Microstructural characteristics and thermal stability of ultrafine grained 6061 Al alloy fabricated by accumulative roll bonding process

An accumulative roll bonding process was employed to introduce a ultrafine grained structure into a commercial 6061 Al alloy. In performing the accumulative roll bonding process, the alloy was rolled with a 50% reduction ratio. Then, the rolled sheet was cut, stacked to be the initial thickness and the stacked piece was rolled again with the same reduction ratio. This procedure was repeated five times so that an effective strain of 4 was accumulated into the alloy. By 5-passes rolling, the grain size of ∼0.4 μm was obtained when the grain size was measured on the rolling plane and a remarkable enhancement in hardness was achieved, over twice than that before rolling. The microstructural examination revealed that, at relatively low strain, fine dislocation cells were formed by an operation of multi-directional slip. With increasing strain, dislocation cells were developed into ultrafine subgrains. At large strains, the microstructural change was dominated by a conversion of low-angled subboundaries to high-angled boundaries, rather than grain refinement. The 1 h static annealing treatment was carried out at temperatures of 373–773 K in order to examine the thermal stability of ultrafine grained 6061 Al alloy. The present ultrafine grained 6061 Al was found to be thermally stable up to 473 K. The microstructural change of 6061 Al alloy during accumulative roll bonding was compared with that observed in ultrafine grained Al alloys fabricated by the equal channel angular pressing technique which is another representative technique for fabricating ultrafine grained bulk materials. In addition, thermal stability of ultrafine grained 6061 Al alloy was discussed in terms of the grain growth kinetics.