Heat Treatment of Recycled Battlefield Stainless-Steel Scrap for Cold Spray Applications
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https://doi.org/10.1007/s11837-020-04259-5 Ó 2020 The Author(s)
ADDITIVE MANUFACTURING: BEYOND THE BEAM TECHNOLOGY
Heat Treatment of Recycled Battlefield Stainless-Steel Scrap for Cold Spray Applications CHRISTOPHER MASSAR,1 KYLE TSAKNOPOULOS,1 BRYER C. SOUSA,1 JACK GRUBBS,1 and DANIELLE L. COTE
1,2
1.—Worcester Polytechnic Institute, Worcester, MA 01609, USA. 2.—e-mail: [email protected]
This work explores the impact of thermally preprocessing recycled austenitic stainless-steel powder for solid-state cold spray metal additive manufacturing with a focus on increasing deposition quality and coating density while maintaining mechanical integrity. The recycled stainless-steel scrap was gasatomized using a novel mobile foundry manufactured by MolyWorks Materials Corporation. The powder was thermally treated based upon thermodynamic modeling using Thermo-Calc. The powder and sprayed specimens were characterized using particle size–shape analysis, microscopy, x-ray diffraction, and nanoindentation. Diffraction results highlighted the presence of both austenite and ferrite phases in the powder. Nanoindentation confirmed that thermally processing the feedstock powder at the austenitization temperature decreased the amount of ferrite present, which was consistent with the porosity observed in the deposits due to the lower yield strength of austenite relative to ferrite. The untreated powder deposits exhibited extensive porosity and microcracking, as opposed to the virtually fully dense deposit from the heat-treated powder.
INTRODUCTION Cold-gas dynamic-spray (cold spray) is a solidstate materials consolidation technology that utilizes particulate feedstock, which is transported via a heated carrier gas stream until exiting a de Laval nozzle and supersonically impacting a substrate. While cold spray was originally conceptualized as a tool for achieving coatings with unique and application-specific properties, the process was adopted by the remanufacturing and repair, and the additive manufacturing (AM) communities. As ballistically impinged particles adhere to a substrate, particle–particle bonding occurs as the process continues to deposit powder layer by layer. Successful consolidation requires particle–substrate and particle–particle metallurgical and mechanical bonding.1 Cold spray parameters vary from the nozzle type to the selection of powder composition and gas source. Typically, inert gases are used, such as helium or nitrogen. Feedstock powder for cold spray typically has a diameter range from approximately
5 lm to 100 lm. Particles achieve velocities between 300 m/s and 1200 m/s with associated pressures under 300 psi for low-pressure cold spray (LPCS) and above for high-pressure cold spray (HPCS). Many materials can be cold sprayed, including polymers, composites, copper, aluminum, steel, and titanium, and are chosen according to the application and the necessary part performance.1 Since cold spray is a solid-state process, the properties of the feedstock directly influence the properties of the component.2,
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