Superplastic forming ability of as-cast AZ91 Mg alloy prepared by twin roll casting

In this study, the twin roll casting process has been utilized to prepare initial as-cast strip of AZ91 alloy that has been further tested in uniaxial tension at 325, 350 and 375 oC, and at strain rates from 10–2 to 10–4 s–1. The ability of AZ91 strip to undergo superplastic or superplastic-like deformation in longitudinal direction, in transverse direction, and in 45 degrees to longitudinal direction was investigated with no further thermal or thermomechanical processing applied prior to the testing. At temperature 350 oC and medium strain rate the maximum elongation reached almost 200% in direction parallel to the strip casting, however in the transverse and 45 degrees direction, different temperature, and at higher strain rates the maximum elongation was lower. Based on microstructural investigation by means of optical microscopy and scanning electron microscopy (SEM) it could be seen that continuous dynamic recrystallization of initial coarse dendritic as-cast microstructure during elevated temperature deformation is active and responsible for high elongations. Additional analysis is provided by means of a strain rate sensitivity characterization and its evolution with increased level of strain. Strain rate sensitivity of AZ91alloy increased with increasing level of strain from 0.20 to 0.33.