What would be the most appropriate parameters, namely, gas temperature and gas pressure, for cold spraying of a given feedstock material? This question is the focus of the present contribution. Initially, it is shown that main coating characteristics can be described as a unique function of a dimensionless parameter, defined as the ratio of particle velocity to critical velocity. Subsequently, these velocities and the respective ratio are worked out and expressed explicitly in terms of key process and material parameters, such as gas temperature and particle size. In this way, final properties of cold-sprayed deposits are linked directly to primary cold-spray parameters. Moreover, it is shown that the window of deposition, as well as the relationship between final properties of the deposit and the spraying conditions, can be incorporated conveniently into simple 2-D diagrams, showing contours of the velocity ratio, or the desired coating property, on the plane of the primary process parameters. Based on these diagrams, the process parameters related to a given coating property can be identified and selected, without a need to refer to intermediate variables such as particle velocity. The paper includes examples of the application of these maps for cold spraying of copper.

In the past years a number of publications reported about Titanium coatings cold sprayed with a nominal power input between 17 to 47 KW (e. g Kinetiks 4000) reaching gas temperatures of maximum 850 °C and gas pressure of maximum 4 MPa. In a recent study at Helmut-Schmidt University (HSU), a Kinetiks 8000 prototype was used to spray titanium, employing a nominal power of about 92 KW to increase the gas temperature up to 1000°C at a pressure of 4 MPa. Under these parameters, a high tensile strength of over 480 MPa and a deposition efficiency (DE) close to 100% were achieved. The present study focuses on further enhanced gas and particle velocities by optimized nozzle designs. The increased particle velocities in comparison to that obtained by using commercial nozzles (types 24, 51) result in better coating performance and allow deviations from ideal (90°) impact angle without significantly reducing coating strength. The influences of process conditions are evaluated and discussed on the basis of coating strengths by Micro Flat Tensile and Tubular Coating Tensile tests, as well as electrical conductivities, nitrogen and oxygen contents.

Cold spraying has a high potential for building up thick coatings or structures, like for rapid prototyping or production of free-standing parts. This method is particularly interesting for high strength materials that are difficult to process or to shape, e.g. Titanium for special applications in aviation industries. In this contribution, the two major challenges are addressed: (I) optimizing mechanical properties by systematic variation of process parameters, and (ii) evaluating the influence of the spray angle with respect to complex geometries as requested by aviation industry. While in the past, high quality titanium coatings were only achieved using Helium as process gas, Nitrogen is used in this study to reduce costs. High deposition efficiencies of more than 95 % can be obtained and the coatings show very low porosities as well as high tensile strength of over 480 MPa. The influence of process conditions on the mechanical properties is discussed on the basis of coating microstructures, Micro Flat Tensile (MFT) and Tubular Coating Tensile tests (TCT-test).