Effects of high-temperature solutionizing on microstructure and tear toughness of A356 cast aluminum alloy

Abstract

A356 alloy was cast into permanent molds and subjected to T6 heat treatment. Two solutionizing (solution heat treatment) temperatures (540°C and 560°C) were used, with the holding time for each varied between 0 min and 240 min. Tear test specimens of 4 mm in thickness were machined from the castings. Tear toughness (UEp: unit crack propagation energy) was obtained from load-displacement curves. With increasing solutionizing holding time, eutectic silicon particles were found to become larger and mean interparticle distance was found to increase. The UEp of specimens solutionized at the higher temperature (560°C) increased with holding time until 120 min but decreased with longer holding times. SEM observation of fracture surfaces after tear testing revealed dimples originating from dispersed silicon particles within the eutectic phase. Fine dimples were also observed in the eutectic aluminum region in specimens with reduced UEp. These fine dimples were different from the dimples found to originate from the eutectic silicon particles. © 2011 The Japan Institute of Light Metals.