Forming limits of Mg alloy ZEK100 sheet in preform annealing process

Abstract

Preform annealing (PFA) is a new two-stage forming technology with an intermediate annealing step, which extends the formability of light-weight (Al and Mg) alloys that comparable to deep drawing steels. ZEK100 is the only rare-earth containing magnesium sheet alloy that available commercially. The objective of this research is to understand the effect of preform and anneal on the forming limits of ZEK100. Various pre-strain paths and levels were introduced in the preform step followed by an optimum annealing treatment. The total strain-based forming limits of the PFA material are strongly dependent on the pre-strain level and annealing conditions. Based on the strain energy principle, an equivalent strain is employed to evaluate the effects of the pre-strain and anneal on the total limit equivalent strain of the PFA material, which is fitted as a function of the equivalent pre-strain and strain ratio of major strain to minor strain; and there is a critical equivalent pre-strain that leads to the lowest total limit equivalent strain at all strain ratios. Based on this concept, a new forming limit diagram (FLD) is constructed with the equivalent strain and the strain ratio as the horizontal and vertical axes, respectively, to evaluate the formability of Mg sheet in this two-stage forming process with intermediate annealing.