Mechanistic modeling of high velocity micro-particle impacts: Application to material deposition by cold spray process

The cold spray process is an additive manufacturing technology primarily suited for ductile metals, and mainly utilized in coating surfaces, manufacturing of freeform parts and repair of damaged components. The process involves acceleration of solid micro-particles in supersonic gas flow and coating build-up by metal bonding upon high velocity impact (300-1200 m/s) onto a substrate. Coating deposition relies on the kinetic energy (impact velocity) of the particles. The main objective of this research was to investigate the mechanics of metal bonding upon high strain-rate deformation in order to improve the technological implementation of the process. The research covers a wider range of impact conditions than those directly associated with cold spray, in order to reveal the governing mechanisms of particle impact and rebound.;Finite element (FE) method has been used to simulate particle impacts due to the complexity of making in-situ observations. Simulations included the effects of high strain-rate plasticity, heat generation and dissipation, material damage, surface roughness and surface interactions in two and three dimensions. It was found that quasi-static approaches hold at low velocities where plastic deformation is limited; however, as the impact velocity increases inertial forces become substantial, leading to extensive plastic deformation and significant temperature rise. Impact behavior was found to be independent from the particle size, except in the presence of surface roughness. Elastic springback of the substrate was found responsible in rebound of the particle, where the exact mechanism depends on the extent of deformation.;Particle bonding process was simulated by defining interfacial cohesion in FE analysis. Critical velocities in the same range as the experimental results were predicted. Metals with low mechanical strength and high density were shown to be more suitable for cold spray operations. Correlation with experimental findings revealed that use of clean powder with low oxide content is crucial in lowering critical velocity and achieving higher quality coatings. Normal impacts were shown to be the most effective in terms of bonding. Preheating the particles and using spherical particles are also beneficial, whereas material brittleness was found to be destructive.