Thermoelectric magnetohydrodynamic control of melt pool flow during laser directed energy deposition additive manufacturing

Experimental Quantification of Inward Marangoni Convection and its Impact on Keyhole Threshold in Laser Powder Bed Fusion of Stainless Steel

“…For most metals and alloys, the temperature coefficient of the surface tension is negative, resulting in an outward convection of the melt pool, which flows from high temperature to low temperature regions on the melt pool surface. However, different from the previously reported outward melt flow patterns during laser AM [11,15,16,[23][24][25], the melt pool dynamics of all samples in this study are featured with strong inward Marangoni convection. In fact, the inward convection is an important phenomenon for iron-based alloys in the welding community [22,54,55], but few experimental observations of this phenomenon have been reported in the field of laser AM [30].…”

“…By automatically tracing hundreds of moving particles in the melt pool from thousands of consecutive frames, the fluid flow is visualized and quantified with an exceptional lateral resolution of approximately 10 µm. The findings reveal the presence of a prevalent inward fluid flow (from cold area to hot area on the melt pool surface) in the melt pool, which differs from the previously reported outward flow with other commercial LPBF materials [16,[23][24][25], along with the coexistence of outward and inward convections in the keyhole mode. This study primarily centers on gaining a deep understanding of the mechanisms underlying inward Marangoni convection and its dual impact on the behavior of the melt pool and the conduction-keyhole threshold within laser-based AM.…”

“…The addition of 3 wt% tungsten equals 1.3 vol% tungsten in the melt pool of SS316L, ensuring a sufficient number of visible tracers without having a significant impact on the melt pool properties. Similar volume fractions of tungsten tracers have been employed in previous studies on melt pool dynamics [23][24][25]. Thin-wall samples of 300 μm in thickness were produced on the build plates with the mixed powder using a SISMA MySint 100 (SISMA, Italy) LPBF machine before the synchrotron measurements.…”

“…However, the melt pool dynamics in the nearly pore-free regime have not been fully explored experimentally, particularly in the conduction mode, as it is more challenging to characterize the melt flow behavior without the contrast of keyhole and pores. Micro-particles with high melting point, such as tungsten and tantalum, have been implemented in in-situ measurements as flow tracers to visualize the liquid flow in the melt pool [16,[22][23][24][25]. However, the traced movements of a few particles within a few radiographs cannot serve as a reliable reference for the high-fidelity mechanistic modeling of the melt pool dynamics in this regime.…”