| Al-Mg-Si series aluminum alloys for automobile applications are capable of obtaining high strength due to age hardening during paint bake process. However, auto body sheet should go through storage procedure. During storage, hardness is increased by natural aging. Therefore, it is hard to get formability at pressing process and get the sufficient effect of age hardening at the paint bake treatment because of short process time. In order to prevent an adverse effect of natural aging, the effect of alloying elements and pre-aging treatment on the age hardening behavior was investigated in this study. The hardness increment during natural aging was greatly suppressed by addition of Cu+In, Cu+Cd and Cu+Sn. These results might be explained by the fact that In, Cd and Sn can pick up vacancies during the natural aging. Therefore, the diffusion rate of Mg and Si is slowing down gradually and age hardening is also delayed. Hardness during natural aging was stabilized and Vickers-microhardness over 130 was achieved with short artificial aging(paint baking) time by pre-aging treatment. Elongation was over 30% which gives enough formability at press stage by addition of Cd and Sn. The formability (LDH(Limiting Dome Height) and FLC(Forming Limit Curve)) test was performed by the punch stretch test. It is considered that alloying element of Sn is effective to enhance formability. |
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