A Simplified Approach for the Determination of Critical Velocity for Cold Spray Processes
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Abdulaziz S. Alhulaifi and Gregory A. Buck (Submitted November 23, 2013; in revised form July 6, 2014) A simple technique employing the first law of thermodynamics was used to predict the critical impact velocity for cold spray processes based on material properties of the particles and substrates. It has been shown that during its interaction with the substrate, a particle should reach around 70% of its melting temperature to obtain good mechanical bonding. To characterize the results in a general way, a nondimensionalization of the relevant parameters was conducted and validated to determine the combination of cold spray process variables required for the particle to reach the critical impact velocity.
Keywords
cold spray, critical impact velocity, non-dimensionalization
1. Introduction Among the myriad applications of thermal spray processes, the deposition of hardened coatings to turbine blades and pump shafts is a common technique that has been used to prevent excessive wear and corrosion of these surfaces and prolong their useful life. Unfortunately, in typical applications of the thermal spray process, the feedstock materials experience high temperatures that can lead to undesired phases and oxidation in the coating material. To prevent the coated material from experiencing this undesired phase transformation, the cold spray process offers an attractive alternative. The cold spray deposition process involves the acceleration of solid particles which are maintained at temperatures lower than the melting point of the particle material, to high velocities using a carrier gas which is often supersonic, and the subsequent impingement of this high velocity, particle laden jet onto a substrate as shown in Fig. 1. In order for deposition to occur, i.e., in order for the solid particles to bond to the substrate, the impact velocity of the particles must be equal to or greater than some critical value, referred to as
the critical velocity for the cold spray deposition. Typically, inert gases such as helium or nitrogen are used as carrier gases to accelerate the sprayed particles of 5-100 lm diameter to high velocity through a convergent divergent nozzle, attaining exit velocities in the range of 200-1200 m/s in a free jet. The free jet subsequently impacts the substrate and, through the conversion of the particle kinetic energy to plastic strain energy, creates a strong bonded coating. Papyrin et al. (Ref 1) developed and patented (Ref 2) the cold spray process in the mid-1980s at the Institute of Theoretical and Applied Mechanics of the Russian Academy of Sciences, and he and his colleagues have deposited a remarkably wide variety of pure metal, metal alloy, and composite materials on a host of different substrates previously not possible. Critical velocity is the key parameter that influences the cold spray deposition efficacy and defines a physical limit between erosion of the substrate and the desired particle deposition onto the substrate surface. Assadi et al. (Ref 3) have studied the particle deformation on
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