Bendability of Al-Mg-Si sheet alloys for automotive closure applications

In an effort to reduce the weight of vehicle body structures, the automotive industry has looked to alternative materials other than steel. One such material that can offer weight savings and is consistent with existing manufacturing processes is aluminum. To date, the largest application of aluminum sheet in high volume is horizontal and vertical outer body panels. The current trend for these applications has been the use of a heat-treatable Al-Mg-Si alloy (AA6xxx) which benefits from artificial aging during the paint bake cycle. One important requirement of these materials is their ability to resist fracture in severe bending operations, such as hemming. The hemming process entails bending the outer panel around an inner panel to create the closure system. While in steel this process can result in a clean flat hem with good craftsmanship, aluminum often cannot be flat hemmed and a larger radius of curvature must be used to produce the hem. While this type of hem, often referred to as a rope hem, helps to alleviate some of the severe bending, it is not the preferred method because it results in a less robust process than the flat hem and can lead to surface defects. In this study several factors that affect the bendability of AA6xxx alloys are quantified. Critical elements regarding natural aging, artificial aging, and composition will be discussed. A parameter of particular interest is the inclusion of copper in these alloys. Copper has been added to some of these alloys in an effort to increase the kinetics during the paint bake response, which in turn increases the post-painted yield stress of the panels. While this increase in yield stress can lead to further weight and cost savings through down-gauging, it is believed by some automotive manufacturers that the inclusion of copper can have a negative effect on formability and bendability. In this paper, the effect of copper on the bending performance of aluminum will be discussed.