Microstructural evolution and rheological behaviour of aluminium alloys A356, and A356 + 0.5% Sn designed for thixocasting

One of the most important parameters in any thixoforming process is the viscosity of the semi-solid raw material. The reduction of the value of this property contributes towards a more complete filling of thin mould sections and decreases the incidence of defects. The present work deals with the influence of Sn on the viscosity of an A356 alloy (A17SiO.3 Mg) in the semi-solid state (SSS). A thermomechanical treatment was employed in order to obtain a non-dendritic microstructure, and the A356-base composition was modified by adding 0.5% Sn to the alloy. Thus, two A356 alloys, one conventional and the other containing Sn were compared in terms of microstructural evolution in the semi-solid state at a solid fraction of 0.5 and up to 3600 s soaking time. Solid fraction, particle size, contiguity and shape factor were measured by optical microscopy. Wettability was evaluated by separately dipping an Al–1.5% Si plate in an Al–12% Si and in an Al–12% Si–l% Sn baths (thus simulating the solid and the liquid phases of the A356 and A356 + Sn alloys, at 580 °C, respectively) and measuring the grain boundary penetration by the liquid. Rheology was measured with a Searle type viscosimeter and by the back-extrusion technique. It was shown that Sn increases the wettability without interfering in the morphology and coarsening rate of the solid phase. In addition, it reduces considerably the viscosity and thixotropy of the A356 alloy, leading to better flow behaviour.