Influence of microstructure and casting parameters on impact strength of low-pressure cast A356 wheels

Instrumented impact strength tests have been performed on different wheels, cast with an Al-Si alloy. Total and instantaneous impact energy has been evaluated using an impact hammer instrumented with a strain gauge. The aim was to investigate the microstructural effect on impact properties of low-pressure cast Al-Si alloys. In hypoeutectic Al-Si alloys, microstructure and defects play a significant rule in the definition of this property. On the other side, instrumented Charpy impact energy tests give the opportunity to investigate crack nucleation and growth mechanisms. Plotting the force between the impact hammer and the specimen over the displacement, a useful evaluation of the material’s behavior can be done. Processing conditions (alloy modification level, die temperatures, furnace pressurization steep slope, cooling conditions, wheel geometry) influence microstructural features of Al-Si alloys such as: secondary dendrite arm spacing (SDAS) and eutectic silicon. Instrumented impact tests have been performed on three different A356 17-inch wheels, in order to evaluate variability in impact toughness due to thickness, type of wheel and processing conditions. Several KV sub-size specimens (5x10x55 mm) have been tested for each wheel. SDAS have been evaluated, in order to evaluate the influence of -aluminium dendrites morphology on crack nucleation and growth. Porosity is a critical feature in impact strength of Al-Si alloys. Its presence overcomes the effect of microstructure itself. Radioscopic investigation has been performed on KV samples, in order to evaluate the effect of porosity. Microstructural features have been compared to impact properties: absorber energy, maximum force, nucleation and propagation energy. The increasing in cooling rate results in different distribution of nucleation and propagation energy.