Search Results for three-dimensional single-particle model

The flame spraying process has been analysed using three-dimensional Computational Fluid Dynamics (CFD) simulations. The process employed at the Volvo Aero Corporation for the coating of fan and compressor housings has been modelled. The gas combustion was simulated as a multi-component chemically reacting flow. The standard, two equations, k-ε turbulence model was employed. A statistical analysis of the computer simulation experiments revealed that particle velocity and particle temperature were dependent on four process parameters, namely the acetylene flow rate, the carrier gas flow, the powder feed rate and the spray distance. The most important factors influencing particle velocity and temperature were the acetylene flow rate and the carrier gas flow. The carrier gas flow rate was shown to have an unexpectedly large influence on particle in-flight properties. Simulations were repeated with particles of different median diameters. The study revealed that a very high correlation existed between particle temperature and particle velocity for particles of the same median diameter. Furthermore, the particle median diameter, when compared with the investigated process parameters, was found to have a more pronounced influence on both particle temperature and velocity. It would appear that the use of powder lots comprised of sufficiently fine-grained powders is the most promising single contribution towards increasing deposition efficiency that can be applied to the current process.

The fluid and particle dynamics of a High-Velocity Oxygen-Fuel Thermal Spray torch are analyzed using computational and experimental techniques. Three-dimensional Computational Fluid Dynamics (CFD) results are presented for a curved aircap used for coating interior surfaces such as engine cylinder bores. The device analyzed is similar to the Metco Diamond Jet Rotating Wire (DJRW) torch. The feed gases are injected through an axisymmetric nozzle into the curved aircap. Premixed propylene and oxygen are introduced from an annulus in the nozzle, while cooling air is injected between the nozzle and the interior wall of the aircap. The combustion process is modeled using a single-step finite- rate chemistry model with a total of 9 gas species which includes dissociation of combustion products. A continually-fed steel wire passes through the center of the nozzle and melting occurs at a conical tip near the exit of the aircap. Wire melting is simulated computationally by injecting liquid steel particles into the flow field near the tip of the wire. Experimental particle velocity measurements during wire feed were also taken using a Laser Two-Focus (L2F) velocimeter system. Flow fields inside and outside the aircap are presented and particle velocity predictions are compared with experimental measurements outside of the aircap.

A contact model that accounts for interfacial cohesion and thermal conduction is developed to investigate the influence of bonding on the final residual stresses build-up in cold spray. The residual stress evolution in the cold sprayed Al-6061 coating on an Al-6061 substrate is investigated via three-dimensional (3D) single-particle and multi-particle impact simulations. It is shown that the interface bonding mainly affects the local residual stress distribution near the interfaces. The residual stresses are largely due to the kinetic peening and bonding effects. The thermal cooling has negligible influence. In general, the peening effect introduces a compressive stress while the bonding effect results in a relaxation to this compressive stress. This work suggests that the interface bonding should be considered as one of the essential factors in numerical modeling of the residual stresses evolution in cold spray.

Numerical studies directly quantifying particle-substrate adhesion under different spray process parameters during impact remain scarce, primarily due to the lack of consideration for adhesion models. This study addresses this gap by investigating bonding behavior under varying spray conditions using a single-particle model with the PD numerical method, incorporating adhesion forces.

A three-dimensional model of particle impact and deformation on rough surfaces has been developed for HVOF sprayed polymer particles. Fluid flow and particle deformation was predicted by the Volume of Fluid (VoF) method using Flow-3D® software. The effect of roughness on the mechanics of splatting and final splat shapes was explored through the use of several prototypical rough surfaces, e.g. steps and grooves. In addition, a numerical representation of a more realistic rough surface, generated by optical interferometry of an actual grit blasted steel surface, was also incorporated into the model. Predicted splat shapes were compared with SEM images of Nylon 11 splats deposited onto grit blasted steel substrates. Rough substrates led to the generation of fingers and other asymmetric three-dimensional instabilities that are seldom observed in simulations of splatting on smooth substrates.

Cold spray can substitute for several coating processes for various applications, due to a high efficiency coupled with high properties for the sprayed product. The use of a composite powder rather than a powder blend was shown to be beneficial, especially for the cold spray of electrical contacts. The objective of this work is to optimize a composite powder (Ag-14wt% SnO 2 ) using numerical simulation of the deformation of the particle at the impact onto the substrate (Cu). Every elementary composite particle was made of an agglomerate of Ag and SnO 2 smaller particles, which exhibited more or less porosity depending on the powder processing conditions. The first step was to study the distribution of these various constituting phases plus porosity. Three types of powders which showed different phases and porosity characteristics deliberately were developed. Three-dimensional images of the agglomerate were acquired using microtomography which exhibited the porosity network well in the dual-phased particle material. These actual 3D images were used to feed a simulation of the impingement of a particulate agglomerate to result in a splat onto the substrate. For this, a two dimensional deformation model was developed on the route to a three-dimensional model which is expected to be more powerful. The influence of agglomerate characteristics, primarily porosity, on the deformation behavior was studied. Consequences on splat-substrate adhesion and deposition efficiency could therefore be investigated in the light of direct observation of the cold-sprayed material.

The behaviour of plasma sprayed powder particles at impact is not adequately understood so that the microstructure of thermal barrier coatings cannot be predicted with the required accuracy. This is due to the complexity of the spraying process in which the shape of the impacting particles is influenced by the spraying conditions and the structure of the previously formed parts of the coating. In this paper we present some techniques to improve the understanding of coating formation. We investigate the impact of a single molten powder particle on a surface by performing splat tests, measuring the particle’s temperature, velocity, and diameter at the appropriate stand-off distance before the spraying process starts. The measured values are assigned to the splats. SEM-images of the splats are analysed by means of a image analysis algorithm. Thus we are able to evaluate the influence of particle properties on the splat shape. In order to get a better understanding of the mechanism of coating build-up, a three-dimensional series of cross sections is created. With this kind of examination the shape of splats can be characterised in three dimensions.

This paper presents a simulation of the simultaneous spraying of a metal and a ceramic powder with different configurations for the injection of the powder into the plasma jet. The plasma jet and the behavior of the injected particles were modeled with a commercially available computational model of the dynamics of liquid bodies. The particles are modeled as discrete Lagrangian objects. Three series of numerical tests were carried out: simultaneous spraying of the powder in a three-dimensional plasma jet in a stable state; simulation of the 3-D plasma flow, assuming that it fluctuates at the same frequency as the arc voltage; and simulation of the effect of the current fluctuation on particle behavior. A pre-calculation with an analytical model made it possible to determine the suitable gas flow rate so that the "average" trajectories of the metal or ceramic powders coincide at the same point on the surface. Paper includes a German-language abstract.

Powder injection parameters like gas flow, injection angle and injector position strongly influence the particle beam and thus coating properties. The interaction of the injection conditions on particle properties based on DPV-2000 measurements using the single-cathode F4 torch is presented. Furthermore, the investigation of the plasma plume by emission computer tomography is described when operating the three-cathode TriplexPro torch. By this imaging technology, the three-dimensional shape of the radiating plasma jet is reproduced based on images achieved from three CCD cameras rotating around the plume axis It is shown how the formation of the plasma jet changes with plasma parameters and how this knowledge can be used to optimize particle injection.

A short survey is given of various techniques of 3D visualization of voids and other structural defects that are inherently present in thermally spray structures. It is proven that the only condition for visualization is possibility to set up a distinguishable threshold based on the gray scale of the microscopical image of the structure. Segmented images are then further processed by commercial software for image analysis, animations and anaglyph files creation. Several softwares were applied and combined, such as Lucia, Voxblast, Amira and Voxler. The visualization technique was demonstrated on a ceramic plasma sprayed coating, where two kinds of voids – semi-globular pores and large interlamellar pores could be separated. Similarly, this visualization technique can be used to distinguish between pores and oxide particles in case of metallic coatings. The visualization is done in a real Cartesian space without any transformation. The results can be useful mainly for: a) teaching to get a better insight into the microstructure of thermally sprayed coatings, b) research, where in combination with mathematical quantification the visualized structure can show important distinctions between coatings sprayed by different techniques and with variety of process parameters.

To obtain good quality coatings, spray parameters must be carefully selected. Due to the large variety in process parameters, it is difficult to optimize the process for each specific coating and substrate combinations. In this paper, the process of particle in-flight, coating growth and pore formation has been analyzed by numerical simulation. Three sub-models, plasma-particle, splat formation and coating buildup, have been developed. A comprehensive three-dimensional computational code (LAVA3D-P) is used to predict the plasma flame formation, flame-particle interaction, and particle state and trajectory. Assuming each splat is disk-like shape and the flattening ratio, a ratio between the splat diameter and droplet diameter, can be calculated based on the interplay among particle kinetic energy, dissipation energy and solidification. A set of rules of coating buildup are proposed to predict the coating deposition and pore formation, considering the influences of particle size, velocity, temperature and location related to the substrate. Using this model we can obtain the porosity, surface roughness and thickness of the coating considering splat adhesion and quenching stresses. Although the current model has server restrictions which attributes to many assumptions, it however, forms a foundation for further improvement of an advanced ceramic coating buildup model.

Thermal spray processing of functionally graded materials requires that the spray patterns of different particle types coincide at impact and that each particle type arrives with the appropriate temperature and degree of melting. Measurements of particle velocity, temperature, and size along with spray pattern characteristics have been obtained for co-injected NiCrAlY and zirconia powder. The plasma and particle flow fields were also simulated with a pseudo 3-D model using the LAVA computer code. The model assumes that the gas flow is axisymmetric while the particles are treated in a fully 3-D manner. A stochastic discrete-particle model that includes turbulent dispersion dictates particle behavior. The simulation produced reasonably accurate velocities and particle trajectories, although, particle temperature is consistently over predicted. Comparisons between the calculated and measured velocity and temperature statistical distributions and calculated molten fractions are discussed.

On the base of gases molecular and kinetic theory a mathematical model of interaction between powder particles and plasma jet is developed. Three-dimensional description of plasma forming gas density distribution as well as particle motion in the plasma jet are a characteristic property of the model. A software for practical realization of the mathematical model is created. Said software provides the possibility to investigate an effect of low-pressure plasma spraying parameters on particle velocity and coordinates in the plasma jet. Computer simulation of particle velocity for powders from aluminium and tungsten oxides in argon plasma under 60 Mbar is conducted. A "Plasma-Technik" VPS unit is used for testing the developed model. Particle velocity measurement is made by a specially developed optical-electronic unit.

This paper aims to develop a multiparameter-based three-dimensional simulation tool for cold spray, designed to predict material deposition behavior in real-world processes under varying process parameters and workpiece geometries.

The numerical forecast constitutes an interesting way for plasma spraying to minimize the number of experiments to achieved the optimum spraying conditions. Many computational codes have been developed to predict the properties of the plasma jet (velocity, temperature) and the particles behavior within (temperature, velocity, melting state). According to the particle injection orthogonally to the plasma jet, the models have to be 3D. However, such codes need several hours if not several days of calculations to obtain the results of one condition. This is the main drawback of the existing sophisticated codes. The computing time is not compatible with industrial needs. Various clever numerical methods were developed in the past to simulate 2-D parabolic gas flows for laminar boundary layers or jets. For example, the Genmix 2-D axi-symmetric algorithm developed by Spalding and Patankar, and known as the Bikini method requires a very low-cost memory and computing time. This algorithm makes it possible, when using the proper thermodynamics and transport properties of plasma gases and the whole equation of Boussinesq-Oseen-Basset and taking into account the thermophoresis and non continuity effect for small particle, to predict in a fast and rather realistic way, the velocity and temperature fields of the plasma jet.

A three dimensional model of coating formation has been developed. Using the model we are able to simulate coating formation by deposition of large numbers of droplets. The properties of impacting particles are assumed to vary stochastically using a normal probability density function. Splat curl up is assumed to be the source of porosity formation. The model is able to predict coating porosity, thickness and roughness as a function of spray parameters.

Computational modeling is used to systematically examine many of the sources of statistical variance in particle parameters during thermal plasma spraying. Using the computer program LAVA, a steady-state plasma jet typical of a commercial torch at normal operating conditions, is first developed. Then, assuming a single particle composition (ZrO2) and injection location, real world complexity (e.g., turbulent dispersion, particle size and density, injection velocity and direction, etc.) is introduced "one phenomenon at a time" to distinguish and characterize its effect and enable comparisons of separate effects. A final calculation then considers all phenomena simultaneously, to enable further comparisons. Investigating each phenomenon separately provides valuable insight into particle behavior. For the typical plasma jet and injection conditions considered, particle dispersion in the injection direction is most significantly affected by (in order of decreasing importance): particle size distribution, injection velocity distribution, turbulence, and injection direction distribution or particle density distribution. Only the distribution of injection directions and turbulence affect dispersion normal to the injection direction, and are of similar magnitude in this study. With regards to particle velocity and temperature, particle size is clearly the dominant effect.

A three-dimensional model of free-surface flows with heat transfer, including solidification, was used to model the build-up of a coating layer in a thermal spray process. The impact of several nickel particles on a stainless steel plate in different scenarios was considered. Particles diameter ranged from 40 to 80 µm and their impact velocity ranged from 40 to 80 m/s. Particles were initially super-heated; their temperature ranged from 1600 to 2000°C. Fast growth of solidification was found to be one cause of particle splashing in thermal spray coatings. Different splat morphologies obtained from the numerical model were comparable with those obtained from the experiments. Simulation of the sequential impact of two nickel particles showed side-flow jetting and particle splashing observed in experiments. The numerical model proved to be capable of simulating different impact scenarios that occur in a thermal spray; this was demonstrated by simulating nine consecutive particles during their impact on the substrate. Several characteristics of a coating layer build-up such as particle splashing and formation of small satellite droplets and rings around the splat could be seen in the numerical results. Particle splashing is one possible cause of porosity formation in thermal spray coatings.

The disadvantage of plasma torches using conventional single cathode techniques is the occurrence of azimuthal and axial instabilities inside the plasma torch. This causes electrical power fluctuations which result in inhomogeneities of the plasma jet enthalpy and with that an uneven plasma particle interaction. Hence, variations in particle properties occur and consequently an uneven coating quality is produced. Using the triple-cathode technique these electrical power fluctuations were successfully reduced, resulting in a stationary plasma flow. Thus this technique appears to offer the potential to homogenize coating properties. Similar results have been shown for plasma torches with triple anode arrangements. The goal of this research group is to homogenize properties of plasma sprayed coatings using of 3-cathode and 3-anode technologies based on numerical simulations. The approach used is to subdivide the complete APS process into the areas plasma torch, free jet as well as coating formation and characteristics. By simulation of the individual areas and combination with experimental results the corresponding process parameters will be obtained for the desired coating properties.

The present study is devoted to the modelling of the arc formation in a direct current plasma gun newly commercialized by Saint-Gobain Coating Solutions (Avignon, France). The CFD computations were performed using the FLUENT code and the electromagnetic coupling was taken into account on the basis of a three dimensional model. The main advances compared to previous works performed on the same subject are numerous. First of all, whereas most of earlier models include the arc region only, the CFD domain was here extended to the gas injection region (i.e. upstream part of the gun, including the gas injection sleeve), thus allowing a better description of the effect of the gas injection on the plasma flow. Second, whereas earlier works include the fluid domain only, the present model includes a fluid/solid coupling in the anode. In particular, the thermal and electromagnetic equations are solved not only in the fluid parts but also in the tungsten and copper parts of the anode. This change was found to be important because the internal surface of the anode is no more a boundary of the domain. Thus, its temperature (and electric potential) becomes variable and is thus not necessarily imposed. Finally, the implemented model provides interesting results describing the arc behaviour inside the plasma gun.