Characteristics Of Laser Brazing Si_p/LD11 Composites And Crystallization Behavior Of Si

Silicon particles reinforced aluminum matrix composites is a new type of composite materials. With advantages of light weight, low expansion, high thermal conductivity, environmental protection and good recyclability, it has broad application prospects in the aviation, the spaceflight, the electronic packing and other fields. However, as it has a large proportion of silicon particles in this kind of materials, which leads to a high content of silicon in the molten pool once melted, there remain a lot of shrinkages and very coarse lath-shaped Si in the material after the solidification, deteriorating the performances of the material significantly and losing the application values.In this paper, the welding characteristics of silicon particle reinforced aluminum matrix composites were investigated systematically, using the laser brazing filler wire technology, with the 4047 Al-Si (12% silicon content) flux cored wire as a filling material. First, the types of the laser spot and the groove, both of which are particularly important to the laser soldering process, were established. The effects of various processing parameters on the forming and organizations were studied through experiments, considering in the perspective of the surface forming and the uniformity of structure morphology. Optimized parameters, and the stable joints with smoothing surfaces and a high content of reinforcements in the weld, were obtained.The dissolution and diffusion of silicon during the welding has significant impact on its crystallization behavior and the structure of the material. The distributions, existing forms and structure characteristics of Si in different areas of the welds under various processing conditions were studied in this paper, based on the analysis of the dissolution, diffusion and crystallization behaviors of Si. The results show that the silicon exists mainly in two forms, namely, primary silicon and eutectic silicon, in the welds. Granular primary silicon dispersively distributes in the weld. The eutectic silicon in different locations shows different morphologies, for instance, the slender needle-like dendrites in the interfaces and the small short rods in the weld center. Studies find that the short rods of the eutectic silicon grow up in the way of eutectic clusters. Finally, the heat treatment (550°C, holding for 8h, with furnace cooling) were carried out, to improve the primary silicon's sharp corner and the morphology of the needle eutectic silicon to prevent them from splitting the matrix and deteriorating performances. The results show that both the needle and short rod eutectic silicon have broken and assembled into granules, and the sharp-cornered primary silicon has also been passivated, making the structure of the weld uniform.