Abstract
The fabrication of gamma prime (γ′) strengthened nickel-based superalloys by additive manufacturing (AM) techniques is of huge interest from the industrial and research community owing to their excellent high-temperature properties. The effect of post-AM-processing heat treatment on the microstructural characteristics and microhardness response of a laser powder bed fused (LPBF) γ′ strengthened nickel-based superalloy, MAD542, is systematically investigated. Post-processing heat treatment shows the significant importance of tailoring the γ′ morphology. With insufficient solutioning duration time, coarse γ′ formed in the interdendritic region heterogeneously, due to the lack of chemical composition homogenization. The cooling rate from the super-solvus solutioning plays an important role in controlling the γ′ size and morphology. Spherical γ′ is formed during the air cooling while irregularly shaped γ′ formed during the furnace cooling. The following aging heat treatment further tunes the γ′ morphology and γ channel width. After two-step aging, cuboidal γ′ is developed in the air-cooled sample, while in contrast, bi-modally distributed γ′ is developed in the furnace cooled sample with fine spherical γ′ embedded in the wide γ channel between coarse irregular shaped secondary γ′. More than 90% of the grains recrystallized during solutioning treatment at the super-solvus temperature for 30 min. The rapid recrystallization kinetics are attributed to the formation of annealing twins which significantly reduced the stored energy. Microhardness responses from different heat-treated conditions were examined.
Highlights
In recent years, the rapid development of metallic additive manufacturing (AM) techniques, e.g., laser powder bed fusion (LPBF), offers great potential for fabricating complex components of nickelbased superalloys for high-temperature applications
The Differential Scanning Calorimetry (DSC) sample was firstly heated from room temperature to 600 ◦C with a heating rate of 20 ◦C/min, the sample was hold and stabilized at 600 ◦C for 60 min
The γ′ morphology is sensitive to the cooling rates from the supersolvus temperature
Summary
The rapid development of metallic additive manufacturing (AM) techniques, e.g., laser powder bed fusion (LPBF), offers great potential for fabricating complex components of nickelbased superalloys for high-temperature applications. Via various alloy design approaches, newly developed superalloys, such as the MAD542 superalloy [10,11] with higher than 60 vol% γ′ fraction, ExpAM and ExpAM-mod superalloys [12] with 55 vol% γ′ fraction, and ABD-850/900AM [13] with low-to-moderate amounts of γ′, were additively manufactured by LPBF in the crack-free condition Both γ′ precipitation and the grain structure have a significant influence on the mechanical properties like strength, creep resistance, and fatigue performance, at the elevated temperature [14,15]. As examined by Kuo et al [25], the creep rupture life of LPBF fabricated IN718 superalloys are much shorter than the cast and wrought counterpart which is attributed to the subgrain boundaries prohibit mobile dislocation and result in stress concentration at the crack tips Under these bases, to serve at the elevated temperature, the well-recrystallized microstructure is of interest. The γ′ characteristics and grain structure with special focus on the recrystallization behavior correspond to different heat treatment conditions were systematically characterized and discussed
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