Effects of (Pr + Ce) Addition and Heat Treatments on Microstructure and Mechanical Properties of Al-5Si-1.2Cu Alloy

Effects of (Pr + Ce) addition and heat treatments on the microstructure and mechanical properties of Al-5Si-1.2Cu alloys were investigated using scanning electron microscopy, x-ray energy-dispersive spectroscopy, transmission electron microscopy, microhardness and tensile testing. The aspect ratio and average area of Si were determined by PRO PLUS 6 image analysis. By comparing the microstructures of the Al-5Si-1.2Cu alloys with (Pr + Ce) addition concentrations of 0.3 wt.%, 0.6 wt.% and 0.9 wt.%, it was found that the eutectic Si and Fe-rich phases were refined when (Pr + Ce) addition was 0.6 wt.%. After adding 0.6w.% (Pr + Ce), the long rods of eutectic Si and fishbones of the Fe-rich phase were changed into round particles and short rods. The short rod-like Al-Si-RE compounds were formed in the grain boundary. In addition, eutectic Si and Fe-rich phase were refined and spheroidized and more RE dissolved in modified alloy after solution treatment (520 °C, 6 h). After aging treatment (175 °C, 6 h), eutectic Si- and Fe-rich phases were finer and more spherical, and the nano-scale Al₂Cu phases were precipitated. The microhardness (97HV), tensile strength (238.62 MPa) and elongation (5.23%) of the heat-treated Al-Si-RE alloys were 24.35, 24.18 and 50.72% higher than those of the unmodified alloy, respectively. The change in microstructure Si, Fe-rich phase, the formation of Al-Si-RE compounds at grain boundaries and precipitation of nano-scale Al₂Cu led to significant improvement in the mechanical properties of the Al-5Si-1.2Cu alloy modified with Pr + Ce.