Experimental study and numerical investigation of the phenomena occurring during long duration cold spray deposition

Low Pressure Cold Spray process is a relatively new additive technique allowing to create high quality metallic coatings, through the high velocity spraying of particles, on both metallic and non-metallic substrates. The adhesion mechanism of the particles is, to date, not fully understood and the phenomena occurring during long depositions (which are the more interesting for industrial processes) are not known and studied in details. Aiming to fulfill this lack of knowledge, the phenomena occurring during long deposition were studied through both careful experimental evaluations and numerical approach. Particular attention was paid at the working conditions of the De Laval nozzle system which accelerates the particles, a computational fluid dynamics model focusing on both geometrical features and spray behavior was proposed. Micronsized aluminum powders and compressed air as carrier gas were used in this experimentation, numerical simulations are performed to predict the gas flow regime. It has been found that the deposition efficiency tends to decrease due to the critical phenomena occur into the spray nozzle during long time depositions. The sprayed particles tend to stick to the walls of the nozzle and, moreover, shock waves occur inside the nozzle further promoting the particles stick phenomena, making the system inefficient.