Material flow in friction stir welding: A review

The solid-state welding technology known as friction stir welding (FSW) is slowly replacing melting-based welding technologies. Friction stir welds functionality relies on the suitable tool stirring action and consequent material flow with the correct filling of the cavity left while the tool is advancing. However, material flow during the process is chaotic, complex and constantly hidden, limiting direct observation of tool-workpiece interaction and induced thermal and kinetic fields. Hence, critical issues such as the role of probe and shoulder, the mechanisms governing the process, i.e. stirring and/or extrusion, and the contact state between the tool and workpiece, i.e., sliding and/or sticking, are challenging to study, even though their knowledge is fundamental for profoundly understanding the process and building reliable numerical models. Here, experimental attempts carried out by the FSW community since its patent to clarify the material flow are critically reviewed, gathering knowledge built so far concerning material flow in FSW.