Formability Of AA1070 Aluminum Sheets Processed By Accumulative Roll-bonding

Accumulative roll bonding (ARB) is a relatively new severe plastic deformation technique. It can be used to continuously produce ultrafine-grained materials. AA1070 commercial-purity aluminum can not be strengthened by heat treatment, but by plastic deformation. The subject of this study was to use AA1070 as a testing material to change its microstructures and mechanical properties using the ARB technique. The materials processed by ARB and modified ARB techniques were investigated to evaluate the effects of ARB on the formability. The main conclusions are summarized as follows:(1) Effects of ARB processing on the sheet formability and the underlying mechanisms were investigated based by examining the microstructures, textures, mechanical properties in the sheets processed by one to four passes (or cycles) of ARB. The results showed that microstructures and properties of the sheets processed changed significantly during ARB deformation, showing typical cold deformation microstructures. The texture components also changed with the number of ARB cycles. The microhardness increased obviously in the first cycle, and then increased slowly with the number of ARB cycles. After subsequent low temperature annealing at 250°C/30min, recovery and recrystallization took place in the processed sheets. As annealed sheets, the maximum strength was achieved in the first cycle, then decreased obviously with the increase of ARB cycles; the Erichsen number was the largest in the fourth cycle, indicating the best formability. Therefore, to some extent the ARB technique can help to improve the mechanical properties and thus the formability of the Al sheet.(2) Effects of changing the rolling direction on the formability in the ARB-processed sheets were investigated by examining the interface bonding, textures and mechanical properties of sheets processed by two cycles with the rotated ARB (i.e. one of the two layered sheets was rotated by 180°around the sheet normal before next ARB) and conventional ARB techniques, respectively. The results showed that the interface bonding depicted little differences under optical microscopy, but it was relatively better in the case of conventional ARB than rotated ARB under electron back-scattering diffraction (EBSD) observations. In both cases, texture gradient developed through the sheet thickness, with shear-type textures near the sheet surface and rolling textures at the center. In the bulk sheets the rolling textures were stronger in the case rotated ARB. The sheet processed by rotated ARB depicted relatively higher tensile strength and planar anisotropy, but lower normal anisotropy and Erichsen number than those of the sheet processed by conventional ARB, and the latter results indicated relatively poor formability of the sheet processed by rotated ARB. Therefore, changing the sheet rolling direction was not undesirable to the formability of sheets processed by ARB.(3) Effects of stacking number per pass on formability in the ARB-processed sheets were investigated by microstructures, textures, mechanical properties of sheets processed by two-layered ARB and three-layered ARB at room temperature, respectively, with no lubrication and under the same total reduction. The results showed that the bonding between metal layers in the sheets processed by three-layered ARB was better than that of two-layered ARB. There were stronger shear texture components, rolling texture components and an overall better mechanical property with much higher elongation and slightly lower tensile strength. Therefore, within the extreme reduction of sheets, increasing the stacking number per pass sheets can help to improve industrial production efficiency, and to some extent, improve the mechanical properties of materials, and which was worth considering an important aspect for improving the existing ARB processing.