Effect of thermal exposure on mechanical properties of EN AW-2618 and EN AW-4032 piston alloys

The stringent regulations about harmful emissions and the race to boost fuel economy force automotive manufacturers to develop downsized, turbocharged, lightweight engines, which require pistons able to withstand higher pressures and temperatures. Given their high thermal conductivity and specific strength, Al alloys are widely used in engine components. However, since precipitation hardening is the main strengthening mechanism of heat-treatable Al alloys, a progressive strength loss inevitably occurs above 200°C. Based on the above, the present study was focused on two Al alloys, widely used for high performance forged pistons: EN AW-2618 and EN AW-4032. Their thermal resistance was assessed in the range 200÷290°C, up to 168h, through hardness-time-temperature curves and tensile tests. In both cases, a decrease in hardness and tensile strength was observed, but EN AW-2618 performed better thanks to the lower coarsening rate of Cu-based strengthening precipitates. The different tensile behavior was related to their microstructures that led to different fracture mechanisms, as evidenced by fractographic analyse