Thermoelectric Measurements for Temperature Control of Robotic Friction Stir Welding

Sammanfattning: Friction stir welding (FSW) has undergone a rapid expansion in several industrial sectors such as in the aerospace, marine, railway and automotive sectors. Current industrial applications are mainly simple long straight welds, but there is a growth of interest in components with higher geometric complexity. However, welding of geometrically complex components represents a challenging task due to the resulting uneven induced thermal dissipation along the weld, but especially due to the need for suitable equipment, able to accurately follow a complex 3D path under high mechanical loads, while managing the machine deflection. This is the case for robots, where the high process forces result in deflections, which affects robots' compliance, leading to weld failures and poor consistency in mechanical properties.In the presented approach, the rotational speed is controlled during welding in order to maintain the set temperature value along the weld. An innovative method to measure the process temperature, the tool-workpiece thermocouple (TWT), which offers a temperature estimation from the whole tool-workpiece interface (TWT-data), is set as the controlled variable. The overall aim of this thesis is then to demonstrate the industrial applicability of TWT temperature control for joining geometrically complex components using robotic friction stir welding.The TWT-data signal is demonstrated to be fast, repeatable and representative of the welding temperature. Moreover, TWT-data supplies online information during the whole weld procedure, especially during plunging. The shoulder contact with the workpiece is identified by TWT-data, providing for an improved plunging operation, which was demonstrated to significantly improve the use of robotic FSW, overcoming the lack of stiffness inherent to this equipment type at this welding stage.Improved joint performance, low tensile strength variation along the weld path and a reduced number of failed welds were achieved by welding under temperature control. As a result, such a welding approach simplifies the development of a welding procedure, allowing for a decrease in time and material. The concept was successfully validated by performing two welds consisting of two dissimilar materials in a two-dimensional weld path on a geometrically complex component by using robotic equipment. The temperature control approach is not limited to robotic equipment, but also suitable for standard FSW equipment, being of interest to a various range of applications where quality and/or time is an important factor.