| In recent years, with the rapid development of the automobile industry, aluminum alloy has been regarded as the best substitute for steel for weight reduction, which has gained its popularity in application. Die casting aluminum alloy plays an important role in auto parts for its high surface quality, good mechanical property and high material utilization. The development of high plastic die casting aluminum alloys has promoted the application of large aluminum alloy castings in automobile, such as the Aural-2(Al-10Si-0.3Mg) alloy.The thesis studied the vacuum die casting Al-10Si-0.3Mg plates with thickness of 2.5mm and 5.0mm. At present, vacuum die casting Al-10Si-0.3Mg has been used to fabricate auto parts of large thin-wall castings with complex geometry, whose wall thickness distribution is inhomogeneous. Therefore, the microstructures and mechanical properties of the alloys with different thicknesses were firstly investigated. Then, in order to improve the tensile properties, especially ductility, of the alloys, the vacuum die casting Al-10Si-0.3Mg alloy has been heat treated. Through experimental methods such as microscopy(OM), scanning electron microscopy(SEM), transmission electron microscopy(TEM) and tensile test, the heat treatment microstructure, tensile properties of the studied alloys were investigated. Furthermore, the influence of microstructure features on the fracture behavior were also discussed. The conclusions are summarized as follows:(1) The microstructure distributions along thickness of plates with thickness of 2.5mm and 5.0mm are inhomogeneous. The microstructure in 2.5mm plate changes from surface to core gradually,, while the microstructure in 5.0mm plate presents as ?sandwich structure?, which consists of two surface zones, a core zone. With the increase of cooling rate, the microstructure gets finer. The area ratio of fine grain in surface zone in 5mm plate is 35%, while that in 2.5mm plates is about 40%. Additionally, the eutectic silicon in 2.5mm plate is finer than in 5mm plate. The hardness, tensile strength, yield strength and elongation of 2.5mm as-cast plate are 106.9HV ? 303.1MPa ? 164.2MPa ? 7.6%, respectively. The hardness, tensile strength,yield strength and elongation of 5.0 mm as-cast plate are 91HV?227.4MPa?115.3MPa?5.8%.(2) The content of Mg and Si in the matrix increased after solid solution treatment. With solid solution treatment, eutectic silicon particles were transferred from fibrous to spherical/granular. After aging treatment, the precipitates in the 2.5mm alloy were obtained, which mainly consists of fine needle ?? phase, rod shape ?' phase and Si particles. The diameter and length of ?? phase are about 10-18 nm and 200-800 nm, respectively, while the diameter and length of the ?' phase are about 20-38 nm and 600-1400 nm, respectively. Both of them are distributed along <001> direction in the matrix.(3) The mechanical property of alloy was significantly improved through artificial aging. For both of these two kinks of plates, their heat treatment parameters were optimized. After 450 ? /1.5h+225 ? /1h heat treatment, the tensile strength, yield strength and elongation of 2.5 mm plate are 212.2MPa?113.1MPa?16% respectively. With 450?/1.5h+225?/2h heat treatment, the tensile strength,yield strength and elongation of 5.0 mm alloy are 184.5MPa?118.7MPa?8.73%.(4) According to the analysis of appearance and microstructure of the area near the fracture surface, the model of fracture in the alloy that after heat treatment is ductile fracture. The size and orientation of eutectic silicon particle affected the tensile property of the alloy. During of the tensile process, there are the crack of the second phase, the initiation and propagation of the microcrack and the connection of the microcracks. The way the alloy fractures is the combination of intergranular fracture and trans-granular fracture. |
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