Introduction to water-based aluminum casting coatings

With the development of lightweight automobiles and the development and production of all aluminum engines and cars, the amount of aluminum used in individual cars is constantly increasing. Due to its good adaptability in the production of complex castings such as aluminum alloy gasoline engine cylinder heads, low-pressure casting is adopted by many casting manufacturers to produce aluminum alloy castings. Under low-pressure casting conditions, castings are filled and crystallized under pressure. For low-pressure metal casting with simple structures and without the use of complex sand cores, there is no sand sticking or other problems caused by it. Production practice has shown that when casting water-cooled cylinder heads with complex internal cavity structures and requiring airtightness tests, if high-performance sand core coatings are not used, it will not only cause serious sand sticking, but also Defects such as sand inclusion make it difficult to clean the sand, increase the workload of cleaning and polishing, seriously affect the quality of castings, and even cause them to be scrapped. It can also affect the internal quality of castings, resulting in defects such as porosity and porosity. Causing the castings to fail the airtightness test after processing, resulting in leakage during the compression test. In addition, sand sticking to castings can also increase tool wear during machining, and even damage the tool, increasing processing costs.

Water-based cast aluminum coating method

1. Dip coating method

At present, the most commonly used and convenient water-based aluminum casting coating is still immersion coating. That is to immerse the sand core or sand mold completely or partially in a tank containing paint, and after a short period of time, remove it from the tank and allow the excess paint to flow back into the tank. This method has high production efficiency, high coating quality, low material consumption, and no high requirements for operators.

2. Spray coating method

The use of compressed air and a spray gun to atomize the coating is characterized by uniform coating quality, which can overcome the phenomenon of brush marks during brushing. Moreover, the production efficiency is high (5-6 times faster than brushing), especially when spraying large areas of sand molds or cores, which is fast and effective and suitable for flow production lines.

3. Flow coating method

In recent years, the development of film coated sand molding production lines in China has been rapid, and the vast majority of them must be coated with paint. This makes coating an indispensable process after molding. As mentioned earlier, spraying has the advantages of high coating efficiency and few coating marks, but its shortcomings are thin coating and not suitable for complex sand cores with deep grooves. Flow coating refers to the ability of a coating to flow downwards under the influence of gravity, which controls the fluidity, leveling, droplet resistance, coating size, penetration depth, and coating uniformity. Through understanding the production site, the coating after flow coating is made uniform and smooth.

Performance and testing of water-based aluminum casting coatings

1. Drying and cracking resistance of water-based cast aluminum coatings

Add a certain amount of easily collapsible film coated sand to the standard breathable mold, and use SYC double wheel hammer type prototype to hammer three times. Then, place the mold together at 700 ℃ for 2 minutes of insulation. After cooling to room temperature, remove it and observe the coating surface with a magnifying glass without any cracks. After drying, the coating is off white and the surface is flat. Use your fingers to vigorously wipe off any powder.

2. The effect of water-based cast aluminum coating on air permeability

The dry crack resistance sample and the sample dried simultaneously without immersion in the coating were used to determine the effect of the coating layer on the permeability. The permeability test was conducted using a SYZ type direct reading permeability tester, and the results are shown in Table 1. It can be seen from Table 1 that different coatings have a significant impact on the permeability of the sand core. The water-based aluminum casting coating reduced the permeability of the sand core by 13%, and after two coats, it decreased by 65%.

Table 1 Effect of water-based cast aluminum coating layer on the permeability of sand core

After immersion coating, the air permeability of the working surface of the sand core decreases, which is particularly beneficial for improving the quality of castings using sand cores. Exhaust and achieving sequential solidification are the two key technologies of low-pressure metal mold casting, which cannot be effectively discharged. This not only forms backpressure in the upper part of the mold cavity, causing defects such as insufficient pouring and cold shut, but may also seriously affect heat transfer and cause local porosity defects, leading to casting scrap. After the sand mold is impregnated, the permeability of its working surface decreases, which is equivalent to a decrease in the amount of gas generated by the sand core heating entering the mold cavity. This gas will be discharged from the core head through the exhaust hole, significantly reducing the harmful effects of the gas.

3. The influence of water-based cast aluminum coating on the strength of coated sand

The production process of the strength specimen is the same as that of the crack resistance specimen mentioned above. After the specimen is cooled in the furnace to room temperature, it is taken out and subjected to strength testing on an SQY hydraulic strength testing machine. The results are shown in Table 2

Table 2 Effect of Coatings on the Strength of Coated Sand

From Table 2, it can be seen that the bending and tensile strength of the samples have been improved after using water-based aluminum casting coatings. In the actual low-pressure casting process, due to the filling of the metal liquid from bottom to top and the fixing effect of the core head, the erosion and buoyancy of the metal liquid flow are tangential forces on the core head. The improvement of bending strength is particularly beneficial for preventing core breakage and other issues.

4. Coating thickness and permeability

After tensile fracture, the sample was placed under a microscope for observation, and the coating thickness and penetration depth were measured. The results are shown in Table 3

Table 3 Coating Thickness and Penetration Layer Depth

Research has shown that water-based cast aluminum alloy coatings have good thermal insulation, moderate air permeability, good insulation performance (especially for thin aluminum alloy castings), and strong ability to adsorb liquid pyrolysis products. After using water-based aluminum casting coating, the quality of castings is significantly improved, and the number of castings that are scrapped due to defects such as insufficient pouring and pressure leakage is significantly reduced. The coating layer peels off in a sheet-like manner during sand cleaning, with good film-forming properties, making it easy to clean.