Effect of Squeeze Pressure on Aging and Mechanical Properties of AA2218-5 Wt Pct Al₂O₃ (TiO₂) Composites

Metal matrix composites (MMCs) based on AA2218 alloy have been successfully developed by stirring powder mix of TiO₂ particles along with Al₂O₃ particles prepared by milling into the alloy melt. The chemical reaction of TiO₂ particles with molten Al alloy results in alumina and simultaneously releases into the alloy elemental titanium, which forms intermetallic phase Al₃Ti in the last freezing liquid at the dendrite cell boundary. Application of squeeze pressure during casting of melt-particle slurry in the pressure range from 100 to 220 MPa results in decreasing the cell size and porosity. Following T61 heat treatment recommended for AA2218 alloy, the composites have been solutionized; their aging response at 170 °C shows significant increase due to squeeze pressure, but the peak hardness at 10 hours is relatively insensitive to variation in squeeze pressure. The 0.2 pct offset proof stress and the UTS of squeeze-cast composites after solutionizing and those aged following solutionizing increases with increasing squeeze pressure. The influence of aging as evident from the comparison of their properties starts increasing significantly in composites squeeze cast at 140 MPa but becomes steady after 180 MPa. The ductility of the composites also improves across the pressure range between 140 and 180 MPa, and there is increased improvement in ductility after aging at higher squeeze pressures. The scanning electron microscopy (SEM) micrographs show that squeeze-cast composites show more dimpled regions than those observed in gravity-cast composites.