The process parameters to manufacture structural aluminum alloys are critical to their ductility. In particular, quench rate after solution heat treatment impacts the extent of grain boundary precipitation and the formation of precipitate free zone (PFZ) during later artificial aging. Cu-containing 6XXX alloys used for high strength automotive applications are quench sensitive as the Cu addition leads to Q-phase precipitation at grain boundaries, resulting in loss of ductility, which can negatively affect downstream manufacturing steps such as automotive joining and forming processes. Self-piercing rivet (SPR) joining, is a single step, spot joining process used to mechanically connect sheet materials together in automotive body structures. Ductility has been identified as an important metric of material rivet-ability or the ability to make a successful, crack-free SPR joint.
Through a full-scale industrial trial on a high strength 6111 aluminum alloy - varied quench rates ranging from slow to fast have been used to produce different grain boundary precipitation and PFZ characteristics. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize grain boundary precipitation and PFZ width as a function of quench rate. Multiple mechanical characterizations, including SPR joint evaluations were done as a function of quench rate. It is shown that uniform and total elongation metrics are not suitable to understand rivet-ability. Instead, ductility metrics from both bending and reduction of area measurements from tensile tests were found to correlate with SPR joint quality.
