Development of a high strain rate superplastic Al-Mg-Zr alloy

For superplastic forming of aluminium to break out of the niche market that it currently occupies, alloys will be required to possess a higher strain rate capability, appropriate in service properties, and a significantly lower price and to be capable of volume production. This paper describes an approach that has been developed in an attempt to address these fundamental requirements. A series of Al-Mg-Zr alloys with increasing levels of zirconium (0-1 wt-%)has been prepared via extrusion consolidation of cast particulate (solidification rate ∼10 3 K s -1 ). The superplastic properties of the resultant cold rolled sheet have been evaluated as a function of thermomechanical treatment and zirconium addition. It has been found that increasing the level of zirconium has the twofold effect of improving the superplastic properties of the alloy while significantly decreasing the concomitant flow stress. At present the optimum superplastic behaviour has been obtained at strain rates of 10 -2 s -1 , with the 1%Zr material exhibiting ductilities in excess of 600%. The manufacturing route produces a bimodal distribution of Al 3 Zr comprising >1 µm primary particles in combination with nanoscale solid state precipitates. The current postulation is that this high strain rate superplasticity is conferred by a combination of particle stimulated and strain induced recrystallisation.