Multiscale Simulation And Optimization Design Of Semi-Solid Slurry Processing For Aluminum Alloy

Semi-solid metal process including rheocasting and thixoforming has been paid much attention because of its fine characteristics such as high efficiency, high performance, low cost, energy conservation and environmental protection etc. Thereinto, thixoforming has well developed and widely applied in industry. Near liquidus casting is a simple and efficient technique to make semisolid slurry for thixoforming. In fact, not all alloys are suitable for semisolid metal forming. In addition, different technical parameters will lead to the different performance of slurry. In order to achieve fine performance for forming and application, a lot of experiments have been made in the past near 40 years, which wasted much manpower and resource. Therefore, computer simulation became a way to study the semi-solid metal forming. In the aid of the National Natural Science Foundation of China, this paper studied the processing of semi-solid slurry of aluminum alloys for thixoforming. A new method is presented in this paper for semi-solid alloy design and technique parameter optimization by multiscale simulation.In the paper, theoretical models for the thermal field, concentration field, nucleation and growth of liquid-solid phase transformation were established concerning the semicontinuous casting process. The change of solid fraction was used to couple calculations on macroscale with mesoscale. The solidification microstructures of ZL201 and A356 alloys were simulated by multiscale simulation method under different conditions. The simulated morphology and size of the grains are consistent with the experimental results, which verifies that the established models are correct.The thermal field for ZL201 alloy in the process of semicontinuous casting was made under different pouring temperature, casting velocity and cooling intensity. The microstructural evolution of ZL201 was simulated for the casting at velocity 2.0 mm/s, pouring temperature 922K and the heat exchanging coefficient 1000 W/(m2.K). The manner of grain growth and the evolvement of solute concentration field were gained by multiscal simulation. The simulated results are consistent with that of Dustin-Kurz, Rappaz-Thevoz and Kanetkar-Stefanescu model.The phase evolvement in Al-7wt%Si and Al-10wt%Si were simulated under the conditions that casting velocity is 2.0 mm/s, pouring temperature is 5K above liquidus, the heat exchanging coefficient of cooling water is 1000 W/(m2K). The grain grows dendritically in the process of solidification. The concentration around the dendrite turns high because of the solute discharged from the growing grain. Eutectic phase comes into being around the dendrite with the decrease of temperature. The eutectic phase is dispersedly distributed in Al-7wt%Si, but continuously distributed along grain boundary in Al-10wt%Si. The eutectic phase in Al-10wt%Si was much more than that in Al-7wt%Si. Up to now, the multiscale simulation of the evolution of alloy phases has not been reported by other researchers in the field of semi-solid metal forming.This paper presented a standard formula for evaluating the microstructures of semi-solid alloys. It provides a reference to the semi-solid alloy design and process optimization. The microstructures of Al-Cu alloy for different solute concentration was simulated at different pouring temperature, casting velocity and the heat exchanging coefficient of cooling water. The results of average grain size and roundness was evaluated by the presented standard formula. The best alloy suitable for semi-solid forming is Al-8wt%Cu and the corresponding processing parameters are that casting velocity is 2.0 mm/s, pouring temperature is 922K, the heat exchanging coefficient of cooling water is 1000 W/(m2K). The microstructures of Al-Si alloy for different solute was simulated at different pouring temperature, casting velocity and the heat exchanging coefficient. The results of average grain size and roundness was also evaluated by the presented formula. The best alloy suitable for semi-solid forming is Al-7wt%Si and the corresponding processing parameters are that casting velocity is 2.0 mm/s, pouring temperature is 896K, the heat exchanging coefficient of cooling water is 1000 W/(m2.K).