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Size effects, influencing a material's strength, elongation, fatigue limit, and longevity, depend on the operative and dominant deformation and failure mechanisms. This study explores the size effects in additive manufactured (AM) GRCop-42 (Cu-4at%Cr-2at%Nb) thin wall structures fabricated via laser-powder bed fusion (L-PBF) and their impact on fatigue life. The influence of internal defects and surface topography on the fatigue life of specimens in both as-built and hot isostatic pressed (HIP) conditions across different thicknesses is investigated. Where micro-computed tomography (μCT) was used to quantify the internal porosity of as-built, pristine HIP'd, and fatigued HIP'd specimens, and laser microscopy was employed to quantify the surface topography of specimens prior to fatigue. Additionally, quasi-static tests were used to establish baseline mechanical properties (i.e. yield strength (YS), ultimate tensile strength (UTS), and elongation) to frame fatigue testing conditions. Results indicate a significant enhancement in fatigue life for HIP'd specimens for both thicknesses, with internal defects depicting a greater impact than surface topography. Furthermore, fractographic analysis suggests that thicker specimens exhibit higher resistance to crack propagation during fatigue testing in the absence of substantial porosity. Thus, the size effects observed on the fatigue life of L-PBF GRCop-42 appears to be dominated by internal defects.

This study investigated the microstructure and tensile properties of the NASA HR-1 (Fe-Ni-Cr) alloy using the laser powder directed energy deposition (LP-DED) process. Laser power and spot size were varied for deposition, and it was noted that low laser power could result in higher cooling rates and finer microstructure. The melt pool depth and width and the defect content increased with the increase in laser power. Out of several laser power (i.e., 350, 750, 1070, 2000, and 2620 W), the samples deposited using 750 W demonstrated the highest material density. In addition, the effects of heat treatment on the microstructure and tensile properties were investigated. It was found that a heat treatment schedule consisting of stress relief, homogenization, solution treatment, and aging could thoroughly homogenize the microstructure (i.e., remove the as-solidified dendritic microstructure), result in relatively uniform, equiaxed grains, and increase the material hardness. Importantly, this heat treatment schedule efficiently reduced titanium segregation, preventing the formation of deleterious needle-shaped η (Ni3Ti) precipitates in NASA HR-1, known to be detrimental to mechanical properties and resulted in almost comparable tensile properties with varying laser power.

Laser Powder Direct Energy Deposition (LP-DED) allows for manufacturing of large components while still maintaining internal thin walls for heat exchanger applications. The LP-DED process has been matured for alloys including stainless steels, superalloys, and titanium, but has had very limited research using copper-based alloys, which are important for applications that require high thermal conductivity. This study quantifies the size effects on microstructure, surface metrology, microhardness, and mechanical response to tensile loads for different thicknesses and powder compositions of hot isostatic pressed (HIP) LP-DED Copper-Chromium-Niobium alloy, specifically GRCop-42. To accomplish this, tensile specimens were sectioned from single track build walls in both horizontal and vertical orientations to also investigate possible anisotropic behavior in the part. Results show that microstructure, hardness, surface metrology, and porosity are independent of wall thickness. Uniaxial loading response showed some variations with specimens orientation and size effects. For vertical specimens, thicker specimens showed a 6% higher elongation than thinner specimens. Horizontal specimens showed close to double the elongation when compared to vertical specimens, where thinner specimens had a higher reduction in elongation than thicker specimens. Additionally, removing the surface effects through polishing practically eliminated the anisotropic behavior between horizontal and vertical specimens along with the observed size effects. To demonstrate size effects dependency on the manufacturing process comparison is made between GRCop-42 alloy produced by Laser Powder Bed Fusion (L-PBF) and LP-DED.