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.

In order to investigate the effect of droplet impact on molten pool dynamics in hybrid laser-MIG (metal inert gas) welding, a three-dimensional mathematical model was developed, and a new approach to simulate the droplet impact was proposed. In the model, the droplet heat and impact under the influence of laser was considered, and the deformation of molten pool was calculated with considering droplet impact and arc pressure. To simplify the model, a modified body heat source model was presented for laser source to take into account multiple reflections and Fresnel absorption. Surface tension, electromagnetic force, and buoyancy were considered to calculate the fluid flow pattern. Furthermore, several dimensionless numbers were used to recognize the importance of the driving forces in molten pool. It was found that molten pool dynamics and final weld bead geometry were strongly affected by the droplet impact. To demonstrate the importance of droplet impact, the temperature, velocity distribution, and the deformation of molten pool were analyzed and compared under different conditions. With considering droplet impact, the penetration was bigger, and the width was smaller. Furthermore, the simulated weld bead geometry was in good agreement with experimental measurement.

Laser keyhole welding of 42CrMo in air and argon atmospheres has been studied both experimentally and numerically. Significant macroscale difference is observed for welding carried out under argon and air shielding. Fusion zone of welding under argon shielding has a “▽” shape, while it has a “U” shape for welding in air. The surface of the weldment is smoother for welding in argon when compared with that in air. Oxygen effect is proposed to account for the experimental results. A three-dimensional heat transfer model with a predefined keyhole is developed to study the heat transport and fluid flow in laser welding process. The variation of weld pool geometry and temperature history is investigated for different oxygen concentrations. It is found that a small amount of oxygen could significantly modify the weld pool dimension, while keeping the temperature history of materials, and thus the microstructure, in the fusion zone and heat-affected zone unchanged.