With the continuous development of industrial technology and production technology, and the intensification of competition in large industrial markets at home and abroad, especially since China's accession to the WTO, the requirements of the quality of castings, especially the surface quality of castings, have become higher and higher in the machine building industry and engineering industry. Casting coating refers to a coating that is applied to the surface of the mold cavity or core to improve its surface fire resistance, chemical stability, resistance to metal erosion, and sand sticking resistance. Casting coatings are closely related to the surface quality of castings. This article discusses the effects of casting coatings on the surface quality of castings from the aspects of the role and performance of casting coatings.
First, the role of coatings
1. Reduce the mechanical and chemical sticking of the casting surface
The mold and core have many pores. During the casting and solidification process, the metal liquid with static pressure and dynamic pressure will penetrate the pores, forming a metal sand shell that is difficult to clean on the surface of the casting. It is called "mechanical sand sticking." ". The application of casting coating can close the pores between the sand particles of the mold and the surface layer of the core, block the passage of the metal liquid, and reduce the mechanical sand sticking of the casting. At the temperature of the molten steel being cast or lower, a metal oxide film is continuously formed on the surface. This metal oxide film can chemically react with silica sand, resulting in "chemical sand sticking" on the surface of the casting. The use of casting coating can isolate the metal liquid from the surface of the mold or core, inhibit the chemical reaction between them, and reduce or eliminate the chemical sticky sand on the surface of the casting.
2. Reduce sand inclusion and sand blasting on casting surface
During the casting process, the high-temperature metal liquid has a strong heat radiation effect on the surface of the mold and core. The thermal pressure and hot wet tensile strength generated by the mold and core after heating will cause sand inclusions in the casting. Casting coatings can slow the radiant heating of the mold or core, thereby reducing or eliminating the occurrence of sand inclusion defects. Coatings with a certain bonding ability can also penetrate between the mold and core surface sand particles, thereby enhancing the surface strength and erosion resistance of the mold or core, and reducing the sand blasting defects of the casting. 铝合金重力，浇铸 铝合金低压。 Aluminum alloy liquid extrusion , aluminum alloy gravity, cast aluminum alloy low pressure.
3.Improve the surface properties and internal quality of castings
By adding thermal insulation materials and chilling materials to the coating, it can improve the temperature distribution of the mold cavity and control the solidification and crystallization process of the alloy, thereby reducing the occurrence of cold cracking and hot cracking on the casting surface. The addition of certain inoculants or alloying elements in the coating can also play a role in local inoculation or surface alloying, thereby improving the metallurgical properties of the casting.
Second, the performance of coatings
During the storage or use of the coating, the solid particles should be suspended as much as possible, without delamination, precipitation, or agglomeration, to ensure uniform coating performance and coating quality.
The performance of the paint with good brushability is: the brush pen is smooth, non-sticky, and does not bring up sand particles when the paint is dipped in paint. Loss of paint.
High-quality coatings should have the property of penetrating into the mold and core at an appropriate depth. Generally, the penetration depth is 2 to 3 times the diameter of the sand particles. The penetration of the coating into the sand pores can also enhance the affinity between the sand particles. 4. Surface strength
The ability of the coating to be scratched by external force after curing, leaving no traces and particles after rubbing is called the surface strength of the coating. Sufficient surface strength can prevent mold damage during handling, lowering the core and closing the box. Good coatings should also have good properties such as sand resistance, crack resistance, preservation, and small gassing properties.
Third, the principle that casting coatings affect the surface quality of castings
1. Gas model and carbon film isolation
Graphite powder coating has good peelability when used in cast iron parts.
Graphite generates carbon monoxide gas film and bright carbon film during casting, solidification and cooling, preventing the metal liquid from interacting with the surface of the sand mold or core, and preventing sand sticking. On the surface of the wet cavity, a coating made by mixing waste engine oil with graphite powder and spraying it, and relying on the reduction gas film and carbon film produced by the high temperature casting, can obtain thin-walled cast iron with an abnormally smooth surface.
2. Slag isolation
After the casting, a slag layer is formed on the interface between the coating layer and the casting. The slag layer is formed with a relatively high viscosity, which blocks the refractory aggregates and blocks the penetration of the molten metal. The slag layer has the best effect when it has no chemical reaction with the metal liquid without silicate, and it does not wet with the metal liquid. During cooling, the shrinkage coefficient of the slag shell layer and the casting metal wire is greatly different. A large shear stress is generated at the interface between the slag shell layer and the casting, which causes the coating shell to peel automatically. The theory is called "slag isolation theory". The coatings are formulated with easily reactive metals and metal oxides, and undergo a thermal reaction to form a new refractory oxide, which can prevent the penetration of molten metal. 铝合金重力，浇铸 铝合金低压。 Aluminum alloy liquid extrusion , aluminum alloy gravity, cast aluminum alloy low pressure.
The theory believes that whether the casting sticks to sand depends on whether a critical thickness of iron oxide layer exists between the casting and the coating layer, and its thickness is about 100 μm. In order to prevent chemical sand sticking, the oxide thickness should be increased. When it exceeds the critical thickness, the coating or the sintered layer of the molding sand and the metal oxide is easily peeled from the casting. Limestone sand molds have the following reactions when casting steel parts: CaCO3 = CaO + CO2, which results in a strong oxidizing atmosphere inside the mold cavity, which intensifies the surface oxidation of the steel parts. Therefore, the limestone sand has an excellent anti-sticking effect.
The coating with good effect on the casting of steel castings is easy to stick to the sand of the cast iron, or even sticky, which can be explained by the easy formation of iron oxide layer on the surface of the casting. Cast iron has a high carbon content, and the carbon element oxidizes before iron. Therefore, it is difficult to form an iron oxide layer that reaches a critical thickness when casting cast iron. Therefore, the "oxidation" theory is not suitable for anti-stick sand coatings of cast iron parts.