Surface Tension-Driven Flow and Its Correlation with Mass Transfer during L-DED of Co-Based Powders

Abstract

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 potential use of laser cladded functionally graded materials to mitigate degradation in boiler tube heat exchangers for power plant applications: A review

“…This might be explained by the Marangoni effect, which is a mass transfer caused by surface tension, driven by thermos-capillary forces and elemental diffusion. As shown in Figure 30, this causes a shallow melt pool to develop as the surface tension changes from low to high, suggesting that the deformation of the molten pool grows as the laser power increases [138]. They further revealed that the variation of velocity during deposition could result in unstable powder interactions with substrate, which often leads to processing defects such as cracks or pores if the movement of heat source is not controlled.…”

“…They further revealed that the variation of velocity during deposition could result in unstable powder interactions with substrate, which often leads to processing defects such as cracks or pores if the movement of heat source is not controlled.

Figure 30 Laser cladding temperature field and velocity distribution [138].

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“…Equation 1 is based on the dilution rate method, indicating that X mix represents the properties of the melt-pool material containing a mixture of powder and substrate materials. It is demonstrated that the melt pool generated during laser deposition is a mixture of melted powders and melted substrates, where the powder and substrate properties are depicted numerically as (X pow ) and (X sub ), respectively [138]. Where α is the mixture fraction governed by the dilution.…”

Design of dissimilar material joint for defect-free multi-material additive manufacturing via laser-directed energy deposition

“…This obstruction hinders powder jetting, causing the molten metal to flow downward and stretch the surface in contact with the obstacle due to adhesion. The surface in contact with the base metal contracts upon solidification, while other surfaces contract due to surface tension [ [28] , [29] , [30] , [31] ]. Consequently, pores form between the different metals due to convection flow within the molten metal.…”