TWI

A friction stir spot welding process variant was evaluated, which was intended to remove the hole left in the weld when the probe is removed, and consequently improve weld properties. In the two-step procedure, first a backing plate was used that had a round dent on its surface so that a bump formed at the back side of the weld, then a probe-less rotating tool was used with a smooth flat backing plate, to remove both the bump on one side and the hole on the other. Similar welds were made for lap joints in 6061 aluminium alloy of 1 mm in thickness and dissimilar lap joints were made to weld 6061 to mild steel sheet (aluminium alloy on top). The back plate dent used was of 8 mm diameter, 1 mm depth. The first FSSW tool used had 12 mm diameter shoulder and a probe of 4 mm diameter, 1 mm length. The tool for the second step had a flat surface of 15 mm diameter. Weld surface appearance, microstructure (particularly of the interface area in the dissimilar weld), tensile shear strength and locus of failure were determined. Material flow under pressure during welding is discussed.

9th International Symposium, 15-17 May 2012, Session 9B: FSSW, Paper 02