Improving Local Formability of Aluminum 6xxx and 7xxx Alloy Sheets using Friction Stir Processing for Automotive Applications

Abstract

Friction stir processing (FSP) on a gantry setup was implemented on various high-strength commercial aluminum sheet metals, including 7085-T76, 7075-T6, 7055-T6, and 6111-T6, to improve the local formability of each alloy. Samples subjected to FSP were bend tested over a safe-to-fracture range by incrementally decreasing the radii of bending to find the radius to thickness ratio (r/t) at fracture. The locally processed region showed a formability improvement of 20% in 7075, 37% in 7055, 41% in 7085, and 71% in 6111 compared to the base alloys. A natural aging study on the FSP alloys showed only 12 hours of natural aging had lowered the elongation-to-fracture back to the base level for 7075 alloy, showing limited shelf life for forming applications for that alloy, whereas 6111 and 7055 alloys retained their elongationto-fracture improvements above the base alloy even after 2 weeks of natural aging. Effects of surface roughness on bending performance was tested on the 7085 alloy where a polished surface after FSP showed a slight improvement in r/t at fracture. Furthermore, the top and bottom surfaces of the processed sheet were bend tested at tension and compression, and the results showed that specimens processed at lower tool-head rotational speeds tended to fracture under tension but not under compression. Finally, FSP was incorporated into a robot tool head to evaluate the feasibility of local FSP for commercial applications at higher welding speeds of up to 5 m/min compared to the current gantry setup with its maximum welding speed of 2.9 m/min.