A proposed diffusion-controlled wear mechanism of alloy steel friction stir welding (FSW) tools used on an aluminum alloy

A study of diffusion wear mechanism in 1.2344 X40CrMoV5-1 steel FSW tool has been carried out from the standpoint of tribological layer generation and interaction with the tool׳s metal. It was shown that during FSW of AMg5M aluminum alloy the latter covers the tool׳s working surface and then iron/aluminum reaction diffusion is initiated under the conditions of high mechanical stress and temperature. Since diffusion by the former austenite grain boundaries is much faster than that of by volume an intermetallic compound is formed inside the tool׳s metal thus causing embrittlement and pulling out the tool׳s metal fragments. •We study wear mechanism in steel FSW tool.•Tribological layers are found on the tool׳s surface.•Grain boundary reaction diffusion occurs in the layers.•Intermetallic compound embrittles the steel grain boundaries.•Wear particles detach from the steel tool by fracture.