Particle behavior in supersonic flow during the cold spray process

The cold spray process is a relatively new process that uses high velocity metallic particles for surface modifications. Metallic powder particles are injected into a converging-diverging nozzle and accelerated to supersonic velocities. In this study two-dimensional temperature and velocitiy distributions of gas along the nozzle axis are calculated and the effects of gas pressure and temperature on particle velocities and temperature inside and outside the nozzle are investigated. It was found that acceleration of the gas velocity takes place in the area of the nozzle throat, and it increases and reaches a maximum value at the nozzle exit. Due to compression shocks, irregular changes of the gas jet properties were found in the area after the nozzle and these resulted in the experience of the maximum particle velocity by the change of the particle size at a given gas pressure and temperature.     

Analysis of tantalum coatings produced by the kinetic spray process

Tantalum (Ta) coatings have been produced using a relatively new process, kinetic spray. Ta starting powders having particle diameters greater than 65 μm are injected into a de Laval-type nozzle, entrained in a supersonic gas stream, and accelerated to high velocities due to drag effects. The particles’ kinetic energy is transformed via plastic deformation into strain and heat on impact with the substrate surface. Particles are not thermally softened or melted, producing relatively low oxide, reduced residual stress, high adhesion and low porosity coatings. Analysis of the mechanical and physical properties of these Ta coatings demonstrated increasing hardness, cohesive adhesion, and decreasing porosity as a function of particle velocity. Comparison between kinetically sprayed coatings and coatings produced using conventional coating methods will be discussed.     

Critical particle velocity under cold spray conditions

Cold spray process is an emerging technique that produces high density coatings. Particles (1 to 50 μm in diameter) are carried by a supersonic gas stream through a de Laval nozzle and, finally, impact on a substrate with high kinetic energy. Low gas temperatures (< 600 °C) make it possible to maintain sprayed material in solid state during the whole process. If the particles reach a given velocity, called “critical velocity”, they can bind to the surface and create a coating. This velocity is clearly dependent on both sprayed material and substrate properties. This work presents an imaging technique that allows a fast measurement of critical velocity. The measuring method is first evaluated by comparing the critical velocity of copper (sprayed on copper substrate) found in the literature, with the measured one. Its accuracy is then tested with other materials and, finally, some improvements of the method are proposed.     

高速低温喷涂气固作用行为及喷涂系统研究进展

高速低温喷涂是利用固相或含固相的低温粉末在高速度、高动能作用下碰撞基体表面沉积的喷涂方法,具有氧化轻微、 结合牢固、组织致密、综合力学性能优异等潜在优势,在高性能金属或金属基复合材料涂层制备、增材制造和零件损伤修复等领域获得广泛关注。以粉末低温高速碰撞沉积过程为主线,凝练现有冷喷涂和低温超音速火焰喷涂两种具体工艺的共性特征,阐明喷涂气流与粉末颗粒的气固两相交互作用规律,分析出合理调控颗粒温度和速度是改善沉积体性能的关键。其次分析高速低温喷涂设备系统的构成,详细讨论各核心部件的结构设计策略及对气固流动行为的影响,总结出通过调整工艺参数与喷枪结构,可以实现颗粒温度和速度的按需控制。最后,对高速低温喷涂工艺及设备系统发展目前尚存的关键问题进行展望。总结如何通过喷涂参数与装置设计,最终达成调控沉积体性能的目的,有助于深入理解高速低温喷涂的沉积机理,对研制高性能的喷涂设备系统具有参考意义。     

真空度对真空冷喷涂气固两相流的影响

目的真空度直接影响着真空冷喷涂时气体流动特性和颗粒撞击速度,研究真空度对气体和颗粒流动特性的影响。方法确定真空冷喷涂系统结构,采用FLUENT软件进行真空冷喷涂气固两相流研究,通过数值模拟研究真空度对流场和颗粒撞击速度的影响,并研究相同压力比下的气固两相流特性。结果当入口压力一定时,喷管内的气体轴线速度、密度和温度与环境压力大小无关;而在射流区,环境压力越小,则气体轴线速度波动越小、密度越低,但到达基板后的气体温度均接近喷管入口温度。环境压力对大粒径颗粒的撞击速度影响较大,颗粒撞击速度随环境压力增大而先增后减,最佳环境压力可根据气相云图和气体密度来确定。当进出口压力比相同时,喷管内和射流区域内的气相速度云图基本相同,气体轴线速度曲线基本重合,而基板前的颗粒速度不同,此时环境压力越低,颗粒速度越高,越有利于形成涂层。结论采用计算流体动力学分析方法厘清了真空度对真空冷喷涂气固两相流的影响,为涂层制备奠定了理论基础。     

Gas dynamic principles of cold spray

This paper presents an analytical model of the cold-spray process. By assuming a one-dimensional isentropic flow and constant gas properties, analytical equations are solved to predict the spray particle velocities. The solutions demonstrate the interaction between the numerous geometric and material properties. The analytical results allow determination of an optimal design for a cold-spray nozzle. The spray particle velocity is determined to be a strong function of the gas properties, particle material density, and size. It is also shown that the system performance is sensitive to the nozzle length, but not sensitive to the nozzle shape. Thus, it is often possible to use one nozzle design for a variety of operational conditions. Many of the results obtained in this article are also directly applicable to other thermal spray processes.     

High velocity pulsed plasma thermal spray

The quality and durability of coatings produced by many thermal spray techniques could be improved by increasing the velocity with which coating particles impact the substrate. Additionally, better control of the chemical and thermal environment seen by the particles during flight is crucial to the quality of the coating. A high velocity thermal spray device is under development through a Ballistic Missile Defense Organization Small Business Innovation Research (SBIR) project, which provides significantly higher impact velocity for accelerated particles than is currently available with existing thermal spray devices. This device utilizes a pulsed plasma as the accelerative medium for powders introduced into the barrel. Recent experiments using a particle imaging diagnostic system showed that the device can accelerate stainless steel and WC-Co powders to velocities ranging from 1500 to 2200 m/s. These high velocities are accomplished without the use of combustible gases and without the need of a vacuum chamber, while maintaining an inert atmosphere for the particles during acceleration. The high velocities corresponded well to modeling predictions, and these same models suggest that velocities as high as 3000 m/s or higher are possible.     

Computational study and experimental comparison of the in-flight particle behavior for an external injection plasma spray process

A three-dimensional computational fluid dynamic (CFD) analysis using Fluent V5.4 was conducted on the in-flight particle behavior during the plasma spraying process with external injection. The spray process was modeled as a steady jet issuing from the torch nozzle via the heating of the are gas by an electric are within the nozzle. The stochastic discrete model was used for the particle distribution. The particle temperature, velocity, and size inside the plasma plume at a specified standoff distance have been investigated. The results show that carrier gas flow rate variation from 2 standard liters per minute (slm) to 4.0 slm can increase the centerline particle mean temperature and mean velocity by 10% and 16%, respectively, at the specified standoff distance. A further increase of the carrier gas flow rate to 6 slm did not change the particle temperature, but the particle velocity was decreased by 20%. It was also found that an increase in the total arc gas flow rate from 52 slm to 61 slm, with all other process parameters unchanged, resulted in a 17% higher particle velocity, but 6% lower particle temperature. Some of these computational findings were experimentally confirmed by Kucuk et al. For a given process parameter setting, the kinetic and thermal energy extracted by the particles reached a maximum for carrier gas flow rate of about 3.5–4.0 slm.     

Particle Bonding Mechanism in Cold Gas Dynamic Spray: A Three-Dimensional Approach

Cold gas dynamic spray (CGDS) is a surface coating process that uses highly accelerated particles to form the surface coating. In the CGDS process, metal particles with a diameter of 1-50 µm are carried by a gas stream at high pressure (typically 20-30 atm) through a de Laval-type nozzle to achieve supersonic velocity upon impact onto the substrate. Typically, the impact velocity ranges between 300 and 1200 m/s in the CGDS process. When the particle is accelerated to its critical velocity, which is defined as the minimum in-flight velocity at which it can deposit on the substrate, adiabatic shear instabilities will occur. Herein, to ascertain the critical velocities of different particle sizes on the bonding efficiency in CGDS process, three-dimensional numerical simulations of single particle deposition process were performed. In the CGDS process, one of the most important parameters which determine the bonding strength with the substrate is particle impact temperature. It is hypothesized that the particle will bond to the substrate when the particle’s impacting velocity surpasses the critical velocity, at which the interface can achieve 60% of the melting temperature of the particle material (Ref 1, 2). Therefore, critical velocity should be a main parameter on the coating quality. Note that the particle critical velocity is determined not only by its size, but also by its material properties. This study numerically investigates the critical velocity for the particle deposition process in CGDS. In the present numerical analysis, copper (Cu) was chosen as particle material and aluminum (Al) as substrate material. The impacting velocities were selected between 300 and 800 m/s increasing in steps of 100 m/s. The simulation result reveals temporal and spatial interfacial temperature distribution and deformation between particle(s) and substrate. Finally, a comparison is carried out between the computed results and experimental data.     

Effect of Nozzle Material on Downstream Lateral Injection Cold Spray Performance

In cold gas dynamic spraying, the gas nature, process stagnation pressure and temperature, and the standoff distance are known to be important parameters that affect the deposition efficiency and coating quality. This investigation attempts to elucidate the effect of nozzle material on coatings produced using a downstream lateral injection cold spray system. Through experimentation, it is shown that the nozzle material has a substantial effect on deposition efficiency and particle velocity. It is proposed that the effects are related to complex interaction between the particles and the internal nozzle walls. The results obtained lead to the conclusion that during the particle/nozzle wall contact, a nozzle with higher thermal diffusivity transfers more heat to the particles. This heat transfer results in lower critical velocities and therefore higher deposition efficiencies, despite a noticeable reduction of particle velocities which is also attributed to particle-nozzle interactions.     

Influence of Helium and Nitrogen Gases on the Properties of Cold Gas Dynamic Sprayed Pure Titanium Coatings

This study investigates the effect of propellant gas, helium, and nitrogen during cold spraying of titanium coatings. Coatings were characterized by SEM and were evaluated for their deposition efficiency (DE), microhardness, and porosity. In selected conditions, three particle velocities were investigated in which for each condition, the propelling gases?? temperature and pressure were attuned to attain similar particle velocities for each gas. Observations show that loosely bonded particles can be detached by high-pressure supersonic gas stream. Selected coatings were characterized by XPS to analyze the occurrence of oxidation and nitridation. Although generally accepted that coating characteristics can be affected by particle temperature, results show that for the same particle velocity, DE and coating density are also a function of substrate temperature. In addition, a thick and fully dense cold sprayed titanium coating was achieved with optimized spray parameters and nozzle using helium. The corresponding average particle velocity was 1173 m/s.     

Optical Diagnostics Study of Gas Particle Transport Phenomena in Cold Gas Dynamic Spraying and Comparison with Model Predictions

Cold gas dynamic spraying (CGDS), a relatively new thermal spraying technique has drawn a lot of attention due to its inherent capability to deposit a wide range of materials at relatively low-operating temperatures. A De Laval nozzle, used to accelerate the powder particles, is the key component of the coating equipment. Knowledge concerning the nozzle design and effect of process parameters is essential to understand the coating process and to enable selection of appropriate parameters for enhanced coating properties. The present work employs a one-dimensional isentropic gas flow model in conjunction with a particle acceleration model to calculate particle velocities. A laser illumination-based optical diagnostic system is used for validation studies to determine the particle velocity at the nozzle exit for a wide range of process and feedstock parameters such as stagnation temperature, stagnation pressure, powder feed rate, particle size and density. The relative influence of process and feedstock parameters on particle velocity is presented in this work.     

Particle-in-flight monitoring in thermal spray processes

A diagnostic system based on non-intensified CCD image sensor is applied for particle-in-flight monitoring of different deposition processes: cold gas dynamic spray (CGDS), computer-controlled detonation spray (CCDS) and direct metal deposition (DMD). An additional illumination source for measuring particle velocity in CGDS and DMD processes is used. Particle velocity measurements are carried out aiming optimization of a Cold Spray nozzle with two zones of powder injection for spaying Al powder. In a pulsed-periodic process like detonation spraying, particle-in-flight visualization and velocity measurements are done by synchronizing detonation pulses with the CCD-camera-based diagnostic tool. A significant variation of particles velocity along the detonation plume is observed. In DMD process, dependence between the carrier gas flow rate and particle velocity is found.     

Feedback control of HVOF thermal spray process accounting for powder size distribution

This work presents a novel formulation of the control problem and a feedback control system for the high velocity oxygen-fuel (HVOF) thermal spray process, which explicitly accounts for the effect of powder size distribution. Initially, based on model predictions and available experimental data, the control problem is formulated as one of regulating appropriate averages (with respect to the particle volume distribution) of the temperature and velocity of the particles at the point of impact on substrate (these are the variables that directly influence coating microstructure and porosity, which, in turn, determine coating mechanical and thermal properties) by manipulating the oxygen/fuel ratio and the combustion chamber pressure, respectively. Then, a feedback control system is developed and applied to a detailed mathematical model of the process. Closed-loop simulations show that the average particle velocity and temperature at the point of impact on substrate reach the desired values in a short time, which validates the feasibility of real-time implementation of feedback control on HVOF thermal spray systems. It is also shown that the proposed formulation of the control problem (which accounts for the effect of powder size distribution) leads to a solution of the control problem that is superior (with respect to the achievement of the desired control objectives) to a solution that assumes a monodisperse powder size distribution. Finally, the proposed control problem formulation and the feedback control system are shown to be robust with respect to disturbances in spray distance and particle injection velocity, and variations in powder size distribution.     

Deposition of metallic coatings on polymer surfaces using cold spray

Current coating technologies such as plasma spray, High Velocity Oxygen Fuel (HVOF) or laser cladding involve the delivery of molten materials during the deposition process. However, such techniques are not well suited to the deposition of metallic coatings on polymers and composites. Cold spray (CS) has attracted much industrial interest over the past two decades. In this method, a material in powder form is accelerated on passage through a converging-diverging nozzle to high speeds via a high pressure coaxial carrier gas jet. The high impact kinetic energy deforms the particles, which creates effective bonding to the substrate.This paper presents the results of an initial study on the potential of the CS process to produce metallic coatings on non-metallic surfaces such as polymers and composites for engineering applications. Experimental and Computational Fluid Dynamics (CFD) results when spraying copper, aluminium and tin powder on a range of substrates such as PC/ABS, polyamide-6, polypropylene, polystyrene and a glass-fibre composite material are presented and analyzed.     

New developments in cold spray based on higher gas and particle temperatures

In cold spraying, bonding is associated with shear instabilities caused by high strain rate deformation during the impact. It is well known that bonding occurs when the impact velocity of an impacting particle exceeds a critical value. This critical velocity depends not only on the type of spray material, but also on the powder quality, the particle size, and the particle impact temperature. Up to now, optimization of cold spraying mainly focused on increasing the particle velocity. The new approach presented in this contribution demonstrates capabilities to reduce critical velocities by well-tuned powder sizes and particle impact temperatures. A newly designed temperature control unit was implemented to a conventional cold spray system and various spray experiments with different powder size cuts were performed to verify results from calculations. Microstructures and mechanical strength of coatings demonstrate that the coating quality can be significantly improved by using well-tuned powder sizes and higher process gas temperatures. The presented optimization strategy, using copper as an example, can be transferred to a variety of spray materials and thus, should boost the development of the cold spray technology with respect to the coating quality. This article was originally published inBuilding on 100 Years of Success, Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, OH, 2006.     

Bonding mechanism in cold gas spraying

Cold gas spraying is a relatively new coating process by which coatings can be produced without significant heating of the sprayed powder. In contrast to the well-known thermal spray processes such as flame, arc, and plasma spraying, in cold spraying there is no melting of particles prior to impact on the substrate. The adhesion of particles in this process is due solely to their kinetic energy upon impact. Experimental investigations show that successful bonding is achieved only above a critical particle velocity, whose value depends on the temperature and the thermomechanical properties of the sprayed material. This paper supplies a hypothesis for the bonding of particles in cold gas spraying, by making use of numerical modelling of the deformation during particle impact. The results of modelling are assessed with respect to the experimentally evaluated critical velocities, impact morphologies and strengths of coatings. The analysis demonstrates that bonding can be attributed to adiabatic shear instabilities which occur at the particle surface at or beyond the critical velocity. On the basis of this criterion, critical velocities can be predicted and used to optimise process parameters for various materials.     

Particle Acceleration in a High Enthalpy Nozzle Flow with a Modified Detonation Gun

The quality of thermal sprayed coatings depends on many factors which have been investigated and are still in scientific focus. Mostly, the coating material is inserted into the spray device as solid powder. The particle condition during the spray process has a strong effect on coating quality. In some cases, higher particle impact energy leads to improved coating quality. Therefore, a computer-controlled detonation gun based spraying device has been designed and tested to obtain particle velocities over 1200 m/s. The device is able to be operated in two modes based on different flow-physical principles. In one mode, the device functions like a conventional detonation gun in which the powder is accelerated in a blast wave. In the other mode, an extension with a nozzle transforms the detonation gun process into an intermittent shock tunnel process in which the particles are accelerated in a high enthalpy nozzle flow with high reservoir conditions. Presented are experimental results of the operation with nozzle in which the device generates very high particle velocities up to a frequency of 5 Hz. A variable particle injection system allows injection of the powder at any point along the nozzle axis to control particle temperature and velocity. A hydrogen/oxygen mixture is used in the experiments. Operation performance and nozzle outflow are characterized by time resolved pressure measurements. The particle conditions inside the nozzle and in the nozzle exit plane are calculated with a quasi-one-dimensional WENO-code of high order. For the experiments, particle velocity is obtained by particle image velocimetry, and particle concentration is qualitatively determined by a laser extinction method. The powders used are WC-Co(88/12), NiCr(80/20), Al₂O₃, and Cu. Different substrate/powder combinations for varying particle injection positions have been investigated by light microscopy and measurements of microhardness.     

Particle loading effect in cold spray

Cold gas dynamic spray is a line-of-sight, high-rate material deposition process that uses a supersonic flow to accelerate small particles (micron-sized) above a material-dependent critical velocity. When the particles impact the substrate, they plastically deform and bond to form a coating. The objective of this research is to investigate the influence of the particle mass flow rate on the properties of coatings sprayed using the cold spray process. Varying the mass flow rate at which the feedstock particles are fed into the carrier gas stream can change the thickness of the coating. It was shown that poor coating quality (peeling) was not a result of flow saturation but, instead, the result of excessive particle bombardment per unit area on the substrate. By increasing the travel speed of the substrate, this can be overcome and well-bonded dense coatings can be achieved. It has also been shown that by heating the carrier gas flow poor coating quality is avoided. The original version of this paper was published in the CD ROM Thermal Spray Comects: Explore Its Surfacing Potential, Interational Thermal Spray Conference, sponsored by DVS, ASM International, and HW International Institute of Welding, Basel, Switzerland, May 2–4, 2005, DVS-Verlag GmBH, Düsseldorf. Germany.