Laser direct energy deposition (L-DED) is one of the most promising additive manufacturing methods, which has been paid more and more attention in recent years. An improved heat and mass transfer model was developed here to analyze thermal behavior, driving force, surface tension-driven flow and its correlation with dilution during L-DED of Co-based powders to a 38MnVS substrate. Thermal behavior was firstly studied for its fundamental influence on fluid flow and mass transfer. Next, the roles of capillary force and thermal capillary force were characterized using both the dimensional analysis and simulation methods, and the mechanism of surface tension-driven flow was also qualitatively investigated. Finally, flow characteristics inside the melt pool were studied in detail and their correlation with the dilution phenomenon was analyzed based on the multi-component mass transfer model. The temperature gradient was found to be much larger at the front of the melt pool, and it took about 200 ms for the melt pool to reach a quasi-steady condition. Moreover, sharp changes in the curvature of the solid/liquid boundary were observed. Surface tension was demonstrated as the main driver for fluid flow and resulted in centrally outward Marangoni flow. Capillary force contributes to the reduction of the curvature of the free surface, and thermal capillary force (Marangoni force) dominated the Marangoni convection. Alloy elements from the powders, such as Co and Ni, were added to the front part of the melt pool and mainly diluted at the upper side of the rear region near the symmetric plane of the melt pool. Fundamental results in this work provide a valuable understanding of the surface tension-driven flow and its correlation with concentration dilution during the additive manufacturing process.

The effects of the surface-active element oxygen on the laser welding of 304 stainless steel (304SS) and nickel were numerically and experimentally studied in pure argon and argon–oxygen mixed gas atmospheres containing 21% oxygen (AMO). In this study, the molten pool morphology, thermal behavior, solidification phenomenon, correlation between dilution and convection flow, and microhardness of welding joints were analyzed. As a result of oxygen effects, the molten pool was deeper, the maximum temperature was higher, and the maximum flow velocity was lower in the AMO. The cooling rate (GR) and combination parameter (G/R) were studied by the direct simulation of temperature gradient (G) and solidification growth rate (R). Combined with the solidification microstructure, it was found that oxygen had little effect on grain size. The major elements Fe, Cr, and Ni within the solidified molten pool in the AMO were uniformly diluted, while the distribution of the above elements was non-homogenous in pure argon. Stronger flow and multiple directions of convection inside the molten pool contributed to uniform dilution in the AMO. The distribution of microhardness was similar to the content of Cr, and the microhardness at the substrate interface of the joint was higher in the AMO than in pure argon. The preliminary conclusions of this study provide in-depth insights into the effects of surface-active element oxygen on heat and mass transfer in laser dissimilar welding.

Understanding heat and mass transfer and fluid flow in the molten pool is very helpful in the selection and optimization of processing parameters, and the surface-active element has an important effect on the heat and mass transfer in laser welding of dissimilar metals. A three-dimensional (3D) numerical model coupled with a sub-model of surface tension, which considers the influence of local temperature and the concentration of surface-active element sulfur at the gas/liquid surface, is used to analyze the thermal behavior, driving forces, fluid flow, and solute dilution during laser linear welding of 304SS and Ni. The relationship between surface tension, driving forces, and the temperature coefficient of surface tension with the spatial distribution of temperature and the surface-active element sulfur is quantitatively analyzed. The simulation results show that the molten pool is fully developed at 45 ms, and the collision of inward and outward convection, with the maximum velocity reaching 1.7 m/s, occurs at the isotherm with a temperature between 2200 K and 2500 K. The temperature-gradient term and concentration-gradient term of surface shear stress play different roles in different positions of the free surface. The local sulfur concentration changes the temperature sensitivity of the surface tension at different sides of the free surface and further determines the transition of convection. Complex fluid flow promotes solute dilution, and the distribution of solute becomes uniform from the front to the rear of the molten pool. The Ni element is transferred to 304SS mainly at the rear side. The work provides theoretical support for the control of joint quality by changing the content of surface-active elements in dissimilar welding.