Design and application of wireless tool temperature measurement of friction stir welding (FSW) for process monitoring and control

This paper proposes a wireless friction stir welding (FSW) tool temperature measurement method, FSW process window, and control strategy based on the FSW tool temperature profiles. Tool temperature is important data for FSW process monitoring, optimization, and control because it includes detailed information on the actual FSW process condition where the tool induces frictional heat, softening, and stirring the workpiece material. Conventionally, thermocouples installed at workpieces or thermal imaging, which are indirect measurements have measured temperature data in the FSW process. The proposed wireless FSW tool temperature measurement system consists of a thermocouple-integrated FSW tool, data measurement, wireless data transmission modules based on widely used Internet of Things (IoT) devices, and a conventional tool holder. The developed system reliably measures multiple temperature channels at a high sampling rate in real time. Based on the tool temperature profiles measured under various FSW process conditions, tool temperature profile features indicating welding quality, and process window for stable welding quality are proposed. FSW process control strategy and process setup for the early FSW process stage based on the tool temperature measurement are also proposed. Experimental validation using aluminum (Al6061-T6) workpieces shows that tool temperature profiles significantly correlate with weld quality. A critical tool temperature threshold of 510 degrees C was identified, beyond which ultimate tensile strength (UTS) stabilizes around 220 MPa (70 % of base material strength). The proposed approach can not only improve FSW process monitoring and control but also contribute to data-based manufacturing as well as relieve the tacit knowledge aspect of the FSW process.