Optimisation of the mechanical properties of a Spark Plasma Sintered (SPS) magnesium alloy through a post-sintering in-situ precipitation treatment

The present paper investigates the potential of the SPS route to improve the mechanical properties of the AZ91 alloy by controlling, in-situ within the SPS machine, the precipitation kinetics of the hardening Al12Mg17 precipitates. To this end, an as-atomised AZ91 powder was sintered by SPS and the formation of the Al12Mg17 phase was controlled by introducing an in-situ precipitation treatment at 200 °C in the wake of a conventional sintering treatment cycle at 360 °C. The influence of the sintering and the in-situ precipitation dwell times on the microstructure (grain size, Al12Mg17 vol fraction) and associated mechanical properties (hardness, compressive properties) were investigated. Using longer sintering times at 360 °C improved the material cohesion while keeping fairly fine grain sizes. The subsequent treatment at 200 °C triggered continuous and discontinuous Al12Mg17 precipitations. Compared to the cast AZ91-T6 treated alloy, the SPS material produced by a combination of 2 h of sintering followed by 2 h of in-situ precipitation led to a significant improvement in mechanical properties: hardness +16%, Yield Compressive Strength (YCS) +59%, Ultimate Compressive Strength (UCS) +32% and strain at UCS +12%.