Effects Of Solution Treatment On Microstructure And Mechanical Properties Of Thixoformed Mg₂Si/AM60B Composite

This thesis studied the microstructure and mechanical properties of thixoformed Mg2Si/AM60B composite, especially, the effect of solution treatment on the microstructure and mechanical properties was investigated, The solution behavior was compared with those of permanent mold casting of Mg2Si/AM60B composite and thixoformed AM60B alloy. The results show that the microstructure of thixoformed Mg2Si/AM60B composite is characterized by firstly solidified structures (globularly primary a particles and globularly Mg2Si particles) and secondarily solidified structures (secondarily primary a grains and eutectic structure). The primary a particles appear in three shapes, the a particles halos due to stable growth of secondarily primary a phase on their surfaces, the a particles with gear-like or cellular secondarily primary a phase of unstable growth.The solution treatment process of thixoformed Mg2Si/AM60B composite can be divided into two stages. The first stage is the rapid coarsening of the primary a particles and secondary grains, and the dissolution of eutectic (3phase (0-2h). In this stage, the high concentration gradient around the primary a particles leads the interface to migrate, which makes the primary particles and secondary grains to coarsen. The coalescence between the secondary grains mainly relies on the dissolution of eutectic β with poor thermal stability. Hardness decreases rapidly, ultimate tensile strength and elongation increase quickly during this stage. And then the second stage occurs. The second stage is the normal growth of the primary particles into polygon particles and secondarily solidified grains and the complete dissolution of eutectic P phase (2h-36h). The coalescence between the secondary grains mainly relies on the interfacial energy. As the concentration homogenizes and concentration gradient decreases, the growth rate is greatly reduced. In this stage, the hardness decreases, ultimate tensile strength and elongation increase slowly. The ultimate tensile strength of271MPa and elongation of16.7%can be obtained after being solutionized at415℃for6h. The weak bonding force strength between grains makes the mechanical properties to get worse.The Mg2Si particles hindered the solute diffusion, and thus make the solution treatment progress of the thixoformed Mg2Si/AM60B composite to be slower than that thixoformed AM60B alloy. The effect of short rodlike Mg2Si particles in the permanent mold casting Mg2Si/AM60B composite is higher than that of the globular Mg2Si paticles in the thixoformed Mg2Si/AM60B composite. The eutectic β particles in the permanent mold casting Mg2Si/AM60B composite are larger and have lower interfacial energy, so its solution treatment progress is slower than that of the thixoformed Mg2Si/AM60B composite. The higher the solution treatment is the easier the atom diffusion is. Therefore, the solution treatment progress is significantly accelerated as the temperature rises.