Preparation Of Semi-solid Billet And Reheating Of Hypoeutectic Al-Si Alloy

Semi-solid metal forming (SSF) has become the focus of studies on metal forming techniques in recent years. The components obtained by using this technique have not only high dimension precision and good surface quality, but also good properties. SSF of aluminum alloys is necessary in both automobile industry and space industry. The production of non-dendritic semi-solid billet and reheating control structure of alloy to obtain billets suited to SSF are the key techniques in the SSF.In this paper, liquidus casting and solid-liquid mixing casting were applied to obtain billets suited to SSF, and the influence of different processes to microstructure of alloy have been studied. Semi-solid billet was been reheating with different conditions, and the samples after reheating have been maken metallic phase observation and hardness test. The sample with high hardness and fine microstructure was selected to carry out the tensile testing. The fracture morphology was then observed under scaning electron microscope (SEM).As the pouring temperature decreased, microstructure of test bar changed from dendritic grains to the mixture of rosaceous and spheric grains, while spheric grains were obtained at liquidus temperature. However, abnormal big grains were also obtained during liquidus pouring. By liquidus pouring, the microstructure was not obvious changed with different moulds. SSF billets of Al-7wt.%Si alloy were obtained by putting modified Al-20wt.%Si alloy billet into hypoeutectic Al-6wt.%Si alloy melt at different temperatures. Hardness of these samples is higher than normal Al-7wt.%Si sample.At conventional reheating, the main reason of alloy structures variation was partial melting and coarsening of primary aluminium phase. And the size of primary aluminium phase increased with increasing heating temperature and heating time. The microstructure of those reheating at 590℃and 600℃were not obvious changed, but it was obvious changed at 610℃. Especially, at 610℃and 80μm/s, a rate from cooling zone into high temperature zone, primary a grains jointed together, and silicon particles precipitated in a grains. It had higher composite mechanical property: hardness was higher than normal Al-7wt.%Si sample, whileσ_b=224MPa, 8=8.48%,ψ=20.6%.