The Research On Microstructure And Mechanical Properties Of SiC_p/6061Al Matrix Composite Prepared By Powder Thixoforming

A new processing technology, powder thixoforming, for preparing SiC particle reinforced Al matrix composites (SiCp/Al) has been proposed. Ingots composed by SiCp and Al alloy powder are first obtained by the blending and pressing steps of powder metallurgy (PM), then the ingots are partially remelted and thixoformed. This technology combines the advantages of both powder metallurgy and thixoforming technologies. It can be expected that SiCp/Al composite components with uniform distribution of SiCp and low or without pores can be obtained through using this technology. In order to obtain SiCp/6061Al matrix composite ingots with uniform distribution of SiCp, the parameters of powder mixing, consolidation and partial remelting have been studied in this paper. Furthermore, the effects processing parameters on microstructures and mechanical properties of powder-thixoforged6061aluminum alloy and SiCp/6061Al matrix composite also have been investigated.The study of microstructural evolution during partial remelting of6061bulk alloy prepared by cold-pressing of the atomized alloy powders indicates that an ideal semisolid microstructure with small (about18μm) and spheroidal primary particles can be obtained. The microstructural evolution can be divided into three stages, the dissolution of eutectic phases and the resulted grain coarsening and growth, the structural separation and spheroidization of primary phase, and the final coarsening behavior of primary particles. Compared with the not milled alloy, the ball milling accelerates the first stage of microstructural evolution due to the large energy stored in the powders from ball milling, but the latter two stages are slowed down because of the formation of large-sized powders. In addition, the microstructural evolution of SiCp/6061Al matrix composite can also be divided into the same three stages to that of the6061alloy. However, SiCp can hinder the heat transfer and the diffusion of solute atoms. So the evolution process is slowed down and a semisolid microstructure with finer primary particle is obtained. Furthermore, proper elevating the heating temperature is beneficial for obtaining small and spheroidal primary particles, but too much liquid phase results in the lost of the thixoforming advantages.In addition, the effects of three processing parameters, such as reheating time, mould temperature and reheating temperature, on microstructures and mechanical properties of powder-thixoforged6061aluminum alloy and SiCp/6061Al matrix composite also have been investigated. The results reveal that these three parameters all have large effects on the microstructure and mechanical properties. Different forming parameters result in different microstructures, and thus different mechanical properties and fracture mechanism of the components. In order to obtain a high performance thixoforged component, a compact and uniform microstructure should be obtained as far as possible through adjusting the processing parameters. The ultimate tensile strength (UTS), elongation and hardness of the resulting6061alloy are up to196Mpa,11.0%and55.2HV respectively under the processing conditions, reheating at660℃for80min and the mould temperature of350℃. Because of the strengthening effect of SiCp, the UTS and hardness of the component are elevated with the increase of the volume fraction of SiCp. The UTS, elongation and hardness of the resulting10%SiCP/6061Al matrix composite are up to228Mpa,5.3%and66.4HV respectively under the processing conditions, reheating at660℃for90min and the mould temperature of250℃. That is the UTS and elongation of the composite are obviously higher than those of the alloy, but the elongation is significantly decreased.