Effect of microstructure and overaging on the tensile behavior at room and elevated temperature of C355-T6 cast aluminum alloy

The present study was focused on the microstructural and mechanical characterization of the Al-Si-Cu-Mg C355 alloy, at room and elevated temperature. In order to evaluate the influence of microstructural coarseness on mechanical behaviour, samples with different Secondary Dendrite Arm Spacing (SDAS) (20-25 μm for fine microstructure and 50-70 μm for coarse microstructure), were produced through controlled casting conditions. The tensile behaviour of the alloy was evaluated at T6 condition and at T6 with subsequent high temperature exposure (41 h at 210 °C, i.e. overaging), both at room and elevated temperature (200 °C). Microstructural investigations were performed through optical and electronic microscopy. The results confirmed the important role of microstructure on the tensile behaviour of C355 alloy. Ultimate Tensile Stress and elongation to failure strongly increased with the decrease of SDAS. Larger SDAS, related to lower solidification rates, modify microstructural features, such as eutectic Si morphology and size of the intermetallic phases, which in turn influence elongation to failure. Overaging before tensile testing induced coarsening of the strengthening precipitates, as observed by STEM analyses, with consequent reduction of the tensile strength of the alloy, regardless of SDAS. A more sensible decrease of tensile properties was registered at 200 °C testing temperature.