Study of abnormal grain growth in cold-rolled AA5182 Al-Mg alloy

Abstract

Studies of recrystallization and grain growth phenomena have a long history, but the causes of abnormal grain growth (AGG) are not well understood. We report on the results of the study of the occurrence of AGG in Al-Mg alloy AA5182. The industrially produced hot band underwent various routes of thermo-mechanical processing: inter-annealing, cold rolling with reductions ranging from 40-85% followed by isochronal anneal (1h) in the temperature range 350-520°C or isothermal treatment at 480°C for various times. The microstructural characterization was conducted by optical microscopy in polarized light and FEG SEM, while the EBSD technique was used for microtexture analysis of the selected states. The results showed that the temperature for the onset of the AGG decreases as the degree of cold reduction increases. The abnormal grains start to appear in the regions close to the surface, i.e., within 1.5 mm from it in the inter-annealed 12 mm thick hot band or 700 μm in the case of cold rolled sheet with 64% reduction. Initially equiaxed, abnormal grains show strong growth anisotropy with much faster growth in the rolling than in the normal direction. Growth anisotropy was attributed to the rod-like shape and alignment of Al6Mn dispersoids through Zener pinning. With extended annealing, bands of abnormal grains form parallel to the surfaces. Microtexture analysis of the sample with incipient abnormal grains showed the presence of retained rolling texture components in the form of R-fibre. In contrast, the incipient abnormal grains appear to have orientations of cube variants, the texture components which are at or below random level. Since the texture is similar throughout the sheet, it is likely that the AGG starts first in the surface region due to the non-uniform deformation and distribution of the intermetallic particles.