Effects of a PCBN-Coated Tool and Auxiliary Plasma Arc on Microstructure and Mechanical Properties of Friction Stir Welded 6 mm Thick DH36 Steel Joints

Abstract Friction stir welding (FSW) experiments were conducted to join 6 mm thick DH36 shipbuilding steel using three distinct welding strategies: (i) the conventional FSW process with a conventional WC-10 wt% Co tool, (ii) the conventional FSW process using a PCBN-coated pin/WC-10 wt% Co tool, and (iii) the plasma-assisted FSW process using the PCBN-coated WC-Co tool. We investigated the effects of advanced tool design and auxiliary energy-assisted approach on process outcomes, including thermal history, axial force, tool performance, and the metallography and mechanical properties of the weld joints. Among the three welding approaches, the PCBN-coated pin/WC-Co tool, combined with plasma arc assistance, significantly reduces tool wear due to a reduction in peak axial force of approximately 24% compared to the conventional method using the same tool. The microstructural analysis demonstrated substantial grain refinement, with up to 23% and 36% reductions in grain size of the stir zone (SZ) and thermo-mechanically affected zone (TMAZ), respectively, resulting from the plasma-assisted FSW process using the PCBN-coated WC-Co tool when compared to conventional welds. This grain refinement improves the hardness with a maximum value of 241.6 HV at the TMAZ on the advancing side. According to the tensile test results, the joints from the conventional FSW process with the conventional WC-Co tool resulted in fractures from the SZ, while the joints from the other conditions failed in the parental metal side.