Mathematical modelling of Inconel 718 particles in HVOF thermal spraying

High velocity oxygen fuel (HVOF) thermal spray technology is able to produce very dense coating without over-heating powder particles. The quality of coating is directly related to the particle parameters such as velocity, temperature and state of melting or solidification. In order to obtain this particle data, mathematical models are developed to predict particle dynamic behaviour in a liquid fuelled high velocity oxy-fuel thermal spray gun. The particle transport equations are solved in a Lagrangian manner and coupled with the three-dimensional, chemically reacting, turbulent gas flow. The melting and solidification within particles as a result of heat exchange with the surrounding gas flow is solved numerically. The in-flight particle characteristics of Inconel 718 are studied and the effects of injection parameters on particle behavior are examined. The computational results show that the particles smaller than 10 μm undergo melting and solidification prior to impact while the particle larger than 20 μm never reach liquid state during the process.     

Analysis and optimization of the HVOF process by combined experimental and numerical approaches

Thermal spraying with the HVOF technology is a well known approach to dense metallic, ceramic and cermets coatings with good mechanical properties. Any attempt for improving HVOF coating properties requires a fundamental understanding of the mechanisms that occur during HVOF spraying. Thermal spray processes are not only optimized by empirical testing and by correlation analysis between process parameters and coating properties but also with numerical approaches. Recent attempts to understand the momentum and heat transfer mechanisms between flame and particles, and thus improve the control of the thermokinetic deposition process by analysis of fundamental thermophysical and fluid mechanical processes, have led to computational modeling of the spraying process and verification of simulation results by in-flight particle analysis.This paper focuses on modeling (tracking) of the particle properties during HVOF spraying using alumina powder. The particle properties are sensitive to a large number of process parameters (e.g., gas temperature, gas expansion velocity, pressure, spraying distance, spray powder particle diameter, nozzle geometry, etc.). Variation of the operating parameters of the HVOF process (gas flow rates, stoichiometric oxy/fuel ratio, nozzle design, etc.) is performed during modeling and simulation. The SprayWatch® system for particle in-flight measurement is used for verification of the numerical analysis result.     

Numerical modeling of in-flight characteristics of inconel 625 particles during high-velocity oxy-fuel thermal spraying

A computational fluid dynamics (CFD) model is developed to predict particle dynamic behavior in a high-velocity oxyfuel (HVOF) thermal spray gun in which premixed oxygen and propylene are burnt in a combustion chamber linked to a long, parallel-sided nozzle. The particle transport equations are solved in a Lagrangian manner and coupled with the two-dimensional, axisymmetric, steady state, chemically reacting, turbulent gas flow. Within the particle transport model, the total flow of the particle phase is modeled by tracking a small number of particles through the continuum gas flow, and each of these individual particles is tracked independently through the continuous phase. Three different combustion chamber designs were modeled, and the in-flight particle characteristics of Inconel were 625 studied. Results are presented to show the effect of process parameters, such as particle injection speed and location, total gas flow rate, fuel-to-oxygen gas ratio, and particle size on the particle dynamic behavior for a parallel-sided, 12 mm long combustion chamber. The results indicate that the momentum and heat transfer to particles are primarily influenced by total gas flow. The 12 mm long chamber can achieve an optimum performance for Inconel 625 powder particles ranging in diameter from 20 to 40 μm. At a particular spraying distance, an optimal size of particles is observed with respect to particle temperature. The effect of different combustion chamber dimensions on particle dynamics was also investigated. The results obtained for both a 22 mm long chamber and also one with a conical, converging design are compared with the baseline data for the 12 mm chamber.     

Prediction of powder particle behavior during high-velocity oxyfuel spraying

A mathematical model is developed to predict particle velocity and temperature during high-velocity oxyfuel (HVOF) spraying. This model accounts for internal heat conduction in powder particles; particle heating, fusion, cooling, and solidification; the influence of particle morphology on thermal behavior; and the composite structure of the particles. Analytical results are obtained that describe particle velocity and temperature variations. The dependence of fluid velocity on particle density and volume fraction is shown. The results agree with empirically established HVOF spraying practice. Physical Metal-lurgy—Materials Science     

Study of In-Flight and Impact Dynamics of Nonspherical Particles from HVOF Guns

High velocity oxygen fuel thermal spray has been widely used to deposit hard composite materials such as WC-Co powders for wear-resistant applications. Unlike gas atomized spherical powders, WC-CO powders form a more complex geometry. The knowledge gained from the existing spherical powders on process control and optimization may not be directly applicable to WC-Co coatings. This paper is the first to directly examine nonspherical particle in-flight dynamics and the impingement process on substrate using computational methods. Two sets of computational models are developed. First, the in-flight particles are simulated in the CFD-based combusting gas flow. The particle information prior to impact is extracted from the CFD results and implemented in a FEA model to dynamically track the impingement of particles on substrate. The morphology of particles is examined extensively including both spherical and nonspherical powders to establish the critical particle impact parameters needed for adequate bonding.     

Spraying of Metallic Powders by Hybrid Gas/Water Torch and the Effects of Inert Gas Shrouding

A hybrid DC arc plasma torch, combining water and gas stabilization, offers a high flexibility in plasma characteristics. These can be controlled in a wide range by the torch operational parameters, such as arc current and secondary gas flow rate. In this study, their influence on plasma spraying of tungsten and copper was investigated. To suppress the in-flight oxidation of the metals, inert gas shrouding was applied. In-flight particle diagnostics and analysis of free-flight particles and coatings was performed for spraying experiments in the open atmosphere and with argon shrouding. Both in-flight particle behavior and coating properties were found to be sensitive to the torch parameters. The application of shrouding was found to affect particle in-flight parameters, reduce the oxide content in the coatings and generally improve their properties, such as thermal conductivity. However, a different degree of these effects was observed for copper and tungsten.     

Adapting of the Background-Oriented Schlieren (BOS) Technique in the Characterization of the Flow Regimes in Thermal Spraying Processes

In thermal spraying technique, the changes in the in-flight particle velocities are considered to be only a function of the drag forces caused by the dominating flow regimes in the spray jet. Therefore, the correct understanding of the aerodynamic phenomena occurred at nozzle out let and at the substrate interface is an important task in the targeted improvement in the nozzle and air-cap design as well as in the spraying process in total. The presented work deals with the adapting of an innovative technique for the flow characterization called background-oriented Schlieren. The flow regimes in twin wire arc spraying (TWAS) and high velocity oxygen fuel (HVOF) were analyzed with this technique. The interfering of the atomization gas flow with the intersected wires causes in case of TWAS process a deformation of the jet shape. It leads also to areas with different aero dynamic forces. The configurations of the outlet air-caps in TWAS effect predominantly the outlet flow characteristics. The ratio between fuel and oxygen determine the dominating flow regimes in the HVOF spraying jet. Enhanced understanding of the aerodynamics at outlet and at the substrate interface could lead to a targeted improvement in thermal spraying processes.     

Influence of Processing Parameters on Residual Stress of High Velocity Oxy-Fuel Thermally Sprayed WC-Co-Cr Coating

Residual stress in high velocity oxy-fuel (HVOF) thermally sprayed WC-10Co-4Cr coating was studied based on design of experiment (DOE) with five factors of oxygen flow, fuel gas hydrogen flow, powder feed rate, stand-off distance, and surface speed of substrate. In each DOE run, the velocity and temperature of in-flight particle in flame, and substrate temperature were measured. Almen-type N strips were coated, and their deflections after coating were used for evaluation of residual stress level in the coating. The residual stress in the coating obtained in all DOE runs is compressive. In the present case of HVOF thermally sprayed coating, the residual stress is determined by three types of stress: peening, quenching, and cooling stress generated during spraying or post spraying. The contribution of each type stress to the final compressive residual stress in the coating depends on material properties of coating and substrate, velocity and temperature of in-flight particle, and substrate temperature. It is found that stand-off distance is the most important factor to affect the final residual stress in the coating, following by two-factor interaction of oxygen flow and hydrogen flow. At low level of stand-off distance, higher velocity of in-flight particle in flame and higher substrate temperature post spraying generate more peening stress and cooling stress, resulting in higher compressive residual stress in the coating.     

Modeling and Control of High-Velocity Oxygen-Fuel (HVOF) Thermal Spray: A Tutorial Review

This work provides a tutorial overview of recent research efforts in modeling and control of the high-velocity oxygen-fuel (HVOF) thermal spray process. Initially, the modeling of the HVOF thermal spray, including combustion, gas dynamics, particle in-flight behavior, and coating microstructure evolution is reviewed. The influence of the process operating conditions as predicted by the fundamental models on particle characteristics and coating microstructure is then discussed and compared with experimental observations. Finally, the issues of measurement and automatic control are discussed and comments on potential future research efforts are made.     

超音速火焰喷涂WC-10Co4Cr粒子行为试验研究

采用DPV-eVOLUTION型热喷涂监控装置对超音速火焰喷涂WC-10Co4Cr过程中的粉末粒子特性进行研究,重点分析喷涂工艺参数对火焰中粒子的温度、速率的影响规律。结果表明:粒子速率、粒子温度和粒子直径并不沿火焰中心线对称分布;在喷涂过程中,直径较小的颗粒可以获得更高的速率;当喷涂距离增加时,粒子的速率和温度均存在极大值;气体流量的增加导致粒子温度和速率同时提高。     

On the Response of Different Particle State Sensors to Deliberate Process Variations

Deliberate particle state variations were performed using atmospheric plasma spray (APS) and high-velocity oxy-fuel flame spraying (HVOF) to create a set of first-order process maps. Particle states were measured simultaneously using five in-flight particle sensors: DPV-2000, Accuraspray, SprayWatch, TDS, and SprayCam. While the sensors use similar methods for calculating particle characteristics, absolute values of temperature and velocity were considerably different. Process map trends among sensors are in agreement for the HVOF process, but differ when using plasma spray at high total gas flow conditions. After understanding the stochastic nature of particle detection, an open loop feedback control algorithm was implemented to achieve similar particle states with different hydrogen gas flow rates. The resulting particle state window measured by three different sensors under select fixed hydrogen flow rates was significantly narrowed.     

Plasma Spraying of Copper by Hybrid Water-Gas DC Arc Plasma Torch

Water-stabilized DC arc plasma torches offer a good alternative to common plasma sources used for plasma spraying applications. Unique properties of the generated plasma are determined by a specific plasma torch construction. This article is focused on a study of the plasma spraying process performed by a hybrid torch WSP500^®-H, which combines two principles of arc stabilization—water vortex and gas flow. Spraying tests with copper powder have been carried out in a wide range of plasma torch parameters. First, analyses of particle in-flight behavior for various spraying conditions were done. After, particles were collected in liquid nitrogen, which enabled analyses of the particle in-flight oxidation. A series of spraying tests were carried out and coatings were analyzed for their microstructure, porosity, oxide content, mechanical, and thermal properties.     

Particle In-Flight and Coating Properties of Fe-Based Feedstock Materials Sprayed with Modern Thermal Spray Systems

New developments in the field of thermal spraying systems (increased particle velocities, enhanced process stability) are leading to improved coatings. Innovations in the field of feedstock materials are supporting this trend. The combination of both has led to a renaissance of Fe-based feedstocks. Using modern APS or HVOF systems, it is now possible to compete with classical materials for wear and corrosion applications like Ni-basis or metal-matrix composites. This study intends to give an analysis of the in-flight particle and spray jet properties achievable with two different modern thermal spraying systems using Fe-based powders. The velocity fields are measured with the Laser Doppler Anemometry. Resulting coatings are analyzed and a correlation with the particle in-flight properties is given. The experiments are accompanied by computational fluid dynamics simulations of spray jet and particle velocities, leading to a comprehensive analysis of the achievable particle properties with state-of-the-art HVOF and APS systems.     

In-flight characteristics of plasma sprayed alumina particles: Measurements,modeling, and comparison

The key phenomena controlling the properties of sprayed coatings are the heat and momentum transfer between the plasma jet and the injected particles. Modern on-line particle monitoring systems provide an efficient tool to measure in-flight particle characteristics in such a way that factors that could affect the coating quality can be identified during the spray process. In this work, the optical sensing device, DPV-2000 from Tecnar, was used for monitoring the velocity, temperature, and diameter of in-flight particles during the spraying of alumina with a Sulzer-Metco F4 plasma torch. Evolution of particle velocity, temperature, diameter, and trajectory showed well-marked trends. Relationships between the position of the in-flight particles into the jet and their characteristics were pointed out, thus delivering valuable information about their thermal treatment. Moreover, a numerical model was developed and predictions were compared with experimental results. A good agreement on particle characteristics was found between the two different approaches.     

气体放电热源喷涂技术的数值模拟研究现状∗

热喷涂技术是表面工程领域中极为重要的一种装备强化修复技术，其中以气体放电形式为热源的喷涂技术包括等离子喷涂和电弧喷涂，两者更是占据热喷涂领域的绝大市场份额，采用数值模拟可以解决一些在试验上较为棘手的重点研究问题， 如等离子体流场和熔滴传热传质行为等，以期实现工艺参数的准确调控和优异涂层的制备。研究电弧及等离子喷涂模拟的模型差异化问题及流场速度、温度、电磁性质，归纳相关模拟的发展历程，并调查试验与模拟的吻合程度。结果表明：电弧喷涂中丝材原料会使阴阳极产生温度差，水平速度分布较发散，熔滴模型也多未考虑熔滴群间相互作用；等离子喷涂研究中常用的三维瞬态双温模型已十分贴近实际工况，对熔滴飞行中的加热、加速过程及破碎行为的研究已较为完备，但仍存在湍流模型计算精度不够、对鞘层弧柱区的研究不够深入等问题。后续应重点在电弧喷涂多液滴模型、等离子体电磁作用和等离子丝材喷涂工艺的数值模拟等方面进行深入研究。     

Numerical Study on Plasma Jet and Particle Behavior in Multi-arc Plasma Spraying

Plasma jet and particle behavior in conventional single-arc plasma spraying has been subject to intensive numerical research. However, multi-arc plasma spraying is a different case which has yet to be investigated more closely. Numerical models developed to investigate the characteristics of multi-arc plasma spraying (plasma generator, plasma jet, and plasma–particle interaction models) were introduced in previous publications by the authors. The plasma generator and plasma jet models were already validated by comparing calculated plasma temperatures with results of emission spectroscopic computed tomography. In this study, the above-mentioned models were subjected to further validation effort. Calculated particle in-flight characteristics were compared with those determined by means of particle diagnostics and high-speed videography. The results show very good agreement. The main aim of the current publication is to derive conclusions regarding the general characteristics of plasma jet and particle in-flight behavior in multi-arc plasma spraying. For this purpose, a numerical parameter study is conducted in which the validated models are used to allow variations in the process parameters. Results regarding plasma jet/particle in-flight temperatures and velocities are presented. Furthermore, the general characteristics of plasma jet and particle behavior in multi-arc plasma spraying are discussed and explained. This contributes to better understanding of the multi-arc plasma spraying process, in particular regarding the injection behavior of particles into hot regions of the plasma jet. Finally, an example test case showing a possible practical application area of the models is introduced.     

Numerical Study to Examine the Effect of Porosity on In-Flight Particle Dynamics

High velocity oxygen fuel (HVOF) thermal spray has been widely used to deposit hard composite materials such as WC-Co powders for wear-resistant applications. Powder morphology varies according to production methods while new powder manufacturing techniques produce porous powders containing air voids which are not interconnected. The porous microstructure within the powder will influence in-flight thermal and aerodynamic behavior of particles which is expected to be different from fully solid powder. This article is devoted to study the heat and momentum transfer in a HVOF sprayed WC-Co particles with different sizes and porosity levels. The results highlight the importance of thermal gradients inside the particles as a result of microporosity and how HVOF operating parameters need to be modified considering such temperature gradient.     

Computational simulation of liquid-fuelled HVOF thermal spraying

Liquid-fuelled high-velocity oxygen–fuel (HVOF) thermal spraying systems are gaining more attentions due to their advantage of producing denser coatings in comparison to their gas-fuelled counterparts. The flow through a HVOF gun is characterized by a complex array of thermodynamic phenomena involving combustion, turbulence and compressible flow. Advanced computational models have been developed to gain insight to the thermochemical processes of thermal spraying, however little work has been reported for the liquid-fuelled systems. This investigation employs a commercial finite volume CFD code to simulate the flow field through the most widely used liquid-fuel HVOF gun, JP5000 (Praxair, US). By combining numerical combustion and discrete phase models the turbulent spray flame is captured and the development of supersonic gas flow is revealed. The flow field is thoroughly examined by adjusting the nozzle throat diameter and combustion chamber size. The influence of fuel droplet size on the flame shame shape and combusting gas flow is also examined.     

等离子喷涂ZrO2热障涂层工艺参数优化设计

为了深入研究等离子喷涂ZrO2粒子的飞行特征与涂层性能之间的关系，采用三水平四因素正交试验法对主气、辅气、电流及喷涂距离等4个主要参数进行了优化设计，并采用DPV2000热喷涂在线监测仪测定了ZrO2粒子的飞行特征参数，通过IA32定量金相分析软件对涂层的孔隙率进行了测试。结果表明，影响ZrO2粒子温度的主要因素为主气和辅气，影响ZrO2粒子飞行速度的主要因素为喷涂距离和辅气。     

Mechanical and Thermal Transport Properties of Suspension Thermal-Sprayed Alumina-Zirconia Composite Coatings

Micro-laminates and nanocomposites of Al₂O₃ and ZrO₂ can potentially exhibit higher hardness and fracture toughness and lower thermal conductivity than alumina or zirconia alone. The potential of these improvements for abrasion protection and thermal barrier coatings is generating considerable interest in developing techniques for producing these functional coatings with optimized microstructures. Al₂O₃-ZrO₂ composite coatings were deposited by suspension thermal spraying (APS and HVOF) of submicron feedstock powders. The liquid carrier employed in this approach allows for controlled injection of much finer particles than in conventional thermal spraying, leading to unique and novel fine-scaled microstructures. The suspensions were injected internally using a Mettech Axial III plasma torch and a Sulzer-Metco DJ-2700 HVOF gun. The different spray processes induced a variety of structures ranging from finely segregated ceramic laminates to highly alloyed amorphous composites. Mechanisms leading to these structures are related to the feedstock size and in-flight particle states upon their impact. Mechanical and thermal transport properties of the coatings were compared. Compositionally segregated crystalline coatings, obtained by plasma spraying, showed the highest hardness of up to 1125 VHN_{3 N}, as well as the highest abrasion wear resistance (following ASTM G65). The HVOF coating exhibited the highest erosion wear resistance (following ASTM G75), which was related to the toughening effect of small dispersed zirconia particles in the alumina-zirconia-alloyed matrix. This microstructure also exhibited the lowest thermal diffusivity, which is explained by the amorphous phase content and limited particle bonding, generating local thermal resistances within the structure.