The effect of thermo-fluid flow on solidification conditions in laser additive manufacturing of superalloy IN718

Investigating the fundamental mechanisms that shape the solidification conditions within the molten pool is crucial for elucidating and enhancing the solidification microstructure in laser additive manufacturing (LAM). Solidification conditions are shaped by temperature distribution and transients at the solidification interface, which are intricately linked to thermo-fluid flow behaviors. However, the influence of these flows on solidification conditions remains elusive. Laser powder bed fusion (L-PBF) and laser directed energy deposition (L-DED) represent distinct LAM processes with different levels of energy inputs. This study delves into the thermo-fluid flow behaviors and solidification conditions of these processes using a transient three-dimensional molten pool model. The laser position and melt flow jointly govern the temperature gradient distribution, while melt recirculation shifts the temperature gradient contour at the solidification interface towards the molten pool's front. Varying melt flow driving forces at the end of L-PBF and L-DED molten pools lead to distinct heat transfer mechanisms. In L-PBF, melt flow dominates, while in L-DED, heat conduction prevails. These mechanisms affect the cooling rate distribution, resulting in opposite primary dendrite arm spacing distributions in the two molten pools. The findings of this study provide an insight into understanding the formation of the alloy solidification microstructure in the LAM.