The profile of the laser beam plays a significant role in determining the heat input on the deposition surface, further affecting the molten pool dynamics during laser-based directed energy deposition. The evolution of molten pool under two types of laser beam, super-Gaussian beam (SGB) and Gaussian beam (GB), was simulated using a three-dimensional numerical model. Two basic physical processes, the laser–powder interaction and the molten pool dynamics, were considered in the model. The deposition surface of the molten pool was calculated using the Arbitrary Lagrangian Eulerian moving mesh approach. Several dimensionless numbers were used to explain the underlying physical phenomena under different laser beams. Moreover, the solidification parameters were calculated using the thermal history at the solidification front. It is found that the peak temperature and liquid velocity in the molten pool under the SGB case were lower compared with those for the GB case. Dimensionless numbers analysis indicated that the fluid flow played a more pronounced role in heat transfer compared to conduction, especially in the GB case. The cooling rate was higher for the SGB case, indicating that the grain size could be finer compared with that for the GB case. Finally, the reliability of the numerical simulation was verified by comparing the computed and experimental clad geometry. The work provides a theoretical basis for understanding the thermal behavior and solidification characteristics under different laser input profile during directed energy deposition.

The international journal of Aerosol and Air Quality Research (AAQR) covers all aspects of aerosol science and technology, atmospheric science and air quality related issues.

Laser beam welding (LBW) has been considered an effective fusion welding method for the dissimilar welding of 304 stainless steel and Ni. However, the principles governing the correlations between the heat input, weld dimension, solidified microstructure and mechanical properties have not been fully studied before. Therefore, LBW experiments with variable heat input were carried out. A transient, three-dimensional model considering liquid metal convection was developed, and solidification parameters such as temperature gradient (G), growth rate (R), and cooling rate (GR) were calculated through thermal analysis to validate the experimental results. Then, microhardness tests were carried out to verify the predications made by the simulation. Energy dispersive spectroscopy (EDS) measurements were performed to study the mass transfer. The results indicate that the joints produced by LBW were nearly defect-free. The heat input per unit length is more effective at characterizing the influence of heat input on weld dimensions. The heat input has a greater influence on the cooling rate (GR) than the morphology parameter (G/R). The results demonstrate that both the solidification characteristics and mechanical property are greatly affected by the thermal behavior in the molten pool.

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.

A three-dimensional numerical model is developed to investigate the influence of sulfur content on the transitions of thermo-capillarity and flow dynamics during laser powder bed fusion (LPBF) of 316L powders. The impacts of variations in sulfur contents on thermal behaviors involving heat transfer and solidification characteristics, thermo-capillarity transition, as well as the spatial and directional transitions in flow dynamics, are analyzed through mechanistic modeling techniques. It is observed that transient thermal behaviors, including melt pool profile, track morphology, and solidification processes, are significantly influenced by the contained sulfur concentration. High sulfur concentrations tend to result in finer microstructures and equiaxed grains. Through simulations, it is noted that the transition in the sign of temperature coefficient of surface tension (TCST) is more easily observable in low-sulfur level but disappears as the sulfur concentration is extremely low (0.0001%) With sulfur content increasing, a more homogenized velocity distribution is observed, accompanied with heightened flow complexity denoted by the emergence of additional branch flows and vortices. These findings offer valuable insights into the underlying physics of melt pool dynamics in the LPBF process and present a potential approach for process optimization.