The intergranular corrosion (IGC) behavior of 6061 aluminum alloy extrusions was investigated. After the IGC test in accordance with ISO/DIS 11846 (method B), heavy IGC was observed at the surface of the extrusion. However, little IGC occurred at the center plane of the extrusion thickness. It was considered that IGC was caused by the existence of precipitate-free zones (PFZs) because PFZs were clearly observed in the surface layer of the extrusion but were not clearly observed in the center position of the extrusion thickness. Furthermore, it was considered that the formation of PFZs was associated with the grain boundary characteristics. That is, most of the grain boundaries had random high angles in the surface layer of the extrusion, but 60 pct of the boundaries were lower than 0.26 rad (15 deg) in the center plane of the thickness. To verify this hypothesis, the center plane was cold rolled, recrystallized, and examined using the IGC test. As a result, heavy IGC was observed, while the center plane, in this case, had almost all random high-angle boundaries.

The effects of the copper content on the bendability of Al-Mg-Si alloy T4 sheets were investigated. The Al-Mg-Si alloys with less than 0.01mass%Cu, 0.4mass%Cu and 0.8mass%Cu were prepared, and the time of solution heat treatment was changed to obtain different dispersion conditions of the second phase particles and to obtain different shear band formation conditions by bending. For the samples with less than 0.01mass%Cu and 0.4mass%Cu, no cracks were observed during the bending. For the sample with 0.8mass%Cu, the maximum depth of the crack by bending increased with the time of solution heat treatment up to 75 seconds, and then decreased over 75 seconds. The second phase particles decreased by increasing the solution heat treatment time, while the formation of shear bands by bending increased by increasing the solution heat treatment time and the copper content. The cause of the occurrence and the propagation of cracks by bending are considered to be the combined effect of the shear band formation across some grains and the micro-voids formed around the second phase particles. Improving of the bendability requires a decrease in the size and number of the second phase particles and/or reduced shear band formation during the bending.

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The influence of the iron content from 0.1 to 1.0 mass% on the mechanical properties of AA6016-T4 sheets was investigated. The amount of the Al-Fe-Si second phase particles increased with the iron content, thus the solute silicon atoms decreased. Increasing of the Al-Fe-Si particles lowers the bendability, while decreasing of the solute silicon atoms lowers the paint bake response (PBR) and improves the bendability. The bendability of the samples then became the worst at the 0.5 mass% iron content, while it at 0.8 and 1.0 mass% became better or the same as that at 0.5 mass%. The increasing of the silicon content in the 1.0 mass% iron content alloy improved the PBR and lowered the bendability.

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