Transport Phenomena, Shrinkage, and Pore Formation during Solidification of a Dendritic Alloy

近年来的研究表明，显微疏松是影响铝合金力学性能的重要显微组织特征。铸造铝合金中的微孔在凝固过程中会形核，过饱和度很小。这表明在凝固之前熔体中存在气孔形核剂。这类试剂似乎是氧化物，或者是皮肤的形式，或者是经常在疲劳试件的断口上观察到的薄膜。这项研究涉及以系统的方式将氧化物引入铝熔体以控制和测量氧化物的数密度的新颖实验。然后用熔体制造铸件，测量铸件中的孔隙率，并与熔体中氧化物的数密度进行比较。第二个目标是开发一种计算机模拟器来预测凝固过程中气孔的形成。这项工作需要对凝固过程中的压力与铸件中气孔的数量、大小及其位置的关系进行创新。用随机渗透率来处理凝固合金中糊状区的渗透率。这种方法以前没有被用来模拟枝晶合金的凝固过程。凝固过程中气孔的形成与上述实验中确定的氧化皮的数密度有关。作为该计划的最后一步，将生产有可能避免铝铸件中的氧化物和氧化皮的试验性铸件。%这笔赠款支持亚利桑那大学材料科学与工程学院和航空航天与机械工程学院的跨学科研究。凝固模拟器将使铸造工程师能够预测铸件中气孔的数量和大小及其位置。避免铸造缺陷消除了铝运输铸件产生疲劳裂纹的主要原因。这项研究的预期结果是在抗疲劳性很重要的运输车辆中更多地接受和使用铝铸件。

9901290PoirierRecent research has shown that microporosity is an important microstructural feature that relates to the mechanical behavior of aluminum casting alloys. Micropores in aluminum casting alloys can nucleate during solidification with very little supersaturation. This indicates the existence of pore-nucleating agents in the melt prior to solidification. Such agents appear to be oxides, either in the form of skins or films that are often observed on fracture surfaces of fatigue specimens. The research involves novel experiments for introducing oxides into aluminum melts in a systematic manner so that the number density of the oxides can be controlled and measured. Castings are then made from the melts and the porosity in the castings measured and compared to the number density of the oxides in the melts. A second objective is to develop a computer simulator to predict the formation of porosity during solidification. This effort requires innovation in how pressure during solidification relates to the population and size of pores and their locations in castings. The permeability of a mushy zone in a solidifying alloy is treated with a stochastic permeability. Such an approach has not been used previously for simulating solidification of dendritic alloys. The formation of pores during solidification is related to the number density of the oxide skins as determined in the above-described experiments. As a final step in the program experimental castings will be produced that have the potential for avoiding oxides and oxide-skins in aluminum castings. %%%This grant supports interdisciplinary research involving faculty of Materials Science and Engineering and Aerospace and Mechanical Engineering at the University of Arizona. The solidification simulator will enable casting engineers to predict the population and size of pores and their locations in castings. Avoidance of casting defects eliminates a major culprit in initiating fatigue cracks in aluminum transportation castings. The desired outcome of the research is a greater acceptance and use of aluminum castings in transportation vehicles where fatigue resistance is important.***