Production of titanium deposits by cold-gas dynamic spray: numerical modeling and experimental characterization

Over the past five years, interest in cold-gas dynamic spraying (CGDS) has increased substantially. Considerable effort has been devoted to process development and optimization for such metals as copper and aluminium. This paper describes work undertaken to expand the understanding of the deposition of titanium by cold-spray methods. CGDS deposits have been produced from commercially pure titanium using room-temperature helium gas. The effect of different powder particle size ranges, types of substrate, substrate preparation methods, and spray parameter conditions on powder deposition have been investigated. Microhardness testing of deposits was conducted, and their microstructures have been examined by scanning electron microscopy. Samples for pull-off bond-strength tests have been prepared from a number of the more promising sets of spray parameters and adhesive strengths determined. A one-dimensional numerical model of particle acceleration, employing isentropic gas flow behavior in the nozzle, has also been used to estimate particle exit velocities. This model explicitly addresses the dependence of the drag coefficient on gas compressibility and demonstrates its significance in terms of predicted particle velocities. By linking this model with the measured particle size distributions, estimates of particle velocity distributions at the nozzle exit plane have been computed. These allow an approximate value of the critical velocity for deposition of titanium to be made. Experimental observations on the microstructure and properties of the deposits are discussed in light of powder particle size and velocity distributions and the underlying physical and mechanical properties of the powders and substrates.