Liquid-state miscibility gap enabled crack suppression in functionally graded additively manufactured Ni-Cu alloy systems

This study investigates how the liquid-state miscibility gap enables crack arrest during functionally graded additive manufacturing (FGAM) of Ni-Cu alloy systems, using a combination of experimental characterization and CALPHAD-based thermodynamic modelling. In that regard, IN625-CuCrZr cuboid samples were fabricated using a dual-hopper laser directed energy deposition (L-DED) system, incorporating a gradual compositional transition to mitigate abrupt thermophysical mismatches. Lack of fusion (LoF) defects were substantially mitigated by tailoring the process parameter to achieve an optimal energy density across the gradient, resulting in an overall porosity level <0.25%. Solidification cracks were observed in the composition corresponding to 25 wt.% CuCrZr, where a Cu-rich liquid film in interdendritic spaces was identified as the primary mechanism for crack initiation. The combined results of Scheil solidification simulations, Kou’s crack susceptibility index (CSI), and differential scanning calorimetry (DSC) measurements of the solidification range confirmed the pronounced effect of compositional variation on crack susceptibility. At higher Cu contents (50–75 wt.% CuCrZr), a liquid-state miscibility gap between the two alloys was revealed by CALPHAD simulations and confirmed by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analysis. This miscibility gap led to the formation of Cu-lean spheroids around which the crack paths deflected, as shown by X-ray computed tomography (X-CT). Electron Backscatter Diffraction (EBSD) analysis demonstrated that the miscibility gap also promoted grain refinement by the spheroids acting as nucleation sites, transitioning the grain structure from coarse columnar (~80 µm) to fine equiaxed (~4 µm), which ultimately resulted in complete crack arrest at 75 wt.% CuCrZr. This work identifies a novel crack mitigation mechanism stemming from the liquid-state miscibility gap, demonstrating that controlled development of the miscibility gap can be leveraged to improve processability across Ni-Cu alloys and other systems prone to liquid-phase immiscibility during fusion-based manufacturing processes. • IN625–CuCrZr FGAM alloys were fabricated via L-DED with graded transitions to reduce mismatches. • Lack-of-fusion porosity was reduced to <0.25% by maximizing absorbed laser power, P a . • Solidification cracks at 25 wt.% CuCrZr arose from Cu-rich interdendritic films, confirmed by CSI simulation. • A miscibility gap at 50–75 wt.% CuCrZr formed Cu-lean spheroids, with cracks deflecting around them. • The miscibility gap led to grain refinement, promoting crack arrest at 75 wt.% CuCrZr.