CET by Fragmentation during the Solidification under Natural and Forced Convection of Non-Refined Al-Based Alloys
Résumé
The columnar to equiaxed transition (CET) has been widely studied for many years [1] because this phenomenon is observed in metallurgical applications like castings. In non refined alloys, detachment of dendrite fragments is the most probable mechanism responsible for the formation of an equiaxed microstructure [1]. In this frame, melt convection influences the grain structure evolution by playing a role in the fragmentation phenomena [2].
We present here a comparative study of the fragmentation mechanism in Al-Ni alloys and Al-Si alloys during directional solidification.
In-situ and real time X-ray imaging of the solidification has provided direct access to the dynamics of fragmentation mechanism. By decreasing the temperature gradient during the experiments, we have been able to approach conditions of a casting. These experiments have yielded a better understanding of fragmentation and have shown the existence of a cascade fragmentation phenomenon in non refined Al-based alloys. Moreover, by coupling these experiments with more classical directional solidification experiments in bulk samples, we have observed that fragmentation is easier in Al-Si alloys compared to Al-Ni alloys and we propose that this diverging behaviour is due to the microstructure morphology and extension of the mushy zone of both alloys.
Additionally, we have performed directional solidification experiments with Accelerated Crucible Rotation Technique (ACRT). This study reveals that ACRT, by creating forced convection, can significantly modify the microstructure pattern. Using ACRT, we have been able to provoke CET in non refined Al - 7.0 wt% Si alloy.