Parametric Redesign of a Convergent-Divergent Cold Spray Nozzle Florentina-Luiza Zavalan¹ and Aldo Rona² University of Leicester, Leicester, United Kingdom ¹ flz1@leicester.ac.uk, ² aldo.rona@leicester.ac.uk Abstract The generation of a high velocity carrier gas flow for cold metal particle applications is addressed, with specific focus on titanium cold spraying. The high hardness of this material makes cold spraying titanium difficult to achieve by industry standard nozzles. The redesign of a commercial conical convergent-divergent cold spray nozzle is achieved by the application of aerospace design codes, based on the Method of Characteristics, towards producing a more isentropic expansion by contouring the nozzle walls. Steady three- dimensional RANS SST k-ω simulations of nitrogen are coupled two-way to particle parcel tracking in the Lagrangian frame of reference. The new contoured nozzle is found to produce higher particle velocities with greater radial spread, when operated at the same conditions/cost of operation as the commercial nozzle. These numerical results have shown the potential for extending cold spray to high density and low ductility particles by relatively minor rig modifications, through an effective synergy between gas dynamics and material science. Introduction Cold spraying is a material deposition process developed about 40 years ago by the Russian scientists as a metal coating technology [1, 2]. This technique consists in accelerating powder particles to a high velocity (typically higher than 300 m/s) by a high-speed flow. The particles impinge onto a substrate, where they plastically deform under the action of their own kinetic energy and form a coating, as shown diagrammatically in Fig. 1. Figure 1: Schematic of a high-pressure cold spray system [3]. The major advantage of this metal processing technique is the low temperature involved, which minimizes any potential phase change of the substrate and keeps the particles in their solid state. This is making cold spray suitable for depositing a wide range of traditional and advanced materials on many types of substrate, lately being also used as an additive manufacturing and repair technology [3-5]. Alkhimov et al. [2] demonstrated that the key parameters for the successful bonding of the particles with the substrate are impact velocity and temperature. High velocity is necessary for optimal deposition efficiency and packing density. At low temperatures, particle oxidation is avoided, which can make cold spray depositions more durable and with a better bond strength than plasma spray coatings. Given the key role of particle velocity and of particle kinetic energy on the quality of the deposition, a good understanding of the factors that affect the particle velocity upon impact is crucial for improving the current cold spray practices. One of the most important components of the cold spray system is the supersonic nozzle and its capability of generating high deposition efficiency. In cold spraying, a supersonic gas jet is formed using a de Laval or a similar converging- diverging nozzle. The influence of the nozzle design in the cold spray process is discussed by several authors in the literature, with several studies dedicated to the optimization of the cold spray nozzle configuration. Dykhuizen et al. [6] presented a method for the optimal design of a cold spray nozzle. Alkimov et al. [7] also investigated a model to optimize the nozzle geometry. Later, various nozzle shapes were investigated, such as convergent-divergent nozzles [8- 12], convergent-divergent-barrel nozzles [10], convergent- barrel [10, 13], and bell-shaped nozzles [14, 15]. Tabbara et al. [16] and Yin et al. [17] investigated the effect of the nozzle cross-section on the particle velocity and distribution. Suo et al. [18] and Varadaraajan et al. [19] studied the effect of the nozzle geometry on particle distribution for rectangular nozzles. Jodoin [20] found that an exit Mach number between 1.5 and 3.0 can ensure the sprayed particles achieve a sufficient deceleration on impact to provide a well-bedded metal coating. Grujicic et al. [21] discovered that a relative Mach number of 1.4 leads to the highest particle acceleration, which is conducive to generating high particle kinetic energy. These particles embed well into the target. Lupoi [22] has shown that the effect of nozzle geometry on deposition efficiency is difficult to predict. There are still some unanswered questions concerning the influence of the nozzle design on the discrete phase properties, that is, on the particulate. Therefore, further studies are necessary to determine the optimal design approach for cold spray nozzles. Addressing this knowledge gap, a new workflow for designing contoured axisymmetric nozzles (the so-called bell-shaped nozzles) for cold spraying is presented. Thermal Spray 2021: Proceedings from the International Thermal Spray Conference May 24–28, 2021 F. Azarmi, X. Chen, J. Cizek, C. Cojocaru, B. Jodoin, H. Koivuluoto, Y. Lau, R. Fernandez, O. Ozdemir, H. Salami Jazi, and F. Toma, editors DOI: 10.31399/asm.cp.itsc2021p0221 Copyright © 2021 ASM International® All rights reserved. www.asminternational.org 221 Downloaded from http://dl.asminternational.org/itsc/proceedings-pdf/ITSC 2021/83881/221/487837/itsc2021p0221.pdf by guest on 23 September 2021