Prediction of heterogeneous microstructural evolution in cold sprayed copper coatings using local Zener-Hollomon parameter and strain

Cold spray processing is a solid-state coating technique and an emerging method for additive manufacturing, in which metal powder particles are bonded through high-velocity impact-induced deformation. However, the severe plastic deformation of powder particles at extremely high strain rates, high strain gradients, and localized elevated temperatures yields rather complex and heterogeneous microstructures in materials produced as coatings or bulk forms. A good understanding, and even prediction, of such heterogeneous microstructures is essential for determining the post-processing conditions as well as the final properties of cold-sprayed products. In this study, we employ a cold spray system to deposit copper coatings over a large temperature range from 373 K to 873 K and we systematically investigate the microstructural evolutions of the coatings using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) techniques. Diverse microstructures have been observed, including recrystallized grains, annealing twins, shear bands, submicron grains, deformation twins, and nanometer-sized grains. To understand the formation of such complex microstructures, we obtain local strains, strain rates and temperatures of the cold-sprayed powder particles using the finite element method (FEM). Based on our experimental and simulation results, we created the first deformation mechanism map for cold sprayed coatings to interpret and predict the heterogeneous microstructural evolutions in copper using the local Zener-Hollomon (Z) parameter and plastic strain (strain-Z-microstructure map). Such a map can be used to predict and design the microstructures of cold-sprayed copper samples based on processing parameters and can also be extended to other severe plastic deformation (SPD) processes, such as cutting, extrusion, and solid-phase welding.