On the mechanics and material-response of metal particles in cold spray impacts

• Online Access: http://hdl.handle.net/2047/D20383699

Cold gas dynamic spray (CGDS), also known as cold spray (CS), is a solid-state deposition technique used for repairing, coating and three-dimensional additive manufacturing suitable for implementation by using a wide group of powder materials. In this technique, micron-scale particles are accelerated in a supersonic gas stream to velocities in the range of 400 - 1200 m/s and bond to one-another upon impact. The deposition relies on the high kinetic energy of the particles which experience strain rates reaching 10^8 1/s. While it is possible to observe impact, rebound and bonding of single particles in-situ, it is currently impossible to observe material behavior in detail, during the short duration of the impacts. In this work, continuum level simulations of single particle impacts have been employed to simulate and analyze the particle impact process, by using the finite element method. The general goal of this work was to investigate the relationship between impact mechanics and material-response. To this end impacts of a) solid Al-6061 particles on sapphire, b) solid Al-6061 particles on Al-6061, and c) porous WC-17Co and porous Al-6061 were simulated. The work also included careful calibrations of a high-strain rate material model for Al-6061. The Johnson-Cook (JC) plasticity model, a phenomenological high strain rate (HSR) materials model, was calibrated for the extremely-high strain range (EHSR) and for high temperatures. To this end, single particle impact observations provided by our collaborators from University of Massachusetts, Amherst were used. In order the represent the high (1 - 1000 1/s) and extremely-high (> 1000 1/s) strain rate effects, the term that represents the strain rate effects in the JC-model was split into two parts; the EHSR region was represented by using a slope, C2, that's steeper than the slope, C1, of the HSR region. In general, it was found that C2 is a function of the initial temperature and average strain rate of impact. This work also involved calibrating the cohesion energy by employing a cohesive zone model in the impact interface. To this end a reduced-order model was established in order to predict the rebound behavior of Al-6061 particles from Al-6061 substrates by combining experimentally obtained coefficient of restitution values, FE simulations, principal component analysis and artificial neural networks. With this surrogate model the cohesive zone model parameters, critical strength σ_c and cohesion energy G_c were determined. These two parameters were calibrated by using the UMass Amherst experiments to be σ_c = 240 MPa and G_c = 1 J/m^2. With a calibrated material model, the impact mechanics of micron-scale Al-6061 particles on Sapphire and Al-6061 substrates was investigated with finite element simulations. A significant change of material behavior during Al-6061 on Sapphire impact was found near impact velocity of 500 m/s, which leads to a change in particle rebound behavior. A thorough examination of the evolution of plastic strain, strain rate, temperature and flow stress during the impact process revealed that a small volume of the material internal to the particle becomes unstable. This was attributed to the intense thermal softening caused by drastic plastic deformation. However, for Al-6061 on Al-6061 single particle impacts, no presence of material instability was found in either the particle or the substrate. A more rigorous analysis with a quantitative definition of adiabatic shear instability further validated that adiabatic shear instability is unlikely to be the dominating cause of bonding in CS of Al-6061. The effects of particle porosity and material parameters on single particle impacts were analyzed. Highly distinct responses were predicted for porous tungsten carbide (WC-17Co) and porous Al-6061 particles. At a relatively low level of initial kinetic energy, porous particles of both WC-17Co and Al-6061 demonstrate a wave guide like response where stress concentration is mostly present in the vertical columns between neighboring pores, and strain is channeled in a similar manner. However, at higher levels of kinetic energy, precipitous failure of material following impact leads to the collapse of porous structure in WC-17Co particle, and in turn halts the wave guide mechanism before it can make deeper influence on the rebound and deformation behavior of the particle. Due to its significantly higher elastic modulus and yield stress, WC-17Co is capable of restoring more energy elastically compared to Al-6061 at the same initial energy level and porosity. For WC-17Co particles, with the increase of porosity, more energy is dissipated through plastic deformation attributed to more prominent stress concentration near the pores. Al-6061 particle tends to absorb less impact energy through plastic deformation as porosity increases, at this point, this is attributed to a meta-material behavior.