Effect of thermal exposure on the residual hardness and tensile properties of the EN AW-2618A piston alloy

The present study was focused on the effect of temperature exposure on the hardness and tensile properties of the EN AW-2618A T6 alloy, used for forged pistons. The peak aging parameters were firstly obtained by solutioning, water quenching and aging the alloy in the range 25-220 °C. The thermal stability of the T6 heat-treated alloy was studied in the temperature range 200-300 °C for times up to a week, thus obtaining the corresponding hardness-time-temperature curves. Tensile tests and microstructural characterisation were carried out on specimens subjected to different thermal exposure and therefore characterized by different residual hardness. Proof and ultimate tensile strength decreased with decreasing the residual hardness of the alloy, while elongation to failure and strain hardening exponent increased. Empirical equations able to modelling the tensile behaviour of the alloy, based on its residual hardness, were defined. Microstructural and fractographic analyses on the thermal exposed alloy evidenced coarsening of strengthening phases, mainly at the grain boundaries, with presence of precipitate free zones and large incoherent precipitates.