The effect of average cooling rates on the microstructure of the Al–20% Si high pressure die casting alloy used for monolithic cylinder blocks

The effect of average cooling rates on the microstructure of the hypereutectic Al–20% Si alloy was investigated using the novel Universal Metallurgical Simulator and Analyzer Platform. The quantitative measurements of the primary Si size and the Secondary Dendrite Arm Spacing of the non-equilibrium α-aluminum as a function of the cooling rates was performed for the laboratory test samples. This research was carried out in order to analyze the microstructure of the high pressure die cast cylinder block and to understand its complex solidification process. The Equivalent Diameter of the primary Si decreased from 89.7 ± 17.3 to 16.5 ± 3.8 μm and the Secondary Dendrite Arm Spacing from 22.1 ± 5.9 to 5.1 ± 0.8 μm with an increase in the cooling rate from 4.9 to 82.9 °C/s. Observations of the cylinder block microstructures revealed that the primary Si size was nearly identical at the subsurface and the centre locations of the bore wall. The Secondary Dendrite Arm Spacing of the non-equilibrium α-aluminum phase as well as the eutectic Si size was significantly smaller at the subsurface of the bore wall. Based on the UMSA laboratory measurements it was determined that the primary Si in the engine bore wall (both at the subsurface and the centre) nucleated as a first phase from the liquid melt at a cooling rate of approximately 72–74 °C/s. It was found that the non-equilibrium α-aluminum dendrites at the engine bore wall subsurface nucleated from the semi-solid melt at a cooling rate of approximately 85 °C/s, while at the centre of the bore wall at approximately 49 °C/s. Research revealed that some primary Si particles nucleated from the beginning of the melt pouring into the shot sleeve prior to the injection process while the α-aluminum dendrites and eutectic Si nucleated in the die cavity. Therefore, it was proven that the injected hypereutectic Al–20% Si liquid melt had solid primary Si particles.