Microstructural inhomogeneity and biaxial stretching limits in aluminium alloy AA6016

The ductility of sheet metals in the regime where both principal in-plane strains are positive is strongly dependent on macroscopic inhomogeneity of the sheet. This can be clearly seen when the grain size becomes large compared with the sheet thickness. However, even with quite small grain sizes there is a tendency in aluminium alloys for grains of similar orientation to occur in colonies, and this complicates any simple correlation between grain size and limit strains in stretching. Sheets of the alloy AA6016 Al–Mg–Si were heat treated to give a wide difference in grain size, and the microstructure characterised using electron back scattered diffraction (EBSD) electron microscopy, which can reveal the presence of orientation colonies. Marciniak driving blank tests were used to reveal the development of dimensional inhomogeneity and the limit strains in balanced biaxial tension. An approach using the Marciniak–Kuczynski model with yield functions derived from texture data and the ‘defect’ associated with the measured inhomogeneity of crystallographic texture gives first-order predictions of the limit strains in this material, although issues of texture evolution and adequate quantification of inhomogeneity need further consideration.