Microstructure evolution of dissimilar AISI 304L and AISI 410S stainless steel joints by the friction stir welding

This study investigates the main microstructural characteristics of dissimilar friction stir welds between AISI 304L and AISI 410S stainless steels. Axial forces ranging from 25 to 40 kN were applied, while maintaining a constant rotation speed of 450 rpm and a welding speed of 1 mm/s. The AISI 304L steel was positioned on the retreating side, and the AISI 410S steel on the advancing side. As the axial force increased, refinement of the ferritic grain size was observed in the heat-affected zone (HAZ) and thermomechanically affected zone (TMAZ) of AISI 410S, along with an increase in δ-ferrite content in the HAZ and TMAZ of AISI 304L. Under all conditions, the stir zone (SZ) exhibited more intense grain refinement than the TMAZ and HAZ. The interfacial region between the two steels was significantly affected by the applied axial force. Higher axial forces promoted increased material intermixing between AISI 304L and AISI 410S. In addition, the interfacial morphology evolved from predominantly elongated and rounded material-flow features to sharper, more angular features, accompanied by the formation of secondary material-flow structures. This evolution indicates improved material flow and contributes to enhanced joint consolidation in the dissimilar friction stir welded stainless steels.