CN104399874A - Hexagonal boron nitride type mold shell investment precoat for precision casting of magnesium alloy investment - Google Patents

Abstract

A hexagonal boron nitride shell surface coating for magnesium alloy investment precision casting, using silica sol as a binder and hexagonal boron nitride powder as a refractory raw material for the shell surface coating, the composition of the coating The components and content are: 100 parts by weight of silica sol, 5-10 parts by weight of deionized water, 0.100-0.15 parts by weight of bactericide, 400-600 parts by weight of hexagonal boron nitride micropowder, 20-30 parts by weight of poly Vinyl acetate emulsion, 0.100-0.150 parts by weight of wetting and dispersing agent, and 0.050-0.100 parts by weight of defoaming agent. The shell surface layer prepared by the present invention has the characteristics of low reactivity with magnesium alloys, and because the hexagonal boron nitride is not infiltrated by the magnesium alloy melt, it is beneficial to magnesium alloy castings, especially magnesium alloys with large-area thin-walled structure characteristics The filling of castings, and other performance indicators can meet the requirements of magnesium alloy investment precision casting.

Description

Hexagonal boron nitride shell surface coating for magnesium alloy investment casting

technical field

The invention relates to a paint in the technical field of non-ferrous metal investment precision casting, in particular to a hexagonal boron nitride shell surface coating for magnesium alloy investment precision casting.

Background technique

As the lightest metal structural materials, magnesium and magnesium alloys have the advantages of high specific strength/specific stiffness, stable size, easy processing and forming, good thermal and electrical conductivity, damping and vibration reduction, electromagnetic shielding and easy recycling, so they are known as " 21st Century Green Engineering Materials". Magnesium alloys have become important structural materials in aerospace, automobile, electronic communication and other industrial fields. Magnesium alloys have strong casting process adaptability. Almost all casting methods such as sand casting, metal casting, gravity casting, lost foam casting, low pressure casting, investment casting, die casting and centrifugal casting can produce magnesium alloy castings. Among them, investment casting (also known as investment casting) technology is one of the most competitive technologies for realizing the overall forming of large magnesium alloy castings. As a material near-net shape technology, investment casting plays an irreplaceable role in the forming of high-precision, complex-structure castings. Its processes range from pressing wax patterns, waxing, molding, dewaxing, roasting to final filling. , the process is complicated and the cycle is long, and it is precisely because of this process that the investment casting has a unique charm, so that it can be applied to the casting of magnesium alloys, which can obtain low surface roughness, high surface and metallurgical quality, and size Accurate large, complex, thin-walled magnesium alloy structural parts.

Due to the high reactivity of magnesium alloys, during the casting process, most of the oxide ceramic materials, such as alumina, silicon oxide, zircon powder, etc., will react to varying degrees, resulting in the formation of a reaction layer on the surface of magnesium alloy castings. When the reaction layer of magnesium alloy castings reaches a certain thickness, it will affect the surface roughness and dimensional accuracy of the castings. For large and thin-walled magnesium alloy castings, the control of the thickness of the surface reaction layer is especially important. Magnesium alloys have low reactivity and are conducive to the filling of magnesium alloys. The surface coating system is the key to the success of large-scale, thin-walled magnesium alloy investment casting.

Hexagonal boron nitride is a refractory material with very stable chemical properties. It has a layered structure similar to graphite. It has good temperature resistance and lubricity. It has very stable chemical properties and is chemically inert to molten metals such as aluminum, magnesium, and zinc. , and is not infiltrated by the metal melt, it will not react chemically with water, weak acid, or strong alkali, and the prepared coating is stable and not easy to gel, so boron nitride is used as the raw material of the shell surface coating , the surface of the prepared shell surface layer is not easy to be infiltrated by metal melt, which is especially beneficial to the filling of large-area thin-walled magnesium alloy castings. The reaction layer formed on the surface of magnesium alloy castings is also better than that of ordinary oxides (such as alumina, Silicon oxide, etc.) The shell surface layer formed should be thin, which is beneficial to improve the surface quality and precision of the casting.

After searching the prior art, it was found that the Chinese Patent Document No. CN103084324A discloses a crucible and its coating method, including the following process steps: making a coating, which includes hexagonal boron nitride and pure water; spraying the coating On the inside of the crucible; allow the crucible to air dry. The coating in this technology contains hexagonal boron nitride and pure water, and also includes silica sol or polyvinyl alcohol binder. This technology is applied to the production of monocrystalline silicon/polysilicon materials in the photovoltaic solar energy field and the semiconductor field. The hexagonal boron nitride coating in this technology has poor wettability with silicon liquid, and it is not easy to produce silicon material under the condition of carrying liquid silicon. In addition, due to the good lubricity of hexagonal boron nitride itself, this makes the silicon ingot release well. However, in this technology, it is difficult to uniformly disperse and suspend hexagonal boron nitride in water, so its fluidity is difficult to guarantee; at the same time, the bubbles generated during the stirring process cannot be eliminated, which seriously affects the surface quality of the coating formed by the paint.

Contents of the invention

Aiming at the disadvantages of poor surface quality of castings caused by surface materials such as alumina, silicon oxide and zircon powder commonly used in the magnesium alloy investment casting industry, the present invention proposes a hexagonal boron nitride for magnesium alloy investment precision casting Shell surface coating, using silica sol as binder and hexagonal boron nitride powder as refractory raw material, the shell surface layer has low reactivity with magnesium alloy, and due to the hexagonal boron nitride It is not for the infiltration of magnesium alloy melt but is beneficial to the filling of magnesium alloy castings, especially the magnesium alloy castings with large-area thin-walled structure characteristics, and other performance indicators can meet the requirements of magnesium alloy investment precision casting.

The present invention is achieved through the following technical solutions:

The present invention relates to a kind of hexagonal boron nitride type shell coating, its viscosity is 32-36s, and the component of this coating comprises: silica sol as inorganic binding agent, polyvinyl acetate emulsion as organic binding agent and as Hexagonal boron nitride micropowder as refractory raw material.

The specific components and contents of the hexagonal boron nitride type shell surface coating are: 100 parts by weight of silica sol, 5-10 parts by weight of deionized water, 0.100-0.15 parts by weight of bactericide, 400-600 parts by weight of Parts of hexagonal boron nitride fine powder, 20-30 parts by weight of polyvinyl acetate emulsion, 0.100-0.150 parts by weight of wetting and dispersing agent, and 0.050-0.100 parts by weight of defoamer.

Described bactericide adopts but not limited to: Kasong bactericide;

The defoamer is used but not limited to: n-butanol, preferably chemically pure;

Described silica sol adopts but not limited to: alkaline silica sol;

The particle size of the hexagonal boron nitride fine powder is 200-325 mesh, and the purity is greater than 99.5%;

The solid content of the polyvinyl acetate emulsion is 42 ± 2%, and the pH value is 3-4;

The wetting and dispersing agent is fatty alcohol polyoxyethylene ether.

The present invention relates to the preparation method of above-mentioned hexagonal boron nitride type shell coating, by dividing silica sol, deionized water, bactericide, hexagonal boron nitride micropowder and polyvinyl acetate emulsion, wetting and dispersing agent, defoaming agent into two parts Groups are mixed separately and then combined to make.

Described preparation method specifically comprises the following steps:

1) Mix silica sol, deionized water, and fungicide in the mixing machine, and slowly start the mixing machine for mixing; then add hexagonal boron nitride micropowder, and keep vortex during the preparation process, add hexagonal boron nitride At the same time of micro-powdering, the mixing machine is used for stirring or artificial auxiliary stirring is added to completely disperse the hexagonal boron nitride micro-powder in the silica sol. During the whole process, the temperature of the coating is controlled by the chiller to not exceed 26.7°C;

2) Mix polyvinyl acetate emulsion, wetting and dispersing agent, and defoamer separately: first add polyvinyl acetate emulsion, then slowly add wetting and dispersing agent and defoaming agent, stir for 3-5 minutes, and then mix Add the liquid into the mixing machine in step 1 and stir for more than 2 hours, then move it to the slurry bucket, keep the coating continuously stirred in the slurry bucket, after 6 hours of stirring, the viscosity can reach 32-36s, that is, Qualified hexagonal boron nitride shell coating.

Since the moisture in the paint will volatilize over time, the proportion of each component in the paint will change, and the viscosity of the paint will gradually increase. Therefore, in the subsequent paint maintenance process, when the viscosity is higher than 36s, it can be properly Add silica sol to dilute, so that the viscosity of the paint is 32‐36s.

technical effect

Compared with the prior art, the hexagonal boron nitride type shell surface coating prepared by the method has good suspension and thixotropy; after the coating forms the surface layer, it has good air permeability and high surface strength, which is very good. It satisfies the requirement of surface coating for magnesium alloy investment precision casting. The preparation method of the coating is simple, non-toxic, and because hexagonal boron nitride is chemically inert and non-wetting at high temperature for magnesium alloys, the surface finish of the magnesium alloy castings produced is high, and it is suitable for magnesium alloys of various grades and components. Investment casting of alloy castings. The use of the coating will greatly increase the yield of magnesium alloy castings and has better economic benefits.

Detailed ways

The embodiments of the present invention are described in detail below. This embodiment is implemented on the premise of the technical solution of the present invention, and detailed implementation methods and specific operating procedures are provided, but the protection scope of the present invention is not limited to the following implementation example.

Example 1

This embodiment realizes the preparation of the hexagonal boron nitride type shell surface coating for magnesium alloy precision casting through the following steps:

1) Add 100kg of silica sol, 5kg of deionized water and 100g of bactericide into the mixing machine, and slowly start the mixing machine for mixing;

2) Gradually add 600kg of 200-mesh-325-mesh hexagonal boron nitride micropowder into the slurry mixing tank under the condition of stirring, supplemented by manual stirring, so that the hexagonal boron nitride micropowder is completely dispersed in the silica sol, and pass through the ice The water machine controls the temperature of the paint so that the temperature does not exceed 26.7°C;

3) Mix 20kg of polyvinyl acetate emulsion, 100g of wetting and dispersing agent and 50g of defoamer in another mixing tank: first add polyvinyl acetate emulsion, then slowly add wetting and dispersing agent and defoaming agent, and stir until 3-5 minutes is appropriate, add the above solution into the slurry bucket, and the preparation of the surface coating is completed;

4) After the preparation is completed, the paint is stirred in the slurry mixing machine for more than 2 hours and then moved to the slurry bucket; keep the coating continuously stirred in the slurry bucket, and use the No. 4 Jane's flow rate cup to measure the viscosity of the paint every 2 hours. After 6 hours, the viscosity of the paint has reached 32s, and it can be dipped.

The magnesium alloy casting produced by the ceramic shell prepared by this type of shell surface coating, after testing, the surface is off-white, flat and smooth, no excessive shell inclusions were found through X-ray inspection, the surface finish is good, and the thickness of the surface reaction layer Less than 0.08mm.

Example 2

The present embodiment realizes the production of the stainless steel investment casting mold shell surface layer through the following steps:

1) Add 100kg of silica sol, 7.5kg of deionized water and 125g of bactericide into the mixing machine, and slowly start the mixing machine for mixing;

2) Gradually add 500kg of 200-mesh-325-mesh hexagonal boron nitride micropowder into the slurry mixing tank under the condition of stirring, supplemented by manual stirring, so that the hexagonal boron nitride micropowder is completely dispersed in the silica sol, and pass through the ice The water machine controls the temperature of the paint so that the temperature does not exceed 26.7°C;

3) Mix 25kg of polyvinyl acetate emulsion, 125g of wetting and dispersing agent and 75g of defoamer in another mixing tank: first add polyvinyl acetate emulsion, then slowly add wetting and dispersing agent and defoaming agent, do not add If it is too fast, otherwise it will make the solution gel, it is better to stir for 3-5 minutes, then add the above solution into the slurry tank, and the preparation of the surface coating is completed;

4) After the preparation is completed, the paint is stirred in the slurry mixing machine for more than 2 hours and then moved to the slurry bucket; keep the coating continuously stirred in the slurry bucket, and use the No. 4 Jane's flow rate cup to measure the viscosity of the paint every 2 hours. After 6 hours, the viscosity of the paint reaches 34s, and it can be dipped.

The magnesium alloy casting produced by the ceramic shell prepared by this type of shell surface coating, after testing, the surface is off-white, flat and smooth, no excessive shell inclusions were found through X-ray inspection, the surface finish is good, and the thickness of the surface reaction layer Less than 0.10mm.

Example 3

The present embodiment realizes the production of the stainless steel investment casting mold shell surface layer through the following steps:

1) Add 100kg of silica sol, 10kg of deionized water and 150g of bactericide into the mixing machine, and slowly start the mixing machine for mixing;

2) Add 400kg of 200-mesh-325-mesh hexagonal boron nitride micropowder gradually into the slurry mixing tank under the condition of stirring, supplemented by manual stirring, so that the hexagonal boron nitride micropowder is completely dispersed in the silica sol, and pass through the ice The water machine controls the temperature of the paint so that the temperature does not exceed 26.7°C;

3) Mix 30kg of polyvinyl acetate emulsion, 150g of wetting and dispersing agent and 100g of defoamer in another mixing tank: first add polyvinyl acetate emulsion, then slowly add wetting and dispersing agent and defoaming agent, do not add If it is too fast, otherwise it will make the solution gel, it is better to stir for 3-5 minutes, then add the above solution into the slurry tank, and the preparation of the surface coating is completed;

4) After the preparation is completed, the paint is stirred in the slurry mixing machine for more than 2 hours and then moved to the slurry bucket; keep the coating continuously stirred in the slurry bucket, and use the No. 4 Jane's flow rate cup to measure the viscosity of the paint every 2 hours. After 6 hours, the viscosity of the paint has reached 36s, and it can be dipped.

The magnesium alloy casting produced by the ceramic shell prepared by this type of shell surface coating, after testing, the surface is off-white, flat and smooth, no excessive shell inclusions were found through X-ray inspection, the surface finish is good, and the thickness of the surface reaction layer Less than 0.11mm.

The sources of raw materials involved in the above examples are: the silica sol is alkaline silica sol provided by Nalco (China) Environmental Protection Technology Service Co., Ltd., the brand is 1130C; the fungicide is provided by Heze Changjiang Chemical Co., Ltd. Kasong, the grade is 1127; the hexagonal boron nitride is provided by Yingkou Borda Fine Chemical Co., Ltd., the particle size is 200-325 mesh, and the purity is greater than 99.5%; Provided by Mixture Co., Ltd., the solid content is 42±2%, and the pH value is 3-4; the wetting and dispersing agent is provided by Nalco (China) Environmental Technology Service Co., Ltd., and the brand is 7856; the described disinfectant The foaming agent is chemically pure n-butanol, provided by Sinopharm Shanghai Chemical Reagent Company.

Claims (9)

1. A hexagonal boron nitride type shell surface coating is characterized in that the coating component comprises: silica sol as inorganic binder, polyvinyl acetate emulsion as organic binder and hexagonal nitrogen as refractory raw material boron powder. 2. The coating according to claim 1, characterized in that, the components and contents of the coating are: 100 parts by weight of silica sol, 5-10 parts by weight of deionized water, 0.100-0.15 parts by weight of bactericidal agent, 400-600 parts by weight of hexagonal boron nitride micropowder, 20-30 parts by weight of polyvinyl acetate emulsion, 0.100-0.150 parts by weight of wetting and dispersing agent, and 0.050-0.100 parts by weight of defoamer. 3. coating according to claim 1, is characterized in that, described bactericide adopts: Kasong bactericide; Described defoamer adopts: n-butanol; Described silica sol adopts: alkaline silica sol ; The wetting and dispersing agent adopts fatty alcohol polyoxyethylene ether. 4. The coating according to claim 1 or 2, characterized in that, the particle size of the hexagonal boron nitride micropowder is 200-325 mesh, and the purity is greater than 99.5%. 5. The coating according to claim 1 or 2, characterized in that, the solid content of the polyvinyl acetate emulsion is 42±2%, and the pH value is 3-4. 6. The coating according to any one of the preceding claims, wherein the viscosity of the coating is 32-36s. 7. A preparation method according to the described hexagonal boron nitride type shell coating according to any one of the above-mentioned claims, is characterized in that, by silica sol, deionized water, bactericide, hexagonal boron nitride micropowder and polyvinyl acetate Emulsion, wetting and dispersing agent, and defoamer are divided into two groups, mixed separately and then combined to make. 8. The method according to claim 7, characterized in that, the method specifically comprises: 1) Mix silica sol, deionized water, and fungicide in the mixing machine, and slowly start the mixing machine for mixing; then add hexagonal boron nitride micropowder, and keep vortex during the preparation process, add hexagonal boron nitride At the same time of micro-powdering, the mixing machine is used for stirring or artificial auxiliary stirring is added to completely disperse the hexagonal boron nitride micro-powder in the silica sol. During the whole process, the temperature of the coating is controlled by the chiller to not exceed 26.7°C; 2) Mix polyvinyl acetate emulsion, wetting and dispersing agent, and defoamer separately: first add polyvinyl acetate emulsion, then slowly add wetting and dispersing agent and defoaming agent, stir for 3-5 minutes, and then mix Add the liquid into the mixing machine in step 1 and stir for more than 2 hours, then move it to the slurry bucket, keep the coating continuously stirred in the slurry bucket, after 6 hours of stirring, the viscosity can reach 32-36s, that is, Qualified hexagonal boron nitride shell coating. 9. According to the application of the hexagonal boron nitride type shell surface coating described in any one of the preceding claims, it is characterized in that, when in the subsequent coating maintenance process, when the viscosity of the coating is higher than 36s, by Add silica sol for thinning to maintain paint viscosity at 32-36s.