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Materials Science in Additive Manufacturing Carbon addition in IN738LC

A B C
1 1 1

A B C
2 2 2

A B C
3 3 3

Figure 3. Backscattered electron imaging of specimens after the industrial standard two-step HT at different magnifications for (A –A ) pure IN738LC,
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1
(B –B ) IN738LC with 0.1 wt.% carbon, and (C –C ) IN738LC with 0.3 wt.%. Bright spots are MC carbides and dark precipitates are γ’.
1 3 1 3
For the three-step HT, the pure IN738LC had a vastly trapping sites for the hot-crack-promoting element, boron.
different grain morphology. Columnar grains changed Despite its promising result on hot crack elimination and
into equiaxed grains encompassing straight annealing ease of application, there are several potential problems
twins (Figure 6A ). Such a microstructure is envisioned associated with this method. First, it is the viability of
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to be beneficial for providing an isotropic property when having sufficient γ’ precipitates, as the increased carbon
the material is strained under different loading directions. content will yield more carbides, thus competing for those
With an increment of 0.1 wt.% of carbon within the essential γ’ forming elements. Second, carbides are also
base alloy, the material failed to make this columnar- known for their pinning effect in nickel-base superalloys,
to-equiaxed transition but retained its columnar grain as they prevent the migration of recrystallization grain
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shape, though the grain size enlarged with more <110> boundaries, thus hampering grain recrystallization.
crystallographic orientations (Figure 6B ). As for the Third, while having more carbides should assist in the
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specimen with 0.3 wt.% of carbon, the three-step HT did eradication of hot cracks during solidification, 34,35 when
not yield a huge difference in comparison to the prior two- existing in abundance, their brittle nature might cause
step HT (Figure 6C and C ). The sample was still mostly large delamination cracks under the inherent residual
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made of large columnar <110> orientated grains along the stresses present in the LPBF’s cyclic heating cycles. In
build direction. this study, we addressed each of these potential problems
sequentially.
4. Discussion
As shown in Figures 2–4, the characteristics of γ’ are
The aim of the present work is to explore the feasibility of strongly correlated with the carbon content and the HT
eliminating the hot cracking problem within LPBF-built procedures. In the as-built state, we did not observe γ’ in
precipitation-strengthened IN738LC by increasing its any of the three samples. All as-built specimens have a
carbon content. This approach is mainly inspired by the cellular microstructure with a cell size of around 500 nm,
previous study, which shows that carbides are effective typical for LPBF-built nickel-base superalloys. Carbides
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Volume 3 Issue 1 (2024) 6 https://doi.org/10.36922/msam.2264