Heat and Mass Transfer Behavior of Pulsed Arc Plasma of Duplex Stainless Steel

Duplex stainless steels contain nearly equal proportions of ferrite (delta) and austenite (gamma). Its unique organizational characteristics enable it to combine the good plastic toughness and uniform corrosion resistance of austenitic stainless steel with the high strength and stress corrosion resistance of ferritic stainless steel. Consequently, it is widely used in marine engineering, petrochemical, and other important energy fields. Oil and gas pipelines are the most reliable and cost-effective means for safely transporting energy over long distances. To extend the service life of duplex stainless-steel oil and gas pipelines, arc additive remanufacturing has been developed based on overlay welding technology with special advantages such as high efficiency and low cost, and thus has broader application prospects. Hence, it is necessary to have the capability to effectively control the heat and mass transfer characteristics of the arc plasma and to elucidate the interaction mechanisms between the arc plasma and droplets. These factors have significant effects on the stability, forming quality, and microstructure of the arc additive repair process of duplex stainless steel. However, the high enthalpy, strong arc light characteristics, and non-equilibrium physicochemical reactions of arc plasma impede the quantitative analysis of the heat and mass transfer mechanisms of arc plasma via in situ testing methods. A multiphysics coupling simulation model of arc-droplet integration was established in this study. The model is based on the theories of electromagnetism, fluid dynamics, thermodynamics, and in situ experiments using a high-speed camera and electrical signal acquisition, while considering the behavior of metal vapor in plasma. The Eulerian multiphase flow model was used to improve the computational convergence of the two-phase mixing region of the arc plasma and metallic droplets. The physical fields of the gas and metal were solved separately using two sets of governing equations, and a species transport model was used to calculate the distribution of the metal vapor. This study thus sought to investigate the heat and mass transfer behavior of a pulsed arc plasma with duplex stainless steel and reveal the interaction mechanism between the arc plasma and droplets. A cycle in the pulsed arc additive manufacturing process of duplex stainless steel was selected to study the heat and mass transfer behavior. The temperature field, velocity field, metal vapor behavior, and experimental results at six characteristic moments of the arc plasma additive manufacturing process were analyzed separately. The results indicate that the peak arc plasma temperature was distributed on both sides of the droplet axis and was positively correlated with the current. The temperature distribution on the substrate surface was not uniform owing to the asymmetric effect of the arc plasma during necking to the transition stage of the droplet. In addition, the results of the flow-field distribution of the arc plasma were similar to those of the temperature field. However, the velocity peaks at different instances were not only related to the corresponding values of current but also to the transition states of the droplets. With the droplet transition, both the high-temperature and high-speed regions of the arc plasma were compressed toward the substrate. Before the droplet necking, the iron vapor was gradually compressed toward the axis as the current increased, and the mass fraction below the droplet could reach 100%. Following the droplet necking, the high concentration of iron vapor above and below the droplet increased the electrical conductivity of the plasma, which in turn accelerated the droplet transition. On the substrate surface axis, the mass fractions of iron vapor at different instances of times were between 20% and 60%. The simulation results of the pulsed arc plasma with duplex stainless steel droplets were generally consistent with the experimental results in terms of heat and mass transfer behavior, although complex electromagnetic thermal effects occurred between the arc plasma and the molten metal.