Room Temperature Recovery of Cryogenically Deformed Aluminium Alloys

Abstract

Production of complex lightweight parts for transport applications requires a high level of formability. This has been shown to be achievable via deep-drawing of aluminium alloys at cryogenic temperatures. Although the effect of an extended plastic deformation regime at low temperature is well known, the mechanisms behind it are unclear due to the limits of the characterisation methods applied. In previous studies, the microstructures of samples deformed at low temperatures were examined after reheating to room temperature (RT) and storage. However, after heating the dislocation structure and density of the deformed material change and do not reflect the cryogenic situation. In this work, we investigate the evolution of flow stress during recovery in Al-Mg and Al-Mg-Si alloys. We examine the RT recovery behaviour of samples pre-strained at 77 K to different strain levels, and evaluate the structural stability upon subsequent deformation. We also study microstructural evolution via in-situ synchrotron X-ray diffraction, starting from initial conditions at cryogenic temperatures to long-term RT-recovery. Recovery of cryogenically deformed samples at RT results in reduction of the flow stress, in dependence on RT storage. The recovery process can be divided into three distinct sections, each based on a different mechanism characterised by either the arranging or the annihilation of dislocations. Subsequent further straining at room temperature after cryogenic forming also generates plastic instabilities and premature fracture due to unfavourable hardening and recovery assisted softening interplay.

Keywords: aluminium alloys, cryogenic temperature, recovery, softening, dislocation density