete vacuum, creates a large internal and external pressure difference, which can forcefully and thoroughly remove the micro-bubbles remaining in the finest structures of the wax mold, such as sharp corners, narrow grooves, recesses, and inside the slurry, achieve deep exhaust, improve the uniformity of the slurry coating, and improve the quality of the dipped slurry by 6.1%, as shown in Figure 3 d. The maintenance time of "more than 5 minutes" ensures that there is a long enough stable pressure time under strong negative pressure, so that the exhaust process can be fully completed, and the slurry can be fully penetrated, spread and densified under negative pressure.

[0084] The combination of high negative pressure and long time can eliminate bubbles to the greatest extent, so that the surface layer of the mold shell can achieve extremely high density, and the apparent porosity is ≤27.3%, as shown in Figure 4 The surface layer thickness is increased by 30%, and a dense and smooth coating is formed. High negative pressure suppresses particle sedimentation and bubble regeneration, maintains the rheological stability of the slurry, and the quality fluctuation of the slurry is less than 3%, as shown in Figure 3 d and Figure 3 e, fundamentally eliminating defects such as pinholes and ant holes caused by bubbles, and ensuring that each batch and each mold group undergoes a nearly identical deep degassing and densification process through strong and sufficient processing.

[0085] Specifically, in step S4, the rotation dripping time is 20 seconds.

[0086] Through the rotation module, the slurry just completed vacuum dipping and attached to each part of the wax tree is redistributed by the combined action of centrifugal force and gravity, avoiding the uneven phenomenon of too thick slurry at the bottom and too thin slurry at the top caused by gravity, accurately controlling the amount of slurry finally attached to the surface of the wax mold, avoiding over-thick coating, and reducing defects caused by slurry flow in the transfer and drying initial stage.

[0087] This setting can control the thickness of the coating and prevent defects, and all operators follow the same standard, avoiding the batch-to-batch differences caused by sensory operation, ensuring the stability of the production process and the consistency of the products.

[0088] Specifically, in step S5, the zircon sand particle size for the surface layer in the sand spraying process is 60-120#.

[0089] The zircon sand with a particle size of 60-120# is uniformly sprayed on the wet surface layer just completed with slurry dipping and dripping. The sand particles will be embedded in the un-solidified slurry, and after drying and solidification, these sand particles form a rough surface, providing strong "anchoring points" or "engaging points" for the subsequent back layer slurry and sand particles, enhancing the interlayer bonding force and preventing delamination. It can form a support structure with sufficient strength, and will not affect the fine replication ability of the surface layer slurry due to being too coarse.

[0090] This setting can provide sufficient surface area and mechanical engagement force to ensure excellent bonding strength between the surface layer and the back layer, while ensuring sufficient strength and air permeability, and minimizing the negative impact on the final surface roughness of the casting.

[0091] Specifically, in step S5, the surface layer drying time is 8-12 hours, the drying environment temperature is 22-25°C, and the relative humidity is 45-65%.

[0092] The silicon-yttrium composite sol in the surface layer slurry takes water as a dispersion medium, through slow and thorough evaporation of free water and part of bound water in the coating, the SiO2-Y2O3 network in the slurry is fully condensed to realize physical solidification and preliminary chemical crosslinking, coating defects caused by too fast or too slow evaporation are avoided, and the mechanical strength is improved to withstand the impact and friction in subsequent sanding, handling and back layer coating, and damage or sand particle falling is avoided.

[0093] The setting ensures that the surface layer is fully solidified under stress-free and defect-free conditions by precisely controlling the time, temperature and humidity of drying, and sufficient strength and stability are obtained.

[0094] Specifically, the second layer and the subsequent back layer are prepared by using mullite powder slurry and mullite sand, and the process parameters follow the silicon sol shell preparation process.

[0095] The second layer and the subsequent back layer constitute the main thickness and most of the mechanical strength of the shell, and bear the thermal stress, mechanical stress and metal liquid static pressure in the processes of dewaxing, baking and pouring. Mullite powder / sand has good high-temperature strength and thermal shock resistance, and is an ideal material for building the main structure. Mullite powder and mullite sand are conventional and low-cost refractory materials widely used in investment casting, and their prices are much lower than that of yttrium oxide or even zirconia powder. In addition to the surface layer that guarantees performance, using mullite material as the back layer main body can significantly reduce the total cost of the shell.

[0096] The setting realizes deep optimization of the overall cost by strictly limiting the use of more expensive materials to the thinnest surface layer and returning the back layer which accounts for most of the volume and cost of the shell to the most economical mullite material. The mullite powder / sand shell usually has good air permeability, which helps to exhaust the gas in the shell during pouring and reduces defects such as pores.

[0097] The application also discloses a preparation method of a shell for titanium alloy investment precision casting, which comprises the following steps:

[0098] T1: using the low-cost surface layer slurry in any one of the above aspects for preparation;

[0099] T2: immersing the wax tree module in the surface layer slurry, and using the vacuum slurry immersion process in any one of the above aspects for surface layer coating;

[0100] T3: performing 60-120# zirconium sand spraying, and drying for 8-12 hours in a 22-25 DEG C, 45-65% humidity environment;

[0101] T4: preparing a back layer by using mullite powder slurry and mullite sand;

[0102] T5: performing dewaxing and baking to obtain the shell.

[0103] The method combines two innovations of low-cost surface layer material and precise vacuum impregnation process, and seamlessly connects with the mature back layer process, systematically solving the two major industry difficulties of high cost and poor forming quality of complex thin-walled parts in titanium alloy investment casting. The surface layer realizes high chemical stability and high quality at low cost through the formula of "zirconium dioxide powder + silicon yttrium composite sol" and vacuum impregnation, and the back layer adopts conventional molybdenum material to maximize cost reduction. The overall implementation significantly reduces material cost, reduces the surface layer by 40%, and significantly improves casting quality with a good yield of >96%.

[0104] Specifically, the apparent porosity of the shell surface layer is ≤27.3%, the surface is dense and smooth, and there are no pitting or ant hole defects.

[0105] The extremely low apparent porosity and dense structure mean that the shell surface layer has few pores and narrow channels, forming an effective physical barrier that can effectively block the penetration of high-temperature titanium liquid into the shell interior. By completely eliminating these surface pits and micropores, the direct entry and stress concentration points of metal liquid penetration and "burr" or "titanium clamping" during subsequent pouring are avoided. It ensures that the shell can accurately and perfectly replicate the surface topography of the original wax mold, laying the foundation for the final casting to obtain a high-gloss surface.

[0106] Specifically, the size of the slag inclusion in the casting is ≤0.1mm, the thin-walled and sharp corner parts with a thickness of not more than 0.7mm are completely filled, and the good yield is ≥96%.

[0107] The size of the slag inclusion ≤0.1mm indicates that the non-metallic inclusions such as oxides and shell debris in the casting are controlled at an extremely small scale, ensuring the continuity and purity of the material, and meeting the requirements of complete filling of thin-walled and sharp corner parts with a thickness of not more than 0.7mm, significantly improving the fatigue life and reliability of the casting.

[0108] Example 1

[0109] Low-cost slurry preparation.

[0110] Zirconium dioxide powder as surface layer powder, silicon yttrium composite sol as surface layer binder, zirconium dioxide powder specification 200# zirconium dioxide powder:400# zirconium dioxide powder=70:30, powder liquid mass ratio 4.8:1, room temperature preparation flow cup viscosity value 22s slurry; defoaming agent 5kg silicon yttrium sol plus 15ml. The slurry preparation environment temperature is 25℃, and the humidity is 55%.

[0111] Example 2

[0112] Short-cycle high-efficiency impregnation process applied to a small number of aviation sleeves.

[0113] Adopt Figure 2The steps of the medium-sized No. 2 low negative pressure short cycle slurry impregnation process are as follows: put the module into the slurry tank, completely immerse the wax part in the surface slurry, close the sealing cap to seal, start the vacuum device to negative pressure -0.05MPa and hold the pressure for 3 minutes, then release the air and pressure; open the sealing cap, take out the module, rotate and drip for 20 seconds, then transfer to the surface sand and subsequent processes.

[0114] The test results showed a sample defect rate of 5.2%. Figure 5 As shown, the flexural strength is 4.355 MPa, and the apparent porosity is 28.10%. Figure 4 As shown.

[0115] use Figure 2 The steps of the No. 4 high negative pressure short cycle slurry impregnation process are as follows: put the module into the slurry tank, completely immerse the wax part in the surface slurry, close the sealing cap to seal, start the vacuum device to negative pressure -0.09MPa and hold the pressure for 3 minutes, then release the pressure; open the sealing cap, take out the module, rotate and drip for 20 seconds, then transfer to the surface sand and subsequent processes.

[0116] The test results showed that the flexural strength of the sample was 4.515 MPa, and the apparent porosity was 27.50%. Figure 4 As shown.

[0117] Example 3

[0118] Long-cycle stable slurry impregnation process is applied to batch ball-shaped car series gear tooth connectors.

[0119] use Figure 2 The high negative pressure long-cycle slurry impregnation process for component #5 is as follows: The module is placed in the slurry tank, the wax part is completely immersed in the surface slurry, the sealing cap is closed, and the vacuum device is activated to continuously draw negative pressure within the range of -0.08 to -0.1 MPa. Through the transparent glass sealing cap, the slurry is observed to show dense bubbles rising and churning into large bubbles. The negative pressure is then maintained for another 10 minutes before releasing the pressure. The sealing cap is opened, the module is removed, and after rotating and dripping for 20 seconds, it is transferred to the surface sand coating and subsequent processes. Testing showed that the flexural strength of the sample was 4.600 MPa, and the apparent porosity was 27.30%. Figure 4 As shown. The yield rate remained stable at 96.3%, with a batch fluctuation of 1.8%, as... Figure 7 As shown.

[0120] use Figure 2The middle 3# low negative pressure long cycle slurry immersion process, the steps are as follows: the mold group enters the slurry barrel, the wax part is completely immersed in the surface layer slurry, the sealing cover is closed and sealed, the vacuum pumping device is started to low negative pressure-0.04~-0.06MPa, and the negative pressure is continuously pumped, the slurry presents dense bubbles upwelling and rolls into large bubbles, and the negative pressure is continuously pumped for 10min, then the gas is released and the pressure is released; open the sealing cover, take out the mold group, rotate and drip for 20s, then turn to the surface layer sand and the subsequent process. The slurry of the sample is obviously diluted, and the coating hanging property is poor, and no further detection is made.

[0121] Comparative example 1

[0122] The conventional surface layer preparation process of the aviation sleeve is used to prepare the surface layer by the ladle pouring slurry method, that is, Figure 2 The middle 1# no negative pressure slurry immersion process.

[0123] After detection, the sample bending strength is 4.117MPa, the apparent porosity is 31.30%, as shown in Figure 4 , the concave corner slag inclusion rate is 60.7%, as shown in Figure 6 .

[0124] Comparative example 2

[0125] The conventional surface layer preparation process is used for batch ball package vehicle series gear connecting piece, and the surface layer is prepared by the method of local brushing+normal pressure slurry immersion.

[0126] The titanium bead slag inclusion rate is 89.5%, as shown in Figure 8 .

[0127] In summary, example 1 successfully prepared a low-cost surface layer slurry by replacing yttrium oxide with zirconium dioxide powder. The formula is stable in performance at 25°C and 55% humidity environment. The vacuum slurry immersion solves the problem of "gas retention" and improves the scientific process from empirical operation to parameterization, precision and reproducibility. The short cycle and high efficiency mode of example 2 is suitable for small batch, multi-species and efficiency pursuit production scene. Compared with the "ladle pouring slurry" of comparative example 1, the defect rate is reduced from concave corner slag inclusion rate>60.7%( Figure 6 ) to 5.2%( Figure 5 ). The long cycle and stable mode of example 3 is suitable for large batch, continuous and pursuit of ultimate stability and high yield production scene. The yield is as high as 96.3%, which realizes a qualitative leap compared with the "local brushing+normal pressure slurry immersion" of comparative example 2 (titanium bead slag inclusion rate 89.5%, Figure 8 ). The batch fluctuation is only 1.8%, and the slurry quality fluctuation is less than 3%, which proves the excellent stability and repeatability of the process.

[0128] Therefore, the application not only significantly reduces manufacturing cost, but also realizes comprehensive improvement of internal purity, surface smoothness and geometric integrity of the casting through accurate control, and is especially suitable for complex and precise components in the fields of aerospace and medical devices.

[0129] The above only is the preferred embodiment of the present application, and does not limit the present application, any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application should be included in the protection scope of the present application.

Claims

1. A shell layer slurry for investment casting, characterized by, Comprise: 15~25% of silicon-yttrium composite sol and 75~85% of zirconia powder; wherein the molar ratio of Y:Si in the silicon-yttrium composite sol is 1: (1.2~1.8).

2. The shell layer slurry for investment casting according to claim 1, characterized by, The zirconia powder is composed of 200# zirconia powder and 400# zirconia powder in a mass ratio of 7:

3.

3. The shell layer slurry for investment casting according to claim 1, characterized by, The powder-liquid mass ratio of the slurry is (3.5~6.5):1, and the room temperature flow cup viscosity is 18~25 seconds.

4. The shell layer slurry for investment casting according to claim 1, characterized by, 10~15ml defoaming agent is added to every 5kg of silicon-yttrium composite sol.

5. The shell layer slurry for investment casting according to claim 1, characterized by, The silicon-yttrium composite sol acts as a binder and can form a ternary mixture containing Y2SiO5 phase with zirconia powder at high temperature to provide a chemical inert barrier to high-temperature titanium liquid.

6. A process for vacuum impregnation of a slurry for investment casting, characterized in that, The shell layer slurry for investment casting according to any one of claims 1-5, specifically comprising the following steps: S1: immerse the mold group with wax trees in the shell layer slurry for investment casting; S2: seal the container containing the mold group and the slurry as a whole and vacuumize, so that the container reaches a preset negative pressure; S3: select a pressure stabilization time t according to the negative pressure, and release the air after reaching time t to complete the immersion; S4: after completing the vacuum immersion and releasing the air, take out the mold group and rotate it for a predetermined time; S5: perform sanding, drying, and subsequent back layer preparation procedures.

7. The process of claim 6, wherein the wax pattern is formed by a process comprising: In step S2, the container is provided with a transparent sealing cover for observing the bubble discharge state of the slurry during vacuumization.

8. The process of claim 6, wherein the wax pattern is formed by a process comprising: In step S2, after observing that the slurry presents a state of dense bubbles rising and rolling into large bubbles, continue to draw negative pressure for 5 minutes during vacuumization.

9. The process of claim 6 wherein, In step S3, the preset negative pressure is -0.04~-0.06MPa, and the maintenance time is 3 minutes, which is suitable for small batch high efficiency production.

10. The vacuum slurry impregnation process for investment casting according to claim 6, characterized in that, In step S3, the preset negative pressure is -0.08~-0.1MPa, and the maintenance time is greater than 5 minutes, which is suitable for batch continuous production to improve batch stability.

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