Numerical investigations on the rebound phenomena and the bonding mechanisms in cold spray processes

Cold spray technology is a relatively new additive process allowing to create high quality metallic coatings, on both metallic and non-metallic substrates, without extensive heating of the powders sprayed. Upon impact with a target surface, conversion of kinetic energy to plastic deformation occurs, the solid particles deform and bond together. The actual bonding mechanism for cold spray particles is still not well understood, a high number of works has been carried out during the past two decades, several theories have been proposed to explain the adhesion/rebound mechanisms making the system ineffective for industrial applications. Therefore, the aim of this research activity is to better explain the complex adhesion/rebound phenomena into cold spray impact processes through numerical simulations; for this purpose, on the base of simplified hypothesis and results found in literature, an original 3D Finite Element Method (FEM) model of an aluminium particle impacting on an aluminium substrate was proposed. A cohesive behaviour algorithm was implemented in the particle-substrate contact regions aiming to simulate the bonding between the impacting particle and the substrate under specific working conditions. A rebound coefficient was also defined representing the particle residual energy. Different simulations were performed using a range of impact velocities and varying the interfacial cohesive strength. It was shown that at low impact velocities the rebound phenomenon is governed by the elastic energy stored in the system, meanwhile at high impact velocities, the rebound phenomenon is mainly due to the strain rate effects making the system mechanically stronger; therefore, a specific range of bonding velocities depending on substrate-particle contact area were found.