A hybrid ultrasonic strategy for super duplex stainless steel welding: Coupling arc induced acoustics and mechanical vibration to refine grains and balance phases

Non-equilibrium solidification in arc-based welding and melting-based additive manufacturing often leads to coarse microstructures and segregation under steep thermal gradients. Although ultrasonic assistance has been explored as an additional process control route, its effectiveness in high-density metallic melts is frequently limited by strong acoustic attenuation, elevated cavitation thresholds, and stage-dependent action. This work proposes a hybrid ultrasonic strategy that couples ultrasonic frequency pulsed arc with external mechanical vibration to overcome these limitations. The fundamental principle lies in the phased co-regulation of molten pool hydrodynamics and solidification behavior via the integration of pulsed arc and mechanical ultrasonic vibration. Specifically, the pulsed arc can tailor the molten pool thermal state and thereby enhance the effectiveness of externally applied mechanical ultrasonic, enabling stronger solidification-stage regulation in dense metallic melts. This strategy is demonstrated in autogenous tungsten inert gas welding of 2507 super duplex stainless steel. Compared with the conventional method, the hybrid strategy achieved significant microstructural refinement: the average ferrite grain size was reduced from 143.4 μm to 70.2 μm, while the austenite area fraction increased from 6.9% to a more balanced 20.0%. These microstructural evolutions led to a simultaneous 16% increase in tensile strength and a remarkable 97.4% enhancement in ductility. These findings provide a physically grounded, transferable framework to mitigate coarse microstructure and phase imbalances in phase-sensitive alloys. This strategy can inspire ultrasonic-assisted processing for various heat sources and structural alloys in welding and additive manufacturing, addressing challenges including microstructural tailoring and precision phase regulation.