The thermal-flow dynamics of hydrogen bubbles in coaxial laser wire directed energy deposition of Al-Mg-Sc alloy

Abstract

Hydrogen porosity is one of the common defects in the additive manufacturing of aluminum alloys. The presence of many pores initiates fracture cracks, reducing the strength and ductility of the materials. Understanding the evolution of hydrogen pores will be more conducive to controlling pores. In this study, micro X-ray computed tomography (μ-XCT) was employed to characterize the pore distribution along layers, and a high-fidelity multiphase flow model was developed to investigate the thermal-flow dynamics of hydrogen bubbles in coaxial laser-wire directed energy deposition. The model simulated bubble behaviors including bubble escape, coalescence, and capture. The simulations agreed well with the experimental results on the profile of the single track and pore amount in different layers. Pore formation depended on both melt flow and solidification rate. The criterion of bubble capture was proposed that the local solidification rate was larger than the bubble velocity. The impact of wire feeding on the molten pool caused strong melt flow at the top of the molten pool, and pores were not easy to form. The low-speed region appeared at the lower middle of the molten pool due to a recirculation pair, where hydrogen bubbles were likely to be captured with a large solidification rate. Heat accumulation during multilayer deposition altered the solidification characteristics, leading to an increase followed by a decrease in pore amount along the deposition direction. The hydrogen pores formed at the interface between columnar and equiaxed grains due to a high solidification rate in the region where the columnar-equiaxed transition occurred. Based on the experimental and simulation results, the evolution of grain morphology and hydrogen pores in multilayer deposition was reconstructed.