CMT-WAAM processing of AZ31 magnesium alloy assisted with ultrasound: Evolution of arc, melt pool and microstructure

Ultrasound has recently been employed to assist cold metal transfer wire and arc additive manufacturing (CMT-WAAM) owing to its widely recognized ability to refine crystal structures via acoustic cavitation. However, there has been lesser focus on the contribution of ultrasound to the thermodynamic behavior of the arc and melt pool, which are closely associated with processability and fusion defects. In this work, the ultrasound is carefully designed and applied to assist the CMT-WAAM of the AZ31 magnesium alloy. The arc morphology, droplet spattering, and melt pool behaviors are monitored and investigated. The results reveal that ultrasound contracts the arc size, eliminates large droplet spatters, and improves the melt wettability, thereby enhancing the process stability. With increasing ultrasound intensity, the capillary waves on the melt pool surface are disrupted, generating atomized fine droplet ejections and reducing the material utilization efficiency. By employing optimized processing parameters, the equiaxed grains of the as-built sample are refined. Furthermore, ultrasound reduces the content of precipitated phases in the AZ31 magnesium alloy, transforming their morphology from long rods to granules. The tensile properties of the alloy significantly improve through ultrasonic-assisted grain refinement. The application of 5 μm ultrasound increases the yield strength from 114.2 MPa to 140.8 MPa and the ultimate tensile strength from 208.2 MPa to 250.7 MPa, representing improvements of 23.3 % and 20.4 %, respectively. However, excessive ultrasonic energy is detrimental to grain refinement and mechanical property enhancement in the AZ31 magnesium alloy.