Friction stir processing/transient liquid phase bonding (FSP/TLP) of AISI304 stainless steel

Friction stir processing (FSP) is used to eliminate the defects and modify the microstructure of the joints made using other welding and joining processes. In transient liquid phase (TLP) bonding, the formation of deleterious phases in the bonding zone as well as diffusion affected zone (DAZ) is always challenging. In this study, FSP was applied as a post-bond treatment to modify the microstructure of TLP joints for 304 stainless steel. Microstructural analysis of the joint after FSP showed no sign of the TLP bond line previously visible in the stir zone as well as refined microstructure due to dynamic recrystallization. The eutectic phases, produced by incomplete isothermal solidification of the liquid interlayer during TLP bonding, were coarse and interconnected before FSP. After FSP, these phases became completely distributed in the stir zone, and in the thermo-mechanically affected zone, they were finer and disconnected from each other. In the thermo-mechanically affected zone, a narrow curved shape TLP bonding line was observed, and its width increased into the heat-affected zone. FSP increased the TLP joint strength by about 37% from the pre-FSP state. After FSP, the joint strength reached about 87% of that of the base metal, which is consistent with the microstructural observations showing a lack of defects and discontinuities at the TLP bond line. As a consequence, FSP can be regarded as a complementary process to improve the quality of TLP joints, maximizing their mechanical properties.