【作者】 邓叙燕；

【导师】 周志敏；

【作者基本信息】 东北大学 ， 材料物理与化学， 2008， 硕士

【摘要】 半固态成形技术(Semi-Solid Metal Process,简称SSM)以其高效、高性能、低成本、节能环保等突出特点受到人们的广泛关注。金属半固态成形技术的工艺路线包括流变成形与触变成形,其中触变成形的方法发展较为成熟,工业化程度高,而浆料制备是触变成形的关键,近液相线铸造技术是获得半固态浆料的简单高效的方法之一。实际上,不是所有的合金都适于近液相线铸造,而且不同工艺参数所得到的浆料的性能也不相同。为了获得具有良好半固态成形性能和使用性能的合金,人们需要进行大量的实验研究,浪费了大量的人力物力,因此通过计算机模拟进行半固态合金设计就成为目前人们关注的课题。本文在国家自然科学基金的资助下,对半固态铝合金的浆料制备过程进行了多尺度模拟及优化设计,提出了用多尺度模拟进行半固态合金设计及制备工艺优化的方法。本文针对半连续铸造过程的特点,建立了描述金属凝固过程的传热、传质以及液固相变的形核与长大模型。通过固相率变化将介观尺度和宏观尺度计算进行耦合,实现合金凝固组织演变的多尺度模拟,并对ZL201合金和A356合金的微观组织进行了多尺度模拟,模拟所得晶粒大小和晶粒形貌与实验结果吻合,说明本文所建立的模型是正确的。对ZL201合金的半连续铸造过程的温度场进行了模拟,得到了不同的浇注温度、铸造速度、冷却强度下的温度场。对铸造速度为2.0mm／s、冷却水的换热系数为1000W／(m2·K)、浇注温度为922K条件下ZL201合金的微观组织演变过程进行了模拟,得到了晶粒的长大方式和长大过程、晶粒周围的溶质浓度变化规律,模拟结果与Dustin-Kurz、Rappaz-Thevoz和Kanetkar-Stefanescu模型吻合。对铸造速度为2.0mm／s、冷却水的换热系数为1000 W／(m2·K)、浇注温度高于液相线5K条件下Al-7wt%Si、Al-10wt%Si合金相的演变过程进行了模拟。初始阶段,结晶核心呈枝晶形状生长,并不断向液相排出溶质,导致枝晶边界的溶质浓度较高。随着温度的降低,枝晶边界形成共晶组织,Al-7wt%Si合金的共晶组织呈弥散分布；Al-10wt%Si合金的共晶组织沿着晶界连续分布,同时晶界内也存在大量灰色的共晶相；Al-10wt%Si合金中的共晶组织比Al-7wt%Si的共晶组织多。目前,在半固态成形研究领域尚未见到有关合金相演变过程多尺度模拟的文献报道。本文提出了一个半固态合金组织评价的量化标准,为半固态合金设计及其制备工艺优化提供了参考依据。对不同合金成分的铝铜合金在不同的铸造速度、浇注温度、冷却水的换热系数条件下的微观组织进行了模拟,将得到的11组平均晶粒尺寸、平均晶粒圆度模拟结果用本文所提出的标准进行了衡量,得到了适合半固态加工的最佳合金成分及其成形工艺条件：合金成分为Al-8wt%Cu、浇注温度为922K、冷却水换热系数为1000W／(m2·K)、铸造速度为2.0mm／s。对不同合金成分的铝硅合金在不同的铸造速度、浇注温度、冷却水的换热系数条件下的微观组织进行了模拟,将得到的6组平均晶粒尺寸、平均晶粒圆度模拟结果用本文所提出的标准进行了衡量,得到了适合半固态加工的最佳合金成分及其成形工艺条件：Al-7wt%Si在浇注温度为896K、冷却水换热系数为1000W／(m2·K)、铸造速度为2.0mm／s条件下的微观组织最佳。更多还原

【Abstract】 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).更多还原