Suspensions have shown a great potential for being employed as the spraying materials in flame spraying processes with the aim of producing thin and dense coatings. The internal axial injection of a suspension within processes like the high-velocity suspension flame spraying (HVSFS) offers the advantage of complete suspension entrainment within the gas stream, which therefore results in enhanced momentum and heat transfer to the particles. Experimental assessment of the achieved particle velocities and temperatures within the combustion chamber is nonetheless practically infeasible. A better insight into the process is attainable through employing computational simulations. Following a computational fluid dynamic (CFD) modelling approach for HVOF processes, combustion and gas flow turbulences were simulated for different combustion chamber geometries and ethene/oxygen ratios commonly used in the HVSFS process. Simulations were done with the commercial software ANSYS CFX 16.2. To account for the highly turbulent flow characteristics, the k-ε model and the Shear Stress Transport (SST) turbulence model were chosen, employing an eddy-dissipation-model for fuel gas combustion. Second-order upwind discretization was used to enable a good resolution of flow features like shock diamonds. The results of the simulation using different levels of detail of the combustion reaction were compared to experiments employing the modelled combustion chambers and gas flows. Chamber pressure and positions of the shock diamonds were monitored in order to allow a qualitative evaluation of the calculated values.

The authors performed a time-dependent, three-dimensional numerical simulation of a non-transferred DC plasma spray with externally applied magnetic fields. Compressible Navier-Stokes equations with MHD source terms and Maxwell's equations were used as the governing equations for plasma flows. In the simulation, two operating conditions, electric currents and strength of externally applied magnetic fields, were parametrically varied in a range of 300 A to 500 A and 0.2 T to 0.8 T, respectively. Numerical results show that the application of strong magnetic fields such as 0.4 T and 0.8 T is recommended for an anode arc rotation leading to elongating an anode lifetime. A voltage variation due to the anode arc rotation shows periodic behavior with a small amplitude, which is expected to be good for plasma spraying processes. Lagrangian approach was used to track injected particles in the plasma jet and the particle temperature and position distributions on a cross section normal to the central axis of spray were studied. Swirl flows induced by the arc rotation hinder the particles from reaching the hot plasma jet. Our numerical results demonstrated that injecting particles in the opposite direction to the swirl flow is an effective way to heat the particles.

Air plasma spraying is a variation of thermal spraying that is used, among others, for the production of thermal barrier and wear resistant coatings. High plasma temperatures enable the processing of ceramic powder particles which have a high melting point and cannot be processed otherwise. Due to their low heat conductance, the ceramic particles are not necessarily fully melted during their flight in the free jet and prior to the impact on the substrate surface. Experimental particle temperature measurements by means of particle diagnostics systems deliver merely the surface temperature of the particles while the melting degree of the ceramic particles remains unknown. Therefore, the temperature field within spherical Al 2 O 3 particles is numerically investigated for a commonly used particle size distribution by considering different particle sizes. The model includes a two-way coupled particle-laden free jet model and takes the latent heat of melting and evaporation into account. The effect of the particles size as well as the stand-off distance on the melting degrees of the particles in the given powder size distribution is determined.

In this study, the effect of the substrate roughness and thickness on the heat transfer coefficient of the impinging air jet upon a flat substrate was investigated. A low-pressure cold spraying unit was used to generate a compressed air jet that impinged on a flat substrate. A detailed mathematical model was developed and coupled with experimental data to determine the heat transfer coefficient and surface temperature of the substrate. It was found that increasing the roughness of the substrate enhanced the heat exchange between the impinging air jet and the substrate. As a result, higher surface temperatures on the rough substrate were measured. It was further found that the Nusselt number that was predicted by the model was independent of the thickness of the substrate. The results of the current study were aimed to cover the influential substrate parameters on surface temperature of the substrate that eventually can affect the final quality of the cold-sprayed coatings.

In cold spraying a powder material is accelerated and heated in the gas flow of a supersonic nozzle to velocities and temperatures that are sufficient to obtain cohesion of the particles to a substrate due to plastic deformation. The deposition efficiency of the powder particles is significantly determined by their velocity and temperature. The particle velocity correlates with the kinetic energy of the particles and thereby with the amount of energy that is converted to plastic deformation and thermal heating. The initial particle temperature significantly influences the mechanical properties of the particle. Velocity and temperature of the particles have nonlinear dependence on the pressure and temperature of the gas at the nozzle entrance. Whereas the particle velocity can easily be measured during the process, the particle temperature is not directly accessible by experimental techniques. Generally information about the particle temperature can be obtained based on theoretical models. In this contribution a simulation model based on the reactingParcelFoam solver of OpenFOAM is presented and applied for an analysis of the cold spray process. The model combines a compressible description of the gas flow in the nozzle with a Lagrangian particle tracking. The predictions of the simulation model are verified based on an analytical description of the gas flow, the particle acceleration and heating in the nozzle. Based on experimental data the drag model according to Plessis and Masliyah is identified to be best suited for OpenFOAM modelling particle heating and acceleration in cold spraying.

This study investigated the accelerating behavior of spray particles during cold spraying (CS) by employing a computational fluid dynamics program, FLUENT. Optimization of the dimensions of CS nozzle was conducted to maximize particle velocity. The results show that the expansion ratio, divergent length, particle density and size, operating temperature significantly influence particle acceleration. It is found that the spray particles in nozzles with long divergent length can obtain a relatively higher impact velocity, but too long divergent length will reduce the particle velocity. Besides, the particle impact velocity shows a downward trend with increasing the particle size or density. Hence, the optimal divergent length should increase with the increase of particle mass. Moreover, higher gas temperature leads to a higher particle velocity, but it has no influence on the optimal divergent length.

The gas dynamics of high-velocity oxyfuel (HVOF) spraying can be described by conservation continuum equations of mass continuity, κ-ε models for turbulence, Navier-Stokes, and the equation of state, the latter accounting for density variations in the flow field. In previous work, a reduced kinetic eddy dissipation model has been validated against measured data, but only for constant mass flow. In this study, the equation of state for HVOF spraying is implemented in numerical modeling software in order to correlate variations in mass flow density with pressure and temperature. The results show how the temperature and pressure of the gases at the inlet of the nozzle, prior to entering the combustion chamber, influence combustion pressure as well as the compressibility of the gases before expanding to ambient pressure.

In this study, a numerical model is developed to simulate cold-spray coating profiles based on spray angle, nozzle traverse speed, and scan step. An extension of the model was also developed that predicts coating thickness distributions based on kinematics data obtained using robot trajectory monitoring equipment. Experimental studies were also conducted to validate the numerical models and assess the simulated results.

The objective of this work is to analyze the thermal conductivity of suspension plasma sprayed thermal barrier coatings using experimental techniques and finite element modeling. The results indicate that the scale of the porosity in the coating has a significant influence on thermal conductivity. Smaller grains, higher overall porosity content, and lower columnar density correspond to lower thermal conductivity. It is shown that FEA can be a powerful tool to predict the thermal conductivity of SPS thermal barrier coatings.

Most models used to simulate plasma torch operation consider only the gas domain with the electrodes included as boundary conditions, imposing a current density profile and temperature at the cathode tip. In order to achieve more realistic simulations, it is necessary to include the electrodes and arc column in the calculation domain. This paper discusses some of the issues, factors, and considerations involved in modeling arc-cathode and arc-anode interactions and their effect on the plasma jet. Descriptions of the arc column and plasma jet are based on the coupling of electromagnetic and fluid equations and require thermodynamic, transport, and radiative property values of the gas mixture. In order to capture the stochastic behavior of the arc, it is assumed that the model is 3D and time-dependent.

This paper describes the development of a numerical model and explains how it is used to investigate arc-cathode interactions in a plasma arc torch. The model is based on magnetohydrodynamic (MHD) theory and couples Navier-Stokes equations for a nonisothermal fluid with Maxwell’s equations for electromagnetic fields. The equations account for the internal geometry of the torch as well as arc current and gas type and flow rate. They are solved using CFD code and relevant boundary conditions and are shown to provide insight on arc dynamics and the effect of cathode shape on arc behavior.

In this study, Eulerian finite element analysis is used to explore the deformation and bonding behavior of composite particles deposited by cold spraying. The objective is to account for shear instability and bonding as well as the geometry of the deforming material. An accurate description of the geometry is essential when the amount of deforming material is limited as in composite particles. Another goal is to provide a framework for modeling the impact of agglomerates. In this case, deposition is influenced by bonding and fragmentation or detachment due to plastic rebound. To account for the latter effect, thin layers of nonbonding material are added to particle and substrate surfaces in the model. Simulations of metal-clad ceramic particles show that there is a critical shell thickness beyond which maximum stress in the hard phase abruptly increases.

In this work, CFD simulations are used to evaluate air cap configurations for twin wire arc spraying (TWAS). Investigators employed a design of experiments (DoE) approach to identify air cap parameters with the greatest impact on gas velocity, jet convergence, and pressure distribution. The ones selected for study are the convergence angle, the length and diameter of the throat, and the distance between the air cap outlet and the point where the wires intersect. In all configurations studied, the spray wires deflected the flow of the primary gas and narrowed the cross-section of the plume along one axis. The effects of each air cap parameter are discussed in the paper along with possible design improvements.

In this study, a 3D two-way coupled Eulerian-Lagrangian approach is used to model the plasma jet and droplet-particle trajectory, velocity, and temperature achievable by suspension plasma spraying. A Reynolds stress model is used to account for turbulence and the effect of the substrate on the flow field and a Kelvin-Helmholtz Rayleigh-Taylor breakup model is used to predict the secondary breakup of the suspension. The focus of this work is on particle behavior near the substrate. Flat substrates placed at stand-off distances ranging from 40 to 60 mm are modeled to provide detailed information on particle impact behavior.

In this work, numerical modeling is used to simulate the effects of laser remelting as a post treatment and as an in-situ component of a hybrid plasma spraying process. Initially, a single-pass 2D model is used to simulate the laser post-treatment process in order to obtain relationships between melting pool depth, relative scanning velocity, and laser power. A 3D finite-element model is then used to study temperature variations during multi-layer deposition of a NiCr alloy by plasma spraying with in-situ laser melting. The effects of phase change are taken into account by defining the enthalpy of the material as a function of temperature. Predicted melting pool depth corresponded well with experimental values.

In earlier experiments, plasma-sprayed Al 2 O 3 coatings were deposited on preheated Al 2 O 3 substrates to study the effect of substrate temperature on splat formation and phase transformations. The aim of the present work is to develop a model to better understand the factors that affect phase selection during the solidification of Al 2 O 3 splats. A model based on one-dimensional heat transfer and classic nucleation theory is presented and used to simulate the rapid solidification process and the influence of process parameters on phase selection. The model accounts for under-cooling phenomena, heterogeneous nucleation, and nucleation kinetics. The findings indicate that the relationship between initial substrate temperature and phase selection is primarily based on the catalytic effect of the alumina substrate on the nucleation of Al 2 O 3 phases as a function of contact angle.

In this work, finite element analysis is used to investigate the effect of pore size on the stress intensity factor (SIF) of plasma sprayed coatings. Test samples with different pore sizes were obtained by spraying wollastonite powders with particle sizes of 60-75 μm, 75-95 μm, and 95-150 μm onto Ti 6 Al 4 V coupons. The results show that coating stress varies in proportion to the length of 2D pores and to a lesser extent the diameter of 3D pores. This implies that reducing the length of 2D pores may be considered as a way to increase the fracture resistance of plasma sprayed coatings.

Within Surface and Coating Technologies, the High Velocity Oxy-Fuel (HVOF) thermal spray process generates significant peening stresses due to the impact at high velocity of semi molten particles onto the substrate. The level of high kinetic and thermal energy of impinging particles is a key-parameter to understand how residual stresses build up through the whole system during spraying, and to which extend these stresses influence the resulting coating adhesion strength. While an appropriate combination of thermal and peening stresses is beneficial to the deposit bonding, no systematic study has been carried out to determine their respective amplitudes. A numerical Finite Element Analysis (FEA) has been developed to isolate peening stresses from thermal stresses developed into the substrate target, after successive impacts of single particle. The investigation is performed on Inconel 718 feedstock material HVOF sprayed on Inconel 718 substrate. The relationship between the developed stress state at the substrate interface and the impinging particle temperature and velocity is given a particular interest.

Aiming at clarifying the individual splat formation mechanism in thermal spray process, commercially available metallic powders were thermally sprayed onto AISI304 substrate surface. The splats changed from a distorted shape with splash to a disk-shaped splat in flattening after collision onto substrate surface, through substrate preheating and/or reducing the ambient pressure. Accordingly, both substrate temperature and ambient pressure have an equivalent effect on the shape transition. The observation on the bottom surface morphology of single splat indicated that the ring-shaped initial solidification might play an important role during splat formation process. As a simulation of the real thermal spray process, free falling experiment has been conducted. The thermal history of the free falling metal droplet onto AISI304 substrate indicated that the flattening pattern is decided so quicky just after collision onto solid surface, which is enough earlier to the finalization of the flattening.

Surface treatment has become an effective method to improve corrosion resistance of magnesium alloys which are the lightest commercial structural alloys with superior specific strength and stiffness. Low pressure cold spraying is used to locally deposit aluminium on AZ31 in order to enhance general and galvanic corrosion of magnesium. This study is aimed at numerical estimation of the residual stress profile due to cold spray coating using FEM. The impact of particles on the substrate is modelled in Abaqus Explicit. The challenge of AZ31 simulations is the intrinsic yield asymmetry and anisotropy which results different behaviour of the material along different direction both in tension and compression. On the other hand, there is no precise material model capable of considering the yield asymmetry and anisotropy experienced by AZ31 in Abaqus. This paper studies the effect of anisotropy of the AZ31 on residual stress induced by cold spray. The results are compared with experimental X-Ray diffraction measurements. It is suggested that an isotropic analysis in the dominant stress direction of AZ31 may result in good estimation of the residual stress profile.