Solidification Microstructure And Temperature Field During Direct Chill Casting Of Al-12.5Si Alloy

Eutectic Al-Si alloys have been widely used in automotive industry for their good liquidity and high specific strength, low coefficient of thermal expansion and specific gravity. In this work, Al-12.5Si alloy was DC cast at different casting velocities and in situ temperature measurement was performed, cooling curves were employed to find the effects of temperature gradient on the shape of the sump and microstructure of the billets at different casting velocities, The primary conclusions that may be derived from this work are as follows:(1) Casting velocity had great influence on the shape of the sump. With increasing of casting velocity, the sump was getting deeper. Both sides of the sump could be regarded as straight lines while the bottom was smooth;(2) The eutectic reaction time was longer in the central area of the billets at lower temperature gradient during direct chill casting. The eutectic reaction time was shorter in the central area of the billets at higher temperature gradient during direct chill casting.(3) The typical microstructure of the DC casting Al-12.5Si alloy was greatly refined compared to the ingot slowly solidified and consists of fine primary Si particles, a-Al dendrites and eutectics. From the periphery to the central area of the billets, primary Si particles became more in amounts and lager in sizes; equiaxed α-Al dendrites and eutectics were fine at the periphery while well developed in the centre; the morphology of eutectic Si phase changed from fibrous to fine needle-like.(4) The microstructure of Al-12.5Si alloy casting at higher velocity was finer than casting at lower velocity. The primary silicon particles were less in amounts and the morphology of eutectics was denser at higher casting velocity while more and sparser at lower casting velocity.