With the rapid development of China’s automotive engine industry and the demand for vehicle lightweighting, the extensive application of aluminum alloys in the sedan and engine industries has become an important development trend, and castings are gradually becoming lighter, more precise, stronger, and more composite. The proportion of aluminum castings in engine castings is increasing, especially aluminum alloy cylinder blocks and cylinders The use of covering in car engines. However, due to the complex structure and process of aluminum alloy cylinder heads, casting defects are prone to occur. For example, pinhole defects are common in low-pressure casting aluminum alloy cylinder head castings. This article combines the author’s experience accumulated in the production practice of aluminum alloy castings to discuss the occurrence and prevention of pinhole defects in aluminum alloy cylinder head castings.
The generation of pinholes in cast aluminum alloys
Pinholes are formed during the solidification process of aluminum alloys, where gases dissolved in the aluminum melt escape without completely precipitating
Caused by the incident. Aluminum alloys absorb a large amount of hydrogen gas during melting and pouring, and continuously precipitate due to a decrease in solubility during cooling. The solubility of hydrogen dissolved in aluminum alloy increases with the increase of alloy liquid temperature and decreases with the decrease of temperature. When it transitions from liquid to solid state, the solubility of hydrogen in aluminum alloy decreases by 19 times. Therefore, during the cooling and solidification process of aluminum alloy liquid, when the hydrogen content exceeds its solubility, it will precipitate in the form of bubbles. Hydrogen bubbles formed due to supersaturated hydrogen evolution, if they do not converge or precipitate in time, will form small and dispersed pores during the solidification process, commonly known as pinholes. When melting and pouring aluminum alloys in an atmosphere with high relative humidity, pinholes in the castings are particularly severe. This is why the dry season produces fewer pinhole defects in aluminum alloy castings than the rainy and humid season, and it is also the main reason for the higher proportion of pinhole defects in aluminum alloy castings in Chongqing. For aluminum alloys, if the crystallization temperature range is large, Then a network of pinholes will be generated. This is because under general casting production conditions, castings have a wide range of solidification temperatures, making it easy for aluminum alloys to form developed dendritic crystals. In the later stage of solidification, the residual aluminum liquid in the gaps of dendritic crystals may be isolated from each other and exist in approximately enclosed small spaces. Due to their relatively low external atmospheric pressure and static pressure of the alloy liquid, when the residual aluminum liquid further cools and shrinks, a certain degree of vacuum can be formed, which can cause supersaturated hydrogen gas in the alloy to precipitate and form pinholes.
The number and size of holes within 1cm2 on the machined surface of cast aluminum alloy are called pinhole degree. The influence of pinholes on the properties of aluminum alloys is mainly manifested in the decrease of casting microstructure density and mechanical properties. Therefore, in the production practice of aluminum alloy castings, it is very important to strengthen the research on the influence of porosity grade on mechanical properties and ensure the quality of aluminum alloy castings by controlling the pinhole grade.
The formation of pores in aluminum alloys is due to the gas absorption of aluminum alloys, but molecular gases generally cannot dissolve in the alloy liquid. Only when gas molecules decompose into active atoms can they dissolve. During the aluminum alloy melting process, refractory materials, metal furnace materials, fluxes, and tools in contact with gases are all introduced A certain amount of gas. In addition, newly built furnace linings, refractory materials for furnaces, crucibles, etc. usually require several days or weeks of use for the chemically bound hydrogen to be fully released from the binder. Generally speaking, the composition of furnace gas is determined by the type of fuel and the amount of air.. Aluminum alloy castings are prone to pinhole defects The sinking is not only related to the characteristics of aluminum alloy itself, but also to a series of external factors. To avoid or reduce pinholes in aluminum alloys and ensure excellent quality of aluminum alloy castings, appropriate preventive measures can be taken accordingly. Preheat furnace materials, melting equipment, and tools. Crucibles, ingot molds, and melting tools, Before use, the surface should be cleaned of oil stains, dirt, etc., coated with protective paint, and preheated to remove adsorbed moisture and other chemicals. Strengthen preventive measures for melting during humid seasons. When casting aluminum alloys during rainy or humid seasons, special attention should be paid to taking degassing protection measures. Preheating treatment should be strictly carried out according to specifications for melting tools, ingot molds, crucibles, and furnace materials to prevent the generation of various pinholes caused by excessive moisture and oil stains. At present, the main method of degassing is to produce a large amount of gas by nitrogen or inert gas in aluminum alloy. Using the principle of partial pressure, hydrogen atoms dissolved in the aluminum liquid diffuse towards the bubbles. When the bubbles float to the surface of the aluminum liquid, they rupture and hydrogen gas escapes into the atmosphere, ultimately achieving the goal of removing hydrogen gas. Adopt casting process for degassing. We have implemented a series of improvements to the aluminum alloy cylinder head mold and casting process.
Conclusion
A detailed analysis was conducted on the main factors causing porosity in aluminum alloy cylinder head castings, and a series of targeted preventive measures were taken for our company’s aluminum alloy cylinder head products in actual production. Ultimately, the occurrence of pinholes on the bottom surface of the castings was effectively prevented, resulting in high-quality castings with significantly improved casting quality and significantly reduced production costs. It is expected to save economic value for the factory every year as follows: 1) Prior to improvement, the scrap rate of pinholes on the bottom surface of aluminum alloy cylinder heads was as high as 18%; 2) Improved situation: The pinhole defect on the bottom surface of the aluminum alloy cylinder head has been reduced to within 3%.