Solidification Morphology Of Semi-solid Aluminum Alloy-rare Earth Primary α Phase In Chaotic Convection

Although development period of the semi-solid technology has a few decades old, it has broad application prospects and unique technological advantages, so that developes rapidly inland and abroad. One of the key semi-solid forming technologies is semi-solid alloy melt preparation technology. The transfer of alloying elements, the grain growth, the distribution temperature field and the final alloy microstructure which are influenced by transmission of the melt flow, above them would have a great impact on the availability of qualified semi-solid slurry. Combined chaotic theory with semi-solid technology in this paper, the effect of chaotic characteristics of semi-solid melt on semi-solid melt solidified microstructures was studied.Combined experiments with simulations, the trajectories of the melt particles and melt temperature were simulated in the analogue mold under the electromagnetic filed on Fluent software MHD modules and standard k-ε model. When the electromagnetic stirring paremeter was 30Hz and 15s, the melt temperature distribution was the most uniform, and its paricle trajectories were most complex.The electromagnetic stirring time and stirring frequency were two important factors which influenced the degree of chaos in the solidification system through the calculation and judgment of chaotic characteristics:Lyapunov exponent, Fractal dimension and Kolmogorove entropy. The chaotic advection occured in the melt under the appropriate electromagnetic stirring. The results of Fractal dimension and Kolmogorov entropy would increase by the increasing frequency and extend stirring time, so the phenomenon of chaos is not essential chaotic, its interior still had a controllable factor. When the electromagnetic stirring paremeter was 30Hz and 15s, the chaotic degeree was the greatest inside the melt; meanwhile, the results of Fractal dimension and Kolmogorove entropy was 2.240 and 6722.4nat/s, respectively. Using A356 as the matrix raw material, through the chaotic advection that induced by low electromagnetic stirring frequency, the effect of different electromagnetic stirring frequency and different rare-earth elements La and Yb on primary a phase of semi-solid A356 aluminum was studied. When the electromagnetic stirring paremeter was 30Hz and 15s, the solidified microstructure achieved the best under low superheat casting. Meanwhile, the average equal-area circle diameter and average shape factor of semi-solid A356-Yb aluminum was 60.28μm and 0.78; and the average equal-area circle diameter and average shape factor of semi-solid A356-La aluminum was 65.36μm and 0.75. Obviously, the primary a phase of A356-Yb was better than that in A356-La which influenced by chaotic advection.Reusing the electromagnetic stirring paremeter 30Hz and 15s, and adding different proportions of binary rare-earth element La and Yb, the laws and effect factors of solidified microstructure on different rare-earth elements were studied under the chaotic advection. When the addition amount of the binary RE were 0.8%La+0.5%Yb, the morphology of the primary phase could be observed with the best parameters, the average equal-area circle diameter and average shape factor of it was 62.47μm and 0.72, respectively.The innovation of this paper are using the chaotic theory to systematically explain diffusion and distribution characteristics of rare-earth, and the behavior of primary a phase morphology evolution and theory under the chaotic advection induced by electromagnetic field.This theory and method is guided to explore new technology of high performance semi-solid aluminum and efficient economical controlling theory.