Effects of impeller stirring time and EEM stirring speed on the microstructural characteristics of modified A356 alloy slurries in the RheoMetal™ process

The RheoMetal™ process is an advanced technique designed for the quick production of semi-solid slurries used in casting of various industrial parts. This method employs an Enthalpy Exchange Material (EEM), acting as a cooling agent to absorb heat and facilitate slurry formation. RheoMetal™ process includes two stirring steps: Primary EEM stirring and secondary impeller stirring. In this study, two series of experiments were performed to investigate the effect of these stirring operations on the microstructure of a modified A356 alloy slurry. In the first experiment, the RheoMetal™ process was performed under different impeller stirring durations of 5 s to 25 s by keeping the other parameters constant. The second type of slurry was prepared under an impeller stirring duration of 15 s and with increased EEM stirring speed from 600 rpm to 1200 rpm. A copper mold was used to rapidly quench the slurries to study their microstructures. The microstructural analysis indicated that the prolonged impeller stirring duration significantly increased the average solid fraction of semi-solid slurry. Among the tested conditions, the medium stirring time of 15 s resulted in the smallest average particle size of 20 ± 3.23 μm, the highest shape factor of 0.71 ± 0.03, and the most homogeneous microstructure. Despite variations in primary EEM stirring speeds, the resulting slurries exhibited similar features, indicating that primary stirring speed had a limited influence on the final microstructure. The outcomes of this study highlight the dominant effect of secondary impeller stirring in homogenizing the final microstructure of the semi-solid slurries.