Experimental Analysis and Simulations of the Effects of Deposition Conditions on Deformation Characteristics of Al-6061 Powders in Cold-Spray

摘要

Cold spray is an additive manufacturing method based on impacting metal powders on a substrate with the aid of a supersonic gas, and it is used extensively to rebuild a damaged metallic structures. The characteristics of interaction between the coating and the substrate depend on the impact velocity, particle and substrate temperatures, and metallurgical properties of the initial powder. In this work, micron scale Al-6061 particles were sprayed on flat substrates of the same material under different spraying conditions with low particle density to characterize single particle impacts. Three groups of cold sprayed samples with different types of heat treatments were investigated. Three different driving gas conditions were tested: He-gas at 927 m/s and 547 K (He-V927), He-gas at 767 m/s and 400 K (He-V767), and N2-gas at 617 m/s and 582 K (N2-V617). Initial powder and impacted particles were analyzed in intact and cross-sectioned form by oblique Scanning Electron Microscopy and Atomic force microscopy). Tape test, scraping test, and shear test were conducted to collect deformed particles. Fully bonded, partially bonded, and failed particles were systematically observed. Continuum simulations of impact of spherical and non-spherical particles under the same spraying condition of the samples were carried out.;The powder used in the tests has distinct solidification boundaries on the surface with some having large melted segments. Both of features are attributed to the gas atomization process used in powder manufacturing. The surface roughness of the bottom of deformed particles is considerably reduced in all roughness measures, after the impact, in comparison to that of the initial powder. Each test group with different gas conditions show different deformation trends. Splats of He-V927 were flattened and bonded with coronal jets. Splats of He-V767 appeared bonded but the particle remained convex above the substrate, and the splats with N2-V617 had mostly craters on the substrate and some partially bonded particles. The specially heat treated sample group showed mixed jets from both of the particle and the substrate, on the other hand, the other two groups showed mostly substrate jets. The yield stress of the Johnson-Cook model for the substrate was chosen to be 1.75 times that of the particles based on comparisons with experimental results. Simulations allowed further classification of the deformation characteristics as functions of impact velocity and temperature. It showed that with increasing impact velocity deformation progresses from local deformation at the impact interface to lateral spreading of the particle, to substrate jetting and finally to jetting in both sides. This work advances our knowledge in particle deformation in cold spray applications and provides further calibration tools for mechanics models of high-strain rate impacts of metals.