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Materials Science in Additive Manufacturing Additive manufacturing of NASA HR-1 angled walls

among the samples. All samples printed at 1,070 W, the opposite edge of the sample until complete rupture
regardless of their angles (0°, 20°, and 30°), showed no occurred. This behavior can be attributed to sharp corners
statistical differences, as P-values were greater than 0.05 acting as stress concentrators, which promote fatigue crack
during testing. Furthermore, t-tests indicated no significant initiation. 32,33
difference between the 1,070 W and 2,620 W samples.
This similarity in fatigue behavior can be attributed to 4. Conclusion
the comparable grain size and γ’ distribution across the In this study, LP-DED NASA HR1 angled walls were
samples, as these features can impede the propagation of manufactured using two different parameter sets, 1,070 W
fatigue cracks. 27,31 and 2,620 W, and three deposition angles: 0°, 20°, and 30°.

One sample from each build angle and laser power The samples underwent a comprehensive heat treatment
setting was selected for observation under an SEM to process comprising stress relief, homogenization, solution
evaluate the fractured surface. Figure 16A shows multiple annealing, and aging. Evaluations were conducted
initiation sites (indicated by red arrows) at the edges of following heat treatments, including tensile and low-cycle
the fractured sample. In Figure 16B, striations are visible fatigue testing. The following conclusions were obtained
in different planes at one corner of the sample, resulting based on the results:
from cyclic loading. This observation suggests that cracks (i) Increasing the laser power from 1,070 W to 2,620
initiated at the surface of the flat testing bars, particularly resulted in a higher defect content due to increased
at the corners, and subsequently propagated toward
porosity. This increase can be attributed to gas being
more easily entrapped in the deeper melt pools created
by higher laser power. Despite the rise in porosity,
the mechanical properties of NASA HR-1 appeared
unaffected.
(ii) After heat treatment, all samples formed an austenitic
microstructure, effectively eliminating the dendritic
microstructure typically formed by LP-DED due to
rapid cooling rates. In addition, the heat treatment
successfully produced a homogeneous microstructure
across all samples, with no-η phase observed at grain
boundaries under optical microscopy.
(iii) During tensile testing, the YS of the 20° samples was
slightly lower compared with the 0° and 30° samples.
However, this was the only statistical difference observed
during the study. UTS and elongation values indicated
all samples performed similarly. Microhardness was
Figure 15. Comparison of reversal cycles required for the failure of
laser powder direct energy deposition heat-treated samples printed with also unaffected by variations in processing parameters
different deposition angles or laser power or build angles after heat treatment.
(iv) In terms of LCF life, the number of reversal cycles
required for failure at high stress (~700 MPa) was
A B
not significantly different among the samples. This
suggests that fatigue life remains consistent regardless
of variations in parameters following heat treatment.
Acknowledgments
The authors would like to acknowledge the support
from the NASA Marshall Space Flight Center. We are
grateful to the Keck Center for providing the facilities and
equipment needed to perform the experiments required for
Figure 16. Scanning electron microscope image of the 0° 1,070 W sample. this study. Finally, we appreciate our colleagues and peers
Images captured at (A) ×10 magnification and (B) ×200 magnification from the University of Texas at El Paso (UTEP) for their
showing the formation of striations. Red arrows indicate multiple
initiation sites. Scale bar: (A) 2 mm, (B) 20 µm; magnification: (A) ×10, encouragement and insightful discussions that contributed
(B) ×200 to the successful completion of this project.

Volume 4 Issue 1 (2025) 9 doi: 10.36922/msam.8069