CHALLENGES OF TEMPERATURE MEASUREMENT DURING THE FRICTION STIR WELDING PROCESS

Silke Augustin, Technical University, Ilmenau, Germany, Thomas Fröhlich, Gunter Krapf, Jean-Pierre Bergmann, Michael Grätzel, Jan Ansgar Gerken, Kiril Schmidt, Technical University of Ilmenau, Germany, Technical University, Ilmenau, Germany, Technical University, Ilmenau, Germany, Technical University, Ilmenau, Germany (+2 others)
2020 Measuring Equipment and Metrology  
The exact determination of the process zone temperature can be considered as an increasingly important role in the control and monitoring of the friction stir welding process (FSW). At present, temperature measurement is carried out with the aid of a temperature sensor integrated into the tool (usually thermocouples). Since these cannot be attached directly to the joining area, heat dissipation within the tool and to the environment cause measurement deviations as well as a time delay in the
more » ... perature measurement. The article describes a process and the challenges that arise in this process, how a direct temperature measurement during the process can be achieved by exploiting the thermoelectric effect between tool and workpiece, without changing the tool by introducing additional temperature sensors. Keywords Friction stir welding, Direct temperature measurement, Seebeck-Effect, Measurement errors Introduction Friction Stir Welding (FSW) was developed and patented by Wayne Thomas at TWI (The Welding Institute) in Great Britain in 1991. It is assigned to the group of solid-state welding processes. In contrast to friction welding, the operating principle in FSW is not based on a relative movement of the workpieces, but by means of a wear-resistant rotating tool. One of the most relevant process variables is the axial force between the tool and the component. This force acts orthogonally to the welding direction and causes the tool to be completely immersed in and to remain in the joining area. This welding process is characterized by comparatively low joining temperatures below the melting temperature and excellent mechanical weld seam properties in comparison to conventional welding processes, such as arc and laser welding [1] . Friction stir welding is used in aerospace, shipbuilding, medical technology, and automotive engineering. However, the challenges for possible direct temperature measurement, based on the Seebeck effect, are the spindle speeds of the welding tool and the forces acting in the process. Thus, high demands on the design of the measuring device and the permanent transmission of the electrical voltage are necessary [2] . The measurement of the joining zone temperature during the process is an increasingly quantifiable indicator, as it allows conclusions to be drawn about the heat input and thus the thermomechanical stress on the microstructure [2] [3] [4] . Temperatures are currently measured by thermography or thermocouples, which are integrated into the welding tool [5] . However, the latter method is very costly and inaccurate, as the thermocouple does not contact the friction point between the shoulder of the tool and the workpiece. In addition, various publications have described that the thermocouples were either destroyed or changed in their position during the welding process so that an exact temperature measurement was not possible [3] . Measurement deviations and time delays can occur due to heat conduction in the tool or the heat transfer to the environment. An alternative to inserting thermocouples in the tool is the Tool-Workpiece-Thermocouple method (TWT method), in which the occurring electrical thermoelectric voltage between tool and workpiece can be measured and then converted into a temperature value. However, this method places high demands on the used measuring circuits and the experimental determination of the various influencing parameters, as these have a decisive influence on the uncertainty of the measured temperature. In the following, the application of this method is described using the example of a robotized friction stir welding system and the results achieved are presented and discussed.
doi:10.23939/istcmtm2020.01.034 fatcat:lgcpwp6cnzcctlvf3lpaagg5oa