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™ process to prepare slurry and cast in a vertical high-pressure die casting machine. Particle size and Si concentration in the α-Al particles in the slurry and in the as-rheocast component were investigated. A uniform distribution of Si in the globular α ₁-Al particles was achieved in the slurry. In the rheocast samples, measurement of the α ₁-Al particles showed that these particles did not increase significantly in size during pouring and secondary solidification. The two additional α-Al particles types, α ₂-Al particles and α ₃-Al particles, were identified as being a result of two discrete nucleation events taking place after slurry production. The Si concentration in the α ₂-Al and α ₃-Al particles indicated that the larger α ₂-Al particles precipitated before the α ₃-Al particles. In addition, in the as-rheocast condition, the Si distribution inside the α 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 α ₁-Al particles during secondary solidification occurred concomitantly with the final eutectic reaction and increased with increasing amount of the Al-Si eutectic phase.