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Materials Science in Additive Manufacturing Inconel 718-CoCrMo bimetallic structures

Although oxidation kinetics are well understood
for single material systems, more studies are needed to
understand the oxidation resistance of coated or bimetallic
systems. For single materials, the oxidation resistance can
significantly vary depending on factors, such as oxide
formation rate, defects, and mean free path of oxygen. .
[29]
The oxide layer’s composition also matters significantly due
to the defect density in the said oxide layer. For example,
chromium oxide layers typically exhibit a very low
defect density, preventing oxygen diffusion [30-32] . Material
constants are typically determined experimentally by
fitting a curve to the data. In the case of parabolic-type
oxidation, the rate equation is given by:
dx = Kp
dt x
Figure 6. SEM and EDS images of the oxidation of the Inconel 718 and
CoCrMo bimetallic sample. The Inconel 718 oxide layer is on the left Where Kp is the rate constant, and x is the scale
[29]
of the micrograph, where a strongly adherent oxide layer is seen. The thickness . Most metallic alloys follow this parabolic
CoCrMo oxide layer is on the right, where spalling of the oxide layer can behavior due to the continual formation of the oxide layer,
be seen. while ceramic materials follow a more linear relationship. In
the case of an alloy’s thin coating, oxygen’s mean free path will
the CoCrMo oxide layer. Although it was found that the be significantly higher to diffuse to the bulk material under
CoCrMo showed less oxidation damage compared to the coating. However, the constants could potentially be
Inconel 718, the Inconel 718 coating oxide layer would determined by first determining the oxidation constants for
be able to resist further oxidation damage, but the coating the coating, then using it as a correction curve for the coated
would also be able to keep the CoCrMo oxide layer from structure. In this work, Inconel 718 coating on CoCrMo
being washed away, preventing degradation of the bulk proved to be a better combination to enhance oxidation
structure. In the opposite scenario with a CoCrMo coating resistance of the bimetallic structure due to the strongly
on Inconel 718, this structure would be best suited for
corrosive environments and situations with high wear, adherent oxide layer on Inconel 718 even after 96 h at 800°C.
as the corrosion and wear resistance of CoCrMo are 5. Conclusions
unmatched compared to most alloys.
Inconel 718-CoCrMo was successfully manufactured using
Wen et al. recently published functionally graded the laser DED-based AM. No difficulty was observed in
structures of Inconel 718 and CoCrMo printed using printing these structures, even during optimization, and the
laser PBF . In this study, we produced defect-free and finished parts had low porosity without any cracking. The
[28]
low-porosity structures without any concerns related to interface could not be distinguished using SEM imaging
the compatibility of the base materials. Our results also but was revealed in EDS mapping of elemental transitions.
show a compatible interface between Inconel 718 and
CoCrMo, even with the case of direct bimetallic transition, Hardness testing revealed a smooth yet brief transition
which is much more prone to issues arising from material between the two alloys. Compression testing showed that the
incompatibility due to a sharp change in composition. The CoCrMo had the highest yield strength of the four structures
hardness of the CoCrMo part in our study compared to the tested and determined that the compression behavior was
FGM study differed by about 30 HV (420 HV compared to not dependent on the bimetallic interface. Oxidation tests
400 HV, respectively), while the same for Inconel 718 part showed that the bimetallic interface was not affected by the
differed by 90 HV (360 HV compared to 270 HV). However, high temperatures and revealed that the Inconel 718 formed
this difference is expected as the printing techniques are a firmly attached chromium oxide layer, while the CoCrMo
different and it is evident that different amounts of carbides showed spalling of the same oxide layer.
formed in the FGM parts compared to the bimetallic parts Acknowledgments
due to faster cooling rates in DED. Finally, the stress-strain
plots in each study showed considerable failure strain, The authors thank JCDREAM (Seattle, WA) for a
reinforcing the notion that unwanted intermetallic phases capital equipment grant to purchase the directed energy
are not a concern for these bimetallic structures. deposition (DED)-based metal 3D Printer at WSU.

Volume 1 Issue 3 (2022) 6 https://doi.org/10.18063/msam.v1i3.18