Solidification Sequence and Evolution of Microstructure During Rheocasting of Four Al-Si-Mg-Fe Alloys with Low Si Content

Four Al-Si-Mg-Fe alloys with Si contents varying from 1.6 to 4.5 wt pct were rheocast, using the RheoMetal (TM) process to prepare slurry and cast in a vertical high-pressure die casting machine. Particle size and Si concentration in the alpha-Al particles in the slurry and in the as-rheocast component were investigated. A uniform distribution of Si in the globular alpha(1)-Al particles was achieved in the slurry. In the rheocast samples, measurement of the alpha(1)-Al particles showed that these particles did not increase significantly in size during pouring and secondary solidification. The two additional alpha-Al particles types, alpha(2)-Al particles and alpha(3)-Al particles, were identified as being a result of two discrete nucleation events taking place after slurry production. The Si concentration in the alpha(2)-Al and alpha(3)-Al particles indicated that the larger alpha(2)-Al particles precipitated before the alpha(3)-Al particles. In addition, in the as-rheocast condition, the Si distribution inside the alpha(1)-Al particles showed three distinct zones; an unaffected zone, a transition zone, and in some cases the start of a dendritic/cellular zone. The phenomenon of dendritic growth of globular alpha(1)-Al particles during secondary solidification occurred concomitantly with the final eutectic reaction and increased with increasing amount of the Al-Si eutectic phase. (C) The Minerals, Metals & Materials Society and ASM International 2015