Study On Preparation Of AlSi7Mg Alloy Semi-solid Billets By Melt Isothermal Treatment Process

Semisolid metal processing technology is regarded as one of the near-net shape forming technologies with wide application in 21 century, and the preparation of semi-solid slurry is the key technique. In this study, considering the character of the preparation of semi-solid slurry without stirring, we obtained AlSi7Mg alloy slurry through isothermal heat-treatment, and study influencing factor for the forming of nondendritic semi-solid slurry. Ultrasonic treatment technologies for preparation of semi-solid slurry are also studied in this literature, idealizal processing parameters are obtained in the experiments.Solidification structure transformation of AlSi7Mg alloy holding at different temperature was studied by permanent mould casting. Structure of primary a-Al transformed with temperature changing was studied at different temperature from 600℃ to 650℃ holding for 30min.The results show that: With decrease of temperature from 650℃ to 600℃ primary a-Al transformed from strong dendritic to rosette grainy. Small dimensional and nondirectional grainy primary a-Al can be obtained at 605℃ holding for 30min.When melt held under 605℃, primary a-Al leant to rosette. In addition, structural transformation of AlSi7Mg alloy held at different time at 605℃ was studied. The results show that: Primary a-Al transformed from irregular dendritic to nondirectional equiaxed grain then to rosette structure with holding time increased from 10min to 60min.Add grain refiner in the melt before isothermal heat-treatment had great influence, it may get the microstructure of alloy better and reduce the dimension of grain to nearly a half.Using grain refining and partial remelting prepared AlSi7Mg alloy semi-solid slurry, rule of structure transformation of undermodified and modified alloys during remelting were studied. .The results show that: primary a-Al in undermodified and modified alloys can be both changed from dendritic to grainy structure, while the transformation of modified alloy can be more easy, need less time and better nondendritic grain can be obtained. During the remelting of alloy, with the rising of temperature the changing of a-Al more quickly, the demention of grain more small;with the extension of time a-Al firstly change to grainy then to coaring.Using ultrasonic treatment technologies prepared semi-solid slurry of AlSi7Mg alloy. Infulences of processing parameters for sturucture transformation were studied in the experiments. The results show that: Ultrasonic treatment can hold back the growth of dendritic grain and promote the forming of globular grain. The effect of starting temperature of ultrasonic application is dramatic, the ideal starting temperature is near 680°C. The effect of time of ultrasonic application is also obviously, with the extension of time ot-Al with better microstructure can be obtained.