The development of a new manufacturing route for superplastic magnesium alloy sheet exploiting twin roll casting

Because of their propensity to dynamically recrystallise, superplastic behaviour can be obtained from magnesium alloys considerably more easily than from comparable aluminium alloys. In some cases even as cast magnesium alloys can exhibit reasonable superplasticity and there appears no need for the special alloying additions or complex thermomechanical treatments required by aluminium alloys such as AA2004 or AA7475. The paper describes the superplastic behaviour (in uniaxial tension) and microstructure of sheet processed from strip cast magnesium alloys. The material was tested in the ascast condition and after warm rolling to a number of gauges. Industrially useful superplastic capability was demonstrated in the strip cast alloys. Furthermore good superplastic capability was also demonstrated in sheet subsequently rolled from the cast metal and the ductilities obtained were not significantly influenced by rolling strain. Results are also included on the superplastic behaviour of die cast AZ31 and AZ91 in both as-cast and cast and rolled conditions. Despite both alloys demonstrating superplastic behaviour there were significant differences in their deformation characteristics as defined by their respective strain rate sensitivities. Some preliminary results obtained from twin roll cast AZ91 that had been subjected to severe shearing immediately prior to casting will be presented