Fundamentals of grain coalescence and applications to aluminum alloy industrial solidification processes

近净形铸造为制造形状复杂的轻金属合金部件提供了最经济有效的方法。然而，热撕裂等铸造缺陷降低了产品质量和工艺效率。热撕裂的特征是半固态金属合金由于晶粒之间的液体膜破裂而自发失效，并发生在凝固结束时变形局部化到晶界的地方。热撕裂的发生与半固体从被液体包围的孤立颗粒逐渐演变成一个连贯和渗透的固体网络密切相关，这一过程被称为颗粒聚结。如果晶粒聚结延迟，因此在较低温度下完成，则热撕裂敏感性显着增加。***这项发现基金研究计划的目的是开发晶粒聚结过程的新知识，然后应用新知识来提高用于汽车和航空航天领域的高强度铝铜铸造合金的质量。在接下来的5年中，我的研究团队将利用实验和模拟来量化合金含量和工艺参数对晶粒聚结动态演变的影响，以及这一过程对半固态本构行为和热撕裂的影响。通过这项研究，7 HQP将在开发加工/结构/缺陷现象学模型方面得到培训，并在铝合金铸造过程的数学模型中应用这些关系。***研究方法：作为第一步，将开发一个数值模型来预测颗粒聚结。同时，将创建具有已知微观结构的模型材料，然后将其用作原位（受限铸造试验）和非原位（半固体拉伸试验）表征聚结转变的一部分。然后，利用实验和数值结果提出基于聚结动态演化的半固态力学行为本构律。最后，新工具将应用于潮汐涡轮机用B206（一种工业铝铜铸造合金）生产部件的砂铸造过程的有限元模拟，以证明在工业过程中预测热撕裂的改进能力。***行业影响：热撕裂是降低铝合金铸件和锻件生产率的主要缺陷。在许多情况下（例如，高可靠性复杂形状铸件），由于这种缺陷的高发生率，产品甚至无法生产。这项研究的结果有望大大提高对聚结和热撕裂的理解，并将为工业上用于铝铜铸件宏观过程模拟提供预测工具

Near-net-shape casting provides the most cost-effective way to manufacture light metal alloy components of complex shape. However, casting defects such as hot tearing reduce product quality and process efficiency. Hot tearing is characterized as a spontaneous failure of semi-solid metallic alloys due to the rupture of liquid films between grains, and occurs near the end of solidification where deformation becomes localized to the grain boundaries. The occurrence of hot tearing is intimately linked with the semi-solid's gradual evolution from isolated grains surrounded by liquid to a coherent and percolated solid network, a process known as grain coalescence. If grain coalescence is retarded and therefore finishes at lower temperatures, hot tearing susceptibility is significantly increased.***The objective of this Discovery Grant research program is to develop new knowledge of the process of grain coalescence, followed by application of the new knowledge to improve the quality of high-strength Al-Cu casting alloys that are used in the automotive and aerospace sectors. Over the next 5 years, my research team will utilize experimentation and simulation to quantify the influence of alloy content and processing parameters on the dynamic evolution of grain coalescence, and the impact of this process on semi-solid constitutive behaviour and hot tearing. Through this research, 7 HQP will be trained in developing processing/structure/defect phenomenological models, and in applying these relationships within mathematical models of aluminum alloy casting processes.***Research Approach: As a first step, a numerical model will be developed to predict grain coalescence. Concurrently, model materials with known microstructure will be created and then used as part of in-situ (restrained casting tests) and ex-situ (semi-solid tensile tests) characterization of the coalescence transition. Then, the experimental and numerical results will be used to propose a constitutive law of semi-solid mechanical behaviour based on the dynamic evolution of coalescence. Finally, the new tools will be applied to a finite element simulation of the sand casting process used to produce components made from B206 (an industrial Al-Cu casting alloy) for a tidal turbine application to demonstrate the improved ability to predict hot tearing in an industrial process.***Industrial Impact: Hot tearing is a major defect that lowers the productivity of aluminum alloy casting and wrought products. In many cases (e.g. high reliability complex shape castings), products cannot even be manufactured because of the high occurrence of this defect. The results of this research are anticipated to lead to a greatly improved understanding of coalescence and hence hot tearing, and will provide a predictive tool for use by industry in macro-scale process simulations of Al-Cu castings.**