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Materials Science in Additive Manufacturing Carbon addition in IN738LC

Prior studies illustrate that one potential method to room for improvement to optimize the HT processes for
eliminate the problem of hot cracks in this alloy is through LPBF-built IN738LC alloys.
the addition of carbon, as the formation of carbides serves In this study, we fabricated five sample sets of IN738LC
as trapping sites for the hot-crack-promoting element, that alloys with different carbon contents through the LPBF
is, boron. The role of carbon has long been a debatable approach and subjected the as-built samples to different
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topic within the nickel-base superalloy community. HT cycles. The evolution of the carbide and γ’ precipitation
First, carbon segregation along grain boundaries was was closely monitored after each procedure, including
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reported to improve the grain boundary strength. As their microhardness and tensile properties. Moreover,
a result, the concurrent improvement in the material’s the less-reported issues of delamination cracking during
room temperature and high-temperature tensile strength the fabrication process are also presented and discussed
(5 – 6%) and elongation (30 – 50%) was observed on toward the end.
several occasions. 15,16 However, low-temperature and high-
stress creep tests show that the increase in carbon content 2. Methods
has a detrimental effect. The excessive carbon content leads
to enlarged carbides, which facilitate the formation of gas 2.1. Sample fabrication
pores and subsequently result in a large primary creep A Trumpf TruPrint 1000 LPBF machine was employed for
strain and short rupture life. Second, carbon was found fabricating the IN738LC samples in the present study. The
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to prevent oxide formation during the material’s melting equipment has a laser source with a wavelength of 1070 nm
stage, improving the overall melt’s cleanliness. Yet, and a laser spot size of 30 μm. Five sample sets with different
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during service under a high-temperature environment, carbon (C) concentrations ranging from 0 to 0.4 wt.% were
the oxidation of carbide itself could introduce plastic produced, with a 0.1 wt.% carbon interval between each
deformations and contribute to soft recrystallized regions sample set. The IN738LC powders were obtained from
nearby, serving as crack initiation sites. Finally, metastable Shanghai Truer Industrial Development Co., Ltd, with
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carbides at low temperatures are effective trapping a powder size ranging from 15 to 53 μm. Its chemical
sites, either for hydrogen or boron, to prevent hydrogen composition is listed in Table 1. Graphite powders were
embrittlement and solidification defects. However, brittle purchased from Nanografi Nano Technology, having an
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topologically close-packed (TCP) phases tend to form on average powder size of ~1 μm. The powders were mixed in
the carbide decomposition at high temperatures, leading to a roller mill (BMU-100-3) for 12 h, with a rotational speed
pre-mature material failure under loading. 21 of 180 rpm. Before sample production, the powders were
dried in a vacuum at 60°C for 24 h. A chessboard laser scan
Besides the effect of carbon on the performance of
nickel-base superalloys, its interaction with γ’ precipitation strategy was adopted for the actual production, with an
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is also another subject worth investigating. The γ’ phase individual island dimension of 4 × 4 mm , a layer thickness
within those LPBF-built IN738LC only appears during of 20 μm, hatch spacing of 90 μm, a laser scanning speed of
heat treatment (HT). It is believed that the additional 1200 mm/s, and a laser power input of 115 W and 130 W,
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carbon will induce competitive precipitation between respectively.
the carbide and γ’. Hence, an optimization of the HT Blocks with a size of 10 × 10 × 25 mm³ were fabricated
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process due to the additional carbon addition might be in a graded structure format from 0 wt.% carbon to
necessary. Conventionally, the as-cast IN738LC goes 0.4 wt.% carbon, with each composition having a 5 mm
through a classic two-step HT, with a solutionization HT height (Figure 1A), under an oxygen concentration of
at 1120°C for 2 h, followed by 24 h aging HT at 845°C. about 100 ppm. The different powder compositions were
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Several studies have reported on the inadequacy of this loaded into the powder storage unit within the LPBF
conventional two-step HT toward recrystallization and machine sequentially; therefore, the graded structure
suggested that a stress-relief HT at 845°C for 24 h before could be produced in a single production run.
the two-step HT is required to lower the temperature
threshold for recrystallization. Past literature suggests 2.2. Material characterization and HT
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that for IN738LC alloys, a unimodal fine γ’ precipitation All as-built specimens were removed from the substrate
or a bimodal γ’ precipitation with the size of the finer γ’ through electric discharge machining. The specimens were
below 100 nm is desirable for both the room-temperature mounted in PolyFast carbon resin and grounded with SiC-
and high-temperature (up to 850°C) tensile properties. paper of grit in the descending order of #220, #500, #1000,
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To obtain such a fine γ’, the aging temperature needs to be #2000, and #4000, respectively. Subsequently, a polishing
at least 1120 °C to trigger the mechanism of “precipitate step with MD-Dac (DiaPro Dac 3 μm suspension) and
refinement by solution HT.” Hence, there is still plenty of MD-Nap (DiaPro Nap 1 μm suspension) was carried out,
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Volume 3 Issue 1 (2024) 2 https://doi.org/10.36922/msam.2264