Cold spray modeling and validation using an optical diagnostic method

International audienceThe Cold Gas Dynamic Spray process uses the kinetic energy of unmelted sprayed particles to produce coatings. The most important element of the cold spray system is the nozzle used to accelerate the particles. Consequently, the nozzle design optimisation is a key to improve the coating quality and reduce the spraying costs. In this study, an axi-symmetric two-dimensional mathematical model is presented and used to predict the flow inside a cold spray nozzle as well as the particle velocity in the vicinity of the nozzle exit. The model results are compared with those obtained using the one-dimensional isentropic theory and with particle velocity measurements made on a commercial cold spray system. The study shows that the particle exit velocity depends on the type, stagnation temperature, and pressure of the propellant gas. The comparisons show that the proposed model is more accurate than the one-dimensional theory. It allows predicting accurately the behavior of the particles in the cold spray jet even in the presence of shock waves. Following this work, the design of new nozzles for specific applications using this mathematical model can be considered

Optimisation of the standoff distance in the cold spray process

International audienceLa projection cold spray utilise principalement l'énergie d'impact des particules pour former des revêtements denses. L'étalement et l'adhésion des particules étant liés à l'énergie cinétique et donc à la vitesse des particules, il est primordial de maximiser leur accélération pour obtenir un rendement de dépôt élevé. Ce transfert de quantité de mouvement peut être optimisé en modifiant les paramètres de fonctionnement du système de projection mais également en choisissant une distance tuyère-substrat adéquate. Cet article décrit un modèle 2D mis en œuvre avec le logiciel de CFD Fluent pour simuler le comportement d'un flux de particules de titane dans un écoulement supersonique. Ses résultats montrent que les particules n'atteignent pas leur vitesse maximale en sortie de tuyère mais environ 90 mm en aval de celle-ci. C'est donc à cette distance que le substrat doit être placé pour optimiser la projection. La précision de ces calculs est confirmée expérimentalement et les variations significatives d'épaisseurs des dépôts montrent l'influence drastique de la distance de tir sur le rendement de dépôt

Influence of particle velocity on adhesion of cold-sprayed splats

International audienceIn cold spray, innovative coating process, powder particles are accelerated by a supersonic gas flow above a certain critical velocity. Particles adhesion onto the substrate is influenced by particle impact velocity, which can change dramatically depending on particle position from the core of the jet. In the present work, an original experimental set-up was designed to discriminate the particles as a function of the levels of velocity to investigate the influence of this parameter on adhesion. Particles at given positions could therefore be observed using scanning electron microscope, which showed different morphologies as a function of impact velocity. High pressure and temperature at the interface during impact were calculated from numerical simulations using ABAQUS®. Transmission electron microscope analyses of thin foils were carried out to investigate into resulting local interface phenomena. These were correlated to particle impact velocity and corresponding adhesion strength which was obtained from LAser Shock Adhesion Test