Analysis of the secondary phases in the microstructure of 319 type Al alloy engine blocks using electron microscopy and nanoindentation

Abstract

The most commonly used aluminum alloy for automotive engine blocks is 319, which possesses good strength and castability. In this research, 319 Al alloy engine blocks were analysed in the as-cast, heat-treated and service-tested conditions. Extensive microscopy and hardness measurements were carried out on samples extracted from top to bottom along the interbore region of each cylinder. The results suggest that a variation in cooling rate along the cylinders caused a refinement in microstructure at the bottom of the cylinders. Although the morphology of the secondary phases changed, the increased cooling rate did not lead to the formation of different secondary phases, which is indicated by the fact that no significant difference in hardness of the secondary phases was observed at the top and bottom of the cylinder. Furthermore, the results from nanoindentation indicate that Al₂Cu has a stronger bond with the matrix compared to Si and Al₁₇Fe_3.2Mn_0.8Si₂.