In-situ mechanical investigation of the deformation of splat interfaces in cold-sprayed aluminum alloy

The mechanical properties of cold sprayed deposits, with splats as their building blocks, are affected by de-bonding and relative sliding of splats during mechanical loading. In this study, inter-splat deformation behavior in 6061 Al coatings is investigated by in-situ mechanical characterization for real-time visualization. The role of processing gas type (He vs. Air), splat geometry, particle bonding, and post-spray heat treatment on splat sliding is evaluated. High porosity and poor splat-bonding in air-sprayed coating result in inferior microhardness (917 MPa) and flexural strength (120 MPa), as compared to He-sprayed coating which exhibits microhardness and flexural strength of 1000 MPa and 450 MPa, respectively. In-situ cyclic bending of coatings is performed inside a scanning electron microscope to examine the splat deformation behavior. Digital image correlation (DIC) analysis of the real-time videos is performed to determine local microstructure strains, experimentally demonstrating splat sliding phenomenon for the first time. Heat-treatment of the coatings heals inter-splat microcracks, reduces porosity and consequently improves the mechanical properties. DIC strain maps revealed arrested splat sliding after heat-treatment. A splat sliding factor (SSf) is introduced, which compares cold-sprayed microstructure deformation with the bulk structure like substrate, providing quantitative information about inter-splat bonding. The SSf is found to be higher for air-sprayed coating (~ 1.4) as compared to He-sprayed coating (~ 0.28), and is further reduced after heat-treatment. The correlation between splat sliding and microstructure will enable development of coatings with predictable and improved mechanical properties by cold spray.