Three-Dimensional Friction Stir Welding of Inconel 718 Using the ESAB Rosio FSW-Robot

Robotic Friction Stir Welding (FSW) facilitates for increased welding flexibility, and allows for studies of forces in three dimensions without having the high cost of a stiff 5-axes machine. Recent developments in tool materials and welding equipment motivate this study on friction stir welding of high-strength alloys by a FSW-robot in a three dimensional workspace. New concepts of aircraft engines suggest higher temperatures to increase engine efficiency, requiring more durable materials such as inconel 718. The used ESAB Rosio FSW robot can deliver up to 12kN downforce and 40Nm torque. This is sufficient for welding high-strength alloys of limited thickness. This study focuses on the process forces during friction stir welding of Inconel 718 with thickness up to 3mm in butt-joint configuration. A newly developed threaded PCBN tool with convex shoulder is used in a local argon-shielded atmosphere. Initial tests are performed in a stiff FSW machine in position controlled mode. The measured process forces in position control are later on used as parameters on the force-controlled robot. Different backing bar materials are investigated with the aim to decrease the risk of root defects. Tool steel and inconel backing bars are proven to be too soft for this purpose and alternatives are suggested. The optimal welding parameters are tuned to combine a good weld quality with the process forces that can be obtained by the robot. The effect of an induction heat source is used to further decrease the need of high process forces.