Search Results for cold spraying

Cold spray is continuously expanding for the repair of parts made of aluminum-based alloys. Beyond repair applications, the process is now expected to be exploited efficiently for the additive manufacturing of shaped parts. However, up to now, cold spray is limited to the achievement of rather simple shapes due to a lack of basic knowledge on coating build-up mechanisms to result in dimension-controlled deposition. The objective of this work is to fill that gap through an experimental and modeling study of the coating build-up in cold spray for this specific application. Experimentally, Al-based coatings were deposited for a large range of particle velocity due to the use of low-pressure, medium-pressure and high-pressure cold spray facilities. Particle velocity was monitored as a function of cold spray conditions. Two different types of Al 2024 (Aluminium 2024 Alloy) powders were tested. Coating porosity and microhardness were studied as a function of (both morphological and metallurgical) powder characteristics and spray conditions, primarily in the light of particle velocity. Various correlations could be exhibited. Finite element (FE) simulations of particle impacts were developed, including particle velocity from experimental measurements. These will be used as inputs in an in-house morphological model, the first stages of which could be established successfully.

This study investigates the in-flight behavior of particles during cold spraying by means of high-speed shadowgraph photography using a laser imaging system. It also characterizes the particle jet outside the nozzle for different powder sizes (<10 µm to 155 µm) and densities (copper, aluminum). Observations of the jet reveal two low-pressure cold spray flow regimes, one stable, the other unstable, the effect and control of which are discussed.

In this study, microstructural characterization is conducted on WC-17Co coatings produced via High Velocity Oxygen Fuel (HVOF), High Velocity Air Fuel (HVAF), and Cold Spraying (CS). All coatings prepared were observed to be of good quality and with relatively low porosity content. SEM study showed important microstructural features and grain morphologies of each coating. While composition of feedstock material was approximately similar, elemental composition using EDS showed higher Co content and lower WC in the CS deposited coating. XRD experiment identified formation of more complex oxides and tungsten phases in coatings deposited technologies involving melting of powders such as HVOF and HVAF. These phases consisted mainly of cobalt oxides and brittle phases such as W 3 Co 3 C or W 2 C caused by decarburization of the tungsten carbide particles. Hardness of all coating samples were examined and CS deposited coating exhibited considerably lower hardness compared to the other two coating samples instead of having significantly lower porosity content. It could be contributed to dissociation and physical loss of hard carbide phase during high velocity impact of particles in CS process. It is in good agreement with detection of higher amount of cobalt in CS deposited coating material. It is strongly believed that results obtained from this study can be used for future investigation in thermo-mechanical properties of WC-Co coatings.

In this study, MCrAlY-Al 2 O 3 composite powders were produced by ball milling and deposited by plasma, HVOF, and cold spraying. The results show that Al 2 O 3 fractions can be well controlled using composite powder due to non-preferential impact debonding of the matrix and Al 2 O 3 . The microstructure of spray powders is well retained in HVOF and cold-sprayed coatings due to the unmelted or partially molten condition of the spray particles. In the case of plasma-sprayed coatings, however, most Al 2 O 3 particles segregate at lamellar interfaces, forming a continuous oxide scale on the splat. The cold-spray coatings exhibit the highest hardness due to the work hardening effect of kinetic deposition.

In the cold spraying process, particle velocity is commonly regarded as the key factor that influences the deposition efficiency and properties of the coating. The present paper reports on a study in which the velocity of in-flight particles was measured using a DPV-2000 system. The influences of He and N 2 gas pressure and temperature and particle morphology on the particle velocity and deposition efficiency of the coating using stainless steel 316L powders were studied. The microstructure of the coating was examined using optical microscopy. The critical velocity of stainless steel 316L powders was estimated according to the particle velocity distribution and deposition efficiency of the coating. The experiment results suggested that the gas pressure has a more significant influence on the particle velocity and deposition efficiency of the coating than the gas temperature. The particle morphology also has significant influence on the particle velocity. The critical velocity of stainless steel 316L powders was in the range of 630 to 680 m/s, and it decreased slightly with the gas temperature.

Various metals such as aluminum, copper, zinc, steel, nickel, titanium, and niobium have been deposited on a wide range of substrate materials via cold spraying. This paper provides a detailed overview of the cold spray process and the coatings typically produced. It discusses the powders and gases used, the dynamics of gas-particle flow in spray nozzles, the effect of temperature and pressure, and the concept of critical velocity. It also presents examples of the properties and microstructures recently achieved in cold sprayed aluminum, zinc, NiCr, MCrAlY, and Cu-Al coatings. Paper includes a German-language abstract.

This paper provides an overview of the equipment used in cold gas spraying. It discusses the general design and operation of key components, including the nozzle, the control system, the gas heater, and powder feeder. It also describes a typical gas supply system, the recycling of helium, and the provisions for health and safety that are necessary in a spray booth. Paper includes a German-language abstract.

A number of studies have been conducted over the past few years with the aim of modeling high-speed particle-substrate interactions during cold spraying. This paper summarizes the work conducted and some of the more important findings. Based on experimental measurements and dynamic deformation modeling, it is shown a radial jet of metal forms at the contact zone during cold spraying. The effect is similar to that observed in explosive welding and it leads to corona-shaped ejection of metal at the periphery of the contact zone. Paper includes a German-language abstract.

In previous studies at McGill University, tin was successfully cold sprayed onto carbon fiber reinforced polymers (CFRPs). A “crack-filling” mechanism was described as the deposition mechanism that allowed deposition of tin onto the CFRP. Improving the coating conductivity for lightning strike protection (LSP) purposes was achieved by adding other metal powders (aluminum, copper, zinc) to tin and cold spraying on the CFRP. At the same time, it was noticed that the addition of this secondary component (SC) provided an increase in deposition efficiency (DE), tamping was initially hypothesized to explain this improvement, thus prompting a study solely on the effect of SC hardness, which is reported elsewhere in this conference. However, it is recognised that other powder characteristics may also be influencing the DE. Thus, in this study, SCs with a wider variety of particle sizes, morphologies, densities and hardness values were mixed with tin and sprayed on CFRPs. The effect of SC properties on tin deposition is discussed and an optimal combination of SC properties for cold spraying of tin is suggested.

In this paper, the phenomenological behaviour of gas flow and particles motion during cold spraying has been studied. Observations of particles behaviour show two features: a uniform jet over a short distance ahead of the nozzle exit and then, a progressive dispersion. These behaviours are explained using a computational analysis based on a direct numerical simulation of the gas flow and the kinematic interactions with the particles. The CFD computation demonstrates that the gas stream starts to be unstable inside the nozzle with more turbulence as it moves towards the exit of the nozzle. The flow is self-oscillated along the flow direction and drives the motion of the Cu particles outside the nozzle. The zone of gas flow instability does correspond to the zone of experimental particle dispersion. Outside the nozzle, the particles form a straight jet over a certain distance that corresponds to the zone of the experimental uniform particles jet. Then, they are deviated and become more and more dispersed towards a very sparse jet along the flow direction. This phenomenon is explained by a Magnus lift force that deviates the particles trajectory when the gas flow becomes highly turbulent while developing a vorticity shedding.

Cold spray process was chosen as a good candidate for dimensional restoration and protection of components. Commercially pure aluminum, aluminum-alloy or titanium were recommended for different applications. This paper investigates laser surface texturing association to enhance durability of sprayed coatings. Laser is easy automated, localized and reliable process. It was applied for prior-surface treatment. Textured surfaces were produced and compared to conventional treatments, such as grit-blasting, in terms of deposition efficiency and adhesion bond strength. Patterns promoted direct particle embedment. Particle-substrate interface exhibited significant temperature rate and strain in cavities. Intimate contacts and particle compressive states were assumed responsible for improvement. The particle deformation and bonding behaviors were evaluated and discussed for the different configurations. Thus, window of deposition was increased with laser surface texturing. Anchoring mechanisms increased two fold the adhesion strength compared to conventional pre-treatments. In one case, the interface was stronger than the coating cohesive strength.

In cold spraying, coatings are formed by a high velocity impact of solid particles. The particles are accelerated in a supersonic gas jet at temperatures of only a few hundred degrees centigrade. In contrast to thermal spray processes no melting of the particles and negligible heating of the substrate occurs. A series of spray experiments with copper powders of different particle size ranges were performed to study the effect of various process parameters on microstructure and properties of the coatings. The coatings have been evaluated for their microstructure, density, oxygen content, hardness and bond strength. With nitrogen as process gas and a -25 +5µm powder, dense coatings were obtained within a broad range of gas inlet pressure and gas inlet temperature.

In the cold spray process, the coating material, by using high particle speed, is plasticized when hitting the substrate and is combined with the base material. In principle, the process is similar to explosion welding. Contrary to the conventional thermal spraying methods the material is not melted. This paper reviews the development of equipment and process controls for a comprehensive cold spray system.

TiO 2 photocatalyst in the form of coatings are of many advantages over those in powder form in practical applications. Various processes have been used to form TiO 2 coatings including sputtering, Sol-Gel, vapor deposition, thermal spraying, etc. In all those processes, the coatings are deposited under temperatures from 300 to more than 2000 °C. High temperature during those processing may change the microstructure of the as-received TiO 2 to a less effective one for photocataltyic performance. In the present study, TiO 2 coating was deposited through cold spraying process using two types of powders, which were agglomerated with anatase ultra-fine particles in micro-size and nano-size. Microstructures of both powders and deposited coatings were characterized using X-ray diffraction and scanning electron microscopy. The photocatalytic performance was examined through acetaldehyde degradation under ultraviolet illumination. The results showed that the nanostructured TiO 2 coatings were evenly deposited on stainless steel substrate through cold spraying. The thickness of the deposits reached up to 15 µm. The coating presented a rough surface and porous structure. Owing to the low temperature of spray powders, no change occurred to the phase and grain size of TiO 2 during deposition process. It was also found that the cold sprayed TiO 2 deposits were photocatalytically active for photodegradation of acetaldehyde.

In the present study, the zinc powder (-48 µm) was used to deposit coating by cold spraying using nitrogen as driving gas at different operating temperatures. The microstructure of the deposited coating was characterized by optical microscopy, scanning electron microscopy and transmission electron microscopy (TEM) to reveal the occurrence of fusion during the impacting of spray particles. The selected area electron diffraction analysis was used to examine the microstructural feature at the near interface areas between the deposited particles in zinc coating. Numerical simulation was carried out to estimate the particle temperature increment during the impacting process. The simulation result suggests a possibility of the melting of zinc particles at the localized contact region on impact. The examination of the coating surface provided the evidence for the occurrence of the melting of spray particles on impact. The experimental results showed that the cold-sprayed zinc coating presented a dense microstructure. The nano-structural phase was formed at the near interface areas between deposited particles in zinc coating, while the grains in the size of micrometers similar to that in the powder were retained in the inside of the particles in the coating. Moreover, the TEM observation evidently showed that the amorphous phase was formed at the interface areas between the particles. It can be considered that the amorphous phase in the coating was formed through subsequent rapid solidification of the melted material on impact. This fact provided further evidence to the occurrence of localized melting during impacting of spray particles.

A mathematical model was developed and used to design and optimize a supersonic nozzle for a cold spray system. The objective was to spray 20 micron-size aluminum particles. Conventional and agglomerated nanostructured powders were used and successfully sprayed using a radial injection port. The microstructure of the coatings revealed that the nanocrystalline structure is preserved. An increase of 100% of the coating hardness was found for nanostructured coatings compared to conventional coatings. Further work needs to be done to improve the porosity of the coatings by changing some of the process parameter.

The low temperature characteristic of cold spraying makes it possible to deposit the coating of temperature sensitive materials, such as nanostructured material, without any significant change in the microstructure of feedstock. In the present study, the Fe and Si powders of particle size less than 75 µm were mixed at a composition of 10wt%Si and ball-milled to produce the nanostructured feedstock. Cold spraying process was used to deposit coating with nitrogen as a driving gas at different temperatures. The microstructure of the as-sprayed nanostructured Fe-Si coating was characterized using optical microscopy, scanning electron microscopy and transmission electron microscopy (TEM). The grain sizes of the feedstock and as-sprayed coating were estimated based on X-ray diffraction analysis. The results showed that the nanostructured Fe-Si coating can be deposited by cold spraying using the ball-milled powders as feedstock. The as-sprayed coating presented a dense microstructure. The average grain size of the as-sprayed coating was comparable to that of the corresponding milled feedstock. No significant effect of the temperature of driving gas on the microstructure of cold-sprayed nanostructured Fe-Si coating was recognized. Moreover, TEM analysis showed that the amorphous phase was present in the as-milled powders and the as-sprayed coating along with the nanocrystalline.

The aim of this work is to characterize the performance and durability of Zn-based composite coatings produced by low-pressure cold spraying and evaluate their potential for use in repair and restoration applications. Mechanically blended Zn+Al+Al 2 O 3 , Zn+Cu+Al 2 O 3 , and Zn+Ni+Al 2 O 3 powder mixtures were deposited on grit-blasted carbon steel (Fe52), copper, aluminum, and nodular cast iron substrates using optimized feed rates. In addition, round samples were drilled and the holes were repaired by handheld spraying. Coated substrates are assessed based on microstructural analysis, laser shock adhesion testing (LASAT), and thickness and hardness measurements. Hole repairs are evaluated based on bond strength and gas permeability measurements. The procedures are described and the finding are presented and discussed.

The aim of this work is to fabricate a particle-reinforced FeAl composite coating by cold spraying. Fe, Al, and WC powders were placed in a ball mill and mechanically alloyed for up to 36 h in order to obtain a nanostructured Fe(Al) solid solution reinforced with a high volume fraction of WC particles. The powder was examined and then cold sprayed on stainless steel substrates using N 2 as the accelerating gas. The as-sprayed deposits exhibited rough surface morphology and dense cross-sectional microstructure with dual-scale WC dispersoids distributed uniformly in the Fe(Al) matrix. The coatings were annealed at 650 °C and subsequently reexamined. In-situ phase transformation from the solid solution to an intermetallic compound occurred after the post-spray treatment along with an improvement in microstructure.

In this study, Al-SiC composite coatings are produced by cold spraying ball-milled Al powders with different volume fractions of SiC particles. The morphology and microstructure evolution of the powder during ball milling are evaluated along with the effect of SiC content on the microstructure and wear behavior of the coatings. The results show that dense Al-SiC coatings with different volume fractions of SiC particles can be fabricated by cold spraying and that abrasive wear resistance is improved by raising the volume fraction of SiC particles. Wear surfaces indicate that the predominant wear mechanism is gouging of the soft Al matrix in the early stages and cracking and spalling of SiC particles in the latter stages. The dispersed SiC particles serve to protect the matrix from wear products thus raising the wear resistance of the coatings.