Friction stir welding of 2507 super-duplex stainless steel: feasibility of butt joint groove filling at different process parameters

The present work explores the feasibility of groove-filling welding of 6 mm-thick 2507 super-duplex stainless steel (SDSS) in a similar butt-joint configuration using friction stir welding (FSW). A 60° V-groove with a 3 mm root face was designed and machined. The FSW trials were performed at varying rotation speeds (400–600 rpm), downward forces (15–25 kN), and a constant travel speed of 25 mm/min using two WC tools with two different cobalt (Co) contents of 8 and 10 wt.%. The groove filling achievements of the welded joints were monitored using three non-destructive tests: visual, penetrant, and radiographic. All the welded joints were subjected to transverse tensile testing in addition to hardness measurements across the joint area. Microstructure and fracture surface characterisation were also investigated to gain insights into the impact of heat input on joint performance through a comparison with the as-received SDSS. Among the trial FSW processes, the results indicate that the FSW parameters of 25 mm/min, 15 kN, and 400 rpm using a WC-8 wt.% Co tool were successfully used to completely fill the groove and produce a defect-free SDSS butt-joint. This joint has the highest average stir zone (SZ) hardness (342 ± 5 HV) and tensile strength (1256 ± 21 MPa) compared to partially jointed specimens.