Understanding the Role of the Heat Transfer Coefficient Between Tool/Workpiece Interface During Friction Stir Welding

The heat transfer coefficient is a key parameter in modeling friction stir welding and has yet to be measured experimentally. The importance of this parameter was shown through validating friction stir models on both sides of the tool/workpiece interface. Both a transient plunge and a steady state model were validated by matching experimental temperatures. The steady state tool temperatures were matched (with in 2.5%), but the steady state workpiece temperatures were off by around 20%. The transient model of the FSW plunge showed the effect of varying the ℎ𝑊/𝑇 on workpiece temperatures and tool temperatures. Two methods were looked at to measure this parameter. Using frequencydomain thermoreflectance (FDTR) with a reflective sensor within the tool was initially looked at as a novel way to measure ℎ𝑊/𝑇, but the difficulty of integrating the system with the FSW machine led to its ultimate failure. The 3𝜔 method was the second choice to perform this measurement. The method was not able to measure ℎ𝑊/𝑇, but the method shows promise that after further work it should be able to perform this measurement. Successful thermal conductivity measurements were performed. A initial experimental setup for measuring contact resistance was achieved. Integration of a 3𝜔 sensor with the FSW environment was attempted. High temperature wire bonding was successful; several attempts at sensor integration with the tool failed; and plans for machine integration look promising.