Influence of the Tool Material on the Microstructural, Residual Stress, and Mechanical Property Development in Ti-6Al-4V Friction Stir Welds

One of the main obstacles to Friction Stir Welding (FSW) for the joining of Ti-alloys is the development of a tool material that can withstand the high temperatures (~>1000˚C) and stresses of the process without breaking. In addition it must be inert with Titanium. In this investigation, a number of tools of different materials were used to join Ti-6Al-4V sheets. Although the different tools seemed to produce sound welds, in-depth characterisation was necessary to assess their influence on the weld properties. Optical Microscopy (OM), Scanning Electron Microscopy (SEM), and microhardness measurements were used to assess the microstructural development and the width of the weld regions in the as-welded and post-weld heat-treated conditions. Mechanical properties were also evaluated by performing tensile testing across traverse sections of the joint. Finally, the residual stress development was studied using neutron diffraction. It was found that the microstructure of the weld region displays either acicular α or martensitic α', which was attributed to the local thermal field associated with the FSW tool and the subsequent rapid cooling. Volumetric features (e.g. wormholes) and tool debris were occasionally observed in some of the welds, resulting in a reduction in mechanical properties. Nonetheless, the process parameters appear to play an important role in the welding process, whereby the tool performance can be enhanced through the use of appropriate parameters.