Powder Bed Fusion of Difficult-to-Print Ni-Based Superalloys: Microstructural Evolution and Cracking Behavior

Abstract

IN738 is a precipitation-strengthened nickel-based superalloy that is widely valued in industry due to its excellent creep resistance and good corrosion performance. However, it exhibits poor manufacturability, primarily due to its complex alloy chemistry and the challenges associated with solidification during processing. In this study, a mechanistic process–microstructure–cracking relationship was first established for LPBF-processed IN738LC. It was observed that melt pool geometry plays a critical role in crack formation, with an optimal width-to-depth ratio governing crack susceptibility. Reducing hatch spacing or increasing laser power resulted in grain coarsening, with the effect being more pronounced for hatch spacing reduction. Nevertheless, crack density was significantly reduced with decreasing hatch spacing, which is attributed to improved part densification and a more favorable melt pool geometry. The as-built microstructure was found to be highly non-equilibrium due to the rapid cooling rates inherent to LPBF. It consisted of a γ matrix with cellular/dendritic solidification substructures, submicron carbides located at interdendritic regions, and dispersed oxide particles with Al-rich cores. No γ′ precipitates were detected in the as-built condition. Elemental segregation and oxide formation, combined with thermal stresses, contributed to reduced ductility and promoted crack initiation during processing. In the second stage of the study, heat treatment was employed to develop a high-temperature-capable microstructure, particularly aiming for a controlled distribution and size of γ′ precipitates with spherical, cuboidal, and irregular morphologies. A series of heat treatments with varying solutionizing (S) and ageing (A) temperatures were performed to promote crack healing and to develop an optimized microstructure, particularly γ′ precipitates with desirable size, distribution, and morphology for high-temperature applications. The heat treatment conditions mainly included solutionizing at different temperatures (S), solutionizing followed by low-temperature ageing at 845 °C (SLA), solutionizing followed by high-temperature ageing at 1120 °C (SHA), solutionizing followed by double-ageing at high and low temperatures (SDA), and the industry-recommended standard heat treatment for this material (ST: S1120-A845). It was observed that high-temperature solutionizing promotes a more homogeneous microstructure, whereas at lower temperatures (around or below 1120 °C), homogenization is only partial. It was observed that varying the solutionizing and ageing conditions led to the development of diverse γ′ precipitate size distributions, ranging from unimodal to bimodal and multimodal. Unimodal distributions were dominated by fine secondary γ′ precipitates, while multimodal structures consisted of fine secondary γ′ in conjunction with coarse primary γ′. It was further demonstrated that high-temperature ageing (≈1120 °C) facilitates γ′ coarsening. In contrast, low-temperature ageing (≈850 °C) stabilizes the secondary γ′, resulting in a fine, well-defined, and coherent γ′ distribution within the γ matrix. In addition to γ′ precipitation, other secondary phases were identified. Carbide precipitates, including those enriched in alloying elements such as Ti, were predominantly located along grain boundaries. Moreover, Cr-rich phases were observed to preferentially form at grain boundaries. These Cr-rich precipitates were shown to develop during low-temperature ageing (≈850 °C) and may contribute to the degradation of tensile properties. Solutionizing was identified as the primary factor governing recrystallization. At elevated temperatures, the microstructure underwent full recrystallization, resulting in pronounced grain coarsening. In contrast, at lower temperatures (e.g., ~1120 °C), the grain structure remained largely similar to the as-built condition with minor modifications. Additionally, crack healing was observed at higher solutionizing temperatures and was directly associated with the recrystallization of the material. A solid-state crack healing mechanism was proposed, whereby the high-energy state of the as-built microstructure—characterized by cracks, free surfaces, and high grain boundary density—provides a strong thermodynamic driving force for energy reduction. Upon heating above a critical temperature, this driving force promotes recrystallization and crack closure, leading to a more stable microstructure. In the subsequent phase, mechanical performance was systematically evaluated through room-temperature tensile testing of both as-built and heat-treated samples. Tensile tests at room temperature were performed along the vertical direction (i.e., loading direction parallel to the build direction). The as-built condition exhibited the lowest yield strength, while the standard heat-treated sample showed the highest. Overall, the tensile properties at room temperature were found to be governed by a combination of factors, including dislocation density, LAGB structures, grain size, γ′ precipitation (precipitation strengthening), anisotropy, residual cracking, crack healing, recrystallization, grain coarsening, and the presence of detrimental grain boundary phases. In the as-built state, the relatively lower strength and higher ductility were primarily attributed to strengthening mechanisms dominated by high dislocation density and low-angle grain boundary (LAGB) networks. The superior strength of the standard heat-treated sample in the vertical direction resulted from the synergistic effect of γ′ precipitation strengthening and the retained as-built microstructural characteristics (e.g., columnar grain structure and high LAGB density). In the other heat-treated conditions, strengthening was mainly controlled by γ′ precipitation together with crack healing during high-temperature solution treatment. The higher yield strength of the SLA sample relative to the other highly solutionized conditions was primarily attributed to the finer γ′ precipitates and reduced interparticle spacing. Furthermore, Cr-rich grain boundary phases formed during ageing at 845 °C contributed to intergranular embrittlement and fracture, which was confirmed by EDS analysis. This was consistent with the lower ductility and reduced UTS values observed in the SLA and SDA conditions relative to the SHA condition. The modified heat treatment strategies developed in this study produced a crack-free and nearly isotropic microstructure while providing improved room-temperature mechanical properties compared with the as-built condition. The combination of recrystallization and complete crack healing highlights their potential for high-temperature service, making these heat treatment routes promising alternatives to the conventional industrial heat treatment. In another case study, CM247LC, a non-weldable Ni-base superalloy, was fabricated by electron beam powder bed fusion (EB-PBF) at a wide range of energy levels. For this purpose, variable process parameters were adjusted to investigate their effect on microstructure and crack formation. Samples fabricated at both low and high area energies exhibited pronounced crack susceptibility. At very low energy densities, lack of fusion (LoF) and porosities were observed, while higher energy densities produced denser samples. Adjustments to energy density and process parameters resulted in a grain structure transition from fine-columnar to coarse-columnar and near-single crystal morphologies. Despite these changes, the cracking issue persisted, with micro-cracks observed in low-energy samples and macro-scale cracks, several millimeters long, forming at higher energy densities, highlighting the material’s high sensitivity to crack formation. Both solidification and liquation cracking were identified— the former showing dendritic crack surfaces, and the latter associated with eutectic phases and grain boundary precipitates. Severe recrystallization around cracks was observed at high energy densities, characterized by elevated dislocation densities. EDS analysis revealed hafnium- and silicon-rich precipitates in interdendritic regions and near cracks, contributing to severe hot cracking in the material.