Microstructure and Hardness Analysis of Laser Welded A357 Semi-Solid Cast Alloy

A357.0 alloy was casted by using RheoMetal™ process, and then welded using laser deep penetration welding. After welding, the specimens were subjected to microstructural characterization and microhardness testing. The results indicate that a surface liquid segregation (SLS) layer enriched of alloying elements was formed on the surface, resulting in an increase of Si content in molten pool compared to that in the ingot. The microstructure of the molten pool is mainly composed of fine equiaxed and columnar crystals. A small amount of spherical α ₁ -Al phases were found in the boundary of the molten pool. The eutectic region in the melt pool is mainly composed of Si phases, and small amount of MgSi, and AlFeMgSi phases. Compared to the matrix, the content of Mg and Fe in the molten pool has significantly decreased, which may be the result of burning loss of Mg and Fe under the high laser energy. The hardness testing of the samples in this study showed that the hardness of the heat affected zone (HAZ) was significantly higher than that of the melt pool and SLS layer. The hardness of equiaxed crystals in the molten pool is slightly higher than that of columnar crystals.