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.
In Al-Mg-Si alloys, the negative effect for the artificial age-hardenability occurs by the cluster (1) formation during natural aging following solution treatment and the positive effect occurs by the cluster (2) formation. For the purpose of obtaining information on the constituent elements of these clusters, soft X-ray absorption fine structure (XAFS) measurements of Mg-K edge and Si-K edge were carried out with the liquid nitrogen cooling. From radial structure function calculated from the extended X-ray absorption fine structure (EXAFS) spectra, since average nearest neighbor distance from Mg atom or Si atom decreases by the formation of cluster (1), it is considered that cluster (1) contains both Mg and Si atoms. The absorption edge energy of Si-K shifted to higher energy by the formation of cluster (1). This indicates that the Si valence increased and ion binding property is high for the bonding with neighbor atoms of Si atom in cluster (1). Since the binding force of ionic bond is stronger than that of a metallic bond, cluster (1) is difficult to be decomposed in the artificial aging and the negative effect is shown.
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