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ABSTRACTThis research evaluates Laser Powder Directed Energy Deposition (LP-DED) for producing fine feature internal microchannels. This study is focused on enhancing and characterising the surfaces of microchannels produced using techniques such as abrasive flow machining, chemical milling, chemical mechanical polishing, electrochemical machining, and thermal energy method to modify internal surfaces of microchannels made from NASA HR-1 Fe-Ni-Cr alloy. Flow testing for discharge coefficient measurement is conducted on processed microchannel samples, followed by characterisation through optical microscopy, Scanning Electron Microscopy (SEM), and Computed Tomography. Findings reveal variations in surfaces due to powder adherence, melt pool undulations, and polishing mechanisms. The study emphasises the significance of removing material equivalent to the mean powder diameter to reduce surface roughness and impact the discharge coefficient. The research proposes a ratio for planarising roughness and waviness peak height and density, offering insights for tailored surface adjustments in specific applications requiring reduced flow resistance.HighlightsInternal microchannels with thin-walls were fabricated using the laser powder directed energy deposition process.Various surface enhancements and polishing processes were developed to modify the surface texture of the LP-DED channels.Flow testing was conducted to determine the discharge coefficient.Post-test characterisation was completed to obtain cross sectional area, perimeter, surface texture, and general surface condition to analyse results.Ratio of roughness and waviness peak and density (Spk/Spd and Wp/WPc) is proposed as a relevant surface characterisation parameter.Tailored surface modifications for specific end-use applications.

The tensile and fatigue properties of a newly developed Ni-based superalloy, HAYNES® 233, were investigated. Two different manufacturing processes including hot rolling (wrought plate) and laser-powder directed energy deposition (LP-DED) were considered. In addition, specimens were tested in two heat treatment (HT) conditions, namely solution annealed (SA), and age-hardened (AH). The microstructures of the materials were characterized using a scanning electron microscope. Tensile properties of specimens were evaluated at 21 °C, 538 °C, and 871 °C along with the fatigue properties at 21 °C under two different strain amplitudes of εa = 0.005 mm/mm and εa = 0.002 mm/mm. The LP-DED material in both the SA and AH conditions showed similar tensile strengths to the wrought AH material at all test temperatures, whereas the wrought SA material showed a noticeable deficit in strength at 21 °C and 538 °C, and was only comparable at 871 °C. Fatigue results indicated that there was no significant difference in fatigue life for specimens tested at εa = 0.005 mm/mm, while differences in fatigue life were observed for specimens tested at εa = 0.002 mm/mm, where fatigue life was observed to correlate well with the size of the crystallographic facets at crack initiation sites.

This study involves a comparative analysis of additively manufactured GRCop-42 specimens produced using two processes: laser-powder bed fusion (L-PBF) and laser powder direct energy deposition (LP-DED). The investigation characterizes a range of material attributes, including surface topography, internal defects, microstructural features, quasi-static mechanical properties, and fractographic characteristics. The findings demonstrate that, despite the specimens being fabricated with the same base material, the resulting material properties vary significantly between the two additive manufacturing processes. As such, material properties cannot be presumed to be uniform across different manufacturing methods. Consequently, material characterization must be conducted for individual manufacturing processes based on specific parameters.

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.

Metal additive manufacturing (AM) is being used for mission-critical applications in both developmental and production components, driven by economic and technical benefits. Laser powder directed energy deposition (LP-DED) allows manufacturing of thin wall geometric features for various components at diameters larger than 2 m. The characterization of geometric capabilities and limitations is critical for establishing guidelines for end users of the technology. Within this study, several samples of enclosed vertical tracks were fabricated and characterized using LP-DED, with 1 mm-thick walls and varying inclination angles up to 45° using the NASA HR-1 alloy (Fe-Ni-Cr). The wall thickness, melt pool, and surface texture, inclusive of waviness and roughness, were evaluated and results presented. The experimental results indicate that the wall thickness increases exponentially above 30°. The surface texture was shown to be dependent on 1) excess powder adherence, 2) melt pool irregularities causing material droop, and 3) excess material. The experiment revealed that the mean roughness reduces with increasing wall angle for the downskin surface. The upskin roughness reaches a maximum peak at 20° and slowly reduces as powder adheres within the valleys. Both the downskin and upskin surface textures are dominated by irregular waviness generated by the melt pool.

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.

Metal additive manufacturing involves manufacturing techniques that add material to produce metallic components, typically layer by layer. The substantial growth in this technology is partly driven by its opportunity for commercial and performance benefits in the aerospace industry. The fundamental opportunities for metal additive manufacturing in aerospace applications include: significant cost and lead-time reductions, novel materials and unique design solutions, mass reduction of components through highly efficient and lightweight designs, and consolidation of multiple components for performance enhancement or risk management, e.g. through internal cooling features in thermally loaded components or by eliminating traditional joining processes. These opportunities are being commercially applied in a range of high-profile aerospace applications including liquid-fuel rocket engines, propellant tanks, satellite components, heat exchangers, turbomachinery, valves, and sustainment of legacy systems. This paper provides a comprehensive review of metal additive manufacturing in the aerospace industry (from industrial/popular as well as technical literature). This provides a current state of the art, while also summarizing the primary application scenarios and the associated commercial and technical benefits of additive manufacturing in these applications. Based on these observations, challenges and potential opportunities are highlighted for metal additive manufacturing for each application scenario.