Influence Of Friction Stir Processing On The Microstructure Of Semi-solid Billets Of Al-Si-Fe Alloy

Under the conventional casting, hypereutectic Al-Si-Fe alloy contains coarse primary Si and needle-like Fe-rich phases (Al,FeSi), and prones to defects. The friction stir processing (FSP) can signifiantly refine the hard and brittle phase; the semi-solid thixoforming has low temperature, less casting defects, and improve the mechanical properties of the thixo-formed parts. Therefore, it was combined FSP with Semi-solid remelting to prepare hypereutectic Al-Si-Fe alloy semi-solid thixoforming blank. The law of refinement of hard and brittrle phase by FSP, and the formation of spherical a-Al phase, shape and size evolution mechanism, as well as the second hard and brittle phase coarsening dynamics in semi-solid remelting process have been carefully studied. We can draw the following conclusions:The hypereutectic Al-Si-Fe alloy was stirred at different speeds (750r/min、1050r/min、1500r/min) in the FSP processing, after that the second phase in the cast structure has been significantly refined. With the speed increasing, the average particle size from3.2μm to3.8μm, the refinement effect was slightly diminished. When the hypereutectic Al-Si-Fe alloy was stirred at750r/min respectively a time, two times and three times, the average particle size from3.2μm to2.6μm, the refinement effect was slightly enhanced.After the FSP refinement, the spherical a-Al phase would precipitate as long as it was held for a period of time in semi-solid remelting process. The formation and evolution of spherical a-Al phase:firstly, liquid membrane formed from the Al grain boundaries undergoing eutectic melting; more liquid was achieved as the temperature rise or the time prolonged, and Al grain surface gradually dissolved to form free spherical a-Al; Moreover, as the temperature increased or the holding time prolonged further, the shape of spherical a-Al phase became clear and round with the immigration of grain boundaries.The grain size of Si phase increased sharply, the morphology became clustering and the pointedness becomes more clearly with the time prolonged in the semi-solid remelting process. Moreover, the coarsening process of Si phase follows the LSW theory:dn-d0n=kt. If the stirring speed was750r/min, the index of coarsening is0.83. when the speed increases to1050r/min, the index also increases to0.99. When the speed is1500r/min, the index increased to1.03. As the FSP stirring speed increases, the growth rate of Si phase is relatively slow in semi-solid remelting process. When the stirring speed is750r/min, the grain growth activation energy of Si phase is638kJ/mol; the energy is665kJ/mol in the case of1050r/min, the energy increasing as the speed increasing; when the speed is1500r/min, the energy also increases to707kJ/mol. With the rise of the FSP speed, the activation energy of the coarsening process of Si phase increases. So the effect of FSP on the activation energy of the coarsening process of Si phase gradually decreases in semi-solid remelting process.In semi-solid remelting process, Fe-rich phase is coarsening with the time prolonged. Moreover, the coarsening process also follows the LSW theory. Without the FSP processing, the coarsening exponent n of Fe-rich phase is7.7, but the coarsening exponent n is1.45after the FSP processing. Obviously, after the FSP,the coarsening rate of iron-rich phase increases.The investigation on the evolution of spherical a-Al phase and the coarsening process of the hard and brittle phase in hypereutectic Al-Si-Fe alloy provids theoretical foundation to achieve microstructures with a certain amount of a-Al phase and fined brittle secondary phases. Meanwhile, this study provides a theoretical basis and optimization measures for hypereutectic Al-Si-Fe alloy semi-solid near net forming and micro structure controling, also a reference for shaping and organizational controling of other materials with hard and brittle phase. So it has important scientific and practical value.