Microstructure evolution during extrusion of AA3xxx aluminum alloys

The through process microstructural evolution of aluminum alloys AA3003 (1.27wt% Mn) and AA3102 (0.26wt% Mn) during high temperature extrusion was investigated. The Direct Chill(DC) cast microstructure of both alloys showed interdendritic eutectic regions consisting of rod/plate like constituent particles in the α-Al. Prior to extrusion, the as-cast materials were homogenized for three different conditions, i.e. 500°C for 8h, 550°C for 8h and 600°C for 24h. Optical Microscopy was used to examine the behavior of constituent particles and dispersoids during homogenization. Back Scattered Scanning Electron Microscopy and Image Analysis with Clemex were conducted to study the evolution of the size, aspect ratio, area fraction and number density of constituent particles. Moreover, to investigate the evolution of constituent particles during homogenization at 600oC, samples heated to and soaked at 600oC for different times were studied. It was found that different mechanisms of microstructure evolution occurred during homogenization including the breaking up, growth and coarsening of particles. Extrusion trials were conducted on a laboratory scale extrusion press located at Rio Tinto Alcan’s Avidia R&D Center in Jonquiere, Quebec. Various AA3003 and AA3102 DC billets with different homogenization treatments were extruded at 400oC and 550oC with an extrusion ratio of 130. The extrudates were examined in different orientations to study the through-thickness microstructure profile. Grain structures were revealed by Polarized Light Optical Microscope and Electron Back Scattered Diffraction. Back Scattered Scanning Electron Microscopy and Image Analysis were conducted on the extrudates to quantify the characteristics of the extruded material. It was found that the homogenization treatments have a significant effect on the as-extruded microstructure. In particular, the presence of dispersoids suggested significant pinning effects on the recrystallization behavior. In addition, Gleeble tests were conducted on a low iron AA3102 alloy using a Gleeble 3500 Thermo-mechanical Simulator at strain rates of 0.1s-1, 1s-1 and 10s-1 and deformation temperature of 400oC, 500oC, and 600oC. The yield stress, work hardening and flow stress results were fit into a physically based flow stress model of Kocks and Chen. It was found that the constituent particles had a minimal effect on the flow stress exponent in the model.Applied Science, Faculty ofMaterials Engineering, Department ofGraduat