Effects of Precipitation Hardening on Mechanical Properties of Multistage Stirred Cast AA6063 Composites

Abstract

This work studied the effects of precipitation hardening on multistage stirred cast alumina reinforced AA 6063 composite through production of alumina reinforced AA6063 alloy by four stage stir casting method, determination of effect of precipitation hardening heat treatment and characterization of microstructures and some mechanical properties of as-cast and heat treated samples of aluminium matrix composites. The amounts of AA 6063 and alumina particles required for the production of aluminium composites with 0, 3, 6, 9, 12 and 15 volume percent of alumina were determined by charge calculations. The particles of alumina were initially pre- heated at 300 oC to enhance wetability with aluminium alloy. The AA 6063 alloy was charged into a gas fired crucible furnace and the liquid was allowed to cool to a viscous mass at temperature of about 600 oC so as to introduce alumina particles. Stirring operations were performed manually and mechanically at 750 oC and 300 rpm, to ensure uniform distribution of reinforcement within the matrix, before pouring into permanent metal mould. The samples produced were solutionized at 550 oC for one hour, quenched in water and later aged at 180 oC for 2, 3 and 4 hours. Some samples were reserved for control experimentation. Mechanical responses of the composite were investigated by tensile, hardness and impact test carried out on the samples. The structures of the samples were examined by optical and scanning electron microscopes. The results trend showed that as the volume fraction of alumina and aging time increase, the strength and hardness values also increase with corresponding decrease in impact value. Composite with 15% alumina aged at 4 hours showed highest strength and hardness values of 262.77 MPa and 64.12 BHN, respectively with impact value of 9.86 J. The study showed that multistage stir casting method and precipitation hardening heat treatment are capable of improving the mechanical properties of AA 6063-Al2O3 composites.