Effect of infill passes on the microstructure evolution of inconel 718 thin walls produced using laser-directed energy deposition

Laser-directed energy deposition (L-DED) is an additive manufacturing technique that, in addition to fabricating new parts, is especially adept at creating and repairing thin wall structures. This study explores the effects of the number of infill passes on the microstructure and mechanical properties of Inconel 718 thin walls fabricated by L-DED. The effects of spatially varying cooling rates across both heights above the substrate and through the thickness of the thin wall samples, quantified through a combination of dendrite arm spacing measurements and finite element simulations, revealed that the most rapid cooling occurred at the walls' base under all conditions. The number of infill passes had a significant effect on this behavior, with the no infill pass (P2 sample) condition showing the most uniform cooling rate through its height. Moreover, the microstructure variation resulted in spatially varying hardness ranging from 170 HV in the stainless steel substrate, to similar to 220 HV towards the skin and top of the samples, and finally up to similar to 270 HV in the center of the samples where the cooling rate was the highest. More infill passes (P3 sample with one infill pass and P4 sample with two infill passes) led to higher cooling rates, finer grain structures, and increased hardness, particularly in the central regions of the thin wall samples. Conversely, fewer passes resulted in a more uniform but generally lower hardness throughout the sample. It is concluded that the infill pass strategy is a critical parameter in the L-DED process that directly influences the thermal history and, thus, the resultant microstructure and properties of Inconel 718 thin walls.