Page 104 - ESAM-1-2

Engineering Science in
Additive Manufacturing HIP temperature effects on LPBF Hastelloy X

resistance. As the core of aerospace vehicles, the turbofan be inferred that the low carbide content in the solution-
1-3
engines encounter challenges such as complex structures, treated Hastelloy X specimens restricted carbide formation
prolonged cycles, and high costs with casting and forging and mechanical performance during high-temperature
techniques. Compared with traditional techniques, service. Therefore, it is imperative to devise a new heat
4,5
the laser powder bed fusion (LPBF) technology was treatment method to enhance the carbides of Hastelloy
employed to enable the integrated precision fabrication of alloys.
6-8
complex structures. However, the rapid layer-by-layer In recent studies, hot isostatic pressing (HIP) has been
manufacturing process of as-built structures harmed the considered a thermally activated process that promotes
microstructure and mechanical properties, hindering the grain boundary migration and carbides with limited
practical application of LPBF technology. Therefore, heat cooling rate. A few studies reported that HIP treatment
9,10
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treatment was necessary for as-built structures to enhance was beneficial for the enhancement of deformation capacity
the comprehensive mechanical properties at room and at high-temperature tensile tests. 23-25 It was found that the
high temperatures. 11-13 high-temperature deformation ability of HIP specimens
Recently, to meet the application requirements of was considered to be possibly affected by grain boundary
the above-mentioned specifications for Hastelloy X, the proportion and carbide precipitation. 26-29 Marchese et al.
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solution treatment was widely employed to optimize found that HIP-triggered recrystallization generated
defects, homogenize the microstructure, and enhance equiaxed grains, and the slow cooling rate led to the
mechanical properties. In research on room temperature production of intergranular carbides and intragranular
mechanical properties, Keshavarzkermani et al. analyzed carbides in the 1160°C HIP process of Hastelloy X alloy.
14
the grain misorientation and grain boundary structure of According to a report by Tomus et al., the low number of
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as-built and solution-treated LPBF Hastelloy X samples. finely dispersed carbides segregating at the grain boundaries
The result showed that the pores were eliminated, and grain in the Hastelloy X alloy HIP-treated at 1,175°C showed
recrystallization with varying degrees was induced with minimal effect on its mechanical properties. Sanchez-
different solution treatment methods, accompanied by the Mata et al. proposed that compared to discrete carbides
24
columnar grains transforming into recrystallized equiaxed along the grain boundaries under ST, HIP treatment at
grains. Cheng et al. investigated the strengthening 1,155°C brought more continuous precipitates along grain
15
mechanism between the microstructure and mechanical boundaries in Hastelloy X specimens. Li et al. proved
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properties of the solution-treated Hastelloy X samples. that the chain-like and plate-like precipitates distributed
They found that the full release of distortion energy with at the grain boundaries under the HIP temperature of
the complete grain recrystallization led to a significant 1,100°C – 1,175°C contributed to the different mechanical
reduction in the dislocation density and the appearance properties of Hastelloy X specimens. Sun et al. conducted
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of twins. These studies succeeded in enhancing the HIP treatment at 1,050°C and solution treatment on LPBF-
mechanical properties at room temperature with limited manufactured GH3536 alloy, with results showing that
grain anisotropy and twin boundary transition. both tensile strength and elongation at high temperature
More importantly, the high-temperature mechanical were improved. In summary, the present HIP treatment
properties of the Hastelloy X alloy are critical for the temperature ranges from 1,050°C to 1,175°C for LPBF-
manufacturing of hot-end components for aerospace manufactured Hastelloy X structures, based on casting and
engines. 16-18 Montero-Sistiaga et al. compared the high- forging structures. However, a higher HIP temperature
19
temperature mechanical properties of as-built and solution- was required to activate the recrystallization and carbide
treated Hastelloy X specimens. They observed a substantial precipitation process due to the distinctive microstructure
decline in both strength and ductility of the Hastelloy X characteristics, which are rarely reported in present
alloy, which may be associated with carbide formation at studies. Therefore, by adopting elevated HIP temperature,
the grain boundaries during the high-temperature tensile this study primarily focused on the role of carbide control
test. Agrawal et al. also confirmed that the reduced in enhancing the high-temperature mechanical properties
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ductility with the elevated temperature was attributed to of LPBF-manufactured Hastelloy X superalloy and the
the intergranular fracture caused by the carbides at the mechanism of grain recrystallization and carbides on
grain boundaries. Although the solution treatment of mechanical performance following HIP treatment.
Hastelloy X alloy achieved high mechanical properties at This study fabricated an LPBF as-built Hastelloy X
room temperature, it failed to acquire sufficient mechanical specimen, along with those subjected to HIP treatment
properties in the high-temperature tensile test, thereby at 1,100°C, 1,180°C, and 1,210°C. The effects of HIP
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impeding its practical application. Furthermore, it could treatment on substructure, recrystallization, and carbide

Volume 1 Issue 2 (2025) 2 doi: 10.36922/ESAM025240015