Effects of Z-axis increment on forming quality and microstructure evolution of Inconel 625 fabricated by laser directed energy deposition

During the layer-by-layer stacking of laser directed energy deposition (LDED), interlayer thermal accumulation significantly alters convective flow patterns and actual layer dimensions, making the Z-axis increment (ΔZ) a critical factor for thermal-fluid stability. To elucidate this transient mechanism, this study investigates the thermo-fluidic and solidification behaviors of Inconel 625 alloy under varying ΔZ during fundamental double-layer deposition. A transient 3D model coupling heat transfer and fluid flow was developed using the Volume of Fluid (VOF) method and validated experimentally. Results reveal that an excessive ΔZ amplifies positive laser defocusing and temperature gradients, triggering intense Marangoni convection that destabilizes the melt pool free surface and increases porosity. Conversely, an optimized insufficient ΔZ of 1.10 mm mitigates the layer height loss induced by thermal accumulation, maintaining a stable laminar flow and continuous thermal input. Furthermore, ΔZ strongly dictates the local cooling rate and solidification morphology. An increase in ΔZ elevates the cooling rate, refining the primary dendrite arm spacing (λ) and enhancing local microhardness. Based on the rapid solidification thermal history, a modified Kurz-Fisher model, λ=45.72(G·R) -0.28, was established. The characteristic exponent n=0.28 quantitatively captures the unique solute trapping and intense interfacial thermal constraints inherent to the LDED process. This work elucidates the cross-scale relationship among thermo-fluid flow, layer-height mismatch, and microstructural evolution, providing a mechanistic basis for adaptive Z-axis control in LDED of Inconel 625.