Microstructure and Mechanical Properties of Friction Stir Processed AISI 316 Stainless Steel: Evaluation of the Effect of Cooling Media and Multi-Step Processing on Microstructure and Mechanical Properties of Friction Stir Processed AISI 316 Stainless Steel

The effect of friction stir processing on microstructure and mechanical properties of AISI 316 stainless steel was investigated, where characterizations were performed using optical microscopy, field emission scanning electron microscopy, electron backscatter diffraction, X-ray diffraction, atom probe tomography, and micro-hardness and tensile tests. The results revealed dramatically refined grains in the stir zone after applying the method. The application of two-stage processing in the presence of external coolant resulted in the finest grain size of 161 nm. The formation of induced martensite and segregation of alloying elements were observed as a result of processing. Moreover, the formation of martensite was induced up to 39.4 volume percent. Finally, one-step friction stir processing, with external coolant, resulted in enhancing tensile properties. It was shown that extreme grain size refinement, a large fraction of induced martensite along with segregation of alloying elements into the grain boundaries could be the main strengthening reasons. Graphical abstract