Page 36 - MSAM-3-1

Materials Science in Additive Manufacturing Carbon addition in IN738LC

3. Results (Figure 2A –C ). The carbides were previously found to be
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titanium carbide (TiC), thus appearing bright under the
3.1. Laser processing parameter optimization BSE condition. The size of these cellular structures was
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LPBF fabrications using five different combinations of approximated to be around 500 nm, consistent with the
powders were conducted within the same build platform previous reports. For pure IN738LC, the MC carbides
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(Figure 1A), with the bottom of the build being pure were isolated from one another, and the distance between
IN738LC, followed by IN738LC with 0.1 wt.% carbon. The individual carbides was reduced with the increment of
carbon content sequentially increased, and the top-most carbon content (Figure 2A –C ). For alloy with 0.3 wt.%
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portion was IN738LC with 0.4 wt.% carbon. The OM images carbon, the carbides occupied almost the entire cellular
of samples fabricated using two different laser powers are boundaries and formed a carbide ring (Figure 2C ).
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shown in Figure 1B (115 W) and Figure 1C (130 W). The After the standard two-step HT, more carbides started
specimen made from 115 W depicts uniformly distributed to precipitate out, mostly along the grain boundaries of
pores, with a density value of 99.35% measured through these three specimens, again appearing as bright dots
OM image analysis. The size of the pores is mostly below (Figure 3A –C ). The contrast within individual grains
100 μm, as illustrated from the enlarged view in Figure 1B. became more uniform than their as-built state. The amount
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In comparison, the specimen fabricated with a 130 W laser of γ’ is inversely proportional to the carbon content,
power input had much fewer and smaller pores, with a
density value of 99.95%, obtained from the same analysis with the pure IN738LC and the IN738LC with 0.1 wt.%
carbon samples having densely packed γ’ (black phases in
approach. The effect of carbon content on microhardness Figure 3A and B ), and the sample with 0.3 wt.% carbon
under the two laser power inputs is shown in Figure 1D having a much lower quantity of γ’ (Figure 3C ). The first
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(115 W) and Figure 1E (130 W), respectively. For both 2
builds, the microhardness value increases concurrently two samples demonstrate a bimodal γ’ distribution, with
with the carbon content, ranging from ~380 HV to ~530 the bigger γ’ having a diameter of about 200 to 400 nm
and the smaller γ’ having a diameter of around 100 nm
HV for 115 W and from ~400 HV to ~ 550 HV for 130 W, (Figure 3A and B ). For the IN738LC sample mixed with
indicating that the higher power gives the as-built samples 3 3
a higher microhardness. Due to a larger number of pores 0.3 wt.% carbon, the smaller carbides are not clearly visible
within the 115 W build, its microhardness results show under the current imaging condition (Figure 3C ). It is
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a greater degree of scatter (Figure 1D), with the lowest speculated that the missing γ’ could be deprived of their
microhardness value recorded being <100 HV, which likely essential solute elements due to the competitive nucleation
occurred when the microhardness indents fell near those and growth of MC carbides.
pre-existing pores. Straight annealing twins were present within the pure
IN738LC alloy after the three-step HT process proposed in
3.2. Evolution of γ’ and carbide under different this paper (Figure 4A ). Its grain boundaries were no longer
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processing and HT conditions tortuous but mostly smooth. The presence of MC carbides
Only specimens fabricated with the 130 W laser power (those white dots) was more apparent than its as-built and
input were subjected to subsequent characterization due two-step HT-processed counterparts. For the IN738LC
to their superior density of 99.95%. The γ’ and carbide samples added with 0.1 wt.% and 0.3 wt.% carbon, the
characteristics of the as-built specimens were examined accumulation of carbides (bright spots) along their grain
under the backscattered electron (BSE) imaging condition boundaries was clearly visible (Figure 4B and C ). The
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(Figure 2). Out of the five different compositions, only higher temperature and longer duration designed in the
three alloys were examined in detail, that is, the pure three-step HT caused the γ’ to grow further, reaching a
IN738LC, IN738LC with 0.1 wt.% carbon, and IN738LC peak diameter of around 600 to 800 nm (Figure 4A ). In
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with 0.3 wt.% carbon. This is because the trend on conjunction, the MC carbide also reached a maximum
the effect of carbon addition is consistent among all dimension of around 500 nm. With a higher carbon content,
specimens. In general, the grain boundaries have a zig- the maximum size of γ’ and those intragranular carbides
zag morphology and are interlocking with one another decreased (Figure 4B and C ). While the larger γ’ in the
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(Figure 2A –C ). The grains were not uniform in color due three-step HT is about twice the size of those in the two-
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to the intragranular rotations induced by inherent residual step HT, the smaller γ’ in the three-step HT is about half
stresses incurred during LPBF’s cyclic heating. With the size of those in the two-step HT, having an average size
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the increase in carbon content, the number of carbides of ~60 nm for the pure IN738LC (Figure 4A ). A similar
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(bright spherical dots under the BSE imaging) was found trend in the refinement of γ’ in the three-step HT was also
to increase monotonously along the cellular boundaries observed for the other two alloys (Figure 4B and C ).
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Volume 3 Issue 1 (2024) 4 https://doi.org/10.36922/msam.2264