This work investigates the feasibility of using friction stir welding (FSW) process as a groove filling welding technique to weld duplex stainless steel (DSS) that is extensively used by petroleum service companies and marine industries. For the FSW experiments, three different groove geometries without root gap were designed and machined in a DSS plates 6.5 mm thick. FSW were carried out to produce butt-joints at a constant tool rotation rate of 300 rpm, traverse welding speed of 25 mm/min, and tilt angle of 3^o using tungsten carbide (WC) tool. For comparison, the same DSS plates were welded using gas tungsten arc welding (GTAW). The produced joints were evaluated and characterized using radiographic inspection, optical microscopy, and hardness and tensile testing. Electron back scattering diffraction (EBSD) was used to examine the grain structure and phases before and after FSW. The initial results indicate that FSW were used successfully to weld DSS joints with different groove designs with defect-free joints produced using the 60° V-shape groove with a 2 mm root face without root gap. This friction stir welded (FSWed) joint was further investigated and compared with the GTAW joint. The FSWed joint microstructure mainly consists of α and γ with significant grain refining; the GTWA weld contains different austenitic-phase (γ) morphologies such as grain boundary austenite (GBA), intragranular austenite precipitates (IGA), and Widmanstätten austenite (WA) besides the ferrite phase (α) in the weld zone (WZ) due to the used high heat input and 2209 filler rod. The yield strength, ultimate tensile strength, and elongation of the FSWed joint are enhanced over the GTAW weldment by 21%, 41%, and 66% and over the BM by 65%, 33%, and 54%, respectively. EBSD investigation showed a significant grain refining after FSW with grain size average of 1.88 µm for austenite and 2.2 µm for ferrite.

The aim of this study is to investigate friction stir welding (FSW) to join A304 austenitic stainless steel and low carbon steel A283 Gr. C in-lap configuration to clad the carbon steel with highly corrosion-resistant stainless steel. Thus, a wide range of FSW parameters were investigated such as FSW tool rotation rate from 200 to 400 rpm, tool traverse speed from 25 to 75 mm/min, and vertical forces of 20 to 32 KN. The FSW parameters combination of high welding rotation rate (400 rpm) and high vertical forces (32 KN) results in rejected joints in terms of surface appearance and clear surface defects. On the other hand, rotation rates of 200 and 300 rpm with different welding speeds and vertical forces resulted in some sound joints that were further investigated for microstructure and mechanical properties. The sound lap joints were examined via optical microstructure, SEM, and EDS investigations. For the mechanical properties, both tensile shear testing and hardness testing were used. The transverse macrographs showed intermixing between the two dissimilar materials with an almost irregular interface. The hardness profile in both materials showed a significant increase across the different regions from the Base Material (BM) to the nugget zone, with a maximum value of 260 Hv in the stainless steel and 245 Hv in the carbon steel. This increase is mainly attributed to the grain refining in the weld region due to the dynamic recrystallization and transformations upon the thermomechanical cycle. The tensile shear load of the joints varied from 20 to 27 KN for the FSWed joints, with the highest joint tensile shear load of 27 KN for that produced at 300 rpm tool rotation and 25 mm/min welding speed.