Standard and Strain Path Change Forming Limit Diagrams of AZ31B Magnesium Sheet at Elevated Temperatures

Effect of strain path change on formability and microstructural characteristics of AZ31B automotive magnesium sheet material is studied at 300°C. The standard forming tests are carried out using Nakazima tests within the environmental chamber. The strain path change tests are performed by in-plane uniaxial, plane strain and balanced biaxial pre-strained specimens in the first stage and Nakazima test in the second stage. The enhancement in the activity of prismatic and pyramidal slip as well as a reduction in material anisotropy with increase in temperature is responsible for improvements in formability. Compared to the standard FLD, all uniaxial, plane strain, and balanced biaxial pre-strained materials exhibit higher limit strains. This rise in FLD also incorporates the effect of additional annealing during temperature rise period prior to testing which cannot be avoided with the present experimental set-up and test methodology. The pre-strain effect is reduced by additional annealing effect caused by elevated temperature soaking time. At 300°C, the pre-strained material is almost fully recovered and behaves as the as-received material. Also, significant dynamic recrystallization is observed at 300°C.