Correlation between microstructure andmechanical properties of Al-Si diecast engine blocks

In spite of weight reduction of passenger car, the cylinder block is the heaviest component among many automotive engine parts and plays a key role in fuel efficiency and the drivability of vehicles. In hypoeutectic aluminium alloys, the final mechanical properties are strictly connected to microstructural features such as the distribution, the morphology and dimensions of primary -Al phase and eutectic Si particles, as well as, type of iron-bearings and defects. The microstructure, in turn, depends on filling process and solidification dynamic. In this work, a HPDC 4-cylinders-in-line cylinder block was exhaustively analysed and mechanical properties were correlated to microstructural features. Mechanical properties are affected by microstructure. The best values of UTS and elongation to fracture are obtained for low secondary dendrite arm spacing (SDAS) values and small and more compact eutectic Si particles. If the combined effect of -Al phase and eutectic Si particles is taken into account, a linear correlation between UTS and product of SDAS, equivalent diameter and aspect ratio of Si particles is observed, while the elongation to fracture shows an exponential trend, suggesting an high sensitivity on microstructural variation.