A softening behavior of 3104-H19 aluminum can body materials during the paint baking process in can productions is an impactful factor on can body strength. However, detailed investigations on the mechanism are limited due to simultaneous occurrence of multiple fine precipitations and recovery, as well as difficulty of observing fine precipitates under high-density dislocations. In this research, effects of solid solution of the additional or impurity elements on paint baking softening behavior were investigated by observing microstructural changes with the high resolution FE-SEM, TEM, and synchrotron radiations. Based on this study, the solute atoms are thought to react with lattice defects (dislocations, dislocation cells, subgrain boundaries) to suppress their movement and recovery during the baking process. Then, precipitations and coagulations of the solute atoms occurred as the baking time passed. It could end up with mitigation of the pinning effect and progress of the recovery. The bake softening behavior could be explained with these multiple stages of the precipitations and coagulations of the solute atoms and the recovery progression. On the base of observed microstructural changes during the baking, the softening curve was analyzed using the newly proposed kinetics equation. The results of this analysis support the mechanism mentioned above.

This study investigated the effect of Sn addition on clustering behavior of natural aging (NA) in Al-0.95mass%Si-0.56mass%Mg-0.04mass%Sn alloys by XAFS measurements, first-principles calculation and Vickers hardness measurements. XAFS measurements at the Sn-K edges were carried out at the BL14B2 beamline, at the Si-K and Mg-K edges were carried out at the BL27SU beamline at SPring-8. It was found that Sn addition brought about the retardation of cluster formations. This retardation was ascribed to the suppression of formations of Mg-vacancy (Va) pairs. By Sn addition, the ratio of Sn-Va pairs was increased from as-quench (AQ) to NA for 36 ks, however Sn atoms were not remarkably included in the clusters. The formation energy of the Sn-Va pair is the smallest in Al matrix, and Sn atoms preferentially bind to vacancies. Since the formation energy of Mg-Va pair is higher than that of the Si-Va pair, the formation of Mg-Va pairs is preferentially delayed. Based on the result of Vickers hardness measurements, negative effect of two-step aging was caused by the participation of Mg atoms in the clusters, not that of Si atoms. It was clarified that Mg atoms have a greater influence on negative effect in two-step aging than Si atoms have.

In this study, the possibility of using adhesive for friction stir spot welding was investigated with the aim of improving the strength and rigidity of aluminum and CFRP dissimilar material joining members and improving galvanic corrosion resistance. As a result, it was found that there is a trade-off relationship between the friction stir spot welding, which improves the joint strength in proportion to the frictional heat, and the weld bond, in which the adhesive is thermally decomposed by the frictional heat. When using adhesive together, it is necessary to consider the selection of adhesive and the optimization of frictional heat according to the application and required strength level.