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

最近的研究表明，微孔是一种重要的微观结构特征，与铝铸造合金的机械行为有关。 铝铸造合金中的微孔在凝固过程中可以在非常小的过饱和度下成核。 这表明在凝固之前熔体中存在孔成核剂。 这些介质似乎是氧化物，以皮肤或薄膜的形式，经常观察到疲劳试样的断裂表面。 该研究涉及以系统的方式将氧化物引入铝熔体的新颖实验，以便可以控制和测量氧化物的数密度。 然后由熔体制成铸件，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个计算机模拟器来预测凝固过程中孔隙的形成。 这一努力需要在凝固期间的压力如何与铸件中的孔隙的数量和尺寸及其位置相关方面进行创新。 用随机磁导率处理凝固合金中糊状区的磁导率。 这种方法以前没有被用于模拟枝晶合金的凝固。 在固化过程中孔的形成与在上述实验中测定的氧化物表皮的数密度有关。 作为该计划的最后一步，将生产实验铸件，这些铸件有可能避免铝铸件中的氧化物和氧化皮。该补助金支持亚利桑那大学材料科学与工程学院以及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孔隙的数量和大小及其位置。避免铸造缺陷消除了引发铝运输铸件疲劳裂纹的罪魁祸首。 该研究的预期成果是在耐疲劳性很重要的运输车辆中更大程度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.***