| With increasing oil prices, there is an increasing pressure on automotive manufacturers to create vehicles with greater fuel economy. This has resulted in attempts to reduce vehicle weight, and ultimately fuel consumption. Aluminum powder metallurgy (P/M) processes offer the possibility to create high performance, low-density components, ideal for automotive applications. However the Achilles-heel of these processes is that of residual porosity; typical press and sinter processes result in densities of 90-95% theoretical density obtained. Increasing the sintered density by the application of a post-sintering densification process can improve the mechanical properties (including strength, hardness, and fatigue performance) of the component. This study assessed the benefits obtainable by performing a post-sintering densification (re-pressing) operation on an emerging hypereutectic aluminum-silicon P/M alloy, Alumix-231 with nominal composition of Al-15Si-2.5Cu-0.5Mg.;Alumix- 231 has been investigated to determine the optimum press and sinter processing conditions of the alloy, as well as to obtain a baseline of data to ultimately determine the benefit of introducing a post-sintering densification process to the fabrication procedure. Samples of Alumix-231 were compacted at GKN Sinter Metals, as well as Dalhousie University, and sintered exclusively in a lab setting. Sintered products were evaluated by analyzing the density before and after sintering, apparent hardness, microstructural features, as well the tensile and fatigue properties, and finally the response to heat-treatment.;It was determined that the optimum laboratory processing route for Alumix-231 involved compaction at a pressure of 600MPa. The compacted specimen should then be de-lubricated at 400°C for 20 minutes before being sintered at a set point of 545°C for a period of 60 minutes. The optimum heat treatment involved solutionizing the samples at. 520°C for 1 hour, followed by water quenching, and artificially aging the samples at 160°C for 8 hours.;Re-pressing processes were performed with varying degrees of deformation on sintered compacts subjected to an annealing or solutionizing heat-treatment immediately prior to deformation. The resulting compacts were then evaluated by means of microstructural analysis, tensile testing, density analysis, and fatigue life determination. The annealed samples that were subjected to a height reduction of approximately 6.5% during repressing, displayed a 40% increase in fatigue life over equivalent samples in the as-sintered condition. It was also determined that solutionizing the samples prior to repressing resulted in the formation of micro-cracks within the microstructure, resulting in premature failure during fatigue testing. |
