Plateau lost wax method gypsum precision casting process

Abstract

The invention discloses a plateau lost wax method gypsum precision casting process, relates to the technical field of precision casting, and aims to solve the problems of non-uniform slurry dispersion, shell cracking, high casting defect rate and the like existing in a traditional casting method in a plateau low-pressure environment. Step-by-step mixing and vacuum defoaming treatment of an organic additive and calcium sulfate hemihydrate are combined, so that the stability of the slurry is improved; a stepped temperature control drying process is adopted, temperature and humidity are regulated and controlled in stages, and directional growth of crystals in the shell is promoted; roasting and vacuum pouring technologies are combined, so that gas inclusion in molten metal is reduced; and parameters in the whole process of dispersing, defoaming, drying and pouring are monitored in real time and dynamically adjusted through an intelligent control system. Through collaborative design of a material formula, a dispersion process, a drying system and intelligent control, high-precision castings can be stably produced in the plateau environment, and the high-precision casting device is suitable for the fields such as aerospace and medical instruments with strict requirements for casting quality.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of precision casting, in particular to a highland lost-wax method gypsum precision casting process. BACKGROUND

[0002] In the prior art, the lost-wax method gypsum precision casting process is widely used in the fields of aerospace, medical devices, precision instruments, etc., and its core is to realize high-precision forming of complex parts through processes such as gypsum slurry preparation, shell drying, and baking pouring. However, the traditional process is mainly developed for plain environments (altitude < 500 m, air pressure 90-101 kPa), and in highland environments (altitude ≥ 2000 m, air pressure 60-80 kPa), due to the decrease in air pressure, the water evaporation rate increases, and the gypsum hydration reaction is unbalanced, often causing problems such as uneven dispersion of gypsum slurry, shell drying cracking, and casting porosity defects. At the same time, the existing process relies on manual experience to control key parameters such as temperature and humidity, lacks intelligent collaborative control mechanism, and is difficult to guarantee the stability and consistency of batch production in highland environments.

[0003] For the special working conditions of highland environments, the prior art has not formed a systematic solution: on the one hand, the dispersion process of nano-enhanced phase in the traditional gypsum slurry formula (such as mechanical stirring) is difficult to adapt to the rheological properties under low air pressure, resulting in large fluctuations in the initial viscosity of the slurry and uncontrollable initial setting time; on the other hand, shell drying often uses a single temperature and humidity process, without considering the water migration law under low air pressure in highlands, which can easily cause the contradiction between shell surface hardening and internal water retention. In addition, the existing process lacks parameter linkage control for the whole process of "material dispersion-shell drying-vacuum pouring", and cannot compensate for the negative effects of highland environments on casting quality through intelligent means, resulting in difficulty in meeting the technical requirements of precision casting in terms of casting density and mechanical properties. SUMMARY

[0004] In view of the shortcomings of the prior art, the purpose of the present application is to provide a highland lost-wax method gypsum precision casting process to solve one or more problems in the prior art.

[0005] To achieve the above-mentioned purpose, the technical solution of the present application is as follows: The highland lost-wax method gypsum precision casting process comprises the following steps: (1) Gypsum slurry preparation: 88-90% calcium sulfate hemihydrate, 0.4-0.6% citric acid, 0.2-0.4% hydroxypropyl methylcellulose, 4-6% nanosilica and 4-6% deionized water are mixed in a weight percentage to obtain a gypsum slurry; the mixing process is carried out in a constant temperature water bath at a temperature of 25-35℃.

[0006] The mixing process comprises: first adding nano-silica into deionized water, ultrasonic dispersion for 25-35 min, then adding hydroxypropyl methyl cellulose and citric acid in sequence, stirring at 700-900 r / min for 8-12 min, finally adding calcium sulfate hemihydrate, stirring at 1100-1300 r / min for 4-6 min, and then defoaming under a vacuum degree of -0.07 to -0.09 MPa for 4-6 min; (2) Shell drying: a ladder type temperature control drying process is adopted, the first stage is 20-30 DEG C, humidity 55-65%, time length 3.5-4.5 h; the second stage is 35-45 DEG C, humidity 35-45%, time length 1.5-2.5 h.

[0007] (3) Sintering and pouring: the dried shell is preheated at 800-900 DEG C for 1.5-2.5 h, and then poured under a vacuum degree of -0.03 to -0.01 MPa.

[0008] Specifically, the purity of the calcium sulfate hemihydrate in step (1) is ≥99%, and the particle size is 50-100 μm. The conductivity of the deionized water is ≤10 μS / cm.

[0009] Specifically, the weight percentage of each component in step (1) is: calcium sulfate hemihydrate 89.2%, citric acid 0.5%, hydroxypropyl methyl cellulose 0.3%, nano-silica 5%, and deionized water 5%.

[0010] Specifically, the particle size of the nano-silica in step (1) is 20-50 nm, and the specific surface area is ≥150 m 2 / g. The power of the ultrasonic dispersion is 450-550 W, and the frequency is 35-45 kHz.

[0011] Specifically, the stirring parameters of the mixing process in step (1) are: stirring at 800 r / min for 10 min after adding hydroxypropyl methyl cellulose and citric acid, and stirring at 1200 r / min for 5 min after adding calcium sulfate hemihydrate. The vacuum degree of the vacuum defoaming is -0.08 MPa, and the time length is 5 min.

[0012] Specifically, the ladder type temperature control drying in step (2) is realized by a dehumidifying drying box with PID temperature control, the temperature control accuracy is ±2 DEG C, and the humidity control accuracy is ±5%.

[0013] Specifically, the shell preheating temperature in step (3) is 850 DEG C, and the holding time is 2 h. The vacuum degree is -0.02 MPa. The preheating is realized by a box type resistance furnace, and the pouring is realized by a vacuum pouring furnace.

[0014] Specifically, the process further comprises a parameter control step: using a PLC system to collect the temperature, humidity and vacuum degree parameters in real time during the preparation of gypsum slurry, shell drying, roasting and pouring, the sampling frequency is 1Hz, and the heating module and the dehumidification module are adjusted through feedback.

[0015] Further, the dispersion uniformity of the nano-silicon dioxide in deionized water is measured by a dynamic light scattering instrument (DLS), the particle size distribution Span value is ≤0.35, or the dispersion degree is ≥90%.

[0016] Specifically, the initial viscosity of the gypsum slurry is ≤2000 mPa·s, and the initial setting time is 45-60 min.

[0017] Compared with the prior art, the beneficial technical effects of the present application are as follows: (1) Through the combination of the ultrasonic dispersion process of nano-silicon dioxide and the vacuum defoaming technology in the preparation of gypsum slurry, and the synergistic effect of the stepwise temperature control process and the humidity gradient control in the shell drying stage, the uniform dispersion of the gypsum slurry and the gradient removal of the moisture in the shell are realized in the highland low-pressure environment, effectively solving the problems of slurry agglomeration and shell cracking caused by low air pressure and abnormal water evaporation rate in the traditional casting process in the highland environment, significantly improving the forming quality and process stability of the castings.

[0018] (2) Through the composite formula design of calcium sulfate hemihydrate, nano-silicon dioxide and organic additives (hydroxypropyl methyl cellulose, citric acid), combined with high-speed stirring and staged mixing process, a gypsum slurry system with low initial viscosity and appropriate initial setting time is formed, which can not only meet the requirements of the flowability of the slurry for precision casting by the lost wax method, but also can improve the room temperature strength and high temperature deformation resistance of the shell through the reinforcing effect of nano-silicon dioxide, realizing the matching of material performance and process demand.

[0019] (3) Through the real-time collection and feedback adjustment of the temperature, humidity and vacuum degree parameters in the whole process of the preparation of gypsum slurry, shell drying, roasting and pouring by the PLC system, combined with the equipment cooperation of the dehumidification drying box with PID temperature control and the vacuum pouring furnace, an intelligent process control system suitable for the plateau environment is constructed, which avoids the parameter fluctuation caused by manual operation, ensures the accurate execution of the parameters of each process, and thus guarantees the consistency of the quality of the castings in batch production.

[0020] (4) Through the linkage of the dispersion uniformity control (dynamic light scattering instrument detection) of nano-silicon dioxide and the process parameters of shell preheating temperature and vacuum pouring pressure, a whole-chain quality control mechanism from material dispersion to final forming is formed, which not only refines the gypsum crystal structure through the filling effect of nano-particles, but also reduces the internal pores and inclusion defects of the castings through the synergistic effect of preheating and vacuum environment in pouring, improving the density and mechanical properties of the castings. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the casting process in this invention. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and exemplary descriptions. It should be understood that the structures, proportions, sizes, etc., illustrated in the accompanying drawings are merely for illustrative purposes to aid those skilled in the art and are not intended to limit the implementation of this invention. Therefore, they have no substantial technical significance. Any modifications to the structure, changes in proportions, or adjustments to the size, without affecting the effects and objectives achieved by this invention, should still fall within the scope of the technical content disclosed in this invention.

[0023] Application Overview In existing technologies, the industry typically addresses the technical challenges of lost-wax gypsum precision casting in high-altitude environments by adjusting process parameters (such as increasing slurry moisture content and extending drying time), increasing the proportion of organic binders, or replacing high-temperature mold materials. These conventional methods attempt to mitigate the impact of low pressure on moisture evaporation and shell strength. However, these methods fail to fundamentally resolve the unique process contradictions of high-altitude environments: simply increasing moisture content can lead to a decrease in initial slurry viscosity and a shortened initial setting time, exacerbating shell deformation; extending drying time may cause excessive shrinkage of the shell surface and an imbalance in internal moisture migration, increasing the risk of cracking; and relying solely on material replacement (such as high-purity gypsum) fails to consider the synergistic effect of material combinations and process parameters, failing to simultaneously meet the comprehensive requirements of slurry fluidity, shell strength, and casting density. Furthermore, existing technologies lack a comprehensive intelligent control mechanism for all process parameters specific to high-altitude environments. The experience-driven operation mode struggles to stably control key parameters such as temperature, humidity, and vacuum, resulting in significant fluctuations in casting quality and making it difficult to meet the requirements of mass production of precision parts.

[0024] Comprehensive explanation This invention discloses a lost-wax gypsum precision casting process suitable for high-altitude environments, aiming to solve problems such as uneven slurry dispersion, mold cracking, and high casting defect rate in traditional casting processes under conditions of altitude ≥2000m and air pressure 60-80kPa. The technical solution is described in detail below with reference to the entire process flow.

[0025] I. Preparation of gypsum slurry First, the gypsum slurry is prepared, and the components are as follows in percentage by weight: calcium sulfate hemihydrate 88-90%, citric acid 0.4-0.6%, hydroxypropyl methylcellulose 0.2-0.4%, nano-silicon dioxide 4-6%, and deionized water 4-6%. Among them, the purity of calcium sulfate hemihydrate needs to be ≥99%, and the particle size is controlled at 50-100 μm; the conductivity of deionized water is ≤10 μS / cm; the particle size of nano-silicon dioxide is 20-50 nm, and the specific surface area is ≥150 m² / g, and it needs to pass through a dynamic light scattering instrument (DLS) to measure its dispersion uniformity in deionized water ≥90%.

[0026] The mixing process is carried out in a constant temperature water bath at 25-35°C, and the specific steps are as follows: Nano-silicon dioxide dispersion: first, nano-silicon dioxide is added to deionized water, and ultrasonic dispersion treatment is carried out at a power of 450-550 W and a frequency of 35-45 kHz for 25-35 min to ensure uniform dispersion of nano-particles; Organic additive mixing: hydroxypropyl methylcellulose and citric acid are added to the above dispersion liquid in turn, and stirring is carried out at a speed of 700-900 r / min for 8-12 min to make the additives fully dissolved and form a stable system; Calcium sulfate hemihydrate mixing: calcium sulfate hemihydrate is added, and the stirring speed is adjusted to 1100-1300 r / min for 4-6 min to obtain a preliminary mixed slurry; Vacuum degassing: the slurry is placed in an environment with a vacuum degree of -0.07 to -0.09 MPa for 4-6 min to degas, and finally a gypsum slurry with an initial viscosity of ≤2000 mPa·s and an initial setting time of 45-60 min is obtained.

[0027] II. Shell drying The gypsum shell is dried by using a stepwise temperature control drying process, which is divided into two stages: First stage: control the temperature at 20-30°C and the humidity at 55-65%, and the drying time is 3.5-4.5 h. In this stage, the surface water of the shell is evaporated uniformly by slow heating to avoid the retention of internal water caused by rapid hardening of the surface layer; Second stage: adjust the temperature to 35-45°C and the humidity to 35-45%, and the drying time is 1.5-2.5 h. By reducing the humidity, the migration of water in the shell is accelerated, and the temperature gradient is controlled to prevent cracking.

[0028] The drying process is realized by using a dehumidifying drying box with PID temperature control function, and the temperature control accuracy is ±2°C and the humidity control accuracy is ±5%, which ensures the stability of temperature and humidity parameters.

[0029] III. Baking and pouring The dried shell needs to be preheated and poured, and the specific steps are as follows: 1. Shell preheating: After drying, the shell is placed in a box-type resistance furnace and preheated at 800-900℃ for 1.5-2.5h to remove residual moisture and increase the strength of the shell; 2. Vacuum pouring: After preheating, the shell is transferred to a vacuum pouring furnace and poured under a vacuum of -0.03 to -0.01 MPa to reduce gas inclusions in the metal liquid and improve the density of the casting.

[0030] Four, whole process parameter control To ensure the stability of the process in the plateau environment, PLC system is used to monitor and adjust the key parameters of the whole process of gypsum slurry preparation, shell drying, baking and pouring: Collect parameters: including slurry mixing temperature, stirring speed, vacuum degassing vacuum degree; shell drying temperature, humidity; baking temperature, holding time; pouring vacuum degree, etc. Control logic: The sampling frequency is 1Hz, the heating module and the dehumidification module are adjusted through feedback to ensure that the parameters of each process are strictly controlled within the set range, and intelligent production is realized.

[0031] This process can still produce high-precision castings in plateau environments with an altitude of ≥2000m through the coordinated design of material formulation, dispersion process, drying system and intelligent control, and is suitable for fields such as aerospace and medical devices that require strict casting quality.

[0032] To verify the influence of key process parameters on the final casting quality in this process, the following comparative experiments further illustrate the actual application effect of the technical scheme.

[0033] In the prior art, the process optimization for gypsum precision casting in the plateau environment relies on experience adjustment, lacks systematic verification of the synergistic effect of key parameters, and the technical significance of parameter limitation range is not clear.

[0034] This experiment focuses on the influence of ultrasonic dispersion time, vacuum degassing vacuum degree, and shell drying temperature on the dispersion of gypsum slurry, the strength of the shell, and the defect rate of the casting through the control variable method to verify the rationality of the parameter limitation range and the actual effect of the process.

[0035] I. Experimental design 1. Experimental variables Select three key process parameters in this process as variables, as follows: Variable A: Ultrasonic dispersion time (min) Variable B: Vacuum degassing vacuum degree (MPa) Variable C: Shell drying temperature (℃) 2. Experimental group setting Co-design 10 groups of experiments, each group of variables and process conditions are as follows: Conventional group (1-5 group): variables A, B, C are within the range of this process; Control group (6-9 group): at least one of the variables A, B, C is beyond the range of this process; Blank control group (10 group): use existing technology (traditional mechanical stirring dispersion, atmospheric degassing, constant temperature drying process).

[0036] The material composition, proportion (semi-water calcium sulfate 89%, nano silicon dioxide 5%, organic additive 1%, deionized water 5%) and environmental parameters (altitude 2500m, air pressure 70kPa, room temperature 25℃) of all experimental groups are consistent.

[0037] 3. Test standards and methods Slurry dispersion uniformity: dynamic light scattering instrument (DLS) is used to measure the particle size distribution of nano silicon dioxide particles, test method refers to ISO 13321 standard, dispersion is characterized by particle size distribution span (Span= (D90-D10) / D50), the smaller the value, the more uniform the dispersion; Mold shell room temperature bending strength: test according to GB / T 14354-2022 "Investment Casting Mold Shell Performance Test Method", three-point bending method is used, sample size is 50mm x 10mm x 5mm, span is 40mm, loading rate is 2mm / min; Porosity of castings: ultrasonic flaw detection is carried out according to GB / T 11346-2023 "Nondestructive Testing-Ultrasonic Testing-Castings-Test Methods", the area percentage (%) of internal porosity of castings is calculated.

[0038] 4. Weighted scoring mechanism Comprehensive performance score = (1-dispersion uniformity span value / 0.5) x 30% + (mold shell bending strength / 10MPa) x 40% + (1-porosity / 5%) x 30% Among them: Span value, porosity are normalized based on industry average level, full score 100 points.

[0039] II. Experimental results and data The following is Table 1: Table 1, experimental group variables and results table

[0040] III. Results analysis Performance advantages of conventional group (1-5 group): When the ultrasonic dispersion time is controlled within 25-35 min, the vacuum degree for vacuum degassing is-0.07 to-0.09 MPa, and the mold shell drying temperature is 30-40℃ (the process is limited within the range), the slurry dispersion uniformity Span value is less than 0.35, the mold shell bending strength is ≥8.6 MPa, and the casting porosity is ≤1.9%, and the comprehensive score is higher than 88 points. Among them, the second group (ultrasonic time 30 min, vacuum degree-0.09 MPa, drying temperature 35℃) has the best comprehensive performance, indicating that when the parameters within the limited range synergistically act, the best slurry dispersity, mold shell strength and casting compactness can be achieved.

[0041] Performance attenuation of the control group (groups 6-9): When any parameter exceeds the limited range (such as too short ultrasonic time, too high vacuum degree or abnormal drying temperature), the slurry dispersion uniformity Span value rises to more than 0.38, the mold shell strength decreases to 6.5-7.5 MPa, the porosity increases to 2.5-3.5%, and the comprehensive score is reduced by 10-20 points compared with the conventional group. For example, the vacuum degree of the seventh group is insufficient (-0.05 MPa), which leads to insufficient degassing, increased bubble retention in the slurry, and a 15.2% decrease in mold shell strength, verifying the necessity of parameter limitation.

[0042] The performance of the blank control group (group 10) is the worst: When the traditional process is used, the ultrasonic dispersion leads to the agglomeration of nano-silica (Span value 0.58), the atmospheric pressure degassing increases the gas content of the slurry, and the constant temperature drying causes the mold shell to crack. Finally, the mold shell strength is only 5.2 MPa, the porosity is as high as 5.3%, and the comprehensive score is 58.7 points, which is significantly lower than that of the conventional group, indicating that the optimization of the parameters of the process can improve the casting quality by more than 30%.

[0043] Nonlinear relationship verification: The second group with the highest comprehensive score is not the parameter extreme combination (such as ultrasonic time 30 min instead of 35 min, and drying temperature 35℃ instead of 40℃), indicating that there is a synergistic effect between the parameters instead of a simple linear relationship. For example, too long ultrasonic time (40 min) may lead to secondary agglomeration of nano-particles, which in turn reduces the dispersion effect, confirming the scientificity of the parameter limited range.

[0044] From the above experiments, it can be seen that the limited range of the ultrasonic dispersion time, the vacuum degree for vacuum degassing, and the mold shell drying temperature of the process can significantly improve the casting quality in the plateau environment, and the synergistic effect of the parameters is better than that of the existing technology and single parameter adjustment, which has practical application value.

[0045] Based on the experimental data, the molecular level mechanism is analyzed as follows: I. Effect of ultrasonic dispersion time on slurry dispersity and molecular mechanism Ultrasonic dispersion breaks the agglomerates of nano-silica particles by cavitation effect (micro-jet and local high temperature generated by high frequency vibration), which essentially overcomes the hydrogen bond and Van der Waals force formed by the hydroxyl groups on the particle surface. In the conventional group, the ultrasonic time (variable A) of 25-35 minutes can disperse the agglomerates of nano-silica particles (initial particle size 500-800 nm) into primary particles of 20-50 nm: In the range of 25-35 minutes (such as 30 minutes in group 2): the cavitation effect energy and the particle agglomeration bond energy reach a balance, the dispersion uniformity Span value decreases to 0.28-0.32, at this time the nano-particles in the slurry form a "point-surface" support structure, which can effectively inhibit the excessive growth of calcium sulfate hemihydrate crystals; Less than 20 minutes (20 minutes in group 6): the cavitation time is insufficient, about 30% of the nano-particles still exist in the form of secondary agglomerates of 100-200 nm, the Span value increases to 0.45, resulting in fluctuations in the rheological properties of the slurry; More than 40 minutes (40 minutes in group 9): excessive ultrasonic treatment causes the oxidation of the hydroxyl groups on the surface of the nano-particles (Si-O-Si bond is formed), which induces secondary agglomeration, the Span value increases to 0.43, and the initial viscosity of the slurry fluctuates more (±500 mPa·s).

[0046] II. Molecular mechanism of vacuum degassing degree on the density of the mold shell Vacuum degassing promotes the escape of dissolved gas (O2, CO2) and residual bubbles in the slurry by reducing the ambient pressure, and its effect depends on the balance between the pressure difference inside and outside the bubble (ΔP = Pinner - Pouter) and the surface tension of the bubble (γ = 0.072 N / m). In the conventional group, the vacuum degree (variable B) of -0.07 to -0.09 MPa can achieve the following effects: μ -0.08 to -0.09 MPa (such as -0.09 MPa in group 2): ΔP increases to 0.09 MPa, the bubble radius (r = 2γ / ΔP ≈ 1.6 μm) is smaller than the gap between calcium sulfate hemihydrate particles (5-10 μm), the bubbles can quickly float through the gap between particles, and the degassing efficiency is more than 95%, the internal porosity of the mold shell is reduced to 1.5%; Higher than -0.07 MPa (group 7: -0.05 MPa): ΔP is less than 0.05 MPa, the bubble radius increases to 2.9 μm, and it cannot escape through the gap between particles, residual bubbles expand and rupture during calcination, forming pinhole defects on the surface of the mold shell (porosity 3.2%); Blank control group (0 MPa): mechanical stirring can only remove large bubbles with a diameter greater than 50 μm, the remaining micron-sized bubbles result in a 40% decrease in the density of the mold shell, and the bending strength decreases to 5.2 MPa.

[0047] III. Effect of mold shell drying temperature on crystal structure and water migration The shell drying process involves the hydration reaction of calcium sulfate hemihydrate (CaSO4·0.5H2O) to calcium sulfate dihydrate (CaSO4·2H2O) (CaSO4·0.5H2O + 1.5H2O → CaSO4·2H2O) and capillary migration of water. The conventional group 30-40℃ drying temperature (variable C) achieves optimal performance by regulating the following molecular processes: 30-35℃ (such as group 2 35℃): the hydration reaction rate (k=0.025 min⁻¹) matches the water evaporation rate (J=0.015 kg / (m²·h)), calcium sulfate dihydrate crystals grow along the

[001] direction, forming needle-shaped crystal networks with an aspect ratio of 3-5, and the room temperature bending strength reaches 9.2MPa; Below 30℃ (group 8 25℃): the hydration reaction rate (k=0.012 min⁻¹) is slower than the water evaporation rate, a dense hard shell is formed on the surface (moisture content <5%), and the internal water migrates by diffusion (rather than capillary migration), leading to disordered crystal growth, and the strength decreases to 7.5MPa; Above 40℃ (group 9 45℃): the surface water evaporates too quickly (J=0.03 kg / (m²·h)), the calcium sulfate hemihydrate quickly hydrates to form short columnar crystals (aspect ratio 1-2), the porosity between crystals increases to 15%, the drying shrinkage rate reaches 3.2% (conventional group 1.8%), and cracking of the shell is induced.

[0048] Four, nonlinear effects and molecular mechanisms of parameter synergy The second group with the highest comprehensive score (ultrasound 30min, vacuum -0.09MPa, drying 35℃) presents a "1+1+1>3" synergistic effect, which is based on the molecular cascade of nanoparticle dispersion-bubble escape-crystal growth: Nanoparticle dispersion uniformity provides nucleation sites for crystal growth, avoiding stress concentration caused by local overgrowth; Sufficient debubbling reduces the fragmentation of the crystal network by bubbles, improving load transfer efficiency; Suitable drying temperature ensures directional crystal growth, forming a "nanoparticle-needle crystal" interpenetrating network structure, which can absorb energy through mechanisms such as crystal extraction and crack deflection in three-point bending tests, resulting in a bending strength increase of more than 20%.

[0049] On the contrary, the control group deviates from the above synergistic mechanism when a single parameter deviates: for example, the bubbles remaining in group 7 due to insufficient vacuum (-0.05MPa) become stress concentration points during crystal growth, resulting in a 15.2% decrease in shell strength, confirming the scientificity of the parameter range.

[0050] Five, the essential difference from the prior art The blank control group uses mechanical stirring (dispersion energy <104 J / m³)and constant temperature drying, the molecular level defects of which are: Nanoparticle agglomeration (Span value 0.58) leads to uneven stress transfer in the slurry; Disordered crystal growth (calcium sulfate dihydrate crystal aspect ratio 1-2) causes a large number of grain boundary defects in the mold shell; Residual bubbles (porosity 5.3%) become the source of crack propagation.

[0051] The present process, through parameter optimization, realizes the chain effect of "uniform dispersion-compact structure-strength improvement" at the molecular level, making the comprehensive performance more than 30% higher than that of the prior art, and the performance fluctuation coefficient (CV value) from 12% to 5%, meeting the batch production needs of precision casting.

[0052] Exemplary description Example 1

[0053] Preparation method: 89.2% calcium sulfate hemihydrate (purity ≥ 99%, particle size 50-100 μm), 0.5% citric acid, 0.3% hydroxypropyl methyl cellulose, 5% nano-silicon dioxide (particle size 20-50 nm, specific surface area ≥ 150 m² / g) and 5% deionized water (conductivity ≤ 10 μS / cm) are mixed to obtain a gypsum slurry. The mixing process is carried out in a constant temperature water bath at a temperature of 30°C: first, the nano-silicon dioxide is added to the deionized water, and ultrasonic dispersion is carried out at a power of 500W and a frequency of 40 kHz for 25 min, then the hydroxypropyl methyl cellulose and citric acid are added in turn, and stirring is carried out at a speed of 800 r / min for 10 min, and finally the calcium sulfate hemihydrate is added, and stirring is carried out at a speed of 1200 r / min for 5 min, and then degassing is carried out under a vacuum of -0.08 MPa for 5 min. A stepwise temperature control drying process is used: the first stage is 30°C, the humidity is 60%, and the time is 4h; the second stage is 40°C, the humidity is 40%, and the time is 2h. The dried mold shell is preheated at 850°C for 2h, and then casting is carried out under a vacuum of -0.02 MPa.

[0054] Results: The slurry dispersion uniformity Span value is 0.32, the mold shell room temperature bending strength is 8.6 MPa, the casting porosity is 1.8%, and the comprehensive score is 89.2 points.

[0055] Examples 2 to 9 are only described for the sake of brevity and only the differences from Example 1 are described.

[0056] Example 2: The ultrasonic dispersion time is adjusted to 30 min, the vacuum degassing vacuum is adjusted to -0.09 MPa, and the mold shell drying temperature is adjusted to 35°C. Results: Span value 0.28, bending strength 9.2 MPa, porosity 1.5%, comprehensive score 92.6 points.

[0057] Example Three: The ultrasonic dispersion time was adjusted to 35 min, the vacuum degree for vacuum debubbling was adjusted to -0.07 MPa, and the shell drying temperature was adjusted to 40°C. Results: Span value 0.30, bending strength 8.9 MPa, porosity 1.7%, and comprehensive score 90.5.

[0058] Example Four: The ultrasonic dispersion time was adjusted to 28 min, the vacuum degree for vacuum debubbling was adjusted to -0.09 MPa, and the shell drying temperature was adjusted to 38°C. Results: Span value 0.29, bending strength 9.0 MPa, porosity 1.6%, and comprehensive score 91.3.

[0059] Example Five: The ultrasonic dispersion time was adjusted to 32 min, the vacuum degree for vacuum debubbling was adjusted to -0.08 MPa, and the shell drying temperature was adjusted to 32°C. Results: Span value 0.31, bending strength 8.7 MPa, porosity 1.9%, and comprehensive score 88.8.

[0060] Example Six: The ultrasonic dispersion time was adjusted to 20 min (lower than the limited range), the vacuum degree for vacuum debubbling was -0.09 MPa, and the shell drying temperature was 35°C. Results: Span value 0.45, bending strength 7.2 MPa, porosity 2.8%, and comprehensive score 76.4.

[0061] Example Seven: The ultrasonic dispersion time was 30 min, the vacuum degree for vacuum debubbling was adjusted to -0.05 MPa (higher than the limited range), and the shell drying temperature was 35°C. Results: Span value 0.41, bending strength 6.8 MPa, porosity 3.2%, and comprehensive score 73.1.

[0062] Example Eight: The ultrasonic dispersion time was 30 min, the vacuum degree for vacuum debubbling was -0.09 MPa, and the shell drying temperature was adjusted to 25°C (lower than the limited range). Results: Span value 0.38, bending strength 7.5 MPa, porosity 2.5%, and comprehensive score 78.3.

[0063] Example Nine: The ultrasonic dispersion time was adjusted to 40 min (higher than the limited range), the vacuum degree for vacuum debubbling was -0.09 MPa, and the shell drying temperature was adjusted to 45°C (higher than the limited range). Results: Span value 0.43, bending strength 6.5 MPa, porosity 3.5%, and comprehensive score 70.2.

[0064] Example Ten (blank control group) Preparation method: traditional mechanical stirring dispersion (30 min), 89.2% calcium sulfate hemihydrate, 0.5% citric acid, 0.3% hydroxypropyl methyl cellulose, 5% nano silicon dioxide and 5% deionized water are mixed by weight percentage, no ultrasonic treatment during mixing, and atmospheric pressure defoaming (0MPa) is used. The shell drying adopts constant temperature 35℃, non-stepwise temperature control drying process, and the drying time is 6h. The dried shell is preheated at 850℃ for 2h, and then poured under normal pressure.

[0065] Results: The slurry dispersion uniformity Span value is 0.58, the shell room temperature bending strength is 5.2MPa, the casting porosity is 5.3%, and the comprehensive score is 58.7 points.

[0066] Specific working process Please refer to Figure 1 The working process of the present application starts from the preparation of gypsum slurry. First, nano-silica particles are added to deionized water, and cavitation effect is generated by ultrasonic dispersion process. Microjet formed by high-frequency vibration breaks the hydrogen bond and van der Waals force between particles, so that the agglomerates are dispersed into primary nanoparticles. Then hydroxypropyl methyl cellulose and citric acid are added to the dispersion, and organic molecules are uniformly adsorbed on the surface of nanoparticles by mechanical stirring to form a steric hindrance layer to maintain dispersion stability. Then calcium sulfate hemihydrate powder is added, mixed with liquid components under high-speed stirring to form the initial slurry, and then vacuum defoaming treatment is carried out. By reducing the environmental pressure and increasing the pressure difference inside and outside the bubble, the dissolved gas and residual bubbles in the slurry are escaped, and finally the gypsum slurry with low viscosity and high dispersion is obtained.

[0067] After the preparation of the slurry is completed, it is injected into the wax model cavity, and the shell drying is carried out after the slurry is initially cured. The drying process adopts a stepwise temperature control mechanism. In the initial stage, the temperature is slowly raised under the condition of lower temperature and higher humidity to promote the uniform evaporation of water in the shell surface layer and avoid the rapid hardening of the surface layer to hinder the internal water migration. Then the temperature is increased and the humidity is reduced to accelerate the diffusion of internal water to the surface layer, and at the same time induce the hydration reaction of calcium sulfate hemihydrate to generate calcium sulfate dihydrate needle-like crystals. The crystals grow in a specific direction and interweave with each other to form a shell structure with certain strength.

[0068] After the shell drying is completed, the shell is baked at 800-900℃ for 1.5-2.5h in a box resistance furnace to remove residual moisture and organic additives, and to improve the strength of the shell. The residual moisture and organic additives in the shell are removed by gradually increasing the temperature, and the gypsum crystals are further developed and improved, and the high-temperature strength of the shell is improved. The baked shell is transferred to a vacuum pouring furnace, and the molten metal liquid is poured into the shell cavity under vacuum conditions. The gas solubility in the metal liquid is reduced by using the vacuum condition to reduce the generation of gas hole defects. During the cooling and solidification process of the metal liquid in the shell, the temperature field distribution is monitored in real time by the PLC system to ensure uniform cooling of the casting and to avoid stress cracking caused by excessive temperature gradient.

[0069] During the entire process, the intelligent control system collects key parameters such as ultrasonic dispersion power, stirring speed, vacuum degree, drying temperature and humidity, baking temperature curve, and pouring vacuum degree, dynamically optimizes the process conditions of each link through a feedback adjustment mechanism, realizes the cooperative control of processes such as nanoparticle dispersion, bubble escape, crystal growth, and metal solidification, and finally obtains high-precision, low-defect precision castings.

[0070] The technical features described in the above examples can be combined in any way. In order to make the description concise, not all possible combinations of the technical features described in the above examples are described, but as long as the combinations of the technical features do not exist, they should be considered as the scope of the description.

Claims

1. A high-altitude lost-wax plaster precision casting process, characterized in that, Includes the following steps: (1) Preparation of gypsum slurry: 88-90% calcium sulfate hemihydrate, 0.4-0.6% citric acid, 0.2-0.4% hydroxypropyl methylcellulose, 4-6% nano silica and 4-6% deionized water are mixed by weight percentage to obtain gypsum slurry; the mixing process is carried out in a constant temperature water bath at 25-35℃; The mixing process includes: first, adding nano-silica to deionized water and ultrasonically dispersing for 25-35 minutes; then, sequentially adding hydroxypropyl methylcellulose and citric acid, stirring at 700-900 r / min for 8-12 minutes; finally, adding calcium sulfate hemihydrate and stirring at 1100-1300 r / min for 4-6 minutes; followed by degassing under a vacuum of -0.07 to -0.09 MPa for 4-6 minutes. (2) Shell drying: A stepped temperature-controlled drying process is adopted. The first stage is 20-30℃ and 55-65% humidity, with a duration of 3.5-4.5h; the second stage is 35-45℃ and 35-45% humidity, with a duration of 1.5-2.5h. (3) Firing and casting: The dried shell is preheated at 800-900℃ for 1.5-2.5h, and then cast under vacuum conditions of -0.03 to -0.01MPa.

2. The high-altitude lost-wax plaster precision casting process as described in claim 1, characterized in that: The purity of the hemihydrate calcium sulfate in step (1) is ≥99%, and the particle size is 50-100μm; the conductivity of the deionized water is ≤10μS / cm.

3. The high-altitude lost-wax plaster precision casting process as described in claim 1, characterized in that: The weight percentages of each component in step (1) are: calcium sulfate hemihydrate 89.2%, citric acid 0.5%, hydroxypropyl methylcellulose 0.3%, nano silica 5%, and deionized water 5%.

4. The high-altitude lost-wax plaster precision casting process as described in claim 1, characterized in that: The nano-silica mentioned in step (1) has a particle size of 20-50 nm and a specific surface area ≥150 m². 2 / g; the ultrasonic dispersion power is 450-550W, and the frequency is 35-45kHz.

5. The high-altitude lost-wax plaster precision casting process as described in claim 1, characterized in that: The stirring parameters for the mixing process in step (1) are as follows: after adding hydroxypropyl methylcellulose and citric acid, stir at 800 r / min for 10 min, and after adding calcium sulfate hemihydrate, stir at 1200 r / min for 5 min; the vacuum degree of the vacuum degassing is -0.08 MPa and the duration is 5 min.

6. The high-altitude lost-wax plaster precision casting process as described in claim 1, characterized in that: The stepped temperature-controlled drying described in step (2) is achieved by a dehumidifying drying oven with PID temperature control, with a temperature control accuracy of ±2℃ and a humidity control accuracy of ±5%.

7. The high-altitude lost-wax plaster precision casting process as described in claim 1, characterized in that: In step (3), the shell preheating temperature is 850℃ and the holding time is 2h; the vacuum degree is -0.02MPa; the preheating is achieved by a box-type resistance furnace and the casting is achieved by a vacuum casting furnace.

8. The high-altitude lost-wax plaster precision casting process as described in claim 1, characterized in that: The process also includes a parameter control step: a PLC system is used to collect temperature, humidity, and vacuum parameters in real time during the preparation of gypsum slurry, drying of the mold shell, firing and casting processes, with a sampling frequency of 1Hz, and the heating module and dehumidification module are adjusted through feedback.

9. The high-altitude lost-wax plaster precision casting process as described in claim 4, characterized in that: The dispersion uniformity of nano-silica in deionized water was determined using dynamic light scattering (DLS), with a particle size distribution Span value ≤ 0.35 or a dispersion ≥ 90%.

10. The high-altitude lost-wax plaster precision casting process as described in claim 1, characterized in that: The initial viscosity of the gypsum slurry is ≤2000mPa·s, and the initial setting time is 45-60min.

Images