Butted friction stir forming of AA6061-T6 to low carbon steel

Friction stir welding (FSW) is a popular method for creating dissimilar material joints in the aerospace and automotive industries. This work proposes a new FSW derivative process: butted friction stir forming (BFSF). BFSF has the distinction of joining two abutting workpieces to a lapped third workpiece. Additionally, the process does not require modification to traditional FSW equipment and tooling. In this study, BFSF combines two workpieces of aluminum alloy 6061-T6 in a butt weld configuration throughout the process. As the FSW tool traverses, it travels over preformed, threaded holes in low carbon ASTM A36 hot rolled steel at regular spacing. Downward force combined with the stirring action of the tool displaces material into the hole, filling the threads and forming a mechanical interlock. Testing of various screw sizes and thread pitches allowed the determination of the optimal joint configuration. Aluminum die-cut bolts of each size established a point of comparison for base material strength. Of the sizes tested, the ANSI metric M profile M9-1.25 screw had the highest mechanical strength. In cross-tensile testing, the average failure load was 3915 N. Shear testing produced a failure load of 5749 N. The M9-1.25 formed screws had a 33.6% decrease in strength compared to the die-cut bolts. While this strength reduction is higher than is often experienced in traditional FSW, the results of this study are a valid proof of concept for further development in BFSF methods.