Grain refinement of high purity aluminium by asymmetric rolling

Abstract

For the purpose of grain refinement, development of the microstructure of coarse grain, high purity aluminium during cold asymmetric rolling has been studied by electron backscatter pattern (EBSP) analysis, as well as optical and transmission electron microscopy, and compared with that developed during conventional rolling. In 91.3% asymmetrically rolled sheet, new fine equiaxed grains with an average size of ～2 µm are evolved almost uniformly throughout the thickness. On the other hand, in conventionally rolled sheet, the coarse fibrous structure is predominant. A change of grain boundary misorientation distribution with an increase in reduction shows that the fraction of sub-boundaries below 10° decreases linearly, and that of the high angle boundaries above 15° increases linearly. The fine grain evolution during asymmetric rolling seems to result from the development of sub-boundaries into high angle boundaries promoted by a simultaneous action of two deformation modes, namely compression and additional shear deformation. Fine grains evolved during asymmetric rolling are stable at temperatures below 423 K. Annealing at temperatures above 473 K results in remarkable grain growth.