Cold Spray Conditions Research Articles

Impact of spray angle and particle velocity in cold sprayed IN718 coatings

Cold spray (CS) deposition has potential applications in repair technology or additive manufacturing due to its ability to operate at low temperatures, achieve high deposition rates, and produce thick, dense coatings. For adoption of CS in repair applications, it is crucial to evaluate the influence of spray angles since geometry of repair zones are typically complex and non-flat. In this work, we investigated the influence of spray angles and particle velocities on the cold spray deposition of Inconel 718 (IN718) powder particles using N2 as propellant gas. Three oblique spray angles including 50, 60 and 70° were compared with the 90° normal spray angle for cold spray deposition of IN718 powders on Al6061 substrate. Three sets of cold spray parameters at each spray angle, giving particle velocities from 820 to 920 m/s were employed to study their effect on the coating properties. Particle velocity was found to have greater impact on coating porosity compared to spray angle. Coating with lowest porosity and lowest surface roughness but highest compressive surface residual stress was achieved with deposition at lowest spray angle (50°) and highest particle velocity (920 m/s). Changing the spray angle has greater influence on the deposition efficiency, with thickest coating observed at normal spray angle of 90° across all three cold spray conditions. Finite element method (FEM) simulations were also performed to understand the influence of powder and substrate temperatures on critical angles for bonding. The simulation results suggest that elevating the powder temperature is likely to cause a slight increase in the critical bonding angle.

Effect of He and N2 gas on the mechanical and tribological assessment of SS316L coating deposited by cold spraying process

In this study, simulation and experimental methods were used to investigate the influence of cold spray conditions on AISI 316L stainless steel coatings. The effect of both helium and nitrogen gases used was investigated. The temperature, particle sizes of spraying powder, and distance from the nozzle throat to the impinging point were estimated by using the Kinetics Spray Solutions GmbH software. The 316L stainless steel (SS) coatings were examined by X-ray diffraction, Scanning Electron Microscopy and Energy Dispersive X-Ray Microanalysis. The tribological behavior was evaluated under different loads (2 N and 5 N) in dry conditions. It was found that the nitrogen and helium propellant gas with high speed and fine particles led to produce good coatings with dense microstructures. From the nanoindentation experiments, the Young's modulus and hardness of the SS 316L samples were enhanced of about 8% with helium due to the high particle velocity. It was shown that the wear resistance of SS 316L produced with helium was higher than that of the standard SS 316L coatings. The coatings produced with helium revealed lower friction coefficient (0.65) and wear rate (6.9 × 10–4 mm3/Nm) under 2 N applied load than that obtained nitrogen. It was also found that the SS 316L cold sprayed by helium with dense structure presents high hardness and good tribological performance that can be suggested for several applications.Graphical

Production of TiC – Co and TiC – NiCr Composite Powders and Study of Their Interaction with a Target under Cold Spray Conditions

In the first part of the paper, the effect of mechanical treatment of particles of reacting Ti + C + Me (Me = NiCr or Co) mixtures on the process of self-propagating high-temperature synthesis (SHS) is investigated. It is shown that as a result of SH-synthesis in the free burning mode, a metal-ceramic sinter of high porosity is formed, which is easily destroyed by mechanical action. An increase in the volume fraction of the metal binder (NiCr or Co) leads to a redistribution of the sizes of carbides towards smaller values.In the second part of the paper, the results of cold spraying nickel-chromium alloy particles and cermet particles of the TiC – NiCr composition on an aluminum substrate are presented. Produced NiCr coatings have a high porosity (up to 10 %) and a layered structure with clearly defined contours of individual deformed particles (splats). Due to the high hardness and low plasticity of metal – ceramic particles of the TiC – NiCr composition, during their interaction with the surface of aluminum substrate, continuous coating cannot be produced, only individual adhered particles are observed. With prolonged exposure to the flow of such particles, erosion of the substrate takes place.

Material Model for Predicting Dynamic Response of Copper and Nickel at Very High Strain Rates Under Cold Spray Conditions

Numerous material models have been proposed to describe material properties under impact-induced extreme conditions. All these models suffer from the fact that the tests used to provide the required empirical parameters were performed on bulk material at limited strain rates, much lower than those found in the cold spray process. In this work, recent published results available in the literature that were obtained using advanced imaging techniques are used to modify material plasticity models to accurately predict the material plastic flow stress under cold spray conditions. The capability of two frequently used material models, Johnson–Cook and Preston–Tonks–Wallace models, to predict real-time experimental data for copper and nickel particles is evaluated. A series of finite element simulations are conducted and the comparison is made with the coefficient of restitution from laser-induced projectile impact test data. The original empirical parameters in Preston–Tonks–Wallace model are re-calibrated so that the coefficient of restitution matches the laser-induced projectile impact test data, providing an enhanced quality in the particle impact behavior predictions. Additionally, experiments and modeling are conducted to explore the bonding mechanism and rebound phenomena of copper powder cold sprayed on low carbon steel substrate.

Characterization of mechanical behaviour of aluminum powders under fast dynamic conditions

Cold spray is a process belonging to the thermal spray family, characterized by relatively low temperatures and high particle velocities. Upon impact, particles undergo large plastic deformation at solid state in dynamic regime up to 109 s-1. The knowledge of powder behaviour in such conditions is essential to catch key phenomena in cold spray and a necessary step for a correct process modelling. However, little is known on mechanical behaviour of feed-stock powders when submitted to cold spray conditions. In this study, an approach focused on single particles combining laser shock induced impact, namely LASHPOL (LAser SHock POwder Launcher), and quasi-static compression was therefore developed. This method was applied in this study on spherical Aluminium powders but can work with any other powder material. The mechanical behaviour of powders was characterized and used to fit the parameters of Johnson-Cook constitutive model, by means of finite element inverse method. The combination of static and dynamic tests resulted in an original characterization of powder, which revealed having a different mechanical behaviour than the corresponding bulk material.

Wear and Fretting Behavior of Cold Sprayed IN625 Superalloy

The wear and fretting behaviour of IN625 cold spray coatings was analysed and the results are presented. The cold spray conditions were selected in order to obtain coatings with minimum porosity and maximum particles splat. This leads to compact and hard deposited material able to resist wear damaging and to dissipate energy during fretting. The coating’s strength was evaluated through nanoindentation that revealed an increased hardness from the surface toward the substrate. This different hardening behaviour from the coating surface toward the substrate leads to increased resistance to fretting and wear as the maximum stresses increase. This was revealed by scratch tests performed at linearly increasing loads that allowed identifying of the damage mechanisms acting on the coating as the triaxial stresses increase. The hardening behaviour of the coating also influenced the fretting behaviour revealed by the weight loss experienced by varying the fretting maximum load.

Study of substrate temperature dependence upon nozzle traverse speed at cold spraying

The paper presents the results of investigation of dependence of substrate temperature upon nozzle traverse velocity under cold spray conditions. A model is proposed for prediction of substrate temperature. Experimental and simulated data were obtained in the typical range of change in the nozzle traverse speed (1-400 mm/s) and air stagnation temperatures (200-600°C), encountered in the practice of cold spraying. The simulated data are in good agreement with experimental. An approximate analytical equation is proposed for estimation of substrate temperature during cold spraying in a wide range of gas stagnation temperatures and nozzle traverse velocities.

Cold Spray Coating Diagram: Bonding Properties and Construction Methodology

Cold spraying is a coating technology where the temperatures of the sprayed particles are kept low (below 1000 °C), and compensated by high impact velocity (300–1400 m/s). Although many particle and substrate variables are important under cold spray conditions, the particle temperature, Tp, and impact velocity, Vp, to a large extent control the quality of coatings. In this study, a cold spray coating diagram (CSCD) using Tp and Vp as coordinates is proposed. The cold spray coating process is characterized by a critical velocity, Vcr, beyond which Vp > Vcr adhesion and bonding of particles to the substrate will take place. If Vp is very high and crosses the erosion velocity, Vero, of the substrate, bonding will not occur. Both Vcr and Vero are dependent on the particle temperature Tp. The ratios Vp/Vcr = 1.0, 1.5, 2.0 and 2.5 were drawn as contours in the CSCD. Coating properties like deposition efficiency, cohesive strength, and flattening ratio have unique relationships with the impact velocity, Vp, expressed as the ratio Vp/Vcr; they are also depicted on the contours of Vp/Vcr in the cold spray coating diagram. In addition, the bonding mechanisms responsible for the adhesion of the particles on the substrate were also identified and indicated in the cold spray coating diagram.

A Laser Shock Approach to Cold Spray

Cold spray utilizes supersonic jets of compressed gas to accelerate powder particles at high velocities. A coating is formed on a substrate by the impact and deformation of particles. Laser Shock consists in illuminating a sample with a pulsed laser to generate a high pressure shock. Cold spray and laser shock are extremely dynamic processes (time scales of about 10-100 ns). In this paper, applications of laser shock for the study of cold spray are presented. A powder particle of a given size and morphology can be laser shock accelerated at cold spray velocities, finally hitting a substrate in a controlled experimental simulation of the process. Results allow also the characterization of powder materials, through the comparison of deformed particles with numerical impact simulations and the fitting of a plasticity model. Two main advantages compared to the split-Hopkinson pressure bar emerge: deformation rates are closer to cold spray conditions and powders are directly tested, rather than macroscopic cylinders. Laser shock can also be used to measure adhesion and internal cohesion of cold-sprayed coatings (LAser Shock Adhesion Test, LASAT). Cold LAser Shock Spray (CLASS), consisting in laser shocking a coating to re-spray it, can be used to characterize property gradient within a coating or as a new spraying process. Laser shock techniques can prove beneficial for the knowledge of powder materials, which is key for advances in cold spray and other powder based processes. Moreover, the combination of the two techniques could lead to hybrid processes.

Fatigue behaviour of Inconel 625 cold spray coatings

ABSTRACTCold spray coatings were produced with Inconel 625 powders sprayed on a heat-treated Inconel 718 substrate. The processing parameters were varied to analyse the microstructural and mechanical variations as a function of cold spray conditions. Cold spray can contribute to improve the fatigue properties of repaired components; the best process conditions, leading to low porosity and high adhesion strength, were employed to produce coatings in notched samples to evaluate the fatigue crack behaviour of the sprayed material. The coatings were heat treated to evaluate the material microstructural evolution and mechanical properties. The crack initiation and growth behaviour of the coatings was studied also for the solution and precipitation hardening heat treatment condition to compare the fatigue behaviour with that belonging to the as sprayed one. It was demonstrated that the heat treatment has a beneficial effect on fatigue cracks retardation.

Effect of Cold-Spray Conditions Using a Nitrogen Propellant Gas on AISI 316L Stainless Steel-Coating Microstructures

Cold-spray techniques have been a significant development for depositing metal coatings in recent years. In cold-spray processes, inexpensive nitrogen gas is widely used as the propellant gas in many industries. However, it is difficult to produce austenitic stainless steel coatings with dense microstructures with cold-spray techniques when using nitrogen propellant gas because of work hardening. In this study, the effects of cold-spray conditions using a nitrogen propellant gas on AISI 316L stainless steel coatings were examined. It was found that a higher nitrogen propellant gas temperature and pressure produce coatings with dense microstructures. The measured AISI 316L coating hardness values suggest that AISI 316L particles sprayed at temperatures of 700 and 800 °C soften due to the heat, allowing uniform deformation on the substrate and consequently forming dense coating microstructures. In addition, AISI 316L powder with particle diameters of 5–20 µm resulted in a denser coating microstructure than powder with particle diameters of 10–45 and 20–53 µm. Finally, the standoff distance between the nozzle and the substrate also affected the AISI 316L coating microstructures; a standoff distance of 40 mm produced the densest microstructure.

Preliminary study of cold spraying using radial supersonic nozzle

The paper deals with the problem of coating production on internal pipe surfaces by the cold spraying. It reports on a new assembly design with a radial supersonic nozzle that greatly facilitates the application of aluminium, copper and nickel coatings onto internal pipe surfaces. Visualisation and Pitot tube measurements of the airflow produced by the nozzle assembly assist to predict the range of internal pipe diameters, in which the pipes could be successfully coated from the inside using the designed instrument. The velocity of aluminium particles has been measured at the exit from the radial nozzle and calculated the particle velocities under the actual cold spray conditions. Finally, the mass and energy consumption characteristics of the process and the deposition efficiencies achievable under typical experimental conditions have been calculated. Those calculations have allowed us to evaluate the cost of potential introduction of the cold spray process into pipe production.

Development of 3D Multicomponent Model for Cold Spray Process Using Nitrogen and Air

Cold spray is a unique coating technology that allows for solid state deposition of particles under atmospheric pressure. In this paper, a three dimensional, Computational Fluid Dynamics (CFD) multicomponent model is developed to estimate cold spray gas conditions involving both nitrogen and air. Calibration of the model followed by validation is accomplished by considering the thermal history of substrate exposed to cold spray supersonic jet. The developed holistic multicomponent model is effective in determining the state of gas and particles from injection point to the substrate surface with the advantage of optimizing very rapid cold spray deposition in nanoseconds. The validation of k-ε type CFD multicomponent model is done by using the temperature measured for a titanium substrate exposed to cold spray nitrogen at 800 °C and 3 MPa. Heat transfer and radiation are considered for the de Laval nozzle used in cold spray experiments. The calibrated multicomponent model has successfully estimated the state of propellant gas for the chosen high pressure and high temperature cold spray conditions. Moreover, the multicomponent model predictions are in good agreement with a previous holistic three dimensional cold spray model in which only nitrogen was used as the surrounding as well as the propellant gas.

Damage Study of Cold-Sprayed Composite Materials for Application to Electrical Contacts

This work demonstrated the feasibility of cold spray for the achievement of industrial electrical contacts to be used in contactors and circuit breakers. This resulted from the study of cold spray conditions when applied to two types of silver-based particulate composites. Starting materials, primarily powder characteristics and addition content, and spraying conditions were optimized from thorough studies of microstructures and mechanical properties of the cold-sprayed composites. These involve x-ray microtomography and transmission electron microscopy in particular, in addition to tensile tests. Moreover, thermomechanical laser testing of series of cold-sprayed microstructures was successfully developed to investigate into damage mechanisms in coatings. The whole led to a novel phenomenological approach to damage in cold-sprayed materials. Durability tests of the best coatings were carried out using industrial facilities. Cold-sprayed contacts compared more than well with those from conventional powder metallurgy (P/M), which opens a new and cost-attractive production route for electrical contacts.

Current Status and Future Prospects of Warm Spray Technology

A modification of high-velocity oxy-fuel (HVOF) thermal spray process named as warm spray (WS) has been developed. By injecting room temperature inert gas into the combustion gas jet of HVOF, the temperature of the propellant gas can be controlled in a range approximately from 2300 to 1000 K so that many powder materials can be deposited in thermally softened state at high impact velocity. In this review, the characteristics of WS process were analyzed by using gas dynamic simulation of the flow field and heating/acceleration of powder particles in comparison with HVOF, cold spray (CS), and high-velocity air-fuel (HVAF) spray. Transmission electron microscopy of WS and CS titanium splats revealed marked differences in the microstructures stemming from the different impact temperatures. Mechanical properties of several metallic coatings formed under different WS and CS conditions were compared. Characteristics of WC-Co coatings made by WS were demonstrated for wear resistant applications.

The effects of powder properties on in-flight particle velocity and deposition process during low pressure cold spray process

In cold spray process, impacting velocity and critical velocity of particles dominate the deposition process and coating properties for given materials. The impacting velocity and critical velocity of particles depend on the powder properties and cold spray conditions. In the present study, the in-flight particle velocity of copper powder in low pressure cold spraying was measured using an imaging technique. The effects of particle size and particle morphology on in-flight particle velocity and deposition efficiency were investigated. The critical velocity of copper powder was estimated by combining the in-flight particle velocity and deposition efficiency. The effect of annealing of feedstock powder on deposition and critical velocity was also investigated. The results showed that the irregular shape particle presents higher in-flight velocity than the spherical shape particle under the same condition. For irregular shape particles, the in-flight velocity decreased from 390 to 282 m/s as the particle size increases from 20 to 60 μm. Critical velocities of about 425 m/s and more than 550 m/s were estimated for the feedstock copper powder with spherical and irregular shape morphology, respectively. For the irregular shape particles, the critical velocity decreased from more than 550 to 460 m/s after preheating at 390 °C for 1 h. It was also found that the larger size powder presents a lower critical velocity in this study.

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.

Liquid-fuel/air premixing in gas turbine combustors: experiment and numerical simulation

Premixing of fuel oil and air for the purpose of lean-premixed combustion in gas turbine combustors was studied experimentally with laser-induced fluorescence (LIF) and numerically using a modified KIVA-3 code. A full-scale industrial burner, operated at atmospheric pressure, was investigated as well as a test burner of reduced scale, operated at pressures up to 15 bar. Three improved-accuracy models were used in the simulations to replace the corresponding standard KIVA-3 models: (1) a droplet-vaporization model including multiple components and real-gas effects; (2) a droplet-dispersion model, using a linear filter; and (3) a swirl correction to the k-ϵ turbulence model. As upstream boundary condition for the spray simulation, drop-size distributions measured under cold-spray conditions were used with Sauter mean diameter (SMD) correlated according to Lefebvre. Calculations with different spray models were performed to study the effect of multicomponent vaporization, droplet-internal inhomogeneities, property-estimation methods, dispersion modeling, and swirl correction to the k-ϵ model. Considerable differences were found in fuel-vapor distribution when using different models. The LIF measurements and the numerical simulations were compared in terms of the fuel-vapor distribution. The simulation results were found to be very sensitive to the initial SMD, in particular in the presence of air swirl. Inaccuracies in the estimated SMD, therefore, constitute the primary accuracy limitation of the simulation. By adjusting the SMD, in many cases good agreement between measurement and simulation could be obtained. Remaining discrepancies that cannot be attributed to SMD adjustments are supposed to originate either from a turbulence-modeling error and the occurrence of secondary droplet breakup, which was not modeled, or, on the other hand, to the error by assuming a proportionality between LIF intensity and fuel oil concentration.