Effects of friction stir spot welding parameters on morphology and mechanical property of modified cast nylon 6 joints produced by double-pin tool

A newly designed tool with double pins was utilized in the underwater friction stir spot welding (UFSSW) of modified cast nylon 6 sheets. A circular gasket was added between two sheets, acting as additional filler material to the nugget zone. The effect of welding parameters (rotational speed, plunge rate, plunge depth, dwell time, and waiting time) on weld formation and joint shear strength was investigated. Crown appearance and cross-sectional morphology of joints produced at different welding parameters were observed and compared. Lap-shear tensile tests were carried out to evaluate the joint strength. The results showed that joints performed at excessive rotational speed, large or low plunge rate, high plunge depth, and extreme dwell time displayed extrusion rings accompanied with cavities throughout or near the bonded areas. Cracks were formed at small rotational speed and short dwell time. With the increase of welding parameters except waiting time, the joint strength increased first and then decreased. Increased waiting time exerted a positive effect on the weld formation and strength. Two fracture modes, namely cross nugget mode and pull nugget mode, were observed in the lap-shear tests. Joints failed with cross nugget mode generally showed larger fracture loads and displacement than joints fractured with pull nugget mode. The maximum joint fracture load was 2825.2 N, which was larger than the joint performed by a tool with a conventional cylindrical pin. The maximum shear strength of 30.2 MPa was obtained, which was about 58.4% of the base material.