Prediction of crystal nucleation and growth behavior of Fe/Ni-based multi-materials deposited by laser directed energy deposition using a multi-area, multi-layer, and multi-scale phase field calculation (M3-PFC)

Laser directed energy deposition (LDED), based on the powder feeding technology, possesses the capability to fabricate complex structures within hollow structures. The arbitrary Lagrangian-Eulerian (ALE) method was innovatively applied to derive temporal interpolation functions for the free surface contour F (x, y, t) and the temperature distribution T (x, y, t) of the melt pool from a mesoscopic computational fluid dynamics (CFD) thermal model. These interpolation functions were not simply integrated but specifically designed to impart a quantified impetus for the growth of crystalline solidification within the microstructural phase field, effecting real-time adjustments to the computational domain of the phase field model. The multi-area, multi-layer and multi-scale phase field calculation (M3-PFC), which takes into account the bi-directional coupling of Marangoni flow and the phase field, was established to incorporate the transportation of elements within the multi-layer deposition and previously solidified material. Thermodynamics of multi-layer deposition was integrated into M3-PFC, enabling the prediction of multi-layer dendritic evolution using LDED process. Influence of Marangoni flow on microstructure evolution of multi-layer C276 deposited on 316 L-substrate utilizing M3-PFC was investigated during LDED process. It was observed that the nucleation of equiaxed and columnar crystals occurred at the top and bottom-middle of the melt pool. Both columnar and equiaxed crystals were preferentially produced along the opposite direction of Marangoni flow, impeding the apparent dendrite-growth in the counter direction. Partial remelting of the (N + 1)th layer was induced by the thermodynamics generated in the (N + 2)th deposition layer, promoting re-solidification of the liquid phase in the (N + 1)th layer. However, no liquid-solid transformation occurred in the Nth layer subjected to periodic heating. The partially remelted tissue in the (N + 1)th layer was shown in wider columnar crystals generated through the competitive growth, which were transformed into planar crystals and identified as the initial solidification conditions for the nucleation of the (N + 2)th layer. The columnar crystal growth, nucleated at the planar crystals of the (N + 1)th layer, was dominated in the (N + 2)th layer. Micropillar compression tests revealed that the columnar crystal had a compressive strength of 1102.78 MPa, while the equiaxed crystals displayed a higher compressive strength of 1464.34 MPa.