Effect of a Ni interlayer on microstructure and mechanical properties of WC-12Co cermet / SC45 steel friction stir welds

Abstract WC-12Co cermet was friction stir lap welded with SC45 steel with or without different thicknesses of a Ni interlayer. The visualization of the material flow was rendered easy by the insertion of the Ni layer. Welding gives rise to the formation of Widmanstatten ferrite, low temperature pearlite, bainite, martensite and retained austenite in the nuggets of the joints. Some interdiffusion is noticed at the steel-cermet, steel-Ni and Ni-cermet interfaces, which leads, in particular, to the formation of oversaturated Co solid solutions in Fe or in Ni. The increase in the Ni interlayer thickness is shown to lead to a more pronounced wear of the Si 3 N 4 tool and to a greater amount of retained austenite in the nugget. The mechanical tests finally suggest that the interface of the Ni bearing joints is of better quality than that of the Ni-free joint.