Abstract

Refill friction stir spot welding (RFSSW) is an emerging technology, capable of joining thin sheets of aluminum alloys. The present thesis comprises two studies which were conducted to address two challenges faced by RFSSW: the long cycle time traditionally associated with welding and the poor life of existing RFFSW tools. In the first study, welds were made in AA5052-H36, at various cycle times and with various process parameters. It was shown that RPM, cycle time, and material thickness, all have an effect on the machine response. Decreasing RPM or weld duration leads to increased force and torque response during welding. Welds with cycle times below one second were successfully made without severely impacting joint quality, suggesting that prior work may have been limited by machine capabilities rather than by phenomena inherent to the process. On average, the sub-one second welds caused a peak probe force of 9.81 kN, a plunge torque of 26.3 N*m, and showed average lap-shear strengths of 7.0kN; compared to a peak probe force of 5.14 kN, a plunge torque of 17.3 N*m, and lap-shear strength of 6.89kN for a more traditional four-second welding condition. In the second study, the life of a steel toolset was quantified as consecutive welds were made in AA5052-H36 until the toolset seized from material accumulation/growth. At a one-second welding condition, the toolset was only capable of producing 53 welds before seizure. At a two-second welding condition, the toolset was only capable of producing 48 welds. In subsequent temperature experiments, thermocouples were embedded into welding coupons at various locations near weld center, allowing novel temperature data to be collected for welds with varying cycle times and parameters. The collected temperature data shows that as cycle time increases, so does weld temperature. At weld center, temperatures in excess of 500°C were observed in welds with 4 second durations. At these temperatures, Fe-Al intermetallic growth is anticipated as a mechanism limiting the tool life observed. The results suggest that steel is not an appropriate choice for RFSSW tools, and future evaluation of other materials is merited.

Degree

College and Department

Ira A. Fulton College of Engineering and Technology; Mechanical Engineering

Rights

https://lib.byu.edu/about/copyright/