Semisolid processing of near-eutectic and hypereutectic Al–Si–Cu alloys
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Semisolid processing of near-eutectic and hypereutectic versions of alloy 380 offers to overcome the problems encountered in casting hypereutectic Al–Si alloys and was thus explored in the present work. Experimental near-eutectic and hypereutectic Al–Si–Cu alloys obtained by adding elemental silicon to the 380 alloy were melted and were cooled to within 5 to 15 °C of their liquidus points before they were poured into a permanent mould in order to produce non-dendritic feedstock for thixoforming. This low superheat casting (LSC) process largely replaced α-Al dendrites with relatively smaller α-Al rosettes in all alloys. The slugs machined from the LSC ingots thus obtained were thixoformed after they were heated in situ in the semisolid range, between 568 and 573 °C, for 5 min in a laboratory press. Semisolid soaking sufficed to produce the required globular structure even when some dendritic features were retained in the starting feedstock. The hardness of the thixoformed parts which ranged between 84 and 96 HB have increased to 121–131 HB after the T6 heat treatment, implying a considerable age-hardening potential. The T6 treatment also improved the morphology of the eutectic silicon with potential benefits regarding the ductility of the thixoformed part.