Cold Spray Gas Flow Dynamics for On and Off-axis Nozzle/Substrate Hole Geometries

Cold spray has been proven to be an economic, green and eco-friendly spraying technique. Its remarkable deposition rate, flexibility, and low process temperature make it particularly popular for fast and low thermal impact part repairs. Despite cold spray advantages, deposition in discontinuous geometries such as steps, corners, and holes, remains a challenge as the understanding of the gas jet interaction with these geometrical features is still limited. Experimental works have shown that it is challenging to fill holes as some inside areas and near the hole entrance remain undeposited or exhibit poorly bonded coating. In this work, we study experimentally and numerically the effect of process gas temperature and pressure in two configurations: with and without an offset between the nozzle's central axis and the hole center. For each configuration, holes of different diameters and depths were prepared and the effect of spray parameters on the deposition patterns of a copper powder feedstock was experimentally investigated. Using numerical modeling, these cases were simulated, and particle velocity and distribution upon impact were calculated. The numerical results were compared to spray deposition patterns obtained from the experiments in the same conditions. In particular, particle footprints and deposition in different areas e.g., hole center and edge were evaluated. As expected, it was observed that as the hole diameter decreases or its depth increases, the deposition rate and the quality of the coating inside the hole are significantly reduced. The results of this study show that increasing the gas inlet pressure and choosing the right offset distance can improve deposition inside the hole and fill the regions that remained undeposited in the on-axis geometry.