No abstract

Metal additive manufacturing processes often use gas-atomized powder as feedstock, but these processes use different methods for consolidation. Depending on the consolidation temperature, secondary phases may be retained during processing, making it important to understand powder microstructure prior to consolidation. Commercial alloy compositions are typically used for these powders because they have been widely studied and qualified; however, the microstructure of the powder form of these compositions has not been studied. This paper aims to understand the commercial Al 6061 powder: how the internal microstructure of the powder differs from wrought both in the as-manufactured and thermally-treated conditions. A specific focus is put on the Mg-rich phases and their morphologies. This was accomplished through transmission electron microscopy, scanning transmission electron microscopy, and energy dispersive x-ray spectroscopy. Both the size and morphology of the phases in the powder differ greatly from those in the wrought form.

Cold gas-dynamic spray is a solid-state materials consolidation technology that has experienced successful adoption within the coatings, remanufacturing and repair sectors of the advanced manufacturing community. As of late, cold spray has also emerged as a high deposition rate metal additive manufacturing method for structural and nonstructural applications. As cold spray enjoys wider recognition and adoption, the demand for versatile, high-throughput and significant methods of particulate feedstock as well consolidated materials characterization has also become more notable. In order to address the interest for such an instrument, nanoindentation is presented herein as a viable means of achieving the desired mechanical characterization abilities. In this work, conventionally static nanoindentation testing using both Berkovich and spherical indenter tips, as well as nanoindentation using the continuous stiffness measurement mode of testing, will be applied to a range of powder-based feedstocks and cold sprayed materials.

The role of high strain rate and severe plastic deformation, microstructure, electrochemical behavior, surface chemistry and surface roughness were characterized for two copper cold spray material consolidations, which were produced from conventionally gas-atomized copper powder as well as spray-dried copper feedstock, during the course of this work. The motivation underpinning this work centers upon the development of a more robust understanding of the microstructural features and properties of the conventional copper and nanostructured copper coatings as they relate to antipathogenic contact killing and inactivation applications. Prior work has demonstrated greater antipathogenic efficacy with respect to the nanostructured coating versus the conventional coating. Thus, microstructural analysis was performed in order to establish differences between the two coatings that their respective pathogen kill rates could be attributed to. Results from advanced laser-induced projectile impact testing, X-ray diffraction, scanning electron microscopy, electron backscatter diffraction, scanning transmission microscopy, nanoindentation, energy-dispersive X-ray spectroscopy, nanoindentation, confocal microscopy, atomic force microscopy, linear polarization, X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy and copper ion release assaying were performed during the course of this research.

In this work, three commercially available aluminum alloy systems (Al 2024, Al 6061, and Al 7075) were considered to explicitly capture the differences in material properties associated with a rapidly solidified, gas-atomized particulate feedstock as compared with their conventionally cast counterparts. Differences between the microstructural, thermodynamic, mechanical, and kinetic behaviors associated with gas-atomized and conventionally bulk counterparts have been tacitly assumed by the cold spray community. However, many researchers continue to utilize legacy properties from bulk materials when simulating particle impact phenomena in silico, for example. By way of recognizing the fact that bulk material properties may not serve as substitutes for gas-atomized powder property input parameters for cold spray process simulation and computation in silico, enhanced cold spray research and development will be more easily achieved. Therefore, understanding the feedstock powder characteristics for use in cold spray can lead to fine-tuning the properties of cold spray consolidations. Optical microscopy, scanning electron microscopy, nanoindentation, microhardness, differential scanning calorimetry, elemental analysis, and cooling rate calculations were utilized. This work confirms preliminary findings that powder alloys may not be treated the same way as their bulk counterparts in so far as the enactment of heat treatment processing parameters are concerned. Specifically, vast discrepancies were found in the grain size, secondary phases, and mechanical behavior between the powder and cast versions of each alloy.