Warm redrawing of AA6082 sheets and investigations into the effect of aging heat treatment on cup wall strength

Abstract

In this work, novel warm deep drawing and redrawing test setups were designed and developed, and a process sequence was proposed to enhance the forming depth of AA6082 sheet. The complete redrawing of the cup was ensured by maintaining the temperature of redrawing die and binder at 200 °C and water-circulated cooling punch near to room temperature. The warm redrawing negated the sudden thinning and strain localization at cup corner, producing an overall draw ratio of 3.38. Further, the fully redrawn cups were subjected to solution heat treatment (SHT) and subsequent different artificial aging cycle to evaluate the impact on post-forming strength. The post-forming cup wall strength was characterized using Vickers microhardness test and ring hoop tensile test (RHTT). Approximately 30% improvement in microhardness of redrawn cup wall was obtained as compared to that of the undeformed sheet, and it was due to the significant grain refinement occurred during the warm deep drawing and subsequent redrawing process as characterized by SEM and EBSD techniques. Moreover, it was observed that the heat treatment had a pronounced effect on both the hardness and ultimate hoop tensile strength of the cup wall, and the peak aged cup wall hardness and tensile strength were 119 VHN and 316.8 MPa, respectively. As verified by TEM micrographs, the precipitation of coherent large needle-shaped β′′ precipitate was responsible for the significant improvement in the strength during the aging treatment. However, the presence of plate-shaped incoherent β-Mg2Si precipitate marginally decreased the total elongation at peak aged condition. Finally, the fractographs of redrawn and different artificially aged RHTT samples were analyzed to comprehend the nature of failure.