| In order to change netlike fibrous brittle silicon phases into dispersed fine particles in Al-Si alloy, pre-heat treatment before deformation is necessary for casting alloys. Equiaxed Al grains were observed besides silicon granulation after heat treatment in the DC casting. To explore whether or not the same phenomenon exits in the Al-Si alloy after chemical modification, by means of optical microscope, scanning electronic microscope and EBSD (Electron Backscatter Diffraction), the effects of Sr modification on solidification behavior and solidification structures of the Al-7Si and Al-12.6Si are studied. The evolution of solidification structures of different conditions during heat treatment is also studied. The main conclusions as followed:1. The nucleation temperature and platform temperature of eutectics decrease with the increase of Sr addition. The effect of Sr on the eutectic silicon is the best hen up to500ppm. At this condition, the morphology of eutectic transformed from needle-like to fibrous, worm-like or spot-like.2. Modification treatment is benefit to the granulation of eutectic Si in Al-7Si and Al-12.6Si alloy when they are heat treatment at500℃. Compared to the alloy without modification, eutectic silicon in well-modified alloy is much more granulated and dispersed in the matrix after heat treatment at500℃for4h, with an average size of2-3μm.3. Grain boundary is observed clearly in Al matrix by optical microscope as silicon particles were granulated. After modification, not only the size of grains in tested alloy decreases significantly, but also its morphology transforms into equiaxed.4. EBSD analysis shows that the percentage of small-angle grain boundary in the Al matrix of unmodified Al-12.6Si alloy decreases with the extension of heat treatment time. However, the percentage of small-angle grain boundary in the Al matrix of modified Al-12.6Si alloy increases firstly and then decreases with the extension of heat treatment time. |
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