5083 aluminum alloy plate of 3 mm thickness was friction stir processed under processing conditions of various rotation speeds and travelling speeds from 300 to 2000 rpm and 100 to 500 mm/min, respectively. Microstructure on cross section of the friction stir processed plate was observed by means of an optical microscopy and SEM-EBSP method. High temperature stability of microstructure in stir zone was evaluated by annealing the specimens at temperatures ranging from 523 to 823 K in a salt bath. Grain refinement was effectively achieved in the stir zone by friction stir processing and grain size was decreased as the rotation speed and/or the travelling speed decreased. Minimum grain size of 1.76 μm was obtained when processed with a rotation speed of 300 rpm and a travelling speed of 100 mm/min. Abnormal grain growth appeared in every specimen when annealed at 823 K regardless of the processing conditions. Critical temperatures of the occurrence of abnormal grain growth tended to decrease as the rotation speed was decreased. No significant change in microstructure was observed at these high temperatures in the specimen when the region that was contacted to shoulder of the tool was removed by mechanically. Possibility of a residual strain in stir zone given by the tool was discussed regarding to the high temperature stability of microstructure fabricated by friction stir processing.

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Using thinner sheet metal is effective in making the products with low cost and light weight. It also contributes to saving the energy and resources on earth. However, the rigidity and formability of sheet metal decrease when the thickness of sheet becomes thinner. Emboss process and restoration process can locally change the shape of a sheet metal, and also add partial work hardening. Therefore, there are some possibilities that the formability of thinner sheet metal is improved by these processing, like as the rigidity. In this study, emboss processes and restoration processes were conducted to the aluminum alloy sheet. The variations of the strength and formability of the aluminum sheet by these processing were investigated. The results showed that the yield stress of the sheet changed and the total elongation increased by performing these processing. On the basis of these results, the improvement of the formability of sheet metal was attempted by emboss process, restoration process and combination of these. The Erichsen cupping test and the deep-drawing test were conducted to the sheets having some processing patterns. From the result, it was confirmed that it was possible to improve the formability of sheet metal by adopting emboss process and restoration process properly.

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Interactions between moving grain boundaries and the coherent Al₃Sc precipitates during recrystallizaion in the Al–5 mass%Mg–0.3 mass%Sc alloy was studied by TEM observation. After solution heat treatment and the following artificial aging at 723 K for 8.64×10⁴ s, the alloy specimens contained the coherent L1₂-Al₃Sc particles with an average radius ~75 nm. After compressive deformation at room temperature and anneal at 723 K for 7.20×10³ s, static recrystallization occurred involving grain boundary motions. TEM observation revealed that, in the recrystallized grains, there were relatively large (~75 nm in radius) incoherent Al₃Sc particles, and also small (~10 nm in radius) coherent Al₃Sc particles close to the boundaries. The passage of the recrystallization front seemed to have caused crystallographic rotation of only the matrix, but not of the Al₃Sc particles. Discussion lead to the conclusion that small coherent Al₃Sc particles, with a radius smaller than the critical (~10 nm), could dissolve in the matrix when they came in contact with recrystallization front and could re-precipitate in the recrystallized grain after the passage of the front.

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The Ono's rotary bending fatigue testing and the cantilever rotary bending fatigue testing were carried out on friction-welded 6061 aluminum alloy joints, and the relationship between the deformation heat input in the upset stage or the upset burn-off length and fatigue strength was examined. In the Ono's type testing, sound joints, which fractured in the heat affected zone in the tensile testing, fractured in the heat affected zone also and the fatigue limit of these joints is slightly lower than that of 6061 aluminum alloy base metal. This is because joints fractured in the softened area in both tensile testing and Ono's type testing using smoothed test specimens. While, in the cantilever type testing, the fatigue limit of sound joints was a little more than or a little less than that of 6061 aluminum alloy base metal. It seems that a weld condition and a structure at the weld interface affected fatigue strength in the cantilever type testing using notched test specimens. Judging from the fatigue limit obtained, sound joints can be produced when either the deformation heat input in the upset stage or the upset burn-off length exceeds a certain value.

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Al–Mg–Si alloys are used for automotive closure panels as a weight saving option. Improvement of the Al–Mg–Si alloy sheet bendability is required for better hemming performance. In this study, the effects of the copper content on the bendability of the Al–Mg–Si alloy sheets were investigated. The Al–Mg–Si alloys with less than 0.01 mass%Cu, 0.4 mass%Cu and 0.8 mass%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 0.01 mass%Cu and 0.4 mass%Cu, no cracks were observed during the bending. For the sample with 0.8 mass%Cu, the maximum depth of the crack by bending increased with the time of solution heat treatment up to 75 s, and then decreased over 75 s. The second phase particles decreased with increasing the solution heat treatment time, while the number of shear bands by bending increased with 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 in the grains and the micro-voids formed around the second phase particles. The improvement of the bendability requires a decrease in the size and number of the second phase particles and/or the restraint of the shear band formation during the bending.

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