Influence of cobalt addition in the fusion zone of duplex stainless steel welded with pulsed Nd:YAG laser

With a volume fraction of austenite and ferrite that is roughly equal, duplex stainless steels (DSS) have good mechanical characteristics and a high level of corrosion resistance. On the other hand, for decades, laser welding of sheets has been adopted as a profitable technology for joining many metallic materials. Obtaining a balanced microstructure and ensuring its qualities and uses is the difficulty in DSS laser welding. Generally, when DSS is welded with a technique inherently associated with high cooling rates, a mostly ferritic microstructure results. The addition of austenite stabilizing elements or heat treatments is indicated to balance the microstructure. Before butt welding two UNS S32750 DSS sheets with a pulsed Nd:YAG laser, a 50-µm-thick cobalt layer was electro-deposited onto the edge of one of them with a Watts bath procedure. The microstructural characterization of the weld beads was performed using optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction. The volume fractions were determined using two methods: (1) Image analysis software for SEM images and (2) computerized volume fraction computation from XRD spectra. As the volume fractions of austenite increased, the results demonstrated that adding cobalt in the fusion zone effectively resolved the unwanted unbalanced microstructure that resulted from autogenous laser welding of the DSS. Residual stresses were determined for the austenitic phase and the ferrite matrix in the weld bead, the former resulting affected by tensile stresses, while the latter by compressive stresses.