Evolution Of Solidification Microstructure Of High Purity Al-si Alloys During Heat Treatment

Pre-heat treatment is needed to be applied on casting alloys in order to change fibrous netlike brittle silicon phases to dispersed fine particles. We have observed equiaxed Al grains in the DC casting microstructures during heat treatment and thermal deformation besides silicon spheroidization. Equiaxed Al grains can be related with heat treatment and deformation respectively. In addition, this phenomenon may be related with the formation of eutectic during solidification. In this work, by means of optical microscope, scanning electronic microscope and EBSD (Electron Backscatter Diffraction), the nucleating temperature, solidified microstructures and evolution of solidification microstructure of high-purity hypoeutectic Al-7Si alloy and eutectic Al-12.7Si alloy with and without Sr modification by ingot metallurgy during heat treatment was studied. The following conclusions are obtained:1. Well-modification of Al-Si alloys can not be estimated by the characteristic values of cooling curves of Al-Si alloys. After modification, the eutectic growth temperature is depressed compared with unmodified alloys. When Sr content is not below 110ppm, Al-7Si alloys are well-modified; when Sr content is not below 220ppm, Al-12.7Si alloys are well-modified.2. After heat treatment at 500℃for different holding time, eutectic silicon was broken and spheroidized, however, coarseness did not happen. The most eutectic silicon morphology of unmodified alloys at 500℃for 6h holding time is short-pole, and less is granular. It is not well spheroidized. The eutectic silicon morphology of well-modified alloys is gradually spheroidized with holding time increasing. It is well spheroidized after heat treatment at 500℃for 6h holding time. The size of eutectic silicon particles is 2-3μm.3. As Si was spheroidized, we also observed clear boundary grains by optical microscope. It could be seen only in eutectic Al and did not appear inα(Al) dendrites. The more densely eutectic Si particles distribute in eutectic Al, the more densely grains distribute in the same area, and the finer grain sizes are. The grains in unmodified alloys are large and the scale of grain size is 10-30μm; the grains in well-modified alloys are almost equiaxed and the scale of grain size is 3-8μm. The microstructure change of well-modified alloys during heat treatment proved that the evolution of eutectic of chill modification and chemical modification during heat treatment has the same principle. By EBSD map, we found the reason of the grain boundary appearing in eutectic Al. To well-modified Al-12.7Si alloys, during heat treatment, grain boundary has polygonization effect to generate nearly equiaxed fine grain with Si spheroidization.