Heat treatment of thixoformed EN AW 7075 alloy

Abstract

Heat treatment of thixoformed EN AW 7075 alloy was investigated in the present work. The hardness of the thixoformed alloy is 76±7 HV. This is nearly equivalent to the soft temper for this particular alloy. Zn, Cu and Mg are predominantly in the eutectic phase in the thixoformed state and have no favourable impact on strength. Besides, forming has occurred in the semisolid state with no evidence of plastic deformation. The maximum hardness that can be achieved after a commercial aging cycle, 16 h at 121°C, is 105 HV. The failure of the thixoformed alloy to adequately age harden is attributed to the lack of Zn, Mg and Cu in solution to offer adequate precipitation hardening capacity. It takes at least a 2 h long solution heat treatment at 470°C to solutionise the interglobular eutectic phase and to offer sufficient precipitation hardening capacity. The T6 and T7 hardness values thus obtained are 181 and 170 HV respectively, and demonstrate a remarkable age hardening potential. The superior T6 hardness of the thixoformed alloy with respect to the hot extruded counterpart is attributed to improved aging kinetics in the former, possibly due to a globular matrix morphology which provides relatively shorter diffusion paths.