Unveiling gas–liquid metal reactions in metal additive manufacturing: High-fidelity modeling validated with experiments

Gases in the atmosphere inevitably react with the melt pool during metal additive manufacturing (AM). Oxygen is particularly reactive and excessive uncontrolled oxidation is detrimental, so most machines purge the chamber with inert gases, which can minimize but not eliminate such reactions. Alternatively, some users exploit the gas–liquid metal reactivity as an opportunity to introduce beneficial precipitates into the melt pool (“reactive AM”). However, the gas–liquid metal reaction and mechanisms in both scenarios remain unclear. Experimental works hitherto provide different explanations to the same phenomena. Therefore, this work seeks to clarify the mass transfer process of oxygen during metal AM through high-fidelity modeling by considering the competition between diffusion and chemical reaction, suboxide evaporation, and the influence of the vapor plume. The simulation results, validated with experiments, provide consolidated insights into the oxygen evolution behaviour during metal AM. Counterintuitively, higher melt pool temperatures do not necessarily lead to greater oxidation rates during processing. The melt pool has regions of high and low oxygen gains due to temperature-dependent reaction regimes, with concurrent oxygen loss from evaporation of metal suboxides. Thus, the net oxygen flux varies for different materials, and the oxygen content cumulatively changes as multiple tracks are scanned. Overall, this work provides useful guidance to AM community that seek to ameliorate or exploit the inevitable gas–liquid interaction in metal AM. Cost-saving measures may be possible for determining the purity of shielding gas used in AM, and physics-guided measures can be taken to limit or control gas–liquid metal reactions.