Cold spray: effect of mixing powders

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Cold spray is a seemingly simple method to apply metal coatings to metal, ceramic and polymer surfaces by accelerating metal powders of about 20 to 40 micron particle diameter to supersonic speeds using a carrier gas, usually nitrogen. On impact with the substrate to be coated, the particles severely plastically deform and instantaneously bond to the substrate and each other. The coating of the substrate takes place in air at room temperature. The usual methods to bond powder particles are performed at very high temperatures, between about 80% of the melting point and even upto the melting point, and require very long times, at least a few hours. Clearly the short times and the fact that coatings are done in air are obviously advantageous; room temperature processing means that heat damage is avoided and also means that unlikely materials pairings are possible, such as polymers embedded in a metal matrix. Moreover, very thick 3D coatings can be made, thus elevating the process to one of additive manufacturing.***Usually, only one metal powder is sprayed, but this proposal is concerned with what happens when a mixture of powders is cold sprayed. As noted above, interesting composites' can be made by cold spray and part of this work concerns making composites for various applications: metal metal composites for biodegradable stents, metal coatings for lightning strike protection for aircraft structures made of carbon fibre reinforced polymer composites and metal matrix-tungsten carbide composites for wear resistant coatings. The scientific objective, however, is to understand what happens to the cold spray process when mixed powders are sprayed. In this respect we are mainly concerned with the cold sprayability of the mixtures, or how easy it is for these powders to bond onto the substrate to form a thick coating. Cold sprayability is usually judged by the deposition efficiency, i.e. how much of the sprayed powder sticks, and the level of porosity in the coating, the lower the porosity, the better the coating as a general rule. Previous work by our team has shown that the mixing can significantly affect cold sprayability, but in a very unpredictable manner, belying the simplicity of the cold spray concept. This work aims to understand the cold spray mechanisms associated with powder mixing, and use this to develop accurate predictions of cold sprayability. ***Cold spray is rapidly becoming a key repair' method and, as such, adds to the capacity of technologies for sustainability. As mentioned above, it is one of the additive manufacturing technologies and is therefore of interest to the automotive and aerospace industries. This program is scientifically intriguing and is of great contemporary engineering importance. It represents an excellent training opportunity to generate high quality personnel in the advanced manufacturing sector a vital contributor to the Canadian economy.