Ultrasonic assisted additive manufacturing of Al-Mg alloys : microstructure refinement and mechanical improvement through meltable ultrasonic probe

Abstract

Wire arc additive manufacturing (WAAM) is a cost-effective technique for manufacturing Al-Mg alloys; however, its rapid melting-solidification process tends to cause coarse grains. Hence, a novel meltable ultrasonic probe assisted WAAM (MUPA-WAAM) was developed, while a three-dimensional numerical model of MUPA-WAAM was established and verified. High-speed camera images showed that the short-length arc deflected towards the meltable ultrasonic (MU) probe, and significant capillary waves appeared on the molten pool. Spray transfer and pulsed short circuiting transfer occurred alternately in the wire. Periodically alternating of shallow and large insertion depths of the vibrating MU probe homogenized the material via double-pulsed arc. The ultrasonic energy promoted melting of MU probe, and melted metal resulting from MU probe increased the productivity by 286.9%. Ultrasonic treatment improved microstructure and mechanical properties. The average grain size decreased from 39 μm without ultrasonic treatment to 14 μm with ultrasonic treatment, and the average porosity was declined to 0.29% with ultrasonic treatment. The ultimate tensile strength and yield strength were increased by 29.1% and 6.7% in the vertical direction, respectively, and those were improved by 8.7% and 9.2% in the horizontal direction, respectively. The average micro-hardness was enhanced by 18.6%. The simulation results confirmed that high acoustic pressure mainly distributed on the annular side of the MU probe, and the cavitation effect in the molten pool induced by high-intensity ultrasound was key factor for affecting microstructure. This work elucidates the ultrasonic transmission mechanism regulated by the pulsed arc and the evolution of acoustic pressure under varying status of MU probe, providing valuable guidance for improving microstructure and mechanical properties in WAAM-fabricated components.