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Materials Science in Additive Manufacturing Carbon addition in IN738LC

The room temperature tensile results of the as-built layers did not take place during the production run, likely
pure IN738LC and IN738LC with an addition of 0.3 wt.% due to part distortions triggered by the build-up of residual
are presented in Figure 7B. These results are comparable to stresses. As for regions that were originally intact after
those reported in the literature, with and without carbon LPBF fabrication, ductile dimples were detected (enlarged
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additions. However, one major issue found in the present view within Figure 8E), suggesting the ductile failure mode
study, which is not reported in prior works, is the presence during the breaking of parts after the LPBF production.
of large, horizontal cracks within the as-built IN738LC with Despite the few reports on such delamination cracks in
an addition of 0.3 wt.% carbon (Figure 8A and B). Its tensile the open literature, these cracks pose a significant challenge
property is only measured in those non-cracked regions. It to the LPBF production of precipitation-containing alloys,
should be noted, though, that the present work used powder which often offer superb strength, wear, and conductivity
mixtures between the IN738LC and graphite powders instead properties. During the production of samples with small
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of pre-alloyed powders. Despite this fact, the presence of dimensions (with a cross-sectional area of 10 × 10 mm ,
2
large delamination cracks still warrants further investigation. Figure 1), no cracks were observed. The delamination
Most of the delamination cracks in the existing literature cracks only occurred for large samples, whose residual
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are centered on those carbide-containing tool steels, stress content is believed to be much larger. Hence, the key
whereas the occurrence of cracking is a measure to relieve to fabricating crack-free samples having large precipitation
the inherent residual stresses accumulated during the LPBF content is to minimize the amount of residual stress that
production. We believe that it is a similar mechanism occurs during production. One proven method is to use
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for the crack occurrence in the current material. A small- a heated substrate to reduce the thermal gradient during
sized, representative fracture surface of the crack under fabrication and, hence, lower the residual stress content. 44
OM is shown in Figure 8C. Within this specimen, the top
portion is the cracked surface, showing a bright contrast. 5. Conclusion
The lower portion is the region, which was originally still The present study aims to examine the effect of increasing
intact after production but was broken apart to examine the carbon content within precipitation-strengthened
the fracture surface. Laser melt tracks were observed on nickel-base superalloy IN738LC during the LPBF process.
the cracked surface during LPBF fabrication (Figure 8D). Besides its benefit of reducing the material’s hot cracking
Such a feature indicates that fusion between successive susceptibility, three potential problems associated with

A B C

D E

Figure 8. Large, horizontal cracks present in the as-built IN738LC with an addition of 0.3 wt.% carbon from (A) longitudinal and (B) cross-sectional views.
(C) The fracture surface of the horizontal crack under optical microscopy. Enlarged scanning electron microscopy images of the crack surface focusing on
the (D) delaminated and (E) still intact regions.

Volume 3 Issue 1 (2024) 10 https://doi.org/10.36922/msam.2264