Formability limits, damage and fracture mechanisms in AA5182 Al-Mg sheets formed under subzero temperature conditions

¹ University of Windsor, Department of Mechanical, Automotive, & Materials Engineering, 401 Sunset Avenue, Canada
² University of Waterloo, Department of Mechanical and Mechatronics Engineering, 200 University Avenue West, Canada

Abstract

The applicability of in demand lightweight Al-Mg alloys is constrained by limited formability and surface defects caused by the Portevin-Le Chatelier (PLC) effect, which manifests as serrated plastic flow due to dynamic strain aging. This study investigated the formability limits, damage and fracture mechanisms for commercially available AA5182 sheets subjected to tensile and Erichsen punch forming tests under room (293 K) and cryogenic (77 K) conditions, that latter was achieved using a liquid nitrogen reservoir. The sheet fracture strain and flow strength increased by 47% and 91%, respectively, for the subzero conditions compared to the reference 293 K case, the Considère necking criterion was also globally satisfied and surface wrinkling was fully suppressed. Optical and scanning electron microscopy confirmed that PLC suppression under subzero forming prevented unstable interactions with opposing shear planes, allowing for more even strain distributions and sheet thinning near the fracture zone. The subzero major strains on the forming limit diagram increased by 18%, 43% and 27% for uniaxial, plane strain and equiaxial strain paths, respectively, compared to 293 K. The average surface roughness was also reduced from 1.16 µm to 0.23 µm, representing the difference between Class B and Class A surface finish designations.