Processing-microstructure-property correlations for temperature-controlled friction stir welding of 304L SS plates

Friction stir welding (FSW) is being considered as a potential repair welding technique for various engineering applications. In this study, an effort was made to explore the correlation between microstructure and mechanical properties in FSWed 304L stainless steel (SS) plates. An in-house developed weld temperature control algorithm was used during FSW of 304L SS plates. FSW was performed at two different temperatures, 725 and 825 degrees C, in order to repair simulated cracks introduced by electrical discharge machining (EDM) in the 304L plates. Optical microscopy and scanning electron microscopy revealed ultrafine grained microstructure in some parts of the stir zone (SZ) of the FSW nugget, which is attributed to dynamic recrystallization occurring during FSW. A significant decrease in the fraction of Sigma 3 special boundaries with concomitant decrease in the number of annealing twins was noticed in SZ as compared to the base metal. Vickers microhardness profiles across the SZ showed increase in hardness which was due to significant grain refinement. Tensile tests were carried out across the weld bead, and failure was found to occur in the base metal, indicating an overmatching FSW nugget.