Critical assessment: friction stir welding of steels

The level of activity in research on the friction stir welding of steels is dwarfed when compared with that on aluminium alloys. There are good reasons for this. The relative weakness of aluminium makes it ideally suited for a process which requires, at high strain rates, the permanent flow and mixing of material without melting. In contrast, there are in general no cost effective tools available for steel. The purpose of this article and the associated papers in this special issue of Science and Technology of Welding and Joining is to assess the state of the art, focusing on the knowledge base in the open literature.