A numerical finite difference model has been developed to treat the transfer of heat and momentum between a gas environment and a particle injected into it. The model is based on an explicit solution scheme for the thermal field and explicit treatment of the momentum exchange. The latent heat associated with phase changes is simulated via a post-iterative heat accumulation scheme. Particle-gas heat transfer is represented by a heat transfer coefficient, which is a function of relative gas velocity. The validity of the model is confirmed via comparisons between predicted behaviour and previously-published experimental data for thermal histories and particle trajectories. Comparisons are also presented with predictions from previously-developed models. Results are then presented for the behaviour of hollow zirconia particles, with particular attention being paid to in-flight melting characteristics. It is shown that there is an optimum combination of particle size and wall thickness for the promotion of efficient melting, for a given gas flow and temperature field.

This paper evaluates two CFD tools for predicting particle speeds and temperatures during atmospheric plasma spraying. The investigation is based on varying two parameters: plasma current and gas flow rate. Although the tools differ in accuracy, both proved to be sufficient for optimizing industrial plasma spraying processes. A comparison of the results with experimental data showed the more advanced tool to be capable of predicting variations in particle characteristics when operating conditions change. Paper includes a German-language abstract.

In this work, process models combining statistics, materials science, and mechanics were developed to optimize and qualify HVOF sprayed coatings as if for aerospace application. In an attempt to correlate process variables with typical coating properties, a preliminary microstructure model was developed using statistical data obtained from 34 design-of-experiments runs. A second model, based on fatigue testing, was also developed. The fatigue model was subsequently used to guide the selection of variables for a final model that is shown to aid in both process qualification and the optimization of process-operating parameters.

This paper presents research work aimed at path planning for continuous processes; we treat the general case and examine thermal spraying in particular. The objective is to simulate the process for the offline programming of a sprayer robot. Integrating process models with those of the robot and workpiece in a single CAD environment simplifies the preparation and optimization of spraying tasks. This paper describes the principles and specifications of the application package with emphasis on the development of the process model and motion generation satisfying the constraints of thermal spraying.

Several effective numerical techniques, based on finite element analysis, have been developed and computed independently in this paper. Results are presented describing the impacting process, and the subsequent temperature and residual stress fields of a molten nickel particle impacting onto a flat substrate. Problems of this type, especially the prediction of the thermal residual stresses, are of major practical interest in thermal spray operations as a pioneering approach.

In this study, a new physically-based finite element approach is proposed to model and predict the superficial oxide layer removal and the occurrence of localized metallic bonding during particle impacts. The process physics, based on explosive welding theory and experiments, and method implementation is presented. Prediction of critical velocity of copper is obtained and compared to experimental data to validate the model. Moreover, the model is also able to show the bonding locations at the interface between particles and substrate. The predicted bonding locations are consistent with experimental data from literature for several metals.

The aim of this study is to model a spray process that combines aspects of plasma and HVOF spraying. The process is characterized by its stability over a broad range of fuel-oxidant conditions and ability to produce coatings using relatively little gas with rather low gross heating values The mathematical model developed accounts for the formation of the plasma jet, the combustion process, and supersonic flow issuing from the spray torch. Simulating the new process made it possible to investigate the effect of the plasma on the velocity and temperature of the gas flow inside and outside the gun. The equations were solved using CFD code and predictions were compared with experimental observations. The benefits of the plasma jet are discussed on the basis of predictions and fuel combustion mechanisms.

The high-velocity air-fuel process (HVAF) is an emerging technology used in the thermal spray industry. The Praxair HVAF process combines air and a liquid fuel (e.g., kerosene, diesel) to generate an energy source with extremely high gas velocities. Analytical studies were conducted to investigate gas and particle dynamics in the Praxair HVAF process for coating with WC-l2Co and stainless steel powders. The mass, momentum, and energy conservation equations were first solved, using the TORCH computer program. Typical output from the model includes temperature and velocity profiles as a function of radial and axial position. The PROCESS gas/particle computer program was then used to calculate from these temperature and velocity profiles the dynamics of particles injected into the gas plume. The primary result of the gas/particle code is a description of the injected particle temperature and velocity as a function of position in the plume. A thorough understanding of the process was obtained using this modeling technique. The results of the modeling were confirmed with process diagnostics. Particle temperature measurements for the WC-Co powder system were obtained with a two-color pyrometer; particle velocity measurements were obtained using particle imaging velocimetry. The coatings produced in the study exhibit superior quality, with high-density, high-hardness, low-oxide content, and high-bond strength.

Atmospheric Plasma Spray is widely used for tens of years to elaborate protective coatings on parts for several applications. However, our understanding of the APS process can still be improved, requiring a fine modeling of the process in parallel with some corresponding experiments. In the present work, a complete series of models was applied to reinforce our knowledge of the process: the case of an alumina coating was considered. A 3D CFD model was first used to study the internal arc within the torch. Interactions between the external plasma jet and the injected particles were then computed in a second step. At this level, the predicted in-flight particle characteristics were compared with some corresponding measurements recorded with the DPV 2000 diagnostic tool. A third model was then applied to investigate the particle flattening on the substrate/coating material. SEM pictures of coating cross-sections were then captured and a last model was finally applied to estimate the coating effective thermos-mechanical properties based on calculations performed directly on the SEM micrographs. This set of models allows investigating the APS process from the DC arc within the torch to the coating properties.

Severe plastic deformation (SPD) is the main feature of the Cold Spray (CS) process, which might result in producing metal grain refinement under extensive hydrostatic pressure and high strain rate loading conditions. In this study, an anisotropic strain gradient plasticity model (SGP) is presented to predict materials behavior in CS process. The enhanced dislocation densities produced throughout particle deformation affect coating material properties and modify their thermodynamic characteristics and kinetics of resistance to plastic deformations. This study also demonstrates that the SGP model can describe the experimentally observed trends and account for homogenization of the accumulated strains under dynamic recrystallization conditions. The evolution of statistically stored dislocation density through the characteristic material length scale parameter is in good agreement with experimental results and data reported by other research groups. The proposed SGP modeling is suggested as an express method to evaluate the advanced coating and additively manufactured materials, and powder feedstock used in thermal spray and 3D manufacturing applications.

A supersonic external flow and high particle velocities, which lead to low porosity coatings, characterize the High Velocity Oxygen Fuel (HVOF) process. A numerical study of this process is proposed in the present work. In a first step, the PHOENICS CFD code is used for the computation of the flow inside a CDS gun and within the supersonic external jet: a two-dimensional axisymmetric geometry is used. The combustion model assumes chemical equilibrium whereas the turbulence effect is taken into account using the Chen-Kim k-ε model, which is appropriate for the computation of round jets. In a second step, an in-house code is used in order to simulate the flow/particles interactions. Finally, numerical results obtained on in-flight particle characteristics are compared with measurements obtained with the DPV2000 system.

In the present paper mathematical model of the deformation behavior of a liquid spherical particle upon its impingement onto a solid surface, including flattening and simultaneous solidification is developed. Particle-substrate interactions are investigated for typical thermal spray process. Numerical simulation for the complete Navier-Stokes equations is based on the finite-difference method on rectangular mesh in cylindrical coordinates. The energy equation is solved for both particle and substrate regions using the adjoint conditions for the temperature. In this paper main attention is paid to investigation of the temperature in contact of the particle with substrate. In connection with the oxide films effect on the surface substrate taking onto account thermal resistance of oxide is simulated. Heat transfer process in particle and substrate has been modeled by 2-D problem of heat conduction with influencing hydrodynamic processes into molten particle. Particle solidification and the movement of the solidification front have been described by means of one-dimensional Stefan problem. Numerical results for the heat transfer process and the effect of some important processing parameters such as particle diameter, viscosity, oxide films and temperature of plasma on the flattening and solidification of a single liquid particle have been discussed. Numerical algorithms were realized in the form of applied programs complex.

The warm spray gun was developed to make a coating of temperature-sensitive material, such as titanium, on a substrate. The gun has a combustion chamber followed by a mixing chamber, in which the combustion gas is mixed with the nitrogen gas at room temperature. The temperature in the gun can be controlled in the range of about 1500 - 2500 K by adjusting the mass flow rate of nitrogen gas. The mixed gas is accelerated to supersonic speed through a converging-diverging nozzle followed by a straight passage. In this paper, the performance of the warm spray gun is investigated by the simulation program in order to deeply understand the performance of the warm spray gun. The gas flow as well as the velocity and temperature of titanium particle inside and outside the gun are predicted by the numerical simulation.

The Shock-wave Induced Spray Process (SISP) is a method of applying coatings of various metallic-based materials onto a wide range of different substrates. It utilizes the kinetic and thermal energy induced by a moving shock-wave to accelerate and heat powder particles. A transient axisymmetric model for the process is developed using Fluent. The model is validated with reference to a simplified one-dimensional approximation of the flow field. Values of pressure, axial velocity, Mach number, as well as static and total temperature are carefully examined. It is found that a zone develops in the flow that experiences elevated levels of temperature and velocity simultaneously. This is the main distinction between SISP and traditional CGDS processes. The effects of varying supply pressure and temperature on these flow variables are investigated in detail. Additionally, the effect of changing the driving gas type is investigated using air and helium as examples.

Thermal spray layers are formed on rough surfaces; however, the flattening process on rough surfaces has not yet been clarified. A mathematical flattening model which takes into account the roughness of the substrate or previously coated layers is proposed in this paper. As a result of surface roughness, the flattening degree and the flattening time decrease with increasing surface roughness in this model. In addition, the characterization of surface roughness is introduced for the flattening model. Several calculated cases of the flattening model are shown.

The High-Velocity Combustion Wire Process is a new high-velocity combustion process now being used in the thermal spray industry. This process combines air, oxygen, and a fuel gas to generate a high-temperature, high-velocity plume that is optimum for producing metallic coatings. Analytical studies were conducted to investigate gas and droplet dynamics for the spraying of three different materials: aluminum, stainless steel, and molybdenum. With the relatively low flame temperatures of the process, the feedstock wire is melted by convective heat transfer with no superheating or vaporization of the droplets. When the droplets strike the substrate, their temperature peaks as the high kinetic energy of the droplet is transformed into thermal energy. The conservation equations were solved using the TORCH computer model, yielding the temperature and velocity profiles as a function of location. The PROCESS gas/droplet computer program was then employed to calculate the dynamics of the molten droplets. The results of this modeling was confirmed with process diagnostics. Experimentation included droplet temperature measurements using a two-color pyrometer and droplet velocity measurements using particle imaging velocimetry for the stainless steel material system. The coatings produced in the study exhibit superior quality with high density, high hardness, low oxide content, and high bond strength.

In this work, a numerical model of the cold spray process was developed to reproduce microstructures obtained in coatings by simulating the deformation of impinging particles and resulting coating build-up. The model employs a library of particle images generated by x-ray microtomography. To each image, a velocity is assigned and the deformation that would be produced by particle impact is estimated by means of finite element analysis and stored for later use. Based on the results, the ing approach has good potential for simulating coating microstructures that can be achieved through cold spraying.

In this work, a CFD model is built to investigate gas flow and in-flight particle characteristics in HVOF spraying with hydrogen as the fuel. Gas flow is solved for a particle-free jet, combustion is represented using a simple eddy-dissipation model, and a time-averaged fluid flow (k-ε) model is used to account for turbulence. Acceleration and heating of individual particles are modeled in a "snapshot" of the gas flow. Particle acceleration is shown to be governed by drag and particle heating by conduction and radiation between particulates and gas phases. Modeling results agree well with experimentally obtained data and observations.

Abstract-Solution precursor plasma spray (SPPS) is a relatively new thermal spray process in which chemical precursors are injected into DC-arc plasma spray torch in place of powder. This process is able to make relatively porous (15-25% porosity) thermal barrier coatings with through-thickness cracks that enhance their thermal strain resistance. The SPPS process can also make dense titania and alumina zirconia coatings. The process can make thin and thick coatings, dense and porous coatings, structural and functional preforms, new compositions, and metastable materials. Metastable materials arise because the solution is molecularly mixed and the cooling rate in thermal spray is high. The process has the disadvantage of needing to provide energy to evaporate the solvent and of being a new process where less extensive empirical knowledge and modeling insights exist, compared to air plasma spray with powders. The microstructure process parameter relation is explored. Results from modeling studies concerning evaporation of droplets and related solute concentration gradients, modeling studies of aerodynamic break-up and experimental studies of non-aero-based droplet break-up and of the effects of solution concentration will be described. These studies will be related to experimental results for making dense coatings.

In cold spray, 5-150 μm particles (of metal, ceramic, composite, and other materials) are accelerated to supersonic velocities through a deLaval nozzle with an inert gas (generally He or N 2 ) that can reach 1000 °C. In the process, the gas jet impingement on the target and the extreme plastic deformation of impacting particles cause heat generation in the coating layers and the substrate. The heat generation has been argued to cause residual stress, which may cause coating-substrate delamination. In this study, heat generation due to gas impingement and particle plastic deformation has been predicted from CFD and FEA simulations, respectively. Furthermore, a finite volume method has been presented for transiently simulating the coating buildup and bulk heat generation in the coating and the substrate. The model is intended to assist researchers understand thermal affects in the coating process and help design more informed coating patterns to reduce negative thermal effects.