Cold Gas Dynamic Spray Process Research Articles

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.

Computational and Experimental Studies on Cavity Filling Process by Cold Gas Dynamic Spray

Time-dependent computational simulations on cavity filling process by cold gas dynamic spray and powder jet deposition process ranging from microscale to macroscale were carried out in order to give an insight for their advanced applications to joining, crack repair, and dental treatment. Shock wave appears in front of the substrate due to underexpansion of jet and in-flight particles interact with the shock wave before their impact. The relation between shock wave, cavity configuration, and particle in-flight behavior in supersonic jet has been discussed in detail. Based on numerical and experimental studies, it was found that when the shock wave covers up the cylindrical cavity, the cavity cannot be filled at all by deposited powders. Moreover, under the condition of shock wave appearing inside the cylindrical cavity, conical deposition was formed due to the secondary back flow jet along the cavity side wall. By adopting conical cavity, cavity can be filled completely resulting from the suppression of the secondary back flow jet along the cavity side wall.

Cold Spray Characteristics of Commercially Pure Ti and Ti-6Al-4V

Ti and Ti-6Al-4V coatings were deposited by cold gas dynamic spray process using nitrogen as propellant gas. For Ti, the inlet gas temperature and pressure were varied for two different powder morphologies in such a way that the average particle velocity ranged from ~600 to 850 m/s. In addition, the nozzle traverse speed was varied. For all conditions, the deposition efficiency, the porosity, and the microhardness were measured. It is shown that the porosity level decreased as the gas temperature and pressure increased, whereas the velocity, deposition efficiency, and microhardness increased. Furthermore, it is observed that a lower nozzle traverse speed engendered a softer coating. The coating adhesion on a grit 24 Al2O3 blasted mild steel surface was established to be greater than 78.8 MPa. In addition, a Ti-6Al-4V coating was produced and was determined to be slightly more porous compared to a CP Ti coating.

Mechanical matching and microstructural evolution at the coating/substrate interfaces of cold-sprayed Ni, Al coatings

The effect of mechanical hard/soft matching of raw powder and substrate in the cold gas dynamic spraying process (CDSP) on the formation of intermetallic compounds was examined. Instead of pre-alloyed materials, pure Al and Ni were selected as a soft and a hard material, respectively, and post-annealing was used for compound formation. Most of the aluminide layers were observed in the coated layer, but not in the substrate, along with the entire original interface for both Al coating on a Ni substrate and vice versa. Thickening of the compound layer depended mainly on the creation of defects during spraying and intrinsic diffusivity of atoms moving toward the coating side. When Ni was coated, the compound layer was made thicker by fast diffusion of Al, while the thickness was limited in soft Al coating on hard Ni substrate. However, the composition of the compound can be affected by relative transfer of diffusing atoms toward both the coating and the substrate. So, for Ni coating on an Al substrate, most of the intermetallic compound formed was Ni-rich and conversion of the Al-rich compound was observed after post-annealing above 500 °C.

CoNiCrAlY microstructural changes induced during Cold Gas Dynamic Spraying

The present study is part of an ongoing research project that aims to develop high performance bond coats by means of Cold Gas Dynamic Spraying (CGDS) for the manufacturing of thermal barrier coatings (TBC). The objective of this work is to investigate the microstructure of a CGDS coating and compare it to that of the original feedstock powder in order to determine whether any microstructural changes have occurred during the deposition process. CoNiCrAlY coatings were deposited using the CGDS system developed at the University of Ottawa Cold Spray Laboratory. Scanning electron microscopy, transmission electron microscopy and X-ray diffraction techniques were used to assess the phases and microstructure of the original feedstock powder and coatings produced. Contrarily to the generally accepted theory that the CGDS process does not lead to changes in the deposited material's microstructure and phase, results from the analysis performed in this study demonstrate the occurrence of important microstructural and phase changes. Evidence of grain refinement of the γ-phase matrix down to the nanometre scale as well as partial dissolution of β-phase precipitates was observed. It is believed that these changes are attributed to the severe plastic deformation encountered by the deposited particles.

Computational Simulation on Performance Enhancement of Cold Gas Dynamic Spray Processes With Electrostatic Assist

A real-time computational simulation on the entire cold spray process is carried out by the integrated model of compressible flow field, splat formation model, and coating formation model, in order to provide the fundamental data for the advanced high performance cold gas dynamic spray process with electrostatic acceleration. In this computation, viscous drag force, flow acceleration added mass, gravity, Basset history force, Saffman lift force, Brownian motion, thermophoresis, and electrostatic force are all considered in the particle equation of motion for the more realistic prediction of in-flight nano∕microparticle characteristics with electrostatic force and also for the detailed analysis of particle-shock-wave-substrate interaction. Computational results show that electrostatic acceleration can broaden the smallest size of applicable particle diameter for successful adhesion; as a result, wider coating can be realized. The utilization of electrostatic acceleration enhances the performance of cold dynamic spray process even under the presence of unavoidable shock wave.

Annealing effects on the intermetallic compound formation of cold sprayed Ni, Al coatings

The annealing of Ni and Al coatings under various conditions on substrates fabricated by a cold gas dynamic spray process (CDSP) were investigated. The powder particles were accelerated through a standard De Laval-type nozzle with air used as the main carrying gas. The coatings were annealed at 450–550 °C in either argon or air atmospheres for 4 h. In the case of Ni coatings during annealing both in argon and air atmospheres, intermetallic compound layers such as Al 3Ni and Al 3Ni 2 were observed at the interfaces between the Ni coating and Al substrate. Also, the intermetallic layer formation of Al 3Ni and Al 3Ni 2 at the interfaces depended on the solid-state diffusion and the annealing temperature. The intermetallic compound AlNi was obtained at the interface of Al coating on a Ni substrate by low-temperature annealing under the melting temperature.

Properties of SiC-reinforced aluminum alloy coatings produced by the cold gas dynamic spraying process

SiC-reinforced Al–12Si alloy coatings were produced using the Cold Gas Dynamic Spraying process. Blends of several compositional variations of Al–12Si and SiC particles were created and subsequently sprayed. The microstructural features of the resulting coatings are examined and the mechanical properties, in particular the adhesion strength and the hardness, of the composite coatings are reported. The results show that, depending on the initial SiC volume fraction of the blend, between 50% and 33% of the SiC in the feedstock powder was retained in the coatings. The adhesion strength of the coatings to their substrates was found to slightly decrease with increasing SiC content while the inclusion of SiC particles within the Al–12Si matrix improved significantly the hardness of the coating.

519 ナノ・マイクロ粒子高速流動の衝突過程とキャビティー充填加工の高度化(T07-4 流体及び界面におけるナノ構造と流動特性(4),大会テーマセッション,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

A real-time and realistic computation on the entire cold gas dynamic spray process has been carried out by the integrated model of flow considering shock wave, splat formation, and coating formation. The in-flight behavior of nano/micro particle in the supersonic jet impinging on the substrate with a cavity, and also the interaction between a particle and shockwave has been made clear from this study. By clarifying the effect of cavity shapes on a coating formation inside the cavity, optimum cavity shape was proposed for cavity filling process. Then, the fundamental data has been provided for the advanced application of a high performance cold gas dynamic spray process to a dental treatment or structural repair.

Numerical Studies of the Application of Shock Tube Technology for Cold Gas Dynamic Spray Process

A new method for a combustion-free spraying is studied fundamentally by modeling and simulation in comparison with first experiments. The article focuses on the numerical simulation of the gas-particle nozzle flow, which is generated by the shock reflection at the end wall section of a shock tube. To study the physical fundamentals of this process, at present only a single shot operation is considered. The particles are injected downstream of the nozzle throat into a supersonic nozzle flow. The measurements of the particle velocity made by a laser Doppler anemometry (LDA) set up show that the maximum velocity amounts to 1220 m/s for stainless steel particles of 15 μm diameter. The CFD-Code (Fluent) is first verified by a comparison with available numerical and experimental data for gas and gas-particle flow fields in a long Laval-nozzle. The good agreement implied the great potential of the new dynamic process concept for cold-gas coating applications. Then the flow fields in the short Laval nozzle designed and realized by the Shock Wave Laboratory (SWL) are investigated. The gas flow for experimentally obtained stagnation conditions is simulated. The gas-particle flow without and with the influence of the particles on the gas flow is calculated by the Surface Engineering Institute (IOT) and compared with experiments. The influence of the injection parameters on the particle velocities is investigated, as well.

Selection of the spraying technologies for over-coating of metal-stampings with thermo-plastics for use in direct-adhesion polymer metal hybrid load-bearing components

Abstract The suitability of various polymer-powder spraying technologies for coating of metal-stampings used in polymer metal hybrid (PMH) load-bearing automotive-component applications is considered. The suitability of the spraying technologies is assessed with respect to a need for metal-stamping surface preparation/treatment, their ability to deposit the polymeric material without significant material degradation, the ability to selectively overcoat the metal-stamping, the resulting magnitude of the polymer-to-metal adhesion strength, durability of the polymer/metal bond with respect to prolonged exposure to high-temperature/high-humidity and mechanical/thermal fatigue service conditions, and compatibility with the automotive body-in-white (BIW) manufacturing process chain. The analysis revealed that while each of the spraying technologies has some limitations, the cold-gas dynamic-spray process appears to be the leading candidate technology for the indicated applications.

WC-based cermet coatings produced by cold gas dynamic and pulsed gas dynamic spraying processes

Due to their mechanical properties, WC-based cermet coatings are extensively used in wear-resistant applications. These coatings are usually produced using thermal spray processes. However, due to the nature and the environment of these spraying processes, the feedstock powder structure and properties suffer from decomposition, which subsequently degrade the performance of the coatings produced. The cold gas dynamic spraying process appears to be a promising alternative technique to preserve the properties of the feedstock powder during the coating preparation. Although the latter technique can minimize or eliminate the degradation of the sprayed material, the deposition of cermet using this technique is a difficult task. In this study, two types of cermet powders, the nanocrystalline (WC–15Co) and the conventional (WC–10Co4Cr) powders were deposited using the cold gas dynamic spraying and the pulsed gas dynamic spraying processes. The feedstock powders and coatings microstructures were investigated by OM, SEM and XRD, as well as their hardness. The results revealed the possibility of depositing cermet coatings onto aluminum substrates using both processes without any degradation of the carbide phase of the feedstock powder. The cold gas dynamic spraying process experienced difficulty in depositing and building up dense coatings without major defects. The pulsed gas dynamic process produced thick cermet (conventional and nanocrystalline) coatings with low porosity as long as the feedstock powder was preheated above 573 K.

Numerical Simulation of the Cold Gas Dynamic Spray Process

A computational fluid dynamic (CFD) model of the cold gas dynamic spray process is presented. The gas dynamic flow field and particle trajectories within an oval-shaped supersonic nozzle as well as in the immediate surroundings of the nozzle exit, before and after the impact with the target plane, are simulated. Predicted nozzle wall pressure values compare well with experimental data. In addition, predicted particle velocity results at the nozzle exit are in qualitative agreement with those obtained using a side-scatter laser Doppler anemometer (LDA). Details of the pattern of the particle release into the surroundings are visualized in a convenient manner.

Effect of particle size, morphology, and hardness on cold gas dynamic sprayed aluminum alloy coatings

This work describes recent progress in Cold Gas Dynamic Spraying process of conventional and nanocrystalline 2618 (Al–Cu–Mg–Fe–Ni) aluminum alloy containing Sc. As-atomized and cryomilled 2618+Sc aluminum powders were sieved in two ranges of particle size (below 25 μm and between 25 and 38 μm), and sprayed onto aluminum substrates. The mechanical behavior of the powders and the coatings was studied using the nanoindentation technique, while the microstructure was analyzed using scanning and transmission electron microscopy. The influence of the powder microstructure, morphology and behavior during deposition on the coating properties was analyzed. It was concluded that the hard cryomilled particles do not experience extensive plastic deformation, and therefore failed to form a coating as dense as those produced using the gas-atomized spherical powder, despite the fact that the irregular shape cryomilled particles presented higher flight and impact velocities than the gas-atomized spherical particles. It was also observed that the influence of the particle morphology on the particle velocities is more pronounced for the larger particle size range (between 25 and 38 μm).

Cold spray deposition of thermoplastic powder

The cold spray deposition of polyolefin powder has been investigated, at substantially lower velocities (up to ∼135 m s−1) than those used in the cold gas dynamic spray (CGDS) process for metals. The particles were accelerated at room temperature in air through a cylindrical nozzle, and were deposited on to polyethylene and aluminium substrates. Fully-dense deposits were formed at temperatures substantially below the melting point of the polymer, and the critical impact velocity for deposit growth was ∼100 m s−1. Melting of the polymer particles did not occur. The influences of polymer particle size, impact velocity, nozzle stand-off distance and substrate material on the deposition process were studied. Although deposition occurred readily on to polyethylene substrates at room temperature, it was initiated on aluminium substrates only after a thin film of melted polymer had been formed to act as a bonding layer. The deposition efficiency was much lower than the values typically seen in CGDS of metals.

Substrate Roughness and Thickness Effects on Cold Spray Nanocrystalline Al-Mg Coatings

Nanocrystalline Al−Mg coatings were produced using the cold gas dynamic-spraying technique. Unsieved Al−Mg powder of average nanocrystalline grain size in the range of 10 to 30 nm and with a particle size distribution from 10 to >100 μm was used as the feedstock powder. The resulting coatings were evaluated using scanning electron microscopy (SEM), transmission electron microscopy, as well as microhardness and nanoindentation measurements. Coating observations suggest that the wide particle size distribution of the feedstock powder has a detrimental effect on the coating quality but that it can be successfully mitigated by optimizing the spraying parameters. Nanohardness values close to 3.6 GPa were observed in both the feedstock powder and coatings, suggesting the absence of cold-working hardening effects during the process. The effects of the substrate surface roughness and thickness on coating quality were investigated. The deposited mass measurements performed on the coatings showed that the effect of using different grit sizes for the substrate preparation is limited to small changes in the deposition efficiency of only the first few layers of deposited material. The SEM observation showed that the substrate surface roughness has no significant effect on the macrostructures and microstructures of the coating. The ability to use the cold gas dynamic spraying process to produce coatings on thin parts without noticeable substrate damage and with the same quality as coatings produced on thicker substrates was demonstrated in this work.

Numerical investigations on cold gas dynamic spray process with nano- and microsize particles

The particle velocity in cold gas dynamic spraying (CGDS) is one of the most important factors that can determine the properties of the bonding to the substrate. In this paper, the acceleration process of microscale and sub-microscale copper (Cu) and platinum (Pt) particles inside and outside De-Laval-Type nozzle is investigated. A numerical simulation is performed for the gas-particle two phase flow with particle diameter ranging from 100 nm to 50 μm, which are accelerated by carrier gas nitrogen and helium in a supersonic De-Laval-type nozzle. The carrier gas velocity and pressure distributions in the nozzle and outside the nozzle are illustrated. The centerline velocity for two types of particles, Pt and Cu, are demonstrated. It is observed that the existence of the bow shocks near the substrate prevents the smaller size particles (less than 0.5 μm) from penetrating, thus leads to poor coating in the actual practices. Furthermore, the extended straight section may have different optimal length for different size particles, and even may be unnecessary for sub-microsize particles.

Thin Film Coatings of WO<SUB>3</SUB> by Cold Gas Dynamic Spray: A Technical Note

Dense and adhesive WO3 films were prepared on a silicon substrate by the cold gas dynamic spray process (or cold spray). In contrast to standard metallic coatings, there was no sizable crater formation and plastic deformation. However, the aggregation of raw powder particles of a relatively large size was found to be destroyed upon impact on the substrate, forming a highly irregular surface with very fine secondary particles and providing good interlocking powder and void reduction among the particles in the coating. High-resolution images of the substrate interface showed that particles at the interface were more densely packed and that good adhesion was obtained. There fore, the particle bombardment onto the first layer of the coating could provide enhanced adhesion to the substrate mechanically and/or chemically.

Cold spray modeling and validation using an optical diagnostic method

The Cold Gas Dynamic Spray process uses the kinetic energy of unmelted sprayed particles to produce coatings. The most important element of the cold spray system is the nozzle used to accelerate the particles. Consequently, the nozzle design optimisation is a key to improve the coating quality and reduce the spraying costs. In this study, an axi-symmetric two-dimensional mathematical model is presented and used to predict the flow inside a cold spray nozzle as well as the particle velocity in the vicinity of the nozzle exit. The model results are compared with those obtained using the one-dimensional isentropic theory and with particle velocity measurements made on a commercial cold spray system. The study shows that the particle exit velocity depends on the type, stagnation temperature, and pressure of the propellant gas. The comparisons show that the proposed model is more accurate than the one-dimensional theory. It allows predicting accurately the behavior of the particles in the cold spray jet even in the presence of shock waves. Following this work, the design of new nozzles for specific applications using this mathematical model can be considered.

Adiabatic shear instability based mechanism for particles/substrate bonding in the cold-gas dynamic-spray process

Particles/substrate interactions during the cold-gas dynamic-spray deposition process are studied using a dynamic axisymmetric thermo-mechanical finite element analysis. In addition, the particles/substrate bonding mechanism has been investigated using a one-dimensional thermo-mechanical model for adiabatic strain softening and the accompanying adiabatic shear localization. The results obtained show that the minimal impact particles velocity needed to produce shear localization at the particles/substrate interface correlates quite well with the critical velocity for particles deposition by the cold-gas dynamic-spray process in a number of metallic materials. This finding suggests that the onset of adiabatic shear instability in the particles/substrate interfacial region plays an important role in promoting particle/substrate adhesion and, thus, particles/substrate bonding during the cold-gas dynamic-spray process.