Ultra grain refinement of commercial purity aluminum by a multi-stack ARB process

Multi-layer stack accumulative roll bonding (ARB) process is proposed and applied to commercial purity aluminum for ultra grain refinement. The six aluminum sheets 0.5 mm thick, 50 mm wide and 400 mm long were degreased and wire-brushed. The sheets were stacked together, and cold-roll-bonded by 50%-reduction rolling. The roll-bonding was followed by four-pass rolling so that the final thickness was 0.5 mm. The sheet was then cut to the six pieces of same length and the same procedure was repeated to the sheets. The ARB process up to six cycles (an equivalent thickness strain of 13.2) was successfully performed. The tensile strength of the ARB processed aluminum increases with the number of ARB cycles, and it reaches a maximum of 287 MPa after the third cycle. The tensile strength is lower by the six-layer stack ARB than that by the conventional two-layer stack ARB. The elongation slightly decreases with the number of the ARB cycles. TEM observation reveals that ultrafine grains begin to develop at the first cycle, and cover most field after the 3rd cycle, and that their aspect ratio decreases with the number of cycles. The grain size of the six-layer stack ARB is larger than that of the two-layer stack ARB. The effects of the number of the layers in stacking are explained by the redundant shear deformation. It is concluded that multi-layer stack ARB is a suitable process for industrial production of ultrafine-grained coils.