Microstructure and mechanical properties of friction stir welded AISI 316L austenitic stainless steel joints

Rolled sheets of 3 mm thick, AISI 316L austenitic stainless steel was successfully friction stir (FS) welded at different tool rotational speeds in the range from 400 rev/min to 800 rev/min with precisely selected constant values of other process parameters (welding speed of 45 mm/min, axial load of 12 kN and tool tilt angle of 1.5°). The microstructural characterization by orientation imaging microscopy (OIM) and transmission electron microscopy (TEM) revealed that discontinuous dynamic recrystallization was the dominant recrystallization mechanism in the stir zone. The grain size of the stir zone was greatly influenced by both heat generation and material strain rate. The shear bands were observed in the weld zone and the intensity of the shear bands was highly influenced by the deformation temperature. The results of the multi-spot energy dispersive spectroscopy (EDS) analysis within the narrow region of the shear bands depict the presence of tungsten traces for all the cases and, the minimal tungsten concentration was found in the weld joint made at 600 rev/min. The non-existence of secondary phases in the weld zone was owing to lower peak temperatures in FS welding. The base steel and the FSW joints depict a stable pitting potential after the activation controlled anodic region and the weld joints had marginally better pitting corrosion resistance than the base steel.