Modeling of selective evaporation induced composition change during high-power laser metal processing

During high-temperature processing of metallic materials, such as high-power laser welding and additive manufacturing, volatile alloying elements evaporate from the liquid pool. Alloying elements evaporate at varying rates when exposed to the same temperature. Consequently, the chemical composition of welded or printed components can deviate substantially from the original base material. The evaporation rate of an alloying element is determined by its vapor pressure over the liquid alloy, which is influenced by the local temperature and the alloy's composition. The loss of alloying elements and the resulting compositional changes impact the microstructure, corrosion resistance, and mechanical properties of the parts. In this work, first, we provide examples of composition change from the laser welding literature. Then, composition changes due to the selective evaporation of alloying elements during laser powder bed fusion additive manufacturing of nickel-based superalloy, Inconel 718 is modeled. First, a heat transfer model is used to calculate the temperature field which is then used to estimate the vapor pressure of alloying elements. Activities of elements in an alloy computed using a thermodynamic model are used along with the vapor pressure to estimate the partial vapor pressure of elements over the molten alloy. Then, the evaporative flux of elements is calculated using the partial vapor pressure and local temperature. Finally, composition change is computed from the evaporative flux using mass balance. The computed results are tested against independent experiments. Evaporative losses and composition change increase at high laser powers and slow scanning speeds.