Physical mechanisms of conduction-to-keyhole transition in laser welding and additive manufacturing processes

Size and build strategy effects for the L-PBF process applied to Inconel 625 vertical struts: a combined numerical and experimental approach

“…-Radiative and convective losses were imposed at the top surface of build layers during their activation stage, and removed as soon as a new layer was activated -For this global model, convective losses were considered on all the external surfaces in contact with the powder. Due to the very low equivalent heat conductivity of the powder bed (kpowder ~ 0.05 ksolid following previous works by [10], a h = 25 W.m -2 .K -1 convection loss coefficient was considered for the first model…”

“…At a microscale and mesoscale, multiphysics thermal and hydrodynamic modeling provide a physical representation of L-PBF melt pools. This involves for instance the keyhole mode formation including a realistic laser matter interaction with the deformed liquid surface using for instance a ray tracing approach [10], or the overlapping of a few tracks to address lack-offusion or roughness formation. Such models can either consider the powder bed as an equivalent medium or directly consider powder grains as discrete media [11] and address their incorporation in the melt pool.…”

Validation of a Theoretical Model for Laser Welding Thermal Field by Multi-Physics Numerical Simulation

“…This suggests that, in this specific region, a condition occurs in which the phenomenon of thermal conduction is prevalent. This can be explained in terms of the conduction-to-keyhole transition [54]: as long as the phenomenon of the multiple reflection of the laser beam is limited, and is not enough to trap a significant fraction of incident rays inside the cavity, as happens in the region closest to the external surface of the solid body, the transition to the keyhole mode cannot be considered to have occurred locally, Moving away from the upper surface, inside the workpiece, the two models diverge sharply. This highlights the intrinsic limitation of models that do not take into account the fluid dynamics of the phenomenon, and cannot capture the effects previously described, at the origin of the distribution of the solid fraction shown in the figure, with particular regard to the lower rear region of the melt pool.…”

“…This suggests that, in this specific region, a condition occurs in which the phenomenon of thermal conduction is prevalent. This can be explained in terms of the conduction-to-keyhole transition [54]: as long as the phenomenon of the multiple reflection of the laser beam is limited, and is not enough to trap a significant fraction of incident rays inside the cavity, as happens in the region closest to the external surface of the solid body, the transition to the keyhole mode cannot be considered to have occurred locally, and the thermal behavior is limited to the conduction mode. The pronounced flare that characterizes the morphology of the upper FZ cross-section, in both cases of experimental detection and numerical simulation, as highlighted previously (Figure 7), confirms the prevalence of the conduction mode, limited to this specific region.…”