Hybrid ultrasonic arc welding of nickel-based alloys: improved cavitation and grain refinement

Hybrid ultrasonic welding experiments on Inconel 625 alloy were carried out to address the challenges of refining microstructure. The welding process phenomena and resulting microstructures were systematically analyzed. Four distinct welding methods—conventional tungsten inert gas (C-TIG), ultrasonic-frequency pulsed TIG (UFP-TIG), ultrasonic vibration-assisted TIG (UV-TIG), and hybrid ultrasonic TIG (HU-TIG)—were compared in terms of grain morphology, grain size distribution, and overall refinement. In the weld zone adjacent to the fusion line, hybrid ultrasonic treatment demonstrated a superior effect on grain morphology adjustment. The results show that HU-TIG effectively suppresses columnar growth and increases the proportion of equiaxed grains, yielding a finer microstructure than the other methods. The synergy of ultrasonic-frequency pulsing and mechanical ultrasonic vibration enhances cavitation activity in the molten pool, promoting dendrite fragmentation and homogenizing elemental distribution. Moreover, ultrasonic-frequency pulsed arcs alter the physical properties of the molten pool, enabling the melt to respond more effectively to ultrasonic cavitation.