Effect of Overageing Conditions on Microstructure and Mechanical Properties in Al–Si–Mg Alloy

Abstract

Al-Si alloys have occupied significant position in the field of automobile applications. They are mainly used in engine parts where the alloys have to withstand high temperature for considerable length of time i.e., ageing effect. This research work has been carried out to investigate the overageing effect on a series of heat treatable Al-Si-Mg alloy (A355 alloy). The alloys were heat treated at 175°C for different length of time and microstructure and mechanical properties were studied. Considerable changes in microstructure were observed by SEM. Microstructure of moderately aged (1, 2 and 5 hours) alloys showed small precipitated particles, where over aged (1000, 10000 and 100000 hours) alloys showed coarse precipitated particles in grain boundary. Composition of the matrix and precipitated phase were ensured by EDS. These changes in microstructure significantly changed mechanical properties of the alloys over different ageing time. Initially the yield strength and hardness of the alloys increased up to a certain length of heat treatment and then it started to decrease with increasing heating time. Total elongation before fracture reduced initially and then increased with increasing heating time. Initially the dispersed second phase particles increased the mechanical strength. But eventually these properties decreased due to the coarsening of the particles. The study leads to the conclusion that the optimum aged was achieved between 3 to 5 hours of ageing time.