Effects of Minor Alloying Elements and Aging Treatments on Paint Bake Hardenability and Formability of Al-Mg-Si Alloys for Automobile Applications

摘要

Al-Mg-Si series aluminum alloys for automobileapplications are capable of obtaining high strength due to agehardening during paint bake process. However, auto body sheetshould go through storage procedure. During storage, hardness isincreased by natural aging. Therefore, it is hard to getformability at pressing process and get the sufficient effect ofage hardening at the paint bake treatment because of short processtime. In order to prevent an adverse effect of natural aging, theeffect of alloying elements and pre-aging treatment on the agehardening behavior was investigated in this study. The hardnessincrement during natural aging was greatly suppressed by additionof Cu+In, Cu+Cd and Cu+Sn. These results might be explained bythe fact that In, Cd and Sn can pick up vacancies during the naturalaging. Therefore, the diffusion rate of Mg and Si is slowing downgradually and age hardening is also delayed. Hardness duringnatural aging was stabilized and Vickers-microhardness over 130was achieved with short artificial aging(paint baking) time bypre-aging treatment. Elongation was over 30 percent which givesenough formability at press stage by addition of Cd and Sn. Theformability (LDH(Limiting Dome Height) and FLC(Forming LimitCurve)) test was performed by the punch stretch test. It isconsidered that alloying element of Sn is effective to enhance formability.