Mechanistic Insights into Ultrasonic Cavitation in Magnesium Alloy Melt Pools: Effects on Pore Formation and Microstructural Evolution

AbstractUltrasonic assistance is widely employed to refine microstructures in additive manufacturing, yet its fundamental role during melt pool solidification remains insufficiently understood. By introducing high-power ultrasound into the melt pool during cold metal transfer wire and arc additive manufacturing (CMT-WAAM) of AZ31 magnesium alloy, this work reveals the spatially heterogeneous and dual roles of ultrasonic cavitation. Incomplete cavitation effect in the melt pool can lead to the retention of cavitation bubbles, significantly increasing the porosity of the melt pool after solidification, with shrinkage defects preferentially forming around retained cavitation bubbles. A framework describing bubble nucleation, growth, migration, and survival within the WAAM melt pool is established based on combined numerical simulations and theoretical analyses. Moreover, microstructural characterization reveals grain refinement in regions distant from cavitation bubbles, whereas bubble-adjacent regions exhibit grain coarsening accompanied by continuous precipitate networks. These results provide mechanistic insight into the concurrent beneficial and detrimental effects of ultrasonic cavitation, thereby offering guidance for optimizing ultrasonic-assisted additive manufacturing.