Microstructure and rheological behavior of grain refined and modified semi-solid A356 Al–Si slurries

A comprehensive study was carried out to establish the effect of melt treatment using Ti-B grain refiner and Sr modifier additions on the microstructural evolution and rheological properties of A356 aluminum alloy in the semisolid state. Rheocast billets of 75 mm in diameter and ∼140 mm in length, prepared by the SEED process, were deformed and several parameters were investigated to characterize the flow behavior of the billets in a compression test. Strain-time graphs were used to calculate the viscosity and liquid segregation was monitored to identify the optimum conditions for melt treatment. It is shown that non-treated alloy has the highest apparent viscosity. Refined semisolid billets have smaller particle size and more globularity of primary α-Al particles and as a result deform and flow more easily. It is postulated that modifier addition reduces the liquid surface tension and leads to improved deformability. © 2006 Acta Materialia Inc.Shahrooz Nafisi, Omid Lashkari, Reza Ghomashchi, Frank Ajersch, Andre Charett