引張予変形したAl–Mg–Si系合金板材の曲げ加工性に及ぼす合金組成の影響

Recently, the reduction of exhausted carbon-dioxide has been seriously required to avoid global warming. Naturally, this is an issue of great interest in the automobile industry. To achieve a sufficient reduction, lightening the autobody by converting the conventional steel sheets to aluminum alloy sheets is regarded as an effective and feasible method. Some Al–Mg–Si alloys have already been applied to a part of autobodies because these alloys have hardenability during paint baking and do not cause a stretcher strain pattern. However, their bendability is significantly poor compared to that of steels. In this study, the bendability has been assessed in terms of the macroscopic and microscopic appearance of the tension-side surface and measured surface roughness, as a function of the alloy composition, i.e., amount of Mg2Si, amount of excess Si and addition of Cu and Cr. Specimens were solutionized, air-cooled or water-quenched and then naturally aged for 7 d. Before the bending test the specimens were stretched by 10%, since such straining is frequently carried out in the actual autobody sheets prior to the bending process. The results obtained were compared with those obtained in the previous report where stretching prior to bending had not been performed. The effects of the amounts of Mg2Si and excess Si and of the Cu addition were similar to those in the specimen series without prior stretching, that is, bendability was decreased when the amounts of Mg2Si and excess Si were increased, while it was slightly improved by the Cu addition of 0.34%. The effect of Cr addition of 0.21% did not coincide with that in the previous report, that is, Cr addition deteriorated surface roughness and tended to cause cracking despite an improvement in the bendability in terms of macroscopic appearance assessment in the present study, while it totally improved the bendability in the previous study.