A numerical study of pore formation mechanisms in aluminium cold spray coatings

Porosity is critical regarding cold sprayed coatings properties: it can be an asset for certain applications as in tribology, it is generally at the cost of mechanical and anti-corrosion properties. It is thus important to understand the mechanisms at its origin, in order to have a better control over it. Since the build-up process of a cold spray coating cannot be directly observed at a microscopic scale, numerical simulation is the only way to address the mechanisms of pore formation. The main purpose of this study is to understand by dedicated simulations the mechanisms, acting during the build-up of a cold spray coating, giving rise to pore formation mechanisms. Besides, an user-independent method developed to assess porosity from these simulations is presented. The first part of the study, analysing the results of a 500 particles impact simulation, shows that porosity is mainly caused by two intricate mechanisms: interfacial porosity is created by a geometrical effect at particle/particle interfaces, whereas stack porosity is caused by particle density variations in the gas flow. Then, an original method, based on the commutation of successive convex hulls, to estimate porosity from multi-particle impact simulations is presented. Initial particles position has a strong influence on this estimation and the reference volume to compute porosity must therefore be thoroughly chosen. Finally, porosity computed by numerical simulation are compared to 3D X-ray microtomography measurements.