Microgravity experiments on columnar-equiaxed transition in Al based alloys

摘要

The investigation on solidification of metallic alloys under well defined experimental conditions is of fundamental interest for the understanding of the physics involved in microstructure formation. In technical casting processes, these microstructures play an important role in the properties and the quality of the final cast product. The authors have investigated the transition from columnar to equiaxed grain growth (CET) in the framework of the ESA map CETSOL in Al-Si hypoeutectic alloys under microgravity conditions on the sounding rocket flight MAXUS-7. In the directionally solidified rod-like samples, the bulk melt flow is suppressed and diffusive mass and heat transport without sedimentation of refiner particles or equiaxed grains is obtained. The analysis of the processed samples creates benchmark data for validation and development of physical models for the formation of equiaxed grains and the CET. Here the authors present preliminary results from the analysis of two AlSi7 samples, one with and the other without addition of grain refiners (Ti and B) under identical experimental conditions. Critical parameters at the CET for temperature gradient and solidification rate are obtained from the experiments, as well as some characteristics of the grain structure. In a first approach the critical parameters are compared to predictions from Hunt's mechanical blocking model.