Transient dynamics and stability of keyhole at threshold in laser powder bed fusion regime investigated by finite element modeling

Abstract

A Finite element model is developed with a commercial code to investigate the keyhole dynamics and stability at keyhole threshold, a fusion regime characteristic to laser microwelding or to Laser Powder Bed Fusion. The model includes relevant physics to treat the hydrodynamic problems—surface tension, Marangoni stress, and recoil pressure—as well as a self-consistent ray-tracing algorithm to account for the “beam-trapping” effect. Implemented in both static and scanning laser configurations, the model successfully reproduces some key features that most recent x-ray images have exhibited. The dynamics of the liquid/gas interface is analyzed, in line with the distribution of the absorbed intensity as well as with the increase of the keyhole energy coupling. Based on these results, new elements are provided to discuss our current understanding of the keyhole formation and stability at threshold.

Absorptivity measurements during laser powder bed fusion of pure copper with a 1 kW cw green laser

“…A nearly similar approach by Simonds [15] on a 316L powder bed also evidences time-fluctuating absorptance values during L-PBF in near keyhole welding mode. Such a keyhole instability and resulting absorptance fluctuation is due to the destabilization of the rear keyhole wall, and has been confirmed analytically by Fabbro [24] and numerically by Mayi [25].…”

Multiphysics simulation of single pulse laser powder bed fusion: comparison of front capturing and front tracking methods

“…The melt pool is sheared from the center of the interaction zone to the sides as the Marangoni coefficient is negative (Wei et al, 2012). Then, as the laser power increases, the models predict a transition from this flat melt pool in conduction mode to moderate vapor deflection [Figure 6(b)-6(c)], typical of the so-called forced conduction regime (Mayi et al, 2021), where the recoil pressure drives the melt flow. The melt is ejected from the center to the rim, leading to a dig of the melt at the center of the interaction zone.…”

“…More numerical models in bare plate configuration since followed. Notably, Courtois et al (2014) studied pore formation at laser cutoff in spot welding, Tan et al (2013) studied absorption of laser incident power by the vapor plume with potential improvement made by using an assisting gas (Tan and Shin, 2014), Pang et al (2016) investigated correlation between keyhole oscillations and vapor plume swing, Kouraytem et al (2019) investigated keyhole fluctuations, with focus on laser absorption and protrusion dynamic on keyhole front wall and Mayi et al (2020) looked at vapor plume dynamics and beam focusing effect at keyhole threshold (Mayi et al , 2021).…”

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

“…But recent advances in melt pool in situ visualization by X-ray radiography [8] and online absorptance measurement [1,9,10], demonstrated the causal relationship between the KH shape and the coupling mechanisms that were then suggested. These experimental works, in turn, constituted a valuable database for more recent modelers, to validate their numerical simulations and to further analyze and quantify the mechanisms that lead to conduction-tokeyhole transition in stationary laser configuration [11][12][13]. KH stability and induced melt flow have also been widely investigated in the context of welding, and more recently, in the field of LPBF.…”