Effect of injection pressure on particle acceleration, dispersion and deposition in cold spray

https://doi.org/10.1016/j.commatsci.2014.03.055Get rights and content

Highlights

  • Injection pressure significantly influences the flow regime and particle velocity.

  • Powder injection rate and coating thickness rise with increasing injection pressure.

  • Particles disperse more prominent at higher injection pressure.

  • The area of the deposition window reduces with increasing injection pressure.

Abstract

The effect of injection gas pressure on particle acceleration, dispersion and deposition in cold spray process was investigated by both numerical and experimental methods. A computational fluid dynamics (CFD) model was developed which exactly matches the real nozzle in experiment to predict the supersonic gas flow field and particle velocity prior to the impact. Based on the simulation results, it is found that injection pressure significantly affects the flow field of the driving gas. Higher injection pressure leads to higher injection flow rate as well as powder injection rate, producing thicker coating on the substrate. Besides, the particle footprints on the substrate surface at different injection pressures were predicted and compared with the experimental measurements of the single track coating width. The results indicate that particles disperse more widely at higher injection pressure. In addition, the parameter η which refers to the ratio of particle impact velocity to critical velocity is used to evaluate the effect of injection pressure on coating characteristics. With increasing the injection pressure, both η and deposition range have a downward trend, which implies the deterioration of the deposition efficiency and coating bonding strength. For the purpose of validation, the deposition efficiency of copper coatings was also experimentally measured at different injection pressures, which confirms the numerical analysis.

Introduction

Cold spraying is a relatively new coating technique developed in the mid-1980s and has been rapidly developing during the past two decades. In this process, powder particles (typically <50 μm) are accelerated to a high velocity ranging from 300 to 1200 m/s by a supersonic gas flow and then impinging onto a substrate in solid state without significant fusion, undergoing intensive plastic deformation. The ‘low temperature’ in cold spray process can minimize the adverse effect brought by molten or semi-molten state, providing a possibility to coat oxygen-sensitive materials [1], [2]. It has been widely accepted that there exists a material-dependent critical velocity for a given condition (e.g. specific particle size, temperature and material properties), only above which bonding at the particle/substrate interface can take place and the cold spray coating can be formed on the substrate surface [3], [4], [5], [6], [7].

Therefore, much effort was devoted to investigate the particle acceleration behaviour and the consequent particle in-flight velocity. It has been widely accepted that particle in-flight velocity is highly dependent on the gas flow field inside and outside the nozzle. In this respect, a number of studies have been conducted to explore the main factors influencing the flow field of the driving gas. The results revealed that operating parameters and nozzle geometry are two dominant causes that determine the flow field of the supersonic driving gas [8], [9], [10]. Therefore, a large body of works regarding to the optimization of nozzle geometry and operating parameters were further carried out in order to achieve the optimum particle impact velocity during the coating build-up process [8], [11], [12], [13], [14], [15], [16], [17]. Many meaningful conclusions were drawn from those studies. Specifically, at the given working condition, nozzles with optimal expansion ratio significantly reduce the shockwaves outside the nozzle and maximize the particle kinetic energy [8], [10], [11]. Also, nozzle cross-section shape was found to significantly influence the particle velocity and dispersion. Circular and square cross-sections result in higher particle velocity while elliptical cross-section makes the particles more dispersed [12], [13]. Besides, the investigation on the optimization of operating parameters indicates that increasing the inlet pressure, temperature or using helium as the driving gas is able to increase the particle impact velocity [8], [9], [14], [15], [16], [17].

Although there have been sizable literatures concerning the optimization of the nozzle geometry and operating parameters, only few studies focused on the significant influence of powder injection on the particle acceleration and deposition. One of the relevant studies was carried out by Han et al., which suggested that an improved injector configuration can avoid the clogging at the injector tunnel and thus enhance the final coating formation [18]. Lupoi and O’Neill reported that injector with small diameter results in narrow powder beam. Also, they pointed out that turbulence is the main reason for the particle dispersion [19]. Additionally, injection position was also found to affect the particle velocity and temperature [13]. Most recently, a numerical study further revealed that injection position even has some effects on the particle preheating temperature [20]. All the mentioned studies have showed the particular importance of injector and injection conditions in the cold spray process. It is known that, in cold spray practice, the injection pressure must be controlled to be higher than the main gas to guarantee the successful injection of powders. However, so far, the complex influence of injection pressure during the cold spray process is not clarified and a systematic study is still lacking. In this study, therefore, a comprehensive investigation on the effect of injection pressure on the particle acceleration, dispersion and deposition was carried out. CFD technique was employed as the main method due to its less economic and timing consumption. The experimental observations against the numerical results were also performed to further provide some convincing evidences.

Section snippets

Computational domain and boundary conditions

Numerical simulations were performed by using ANSYS-FLUENT 14.5 to predict the gas flow field and particle velocity in cold spray [21]. The commercial MOC nozzle (CGT GmbH, Germany) was used in this study, which exactly matches the real gun in the experiment. The dimensions of the nozzle are listed in Table 1. The two-dimensional axi-symmetric model was employed to save the computational time. The dimensions of the computational domain and boundary conditions were provided in Fig. 1. The

Experimental details

Coatings were produced by using a home-made cold spray system (LERMPS, UTBM, France) with the MOC nozzle. The main gas pressure and temperature are set as 2.5 MPa and 873 K, respectively. Three injection pressures (2.5, 2.6 and 3.0 MPa) were tested, which mach the modeling parameters. The substrate was located 30 mm away from the nozzle exit. A gun traverse speed of 50 mm/s was employed for the coating deposition. Pure copper powders (Sulzer metco, +15–45 μm, spherical) were selected as the

Effect of injection pressure on gas flow field

Fig. 3 shows the temperature and velocity contours of the gas inside and outside the nozzle at different injection pressures. In all cases, after leaving the injector, the low temperature carrier gas suddenly mixes with the high temperature main gas. The momentum and heat exchanges between two streams result in the temperature reduction of the main driving gas at different levels. More specifically, when the injection pressure is 2.5 MPa, that is to say the pressure difference at the injection

Conclusions

The effect of injection pressure on the particle acceleration, dispersion and deposition was systematically investigated. Both CFD study and experiment were carried out to achieve this objective. It is found that gas flow field is significantly affected by the injection pressure. Increasing the injection pressure results in the increment of carrier gas velocity and flow rate. As a consequence, the powder injection rate and final coating thickness show an upward trend with the injection

Acknowledgments

The authors would like to acknowledge the support by Marie Curie FP7-IPACTS-268696 (EU).

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