Formability limits and fracture mechanisms in AA5182 Al-Mg sheets under room and cryogenic temperature conditions

Abstract

The formability of in-demand Al-Mg alloys, such as AA5182, is constrained by the Portevin-Le Chatelier (PLC) effect, which manifests as serrated plastic flow due to dynamic strain aging (DSA) from solute-dislocation interactions. This phenomenon contributes to the premature onset of necking (i.e., reduced ductility) and surface defects, hindering the broader application of these alloys in lightweight structural components. The formability limits and fracture mechanisms for AA5182 sheets subjected to uniaxial tension and Erichsen punch forming were investigated under room temperature (293 K) and cryogenic (77 K) conditions. The cryogenic tensile fracture strain and flow stress increased by 47 % and 91 %, respectively, the Considère necking criterion was globally satisfied and surface wrinkling was fully prevented. Cryogenic forming limit diagrams (FLDs) were obtained for the first time for AA5182 sheet material using the Erichsen punch tests. The major strain limits increased by 18 %, 43 % and 27 % for uniaxial (drawing), plane strain and biaxial stretching strain paths, respectively, compared to room temperature conditions. Optical microscopy of the through-thickness microstructure confirmed that PLC suppression at 77 K prevented unstable interactions with opposing shear planes, promoting more uniform strain distribution and sheet thinning. Scanning electron microscopy of the fractured edges revealed a higher frequency of dimple initiation sites and 29 % greater dimpled area post-forming under cryogenic conditions due to delayed fracture and correspondingly increased void formation and growth. The average post-forming surface roughness was also reduced (improved) by a factor of 5, representing the difference between Class B and Class A surface finishes.