Formability enhancement of ultrafine-grained pure copper sheets produced by accumulative roll bonding aided by electromagnetic forming

Accumulative roll bonding (ARB) is a new and cost-effective process for the production of ultrafine-grained sheets by applying severe plastic deformation. Various mechanical properties of sheets improve after the ARB process. However, the material formability severely decreases in the ARBed sheets restricting their usage in industrial applications. The main objective of this study is to investigate the effect of the high strain rates on the formability improvement of ultrafine-grained sheets manufactured by the ARB process. For this purpose, the electromagnetic forming (EMF) has been performed for applying high strain rates on ARBed pure copper sheets, and its capability to resolve the low toughness issue of the ultrafine-grained products of the ARB process has been investigated. Several sheet samples have been subjected to one, two, and three ARB passes, and their microstructural and mechanical properties such as the bonding quality, evolution of the microstructure and crystallographic texture, microhardness, mechanical strength, and formability have been determined after each pass. The results have indicated a substantial increase in the sheets' hardness and mechanical strength and a significant decrease in their formability, especially after the first pass. The elongation, on the other hand, has dropped abruptly during the first pass, but it has increased slightly in further passes. Afterward, the Nakazima test and the EMF have been used to form the ARBed sheet samples in the quasi-static and high strain rate conditions, and then the forming limit diagram and formability of formed samples have been compared in these conditions. It has been found that the formability of the one-, two-, and three-pass ARBed sheets in plane strain conditions have been, respectively, increased by 102%, 113%, and 99% in the high strain rate with respect to the quasi-static condition.