Additive manufacturing (AM) techniques of various forms are becoming ever more prominent and important in many industries, thanks to their efficiency, versatility, and economy compared to traditional fabrication methods. Before AM can fully complement or even supplant more established manufacturing techniques, however, some persistent problems need to be addressed, such as the increased fragility and reduced structural integrity of some AM-fabricated products. One possible answer is the combination of metal AM technologies with cold spraying (CS), in which a supersonic jet of fine powder is used to coat an object to repair or protect it. Already a popular technique in conventional manufacturing, CS also promises to enhance the durability of AM-produced metal parts, especially those with complex geometries subject to repeated stresses and loading. To investigate the possibilities, experimenters examined the behavior of CS-coated, AM-fabricated stainless steel under cyclical loading using the U.S. Department of Energy’s Advanced Photon Source (APS). The work, published in Additive Manufacturing, shows that the integration of additive manufacturing with the cold spray coating process can offer an intriguing way to overcome such limitations and further increase the versatility of additive manufacturing techniques.
In these experiments, the research team from the University of Massachusetts, the CCDC Army Research Laboratory, the Worcester Polytechnic Institute, and Argonne sprayed a chromium carbide nickel (CrC-Ni) coating onto specimen parts fabricated by direct laser metal sintering (DLMS) of 15Cr-5Ni precipitation-hardening stainless steel (15-5 PH SS) powder stock. These were compared with identical 15-5 PH SS as-fabricated (AM) specimens. The specimens were then studied employing synchrotron x-ray energy dispersive diffraction (EDD) at the X-ray Science Division Materials Physics & Engineering 6-BM-A beamline of the APS (an Office of Science user facility at Argonne) before and after combined cyclic loading to provide in-depth analysis of their fatigue life (Fig. 1).
Cold spraying uses the kinetic energy of the gas-propelled particles to induce bonding with the target substrate, avoiding significant heating or decomposition, unlike thermal spraying techniques which involve some melting of the spray material. Several systems, including aluminum and titanium, have been demonstrated to show greatly improved fatigue performance after the deposition of a CS coating, including better crack resistance. For AM-fabricated metal parts, cold spraying can mitigate issues of surface roughness and residual stresses created by the manufacturing process.
The x-ray EDD studies of the as-fabricated and cold-sprayed specimens showed process-induced stress and strain in axial, radial, and hoop directions. In general, however, the cold spraying of CrC-Ni greatly alters residual stress in all three directions, particularly in the radial direction. In the CS specimens, a significant stress gradient develops at the substrate/coating interface. Residual stress dropped from a tensile to compressive state within the specimen. Surface roughness of the CS-coated specimens is also greatly improved. Together, these factors are expected to markedly enhance fatigue performance.
Under fatigue testing including fractographic analysis, the CS specimens clearly demonstrate substantially improved performance under all loading conditions, resulting from the reduction in overall residual stress. Although damage to CS coating under cyclic loading has been demonstrated to sometimes lead to failure of the substrate, the present work shows that fatigue-crack growth mechanisms are favorably affected by the coating deposition. This appears to be due to reduced residual stresses in the substrate along with a decrease in build-orientation effects. In the as-fabricated specimens, fatigue cracks were seen to originate mostly at areas of surface roughness, from which they propagated into the specimen cross-section. In contrast, in the CS specimens, compressive residual stress at the substrate/coating interface retards crack growth and propagation and even suppresses the generation of secondary cracks. These beneficial effects of the cold spray coating are especially evident in the high cycle fatigue regime.
A persistent limiting factor to the widespread adoption of AM techniques such as direct metal laser sintering has been the increased fragility of AM components that can render them ill-suited for functionally-critical parts with complex geometries and challenging stress environments, relative to conventionally-fabricated pieces. ― Mark Wolverton
See: Davoud M. Jafarlou1, Gehn Ferguson2, Kyle L. Tsaknopoulos3, Andrew Chihpin Chuang4, Aaron Nardi2, Danielle Cote3, Victor Champagne2, and Ian R. Grosse1*, “Structural integrity of additively manufactured stainless steel with cold sprayed barrier coating under combined cyclic loading,” Additive Manu. 35, 101338 (2020). DOI: 10.1016/j.addma.2020.101338
Author affiliations: 1University of Massachusetts, 2CCDC Army Research Laboratory, 3Worcester Polytechnic Institute, 4Argonne National Laboratory
Correspondence: * firstname.lastname@example.org
This research was accomplished through a cooperative research agreement with the U.S. Army Research Laboratory, Contract: W911NF-15-2-0024, “Intelligent Processing of Materials by Design.” This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.
The U.S. Department of Energy's APS is one of the world’s most productive x-ray light source facilities. Each year, the APS provides high-brightness x-ray beams to a diverse community of more than 5,000 researchers in materials science, chemistry, condensed matter physics, the life and environmental sciences, and applied research. Researchers using the APS produce over 2,000 publications each year detailing impactful discoveries, and solve more vital biological protein structures than users of any other x-ray light source research facility. APS x-rays are ideally suited for explorations of materials and biological structures; elemental distribution; chemical, magnetic, electronic states; and a wide range of technologically important engineering systems from batteries to fuel injector sprays, all of which are the foundations of our nation’s economic, technological, and physical well-being.
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