Failure mechanisms of Friction stir welding tools related to process control and tool geometry

Friction stir welding (FSW) is subjected to process-specific challenges including comparatively high process forces and tool wear resulting from thermomechanical stresses. As a result, the acting loads and the geometric-related tool wear can cause tool failure. The tool (shoulder) design, whether it is concave or flat, with or without geometrical elements, is mainly responsible for the related failure mechanism and tool life. Therefore, this study systematically analyzes the failure mechanisms as a function of the process temperature, during FSW of AA-6060 T66 using tools made of H13 tool steel, with different shoulder designs, namely a concave contour and a scroll contour. The mechanism responsible for tool failure was induced by repeated welding at rotational speeds of 4000 rpm and 2000 rpm, at process temperatures within the range of the secondary hardness maximum (552 °C and 555 °C) and below the temperature of the secondary hardness maximum (488 °C and 499 °C). The experimental investigation showed that reducing the rotational speed of the scrolled shoulder from 4000 rpm to 2000 rpm resulted in less wear and therefore an increase in tool life from 474 m to up to 1400 m. In this context, it has also been shown that the shoulder geometry affects the mechanism relevant to failure due to the free length of the probe.