Characterising and Modelling the Non-Isothermal Deformation of Magnesium Alloys for Hot Blank-Cold Die (HB-CD) Sheet Forming

Magnesium, the structural metal with the lowest density, and its alloys have been of great interest to the transportation sector for their great lightweighting potentials. The hexagonal close packed crystal structure and strong texture of magnesium alloy sheets contribute to their severely-limited room temperature ductility when compared to other structural metals; it is therefore that magnesium sheet forming without heating is practically impossible. Isothermal warm and hot forming practices, including superplastic forming, resolve the formability issue, yet at a high cost in terms of both energy and production rate. Recently, hot blank – cold die (HB-CD) forming, a non-isothermal sheet forming approach, has been recently introduced as a means to achieve high sheet formability at reduced energy consumption and high production rates to meet the needs of the automotive sector. But it is also feasible that HB-CD can be used for the economic and efficient forming of magnesium alloy sheets into aerospace components; this work explores those possibilities. The tensile deformation behaviours of AZ31 and ZEK100 magnesium alloy sheets are investigated over a wide range of temperatures and strain rates, to cover the full window for quenching from a high temperature to RT through HB-CD. The transient deformation at various cooling rates is also investigated, and its effects on tensile ductility are evaluated to identify the appropriate range(s) for HB-CD forming. Changes in the deformation mechanisms across the temperature range for HB-CD are discussed, and the development of a constitutive model that can capture material behaviour during the process is also presented.