Research On Preparation Technique For High Strength Casting Aluminum Alloy Of Al-Si Series

Weight reduction is one of the key techniques for the development of high-speed train in China. Presently in our country, however, most of the key components of the high-speed train are made of iron and steel due to their large dimension and complex shape, which makes the train have high weight and limits the technical advancement of high-speed train. Therefore, it is of theoretical and practical significance to explore a kind of new lighter material with high strength to substitute the present iron and steel materials.For its low specific density and high specific strength, aluminum alloy is the optimum material for lightweight. Compared with others, the Al-Si series has been most widely applied, for it has excellent casting properties such as good flowing property, airtightness, low shrinkage ratio and heat cracking trend, and it can have good mechanical properties after modification and heat treatment as well as good corrosion resistant characteristic and machinability. However, the properties of existing Al-Si alloy cannot meet the requirement of key components for the high-speed train, and the traditional casting technique tends to make more casting defects.In this paper, preparation technique of the typical Al-Si alloy A356 are investigated without changing its chemical composition as well as the addition of any modifier and refiner. The material is prepared with vacuum adjustable pressure casting method. Meanwhile, the casting system and the choosing of pouring temperature have been improved, and the vacuum-stirring measurement is applied for degassing. Therefore, the melt fills the mould under vacuum condition and solidifies under adjustable pressure, reducing the harmful influence of gas to the minimal level. As a result, the material's density is greatly improved with the minimal porosity of 0.03%, and the cast tensile strength reaches 230MPa, which is nearly 50% higher than that of A356 alloy prepared with the traditional gravity pouring casting process.The heat treatment process of A356 alloy is also investigated and optimized in this paper. The result shows that the optimum heat treatment process is: solution treated at 545℃for 14 hours, aging treated at 160℃for 10 hours, and the quenching medium is hot water of 70℃～80℃. After T6 heat treatment, the tensile strength can be as high as 335MPa, which is 24% higher than that of traditional A356 alloy prepared by gravity pouring casting process. Furthermore, its ductility still keeps good with the elongation of 3%. Analysis indicates that density has a great influence on the mechanical properties of A356 aluminum alloy. The mechanical properties increase with the improvement of the material's density. Proper heat treatment process can greatly improve the microstructure of the material, so that the mechanical properties can be further improved.Based on the traditional Al-Si alloy A356, the high strength is realized through the improvement of the preparation technique. The study in this paper provides an important technical reference for the material choosing and manufacturing of key components for high-speed train with the purpose of lightweight.