Optimized Design for a Device to Measure Thermal Contact Conductance During Friction Stir Welding

Abstract

Friction stir welding (FSW) is a solid-state welding process that is finding increasing use in a variety of industries, owing to its ability to create high-quality welds with less heat input than fusion welding. While the modeling of FSW has been an active effort for at least 15 years, two input parameters, namely the friction coefficient and the heat transfer coefficient, are still adjustable quantities that are difficult to measure. This lack of information compromises the predictive capability of FSW models. While the modeling of friction between the tool and workpiece remains a complex task, the measurement of heat transfer should be possible, but has not been adequately addressed because of the difficulty of accessing the relevant interface with thermocouples. This paper presents a multi-layered frequency-domain thermoreflectance (FDTR) method and transducer design to measure the heat transfer coefficient between the spinning tool and the workpiece. Due to constraints of the welding process, a multi-layered structure is needed for a useable measurement to maximize the heat flow from the modulated heating surface through the heat transfer interface into the welded workpiece. An analytical 2D thermal quadrupole model is shown to be useful in determining layer properties. A multi-layered structure for a specific tool design is validated using COMSOL and optimized. This process can be used to determine the ideal transducer structure to maximize the signal from an FDTR measurement during a friction stir welding process.