A special coating for lost foam casting of motor housing, its preparation method and application

Abstract

This invention belongs to the field of lost foam casting technology for motor housings, and specifically relates to a special coating for lost foam casting of motor housings, its preparation method, and its application. This invention utilizes a quantitative formulation design method, based on aggregate proportion calculation formulas matching thermal expansion coefficients and particle size distribution correlation formulas based on target permeability, to achieve synergistic compounding of quartz powder and bauxite. Combined with key functional components such as mullite, organic fibers, and flake graphite, this achieves a balance between high permeability and excellent crack resistance in the lost foam coating for motor housings. This invention also provides a highly efficient two-coat gradient drying process matched with the coating, significantly improving production efficiency and reducing energy consumption while ensuring coating performance.

Description

Technical Field

[0001] This invention belongs to the field of lost foam casting technology for motor housings, and in particular relates to a special coating for lost foam casting of motor housings, its preparation method and application. Background Technology

[0002] As a typical thin-walled, multi-cavity, and complex cast iron part, the lost foam casting of motor housings faces two major challenges: first, it requires extremely high high-temperature permeability (usually 95-105) to expel a large amount of foam pyrolysis gas and prevent defects such as porosity and cold shuts; second, it requires excellent thermal shock resistance to prevent the coating from cracking and causing sand adhesion under the scouring of high-temperature molten metal.

[0003] Currently, the solutions on the market are mainly divided into two categories: one is high-performance coatings that use low-expansion special aggregates such as abrasive sand and fused silica, but the cost is high; the other is low-cost coatings that use traditional quartz powder as the main aggregate, but they generally have problems such as a large coefficient of thermal expansion that makes them prone to cracking and insufficient high-temperature strength.

[0004] The core challenge that current technologies have not yet addressed lies in how to improve the crack resistance, refractoriness, and air permeability control of coatings through scientific compounding while ensuring low costs (primarily using quartz powder). Furthermore, traditional coating formulation design relies heavily on trial and error based on experience, lacking quantitative design models based on material thermodynamics and fluid mechanics, resulting in long formulation development cycles and poor reproducibility.

[0005] Bauxite (Al2O3≥80%) has advantages such as high refractoriness (>1700℃) and good thermal stability, and theoretically can complement the properties of quartz powder. However, the two have significantly different coefficients of thermal expansion (quartz approximately 12×10⁻⁶). -6 / K, bauxite approximately 8×10 -6 (K) Simple compounding may lead to interfacial stress concentration, which in turn reduces the coating's crack resistance. Therefore, establishing a scientific calculation model for composite proportions has become a pressing technical challenge in this field.

[0006] In view of this, the present invention is proposed. Summary of the Invention

[0007] The primary objective of this invention is to achieve synergistic compounding of quartz and bauxite through a quantitative formulation design method, thereby providing a low-cost, high-crack-resistant coating specifically for lost foam casting of motor housings.

[0008] The purpose of this invention is also to combine scientific aggregate particle size distribution with key functional components such as mullite, organic fiber, and flake graphite to achieve a balance between high air permeability and excellent crack resistance in the lost foam coating for motor housings.

[0009] On one hand, the present invention provides a special coating for lost foam casting of motor housing, comprising main aggregate, functional aggregate and additives. The main aggregate comprises quartz and bauxite, the functional aggregate comprises mullite and flake graphite, and the additive comprises one or more of sodium bentonite, sodium tripolyphosphate, polyvinyl alcohol, phenolic resin, organic fiber and spodumene.

[0010] The mass ratio of the quartz to the bauxite is determined according to Formula I:

[0011] (I)

[0012] in,

[0013] X Q The mass percentage of quartz in the main aggregate;

[0014] X A The percentage of the bauxite in the main aggregate;

[0015] α Q The coefficient of thermal expansion of the quartz is denoted as .

[0016] α A The coefficient of thermal expansion of the bauxite is given.

[0017] E Q The elastic modulus of the quartz;

[0018] E A The elastic modulus of the bauxite;

[0019] ν Q The Poisson's ratio of the quartz.

[0020] ν A The Poisson's ratio of the bauxite;

[0021] α T The target coefficient of thermal expansion for the lost foam coating specifically designed for the motor housing.

[0022] It is understandable that the target thermal expansion coefficient α of the lost foam coating for the motor housing is... T Substituting the material parameters into Formula I, the ideal mass ratio X of the quartz to the bauxite is calculated. Q / X A (X) Q +X A =1), and then the weight ratio of the two can be fine-tuned by taking into account other factors such as cost.

[0023] In some embodiments of the present invention, the main aggregate comprises 60-75 parts by weight of quartz and 15-30 parts by weight of bauxite, and the functional aggregate comprises 6-8 parts by weight of mullite and 4-7 parts by weight of flake graphite.

[0024] In some embodiments of the present invention, the additives include 1.5-2.5 parts by weight of sodium bentonite, 0.2-0.4 parts by weight of sodium tripolyphosphate, 0.8-1.2 parts by weight of polyvinyl alcohol, 1.5-2.0 parts by weight of phenolic resin, 0.1-0.3 parts by weight of organic fiber and 2-4 parts by weight of spodumene.

[0025] It is understood that the components sodium bentonite, sodium tripolyphosphate, polyvinyl alcohol, phenolic resin, organic fiber and spodumene in the additive can also be replaced by other substances with similar functions, as long as the performance of the coating can be maintained. All of these should be within the scope of equivalent protection of this invention.

[0026] In some embodiments of the present invention, the particle size distribution of the main aggregate is determined according to Formula II:

[0027] (II)

[0028] in,

[0029] P is the target air permeability value of the lost foam coating for the motor housing;

[0030] k is a system constant;

[0031] The median grain size of the quartz is given.

[0032] The median particle size of the bauxite;

[0033] V coarse The volume of coarse particles with a particle size greater than 200 mesh in the main aggregate;

[0034] V total This refers to the total volume of the main aggregate;

[0035] Φ fine The volume proportion of fine powder with a particle size of less than 325 mesh in the main aggregate.

[0036] Understandably, the required coarse particle volume ratio V can be calculated using Formula II. coarse / V total The lower limit and the volume ratio of fine powder Φ fine The upper limit of the particle size can be used to determine the particle size of various components and the weight fraction of components with different particle sizes.

[0037] In some embodiments of the present invention, the quartz comprises quartz sand with a particle size of 100-200 mesh and quartz powder with a particle size of 325-500 mesh. Preferably, the mass ratio of the quartz sand to the quartz powder is 1.5-3:1; more preferably, it is 1.5-2:1.

[0038] In some embodiments of the present invention, the bauxite is calcined bauxite. Preferably, the particle size of the bauxite is 325-350 mesh.

[0039] In some embodiments of the present invention, the mullite is mullite powder. Preferably, the mullite has a particle size of 325-400 mesh.

[0040] In some embodiments of the present invention, the particle size of the flake graphite is 150-200 mesh.

[0041] In some embodiments of the present invention, the spodumene is spodumene micro powder. Preferably, the particle size of the spodumene is 300-500 mesh. Preferably, the coefficient of thermal expansion of the spodumene is ≤2.0×10⁻⁶. -6 / K.

[0042] In some embodiments of the present invention, the phenolic resin is a modified water-soluble phenolic resin. Preferably, the thermal decomposition temperature (T0) of the phenolic resin is... decomp The temperature is 1400-1450 degrees Celsius.

[0043] In some embodiments of the present invention, the organic fiber is an organic matter fiber. Preferably, the length of the organic matter fiber is 0.1-0.5 mm.

[0044] On the other hand, the present invention provides a method for preparing a special coating for lost foam casting of motor housings, with quartz and bauxite as the main aggregates, comprising the following steps:

[0045] (1) Determine the target thermal expansion coefficient α T , will α T Substituting the material parameters into Formula I, the mass ratio X of the quartz to the bauxite is determined. Q / X A :

[0046] (I)

[0047] in,

[0048] X Q The mass percentage of quartz in the main aggregate;

[0049] X A The percentage of the bauxite in the main aggregate;

[0050] αQ The coefficient of thermal expansion of the quartz is denoted as .

[0051] α A The coefficient of thermal expansion of the bauxite is given.

[0052] E Q The elastic modulus of the quartz;

[0053] E A The elastic modulus of the bauxite;

[0054] ν Q The Poisson's ratio of the quartz.

[0055] ν A The Poisson's ratio of the bauxite;

[0056] α T The target coefficient of thermal expansion for the lost foam coating specifically designed for the motor housing;

[0057] (2) Determine the functional components and additives and their dosage;

[0058] (3) The quartz, the bauxite, the functional components and the additives are mixed to obtain the special coating for lost foam casting of motor housing.

[0059] It is understandable that the target thermal expansion coefficient α of the lost foam coating for the motor housing is... T Substituting the material parameters into Formula I, the ideal mass ratio X of the quartz to the bauxite is calculated. Q / X A (X) Q +X A =1), and then the weight ratio of the two can be fine-tuned by taking into account other factors such as cost.

[0060] In some embodiments of the present invention, step (1) further includes:

[0061] Determine the target air permeability value P, and substitute P and relevant particle size parameters into Formula II to determine the volume ratio V of coarse particles in the main aggregate. coarse / V total The volume ratio of fine powder Φ fine :

[0062] (II)

[0063] in,

[0064] P is the target air permeability value of the coating;

[0065] k is a system constant;

[0066] The median grain size of the quartz is given.

[0067] The median particle size of the bauxite;

[0068] The main aggregate is the quartz and the bauxite;

[0069] V coarse The volume of coarse particles with a particle size greater than 200 mesh in the main aggregate;

[0070] V total This refers to the total volume of the main aggregate;

[0071] Φ fine The volume proportion of fine powder with a particle size of less than 325 mesh in the main aggregate.

[0072] Understandably, the required coarse particle volume ratio V can be calculated using Formula II. coarse / V total The lower limit and the volume ratio of fine powder Φ fine The upper limit of the particle size distribution can be used to determine the particle size distribution of various components in the main aggregate and the weight percentage of each particle size component.

[0073] In some embodiments of the present invention, small samples are prepared and tested. If performance deviations exist, the system constant k in Formula II is finely adjusted, and the optimal particle size distribution of the main aggregate is obtained by performing 1 to 2 iterations.

[0074] In some embodiments of the present invention, the functional components and additives and their dosages are determined according to the desired coating performance.

[0075] In some embodiments, the functional component includes mullite. Preferably, the mullite is mullite powder. Preferably, the lost foam coating for motor housings includes 6-8 parts by weight of mullite. Mullite reacts with the surface of quartz powder at high temperatures (above approximately 1100°C), promoting "in-situ mullitization" and generating a needle-like mullite interwoven structure, significantly improving the high-temperature strength and crust integrity of the coating, and compensating for the weaknesses of quartz powder in high-temperature phase transformation.

[0076] In some embodiments, the functional component further includes flake graphite. Preferably, the lost foam coating for motor housing comprises 4-7 parts by weight of flake graphite. The flake graphite is uniformly dispersed in the coating, and its excellent lubricity and resistance to molten metal wetting enhance the coating's resistance to iron penetration and its collapsibility, further preventing sand adhesion.

[0077] In some embodiments of the present invention, the additive may include one or more of the following: suspending agent, binder, dispersant, film-forming agent, toughening agent, high-temperature strengthening agent, and flux.

[0078] In some embodiments, the binder is sodium bentonite. Preferably, the lost foam coating for the motor housing comprises 1.5-2.5 parts by weight of sodium bentonite. The sodium bentonite serves as a basic binder and suspending agent.

[0079] In some embodiments, the dispersant is sodium tripolyphosphate. Preferably, the lost foam coating for motor housings comprises 0.2-0.4 parts by weight of sodium tripolyphosphate. As a highly efficient dispersant and anti-flocculator, sodium tripolyphosphate's anionic groups can adsorb onto the surface of aggregate particles such as quartz and bauxite, enhancing electrostatic repulsion and effectively preventing flocculation and sedimentation of high-solids-content, high-proportion micro-powder slurries. This ensures the slurry has excellent suspension stability and flowability during application, which is the technological basis for achieving a uniform, defect-free coating with just two coats.

[0080] In some embodiments, the film-forming agent is polyvinyl alcohol (PVA). Preferably, the PVA is present in an amount of 0.8-1.2 parts by weight. The PVA serves to form a toughening film.

[0081] In some embodiments, the high-temperature reinforcing agent is a phenolic resin. Preferably, the phenolic resin is a modified water-soluble phenolic resin. Preferably, the thermal decomposition temperature (T0) of the phenolic resin is... decomp The temperature is 1400-1450 degrees Celsius. In some embodiments, the lost foam coating for the motor housing comprises 1.5-2.0 parts by weight of phenolic resin. The phenolic resin provides high-temperature instantaneous strength by matching the casting temperature.

[0082] In some embodiments, the toughening agent is an organic fiber. Preferably, the organic fiber is an organic matter fiber. More preferably, the length of the organic matter fiber is 0.1-0.5 mm. In some embodiments, the lost foam coating for motor housing comprises 0.1-0.3 parts by weight of organic fiber. During the drying and low-temperature stages, the organic fiber forms a three-dimensional network within the coating, effectively transferring and dispersing stress, and greatly suppressing drying cracking and early thermal stress cracking caused by the large thermal expansion coefficient of quartz powder.

[0083] In some embodiments, the flux is spodumene. Preferably, the spodumene is spodumene powder, more preferably spodumene powder of 300-500 mesh. Preferably, the coefficient of thermal expansion of the spodumene is ≤2.0×10⁻⁶. -6 / K. In some embodiments, the lost foam coating for the motor housing comprises 2-4 parts by weight of spodumene. The spodumene, while serving as a flux, can synergistically regulate the overall thermal expansion behavior of the coating and provide low-temperature healing capability.

[0084] On the other hand, the present invention also provides a coating method for lost foam casting of motor housing, comprising the following steps:

[0085] S1: Add water to the above-mentioned lost foam coating for motor housing at a powder-to-water ratio of 1:0.70-0.75 and stir to obtain a slurry with low viscosity. Use the slurry with low viscosity to perform the first dip coating on the lost foam substrate of the motor housing.

[0086] S2: Add water to the above-mentioned lost foam coating for motor housing at a powder-to-water ratio of 1:0.60-0.65 and stir to obtain a slurry with high viscosity. Use the slurry with high viscosity to perform a second dip coating or flow coating on the lost foam substrate of the motor housing after the first dip coating.

[0087] S3: After the lost foam of the motor housing that has undergone the second dip coating or flow coating is air-dried at room temperature for 1-2 hours, it is cured at 40-45℃ for 4-6 hours to obtain the lost foam of the motor housing with a coating.

[0088] On the other hand, the present invention also provides the application of the above-mentioned special coating for lost foam casting of motor housing or the coated lost foam of motor housing obtained by the above-mentioned coating method in lost foam casting of motor housing.

[0089] Compared with the prior art, the present invention has at least the following beneficial effects:

[0090] (1) Excellent performance and low cost: The special coating for lost foam casting of motor housing in this invention has high air permeability (95-105), high crack resistance (thermal shock resistance), good delamination properties and sufficient high temperature strength. The main aggregate uses low-cost quartz powder and bauxite, and the overall cost is reduced by 25%-35% compared with high-end pearl sand-based coatings.

[0091] (2) Scientific design and good reproducibility: The dual-formula quantitative design method pioneered in this invention elevates formula development from empiricism to scientific prediction, significantly shortens the development cycle (reducing the number of trials and errors by more than 50%), and ensures the stability and reproducibility of formulas for different batches and different production lines.

[0092] (3) High efficiency and energy saving: The lost foam coating method for motor housing of the present invention improves efficiency by about 30% compared with the traditional three-layer process through "two-layer coating", and reduces energy consumption by about 20% with the matching gradient drying process, and significantly reduces the risk of coating drying cracking.

[0093] (4) The crack resistance mechanism is clear and the effect is significant: This invention optimizes the thermal expansion matching of aggregates through Formula I and combines the three-dimensional network toughening of organic fibers to inhibit the generation and propagation of cracks from both macroscopic and microscopic levels. The coating’s resistance to drying cracking and thermal shock cracking is more than 50% higher than that of traditional quartz powder coatings.

[0094] (5) Precise and controllable air permeability: This invention establishes a quantitative relationship between air permeability and particle size parameters through Formula II. The formula can be quickly adjusted according to different casting requirements (such as thin-walled parts that require higher air permeability) to achieve precise customization of air permeability in the range of 95-105.

[0095] (6) Improved thermal expansion adaptability and self-healing ability: By introducing spodumene micro powder with low thermal expansion coefficient as a flux, the dimensional stability and microcrack self-healing ability of the coating during the heating process are further enhanced. It is especially suitable for thin-walled complex castings that are sensitive to thermal shock, and the thermal shock resistance of the coating is improved by 15%-20% compared with the formulation using traditional potassium feldspar flux. Attached Figure Description

[0096] To more clearly illustrate the technical solutions in the specific embodiments of this disclosure or the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this disclosure. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0097] Figure 1 The air permeability of the coatings on the three stainless steel ring samples in the example is shown at different temperatures. Detailed Implementation

[0098] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0099] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments according to this application. As used herein, the singular form includes the plural form unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used herein, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof. Unless otherwise stated, all parts, percentages, and ratios used herein are based on mass meters.

[0100] Furthermore, it should be understood that the one or more method steps mentioned in this invention do not preclude the existence of other method steps before or after the combined steps, or the insertion of other method steps between these explicitly mentioned steps, unless otherwise stated; moreover, unless otherwise stated, the numbering of each method step is merely a convenient tool for identifying each method step, and not for limiting the order of the method steps or limiting the scope of the invention. Changes or adjustments to their relative relationships, without substantially altering the technical content, should also be considered as within the scope of the invention.

[0101] The core innovation of this invention lies in proposing a complete quantitative formulation design method, including an aggregate proportion calculation formula based on matching thermal expansion coefficients and a particle size distribution correlation formula based on target air permeability, realizing a leap from "experience-based trial and error" to "scientific prediction".

[0102] In some specific embodiments, the lost foam coating for motor housings of the present invention comprises the following components:

[0103] (1) Composite aggregate (85-90 parts)

[0104] 1) Quartz powder (60-75 parts): adopt "bimodal particle size distribution".

[0105] ① Coarse particles: 100-200 mesh quartz sand, accounting for 45%-60% of the total aggregate weight. Forms the main skeleton and main air permeability channels of the coating.

[0106] ② Fine particles: 325-500 mesh quartz powder, accounting for 15%-30% of the total aggregate weight. Fills the gaps between coarse particles, improving the density and surface smoothness of the coating.

[0107] 2) Bauxite (Al2O3≥80%, 325-350 mesh): 15-30 parts. The precise proportion is determined by calculation using the following formula I, which is used to improve the refractoriness and high-temperature strength of the coating.

[0108] (2) Composite functional aggregate (10-15 parts)

[0109] 1) Mullite micro powder (325-400 mesh, 6-8 parts): Key additive. It reacts with the surface of quartz powder at high temperatures (above 1100℃), promoting "in-situ mutulitization" and generating a needle-like mullite interwoven structure, which significantly improves the high-temperature strength and crust integrity of the coating and compensates for the weakness of quartz powder in high-temperature phase transformation.

[0110] 2) Flake graphite (150-200 mesh, 4-7 parts): It is evenly dispersed in the coating. Utilizing its excellent lubricity and resistance to wettable molten metal, it enhances the coating's resistance to iron penetration and its collapsibility, further preventing sand adhesion.

[0111] (3) Additives

[0112] 1) Composite suspension binder:

[0113] ① Sodium-based bentonite: 1.5-2.5 parts (for suspension and bonding with the base).

[0114] ② Sodium tripolyphosphate: 0.2-0.4 parts (key dispersant). As a highly efficient dispersant and anti-flocculator, its anionic properties can be adsorbed on the surface of quartz powder particles, enhancing electrostatic repulsion and significantly improving the dispersion stability and leveling of high solids content quartz powder slurry. It is the key to achieving uniform "two-coat" application.

[0115] ③ Polyvinyl alcohol (PVA, type 1788): 0.8-1.2 parts (for toughening film formation at room temperature).

[0116] 2) High-temperature strengthening and toughening agents:

[0117] ① Modified water-soluble phenolic resin: 1.5-2.0 parts. Its mechanism of action is the same as before, providing high-temperature instantaneous strength by matching the casting temperature.

[0118] ② Organic fibers (length 0.1-0.5mm): 0.1-0.3 parts (core crack-resistant and toughening component). A key addition; during the drying and low-temperature stages, the fibers form a three-dimensional network within the coating, effectively transferring and dispersing stress, greatly suppressing drying cracking and early thermal stress cracking caused by the high thermal expansion coefficient of quartz powder.

[0119] 3) Flux:

[0120] Thermally expandable flux: Spodumene micron powder (300-500 mesh, coefficient of thermal expansion ≤ 2.0 × 10⁻⁶) / K), dosage 2-4 parts. Spodumene micropowder is used to synergistically regulate the overall thermal expansion behavior of the coating and provide low-temperature healing capability while achieving the fluxing function.

[0121] The key to the success of this invention lies in the construction of a stable system of "high-permeability main skeleton - high-temperature ceramic reinforcement - multi-level stress buffer" through multi-component and multi-scale scientific design and synergistic effect, which perfectly balances the contradiction between low cost, high permeability and high crack resistance.

[0122] (1) The synergistic mechanism of high air permeability and high temperature strength:

[0123] An innovative model of "coarse quartz sand skeleton + bauxite / mullite two-phase micro-area reinforcement" is adopted.

[0124] Skeleton and Channels: 100-200 mesh coarse quartz sand forms a solid main skeleton, and the gaps between them form stable and interconnected macroscopic pore channels, which is the fundamental guarantee for achieving high air permeability (95-105).

[0125] Dual-phase micro-area reinforcement: The synergistic effect of introduced bauxite and mullite micro-powders. Bauxite micro-powder possesses higher refractoriness and thermochemical stability, filling the gaps in the coarse skeleton; mullite micro-powder exhibits greater activity at high temperatures. Under high casting temperatures, complex solid-state reactions and sintering occur at the interface between the two and the quartz particles:

[0126] The reaction between bauxite (Al2O3) and quartz (SiO2) at the interface promotes the formation of secondary mullite (3Al2O3·2SiO2).

[0127] The pre-added mullite micropowder serves as a seed crystal, further guiding and strengthening this high-temperature ceramic bonding process.

[0128] Ultimately, at the contact points and neck region between the coarse particles, high-temperature ceramic "nodes" dominated by mullite phase are formed, firmly welding the loose particle skeleton into a whole. This "spot welding" reinforcement significantly improves the overall high-temperature strength and corrosion resistance of the coating without significantly blocking the macroscopic ventilation channels.

[0129] (2) Systemic suppression mechanism of quartz thermal expansion defects:

[0130] By employing a three-pronged approach of "chemical adaptation, physical buffering, and structural compensation," the inherent defect of high thermal expansion coefficient in quartz powder has been completely resolved.

[0131] Chemical compatibility and stabilization (based on Formula I): This is the core theoretical breakthrough of this invention. By accurately calculating the ratio of quartz to bauxite using Formula I, the scientific goal is to achieve a coefficient of thermal expansion (α) of the composite aggregate matrix. T This is suitable for understanding the thermal behavior of foam models and the solidification process of metals. Bauxite has a low coefficient of thermal expansion (approximately 8 × 10⁻⁶). -6 / K) has a higher coefficient of thermal expansion for quartz (approximately 12 × 10⁻⁶ K) -6 / K) forms a negative regulation, causing the α of the complex to T It falls within the ideal range of 8.5-9.5×10. -6 Within the / K range, internal stress caused by thermal mismatch is reduced at the source.

[0132] Physical buffering and toughening: Organic fibers are distributed in a three-dimensional random pattern in the coating, acting as "micro-reinforcing bars". When microcracks are generated in the coating due to temperature changes or stress, the fibers cross the two sides of the crack through the "bridging effect", consuming the fracture energy, preventing crack propagation, and significantly improving the fracture toughness and thermal shock resistance of the coating.

[0133] Structural compensation: The high-temperature reaction of mullite micro powder consumes SiO2, which has the highest surface activity of quartz particles, and inhibits the rate and extent of quartz quartzization (accompanied by about 15% volume change) at 573℃ (β-quartz to α-quartz transformation) and higher temperatures, thereby improving the volume stability of the coating during the heating process.

[0134] (3) Process compatibility and performance designability mechanism:

[0135] Slurry stability and workability assurance: Sodium tripolyphosphate, as a highly efficient dispersant, has anionic groups that can adsorb onto the surface of aggregate particles such as quartz and bauxite, enhancing electrostatic repulsion and effectively preventing flocculation and sedimentation of high-solids-content, high-proportion micro-powder slurries. This ensures that the slurry has excellent suspension stability and workability. This is the technological basis for achieving a uniform, defect-free coating with just two coats.

[0136] (4) Quantitative design and control of performance (based on Formula II): This invention abandons the traditional trial-and-error method. Through Formula II, the coating air permeability value (P) and aggregate particle size distribution parameters (such as the proportion of coarse particles V) are established. coarse / V total Fine powder volume ratio Φ fine A quantitative correlation model between the two is established. This allows for the reverse derivation of scientific particle size distribution schemes for the air permeability target values ​​required for different casting structures (such as variations in wall thickness), enabling the predictability and designability of the core performance of the coating.

[0137] (5) Thermal expansion adaptation of flux and low-temperature healing mechanism

[0138] This invention innovatively elevates fluxes from their traditional single function of "promoting sintering" to a key synergistic component possessing both "thermal expansion compatibility" and "low-temperature healing" properties. When using spodumene micropowder with a low coefficient of thermal expansion (α≈1×10⁻⁶), -6 When / K), its mechanism of action includes:

[0139] ① Active adjustment of thermal expansion: Based on the mixing principle of composite materials, the extremely low thermal expansion characteristics of spodumene can affect the macroscopic thermal expansion coefficient (α) of the composite aggregate matrix. T Make negative fine adjustments to make it fall more precisely within the target range (8.5-9.5×10). -6 / K), further reducing thermal mismatch stress from the source.

[0140] ② Low-temperature liquid phase healing: Spodumene begins to form a liquid phase of suitable viscosity at approximately 900℃. This temperature window falls precisely after the harmful phase transformation of quartz (573℃) and before the pouring of molten metal. This liquid phase can rapidly fill and "heal" microcracks caused by phase transformation or drying, improving the structural integrity of the coating during the critical heating stage.

[0141] ③ Interface stabilization: The Li2O component in spodumene can react with the surface of quartz and bauxite to form a lithium aluminum silicate phase with higher thermal stability, which enhances the bonding between particles.

[0142] Therefore, the selection and dosage of flux can be incorporated into the overall quantitative design framework to achieve secondary optimization of the coating's thermal expansion properties and low-temperature self-healing.

[0143] To ensure optimal coating performance, this invention establishes two calculation formulas based on the principles of materials science:

[0144] (1) Formula I: Main aggregate matrix proportioning formula based on matching thermal expansion coefficient (to solve crack resistance)

[0145] To achieve the optimal match between the overall coefficient of thermal expansion (CTE) of the coating and the process requirements, and to reduce thermal stress, the mass ratio of quartz (Q) to bauxite (A) should satisfy the following relationship (based on the Turner composite material model):

[0146] (I)

[0147] In the formula:

[0148] X Q X A : Mass percentage of quartz and bauxite in the main aggregate matrix (X) Q +X A =1).

[0149] α Q α A : Coefficient of thermal expansion of material (10 -6 / K), typical value: α Q ≈12, α A ≈8.

[0150] E Q E A Elastic modulus (GPa), typical value: E Q ≈70, E A ≈300.

[0151] ν Q ν A Poisson's ratio, typical value: ν Q ≈0.17, ν A ≈0.22.

[0152] α T The target coefficient of thermal expansion for lost foam casting coatings used on motor housings. Extensive experimental verification has shown that the optimal coefficient of thermal expansion for thin-walled motor housing components is 8.5-9.5 × 10⁻⁶. -6 / K.

[0153] (2) Formula II: Aggregate particle size distribution correlation based on target air permeability (solving air permeability)

[0154] The air permeability of the coating is mainly determined by the interconnected pores and is strongly correlated with the aggregate packing state. Based on Andreassen's close packing theory, a correction factor is introduced to establish an empirical formula:

[0155] (II)

[0156] In the formula:

[0157] P: Target air permeability value of the lost foam coating for motor housing.

[0158] k: System constant, determined through benchmark experiments.

[0159] , : Median particle size of quartz and bauxite.

[0160] V coarse : 100-200 mesh main aggregate volume.

[0161] V total Total volume of main aggregate.

[0162] Φ fine : The volume ratio of fine powder (<325 mesh) in the main aggregate.

[0163] The formula shows that air permeability increases with the increase of coarse particle ratio and decreases sharply with the increase of fine powder content.

[0164] In some specific implementations, the preparation process of the special coating for lost foam casting of the motor housing is as follows:

[0165] (1) Input target: Determine the target air permeability value P (e.g., 110) and the target thermal expansion coefficient α. T (e.g., 9.0×) / K).

[0166] (2) Calculate the main aggregate ratio: α T Substituting the material parameters into Formula I, calculate the mass ratio X of the quartz to the bauxite. Q / X A .

[0167] Determine particle size distribution: Substitute P and relevant particle size parameters into Formula II to deduce the required coarse particle volume ratio V. coarse / V total The volume ratio of fine powder Φ fine .

[0168] (3) Constructing a complete formula: Within the above calculation framework, add a fixed proportion of functional components (6%-8% mullite, 4%-7% graphite) and determine the amount of additives.

[0169] (4) Experimental verification and iteration: Prepare small samples for testing. If there is a performance deviation, fine-tune the system constant k in the formula and perform 1-2 rounds of iteration to obtain the optimal formula.

[0170] (5) Mix the quartz, bauxite, functional components and additives in the specified amounts to obtain the special coating for lost foam casting of motor housing.

[0171] In some specific implementations, the preferred coating process for the lost foam coating for motor housings of the present invention is a "two-coat gradient drying" method, which specifically includes the following steps:

[0172] (1) Two coats:

[0173] 1) First coat (dip coating, base layer): Use a slurry with low viscosity (flow cup 25-30 seconds) (powder to water ratio 1:0.70-0.75) to form a uniform and dense base layer of about 0.3mm.

[0174] 2) Second coat (dip coating / flow coating, reinforcement layer): After surface drying, use a slurry with a slightly higher viscosity (flowing cup in 35-40 seconds) (powder-to-water ratio 1:0.60-0.65) to achieve a total thickness of 0.8-1.0 mm, which can be thickened for hot spots.

[0175] (2) Gradient drying:

[0176] 1) First stage (air drying at room temperature, 1-2 hours): Initial fixation of the organic fiber network and slow dehydration.

[0177] 2) Second stage (low temperature curing, 40-45℃, 4-6 hours): This stage allows PVA and phenolic resin to fully cure, the fiber network to be fully formed, and the best handling strength to be obtained.

[0178] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0179] Unless otherwise stated, all reagents used in the examples are commercially available or synthesized using conventional methods and are ready for use without further processing, as are the instruments used in the examples.

[0180] Example

[0181] 1. Design and preparation of coatings specifically for lost foam casting.

[0182] A special coating was designed for the HT250 gray cast iron motor housing (wall thickness 4-10mm, complex structure).

[0183] Target determination: Based on the characteristics of this casting, the target permeability value P=100 and the target coefficient of thermal expansion α are set. T =9.0×10 -6 / K.

[0184] Calculate the main aggregate proportion: Substitute into formula I to calculate X Q :X A The ratio is approximately 1.52:1, meaning quartz accounts for about 60.3% and bauxite about 39.7%. Considering the total aggregate of 100 parts and cost, the initial ratio is set at 60 parts quartz powder and 30 parts bauxite.

[0185] Determine particle size requirements: Based on Formula II and P=100, calculate the required volume ratio Φ of fine powder (<325 mesh). fine <0.4, and the volume ratio of coarse particles (100-200 mesh) must be >0.5.

[0186] The complete formula is formed, and the components and their weight parts are as follows:

[0187] Aggregates: 45 parts 100 / 200 mesh quartz sand, 15 parts 325 mesh quartz powder, 30 parts 325 mesh calcined bauxite, 6 parts 400 mesh mullite powder, and 4 parts 200 mesh flake graphite.

[0188] Additives: 2 parts sodium bentonite, 0.3 parts sodium tripolyphosphate, 1.0 part PVA, modified phenolic resin (T decomp =1410℃) 1.8 parts, organic fiber 0.2 parts, spodumene micro powder (400 mesh, α=1.2×10 -6 3 copies of / K).

[0189] After weighing and dry mixing the components of the coating, the resulting coating exhibits good properties.

[0190] 2. Determination of high-temperature air permeability

[0191] Three samples were prepared by coating a stainless steel ring (inner diameter Φ50mm, thickness 1.80mm) using the following steps.

[0192] First dip coating: Add water to the dry-mixed paint at a powder-to-water ratio of 1:0.72 and stir for 4 hours. After stirring evenly, immerse the stainless steel ring in the paint to form a uniform and dense layer of about 0.3 mm.

[0193] Second dip coating: Add water to the dry-mixed paint at a powder-to-water ratio of 1:0.65 and stir for 4 hours. After stirring evenly, dip the stainless steel ring that has undergone the first dip coating into the paint again to form a coating with a total thickness of about 0.9 mm.

[0194] Gradient drying:

[0195] After the stainless steel rings were coated twice, they were air-dried at room temperature for 2 hours and then cured at 45°C for 5 hours to obtain three coated stainless steel ring samples: Sample 1, Sample 2 and Sample 3.

[0196] The air permeability of the coatings on each sample was measured under different temperature conditions using a ZTY intelligent air permeability tester (unit: cm). 4 / gram-minute (generally considered a dimensionless value), the result is as follows: Figure 1 As shown.

[0197] 3. Determination of thermal crack resistance

[0198] Three white molds with T-sections (80 / 40*120mm, 15mm thick) were coated using the method described above to obtain yellow molds with a coating. The surface of the yellow molds was observed for cracks, and the coating thickness was measured. The coating thicknesses of the three yellow molds were 0.98mm, 1.0mm, and 0.97mm, respectively. An NBD-M1700-22TI box furnace was heated to 1200℃, and the three yellow molds were placed inside the furnace and held for 3 minutes. Immediately after removal, the state of cracks in the mold shell was observed using a magnifying glass and photographed. No cracks were found in any of the three yellow molds, indicating good resistance to thermal cracking.

[0199] 4. Pouring and Inspection

[0200] The lost foam casting of the coated motor housing was prepared according to the above method. The HT250 gray cast iron motor housing was cast at 1380℃. The resulting casting had a smooth surface, no sand adhering to the inner cavity, and no porosity defects after X-ray inspection. The coating peeled off in large shell-like pieces, with a one-time peeling rate of >95%.

[0201] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An electro-motor shell special coating for lost foam, comprising main aggregate, functional aggregate and additives, characterized in that, The main aggregate comprises quartz and bauxite, the functional aggregate comprises mullite and flaky graphite, and the additives comprise sodium bentonite, sodium tripolyphosphate, polyvinyl alcohol, phenolic resin, organic fiber and spodumene; The mass ratio of the quartz to the bauxite is determined according to Formula I: （I） wherein, X Q is the mass percentage of the quartz in the main aggregate; X A is the mass percentage of said bauxite in said main aggregate; α Q is the coefficient of thermal expansion of the quartz; a A is the coefficient of thermal expansion of the bauxite; E Q E is the elastic modulus of the quartz; E A Elastic modulus of the bauxite; ν Q Poisson's ratio for the quartz; ν A Poisson's ratio for the bauxite; a T target thermal expansion coefficient for the motor housing lost foam special coating; determining a target thermal expansion coefficient a T , and substituting the values of a T and the material parameters into equation I to determine the mass ratio X of the quartz to the bauxite Q / X A ; The particle size distribution of the main aggregate is determined according to Formula II: (I) wherein, P is a target air permeability value of the motor shell lost foam special coating; k is a system constant; D50 is the median particle size of the quartz; D50 is the median particle size of the bauxite; V coarse V is the volume of coarse particles in the main aggregate having a particle size greater than 200 mesh; V total Vt is the total volume of the main aggregate; Φ fine is the volume fraction of fine powder in the main aggregate with a particle size less than 325 mesh. determining a target air permeability value P, and substituting P and related particle size parameters into Formula II to determine the volume proportion of coarse particles V in the main aggregate coarse / V total and the volume proportion of fine powder Φ fine .

2. The lost foam motor case specialty coating of claim 1, wherein, The main aggregate comprises 60-75 parts by weight of quartz and 15-30 parts by weight of bauxite, and the functional aggregate comprises 6-8 parts by weight of mullite and 4-7 parts by weight of flaky graphite.

3. The EMD coating for motor case according to claim 2, wherein The additives comprise 1.5-2.5 parts by weight of sodium bentonite, 0.2-0.4 parts by weight of sodium tripolyphosphate, 0.8-1.2 parts by weight of polyvinyl alcohol, 1.5-2.0 parts by weight of phenolic resin, 0.1-0.3 parts by weight of organic fiber and 2-4 parts by weight of spodumene.

4. The electro-motor case LFT dedicated coating according to any one of claims 1-3, characterized in that, The quartz comprises quartz sand with a particle size of 100-200 mesh and quartz powder with a particle size of 325-500 mesh, the mass ratio of the quartz sand to the quartz powder is 1.5-3: 1, the particle size of the bauxite is 325-350 mesh, the particle size of the mullite is 325-400 mesh, and the particle size of the flaky graphite is 150-200 mesh.

5. A method for preparing a special coating for an electric machine shell lost foam using quartz and bauxite as main aggregates, characterized in that, The method comprises the following steps: (1) determining a target thermal expansion coefficient a T , and substituting a T and material parameters into Formula I to determine a mass ratio X of the quartz to the bauxite Q / X A : （I） wherein, X Q is the mass percentage of the quartz in the main aggregate; X A is the mass percentage of said bauxite in said main aggregate; α Q is the thermal expansion coefficient of the quartz; alpha A is the coefficient of thermal expansion of the bauxite; E Q E is the elastic modulus of the quartz; E A Elastic modulus of the bauxite; ν Q Poisson's ratio for the quartz; ν A Poisson's ratio for the bauxite; a T target coefficient of thermal expansion for the coating; (2) determining the functional components and the additives and the amounts thereof; (3) mixing the quartz, the bauxite, the functional components and the additives to obtain the motor shell lost foam special coating; The step (1) further comprises: determining a target air permeability value P, and substituting P and related particle size parameters into equation II to determine the volume fraction of coarse particles V in the main aggregate coarse / V total and the volume fraction of fine powder Φ fine : (I) wherein, P is a target air permeability value of the motor shell lost foam special coating; k is a system constant; D50 is the median particle size of the quartz; D50 is the median particle size of the bauxite; The main aggregate is the quartz and the bauxite; V coarse V is the volume of coarse particles in the main aggregate having a particle size greater than 200 mesh; V total V is the total volume of the main aggregate; Φ fine is the volume fraction of fine powder in the main aggregate with a particle size less than 325 mesh.

6. A method of coating an electric machine housing lost form, characterized in that The method comprises the following steps: S1: stirring the motor shell lost foam special coating according to any one of claims 1-4 with water at a powder-to-water ratio of 1: 0.70-0.75 to obtain a slurry with a lower viscosity, and using the slurry with a lower viscosity to perform first-time dip coating on the motor shell lost foam matrix; S2: stirring the motor shell lost foam special coating according to any one of claims 1-4 with water at a powder-to-water ratio of 1: 0.60-0.65 to obtain a slurry with a higher viscosity, and using the slurry with a higher viscosity to perform second-time dip coating or flow coating on the motor shell lost foam matrix which has been subjected to the first-time dip coating; S3: after the motor shell lost foam which has been subjected to the second-time dip coating or flow coating is air-dried at room temperature for 1-2 hours, curing it at 40-45℃ for 4-6 hours to obtain the motor shell lost foam with a coating.

7. Application of the motor shell lost foam special coating according to any one of claims 1-4 or the motor shell lost foam with a coating obtained by the coating method according to claim 6 in motor shell lost foam casting.

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