LONG TIME COLD SPRAY DEPOSITION: EXPERIMENTAL INVESTIGATIONS AND NUMERICAL APPROACH

Low Pressure Cold Spray process is a relatively new coating technique which allows 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. To date, the complex adhesion mechanisms of the particles are still being discussed and, what is more, the phenomena occurring during the long time deposition processes, the most interesting ones for industrial applications, are not adequately studied and known. The aim of this paper is to contribute to fill this lack of knowledge through the detailed analysis of the phenomena occurring during long time depositions. To this end a full experimental campaign was carried out. Moreover it was also proposed a numerical model focusing on both geometrical features and fluid-dynamic behaviour of converging-diverging de- Laval nozzle system. Micron-sized aluminium powders and compressed air as carrier gas were used in this experimentation. 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, a shock wave occurs inside the nozzle further promoting the particles stick phenomena, making the system inefficient.