High temperature composite forming limit diagrams of four magnesium AZ31B sheets obtained by pneumatic stretching

Abstract

Formability of four magnesium AZ31B sheet materials, produced either by direct chill or twin roll continuous casting, was investigated at 400°C and 5×10−3s−1 using pneumatic stretching. Sheet specimens were deformed through a series of four elliptical die inserts, with aspect ratios ranging between 1.0 and 0.4, producing ellipsoidal domes with different biaxial strain combinations. Testing was carried out in two scenarios, i.e. with the major strains aligned either along or across the rolling direction of the material. Circle grid analysis was then used to map the planar strains of the deformed specimens; the latter were used to generate comprehensive material forming limits curves (FLCs) that bound the safe, marginal and failure deformation zones. Orientation effects were quantified by constructing a “composite FLD” for each of the four sheets; the diagrams collectively showed that greater formability limits are achieved along the material's rolling direction. Furthermore, detailed comparisons between the four sheets were carried out based on formability limits, deformation uniformity, maximum dome height prior to failure, and fracture surface morphology and chemistry. Disparities in formability were linked to differences in grain structure and material inhomogeneities.