Nanomechanical Characterization of Additive Manufactured GRCop-42 Alloy Developed by Directed Energy Deposition Methods

Introduction: Aerospace applications such as combustion chambers and liners for rocket engines have an aggressive thermomechanical environment. For example, the combustion chambers of Reusable Launch Vehicle (RLV) can reach temperatures as high as 3315⁰C, the pressure of 20 MPa, sizeable thermal gradient between internal and external surfaces, and repeating thermal stress from multiple launches. This environment demands specialized design together with the use of materials with a superior mechanical (high stiffness at elevated temperature, superior low-cycle fatigue life) and thermal (excellent creep resistance, high thermal conductivity) properties. GrCop-42 (Cu-4 wt.% Cr-2 wt.% Nb)  is one such candidate material developed by NASA Labs and manufactured using additive manufacturing (AM). AM or three-dimensional (3D) printing enables easier built up and customization of complex geometries such as RLV combustion chamber lines allowing for time and cost-saving in the long run. However, AM manufacturing also offers a unique processing environment (localized rapid heating-cooling rates, unidirectional material deposition, and high laser energy of deposition) than traditional metal fabrication processes. Here we characterize the mechanical and structural properties of heat-treated GrCop-42 manufactured via blown powder deposition (BPD) and compare its response to standard copper alloys. Methods: We use experimental techniques including surface profiling using laser microscopy, instrumented indentation and reverse analysis for nanomechanical characterization, X-ray diffraction (XRD) and residual XRD for crystal structure and residual stress measurement, and Scanning electron microscopy (SEM) with electron backscatter diffraction (EBSD) for microstructural and composition evaluation. Results: The average surface roughness of as-deposited material was observed to be high (580 mm) compared to traditional manufacturing. Elastic modulus and Hardness values estimated from nanoindentation testing were 129.3 ± 5.81 GPa and 1.1 ± 0.02 GPa, respectively, which is comparable with standard manufacturing. Reverse analysis algorithm from literature using power law and dimensionless functions was applied to extract elastoplastic properties, which showed a high hardening exponent of 0.75. SEM images revealed two different phases in the microstructure, with Cr₂Nb rich precipitates distributed throughout. XRD showed a sharp peak at 50°, indicating a single dominant crystal orientation. The role of electropolished vs mechanical polishing was examined for precise residual stress measurement. Residual stresses by XRD on unpolished after heat-treated samples showed compressive stress with large variation between locations. Continued experiments on the sample will examine its mechanical response under elevated temperature conditions.  Conclusion: The impact of AM manufacturing for copper-alloy was examined for GrCop-42, a material developed by NASA for high-temperature aerospace applications. Nanomechanical characterization of the material revealed the presence of Cr2Nb rich precipitates, which is expected to be responsible for strengthening effect for such allows. The high surface roughness observed in the as-prepared sample indicates significant post-processing and the polishing needs for the sample or tweaking of manufacturing parameters. The sample showed a predominant crystal orientation, consistent with unidirectional deposition. Such detailed analysis will reveal important information on as-deposited AM metal alloys and the effect of post-processing to critically evaluate manufacturing and processing parameters for practical large-scale applications.