Microstructural Characterization of Aluminum 6061 Splats Cold Spray Deposited on Aluminum 6061-T6 Substrate
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THE cold gas dynamic spray process, also known as Cold Spray (CS), is a solid-state spray material deposition process in which solid powder particles, accelerated to velocities 400 to 1200 m/s in a heated gas stream,[1–3] are sprayed on a substrate to create coatings of metals, polymers, ceramics, and composites materials on the substrate.[3–12] CS has the ability to deposit dense thick coatings with high spray yield and energy efficiency, which makes it attractive as a new means of part repair and additive manufacturing as well.[13–18] CS is also a severe plastic deformation (SPD) process in that it can produce ultrafine grains (UFGs) in the deposited material.[19–22]
WENDY C. EVANS is with Teltrium Solutions, LLC, Rockville, MD, 20852. XINGDONG DAN, AZIN HOUSHMAND, SINAN MU¨FTU¨, and TEIICHI ANDO are with the Department of Mechanical and Industrial Engineering, Northeastern University, Boston, MA, 02115. Contact e-mail: [email protected] Manuscript submitted February 4, 2019. Article published online May 31, 2019 METALLURGICAL AND MATERIALS TRANSACTIONS A
Although CS has not been widely adopted in the commercial production of thick coatings or bulk materials due mainly to persistent residual porosity and a lack of complete metallurgical bonding at all positions of the splat interface,[6,23–25] significant progress has been made in recent years in understanding and improving the splat adhesion.[19–21,26–28] It is generally stated that the metallurgical bonding in CS is promoted by adiabatic shear instability, a localized viscous flow caused by adiabatic heating and consequent thermal softening which occurs along the particle/substrate and particle/particle interfaces.[1,5,27,28] However, the exact nature of metallurgical bonding mechanism in CS is still not completely understood. The phenomena that occur at the particle/substrate and particle/particle interfaces in CS are investigated mainly by scanning transmission electron microscopy and transmission electron microscopy (S/TEM) and numerical modeling. In most current numerical models of the particle impact in CS,[1,29–33] the thermal softening that causes the viscous flow is predicted with a constitutive plasticity model, represented by the Johnson–Cook model,[34] usually with extrapolations or modifications for conditions at very high strain rates.[32,33] Although predictions with such models often match experimental observations, such as the occurrence of jetting at splat peripheries,[1] these models,
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being empirical in nature, may not entirely capture the exact structural evolution involved in the localized thermal softening and material flow at the interface. TEM, on the other hand, captures at least the final structures left after the deformation of the powder particle which typically takes only tens of ns [31] and the subsequent cooling where thermal effects may continue for a few ls.[35] Previous TEM studies[19–22,36–38] have revealed nanoscale grains along the particle/substrate and particle/particle interfaces and attribu
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