Effect of the increase in the entrance convergent section length of the gun nozzle on the high-velocity oxygen fuel and cold spray process

Nozzle geometry, which influences combustion gas dynamics and, therefore, sprayed particle behavior, is one of the most important parameters in the high-velocity oxygen-fuel (HVOF) thermal spray process. The nozzle geometry is also important in the cold spray method. The gas flows in the entrance convergent section of the nozzle exhibit a relatively higher temperature and are subsonic; thus, this region is most suitable for heating spray particles. In this study, numerical simulation and experiments investigated the effect of the entrance geometry of the gun nozzle on the HVOF process. The process changes inside the nozzle, as obtained by numerical simulation studies, were related to the coating properties. An Al₂O₃-40 mass% TiO₂ powder was used for the experimental studies. The change in entrance convergent section length (rather than barrel part length or total length) of the gun nozzle had a significant effect on the deposition efficiency, microstructure, and hardness. The deposition efficiency and hardness increased as this geometry increased. On the other hand, the calculated and measured particle velocity showed a slight decrease. This effect on the HVOF process will also be applied to the nozzle design for the cold spray method.     

高速低温喷涂气固作用行为及喷涂系统研究进展

高速低温喷涂是利用固相或含固相的低温粉末在高速度、高动能作用下碰撞基体表面沉积的喷涂方法,具有氧化轻微、 结合牢固、组织致密、综合力学性能优异等潜在优势,在高性能金属或金属基复合材料涂层制备、增材制造和零件损伤修复等领域获得广泛关注。以粉末低温高速碰撞沉积过程为主线,凝练现有冷喷涂和低温超音速火焰喷涂两种具体工艺的共性特征,阐明喷涂气流与粉末颗粒的气固两相交互作用规律,分析出合理调控颗粒温度和速度是改善沉积体性能的关键。其次分析高速低温喷涂设备系统的构成,详细讨论各核心部件的结构设计策略及对气固流动行为的影响,总结出通过调整工艺参数与喷枪结构,可以实现颗粒温度和速度的按需控制。最后,对高速低温喷涂工艺及设备系统发展目前尚存的关键问题进行展望。总结如何通过喷涂参数与装置设计,最终达成调控沉积体性能的目的,有助于深入理解高速低温喷涂的沉积机理,对研制高性能的喷涂设备系统具有参考意义。     

Effect of Nozzle Geometry on the Microstructure and Properties of HVAF-Sprayed WC-10Co4Cr and Cr<Subscript>3</Subscript>C<Subscript>2</Subscript>-25NiCr Coatings

Thermally sprayed hard metal coatings are the industrial standard solution for numerous demanding applications to improve wear resistance. In the aim of improving coating quality by utilising finer particle size distributions, several approaches have been studied to control the spray temperature. The most viable solution is to use the modern high velocity air-fuel (HVAF) spray process, which has already proven to produce high-quality coatings with dense structures. In HVAF spray process, the particle heating and acceleration can be efficiently controlled by changing the nozzle geometry. In this study, fine WC-10Co4Cr and Cr₃C₂-25NiCr powders were sprayed with three nozzle geometries to investigate their effect on the particle temperature, velocity and coating microstructure. The study demonstrates that the particle melting and resulting carbide dissolution can be efficiently controlled by changing the nozzle geometry from cylindrical to convergent–divergent. Moreover, the average particle velocity was increased from 780 to over 900 m/s. The increase in particle velocity significantly improved the coating structure and density. Further evaluation was carried out to resolve the effect of particle in-flight parameters on coating structure and cavitation erosion resistance, which was significantly improved in the case of WC-10Co4Cr coatings with the increasing average particle velocity.     

Optimal design of a novel cold spray gun nozzle at a limited space

Numerical analysis for the accelerating behavior of spray particles in cold spraying is conducted using a computational fluid dynamics program, FLUENT. The optimal design of the spray gun nozzle is achieved based on simulation results to solve the problem of coating for the limited inner wall of a small cylinder or pipe. It is found that the nozzle expansion ratio, particle size, accelerating gas type, operating pressure, and temperature are main factors influencing the accelerating behavior of spray particles in a limited space. The experimental results using the designed short nozzle with a whole gun length of <70 mm confirmed the feasibility of optimal design for a spray gun nozzle used in a limited space.     

Effects of spray conditions on coating formation by the kinetic spray process

The kinetic spray coating process involves impingement of a substrate by particles of various material types at high velocities. In the process, particles are injected into a supersonic gas stream and accelerated to high velocities. A coating forms when the particles become plastically deformed and bond to the substrate and to one another upon collision with the substrate. Coating formation by the kinetic spray process can be affected by a number of process parameters. In the current study, several spray variables were investigated through computational modeling and experiments. The examined variables include the temperature and pressure of the primary gas, the cross-sectional area of the nozzle throat, the nozzle standoff distance from a substrate, and the surface condition of nozzle interior and the powder gas flow. Experimental verification on the effects of these variables was performed primarily using relatively large-size aluminum particles (63–90 μm) as the feedstock material. It was observed that the coating formation is largely controlled by two fundamental variables of the sprayed particles: particle velocity and particle temperature. The effects of different spray conditions on coating formation by the kinetic spray process can be generally interpreted through their influences on particle velocity and/or particle temperature. Though it is limited to accelerate large particles to high velocities using compressed air or nitrogen as carrier gas, increasing particle temperature provides an additional means that can effectively enhance coating formation by the kinetic spray process.     

Particle behavior in supersonic flow during the cold spray process

The cold spray process is a relatively new process that uses high velocity metallic particles for surface modifications. Metallic powder particles are injected into a converging-diverging nozzle and accelerated to supersonic velocities. In this study two-dimensional temperature and velocitiy distributions of gas along the nozzle axis are calculated and the effects of gas pressure and temperature on particle velocities and temperature inside and outside the nozzle are investigated. It was found that acceleration of the gas velocity takes place in the area of the nozzle throat, and it increases and reaches a maximum value at the nozzle exit. Due to compression shocks, irregular changes of the gas jet properties were found in the area after the nozzle and these resulted in the experience of the maximum particle velocity by the change of the particle size at a given gas pressure and temperature.     

Preliminary study on the TriplexPro™-200 gun for atmospheric plasma spraying of yttria-stabilized zirconia

The recently introduced three-cathode TriplexPro™-200 atmospheric plasma spray gun was tested with yttria-stabilized zirconia. The effects on the particle characteristics (velocity and surface temperature) were measured by time-of-flight measurement and two-color pyrometry, respectively (DPV-2000 diagnostic system) while the arc current intensity, the plasma gas flow, and its composition were systematically varied.With typical spray parameters of the Triplex® II, using the TriplexPro™-200 with the 9.0 mm nozzle the particle characteristics were found to be almost the same while the nozzle still provides power reserves. In this case, helium may be dispensed with entirely.Using the 6.5 mm or the 5.0 mm nozzle operation is limited to a maximum 500 A current, which significantly increased particle velocities, but it was not possible to keep the particles fully molten. The investigated range of powder grain size (d₅₀ = 58 μm) is to be considered with these parameters. Further possibilities are to be expected with the 6.5 mm and 5.0 mm nozzle using smaller-sized or lower-melting powders.Operating the TriplexPro™-200 gun with the 11.0 mm nozzle and/or nitrogen as secondary plasma gas instead of helium, increased particle temperatures and velocities are achievable. This offers extended possibilities to spray high-melting oxide ceramics like yttria-partially stabilized zirconia.     

Theoretical and Experimental Particle Velocity in Cold Spray

In an effort to corroborate theoretical and experimental techniques used for cold spray particle velocity analysis, two theoretical and one experimental methods were used to analyze the operation of a nozzle accelerating aluminum particles in nitrogen gas. Two-dimensional (2D) axi-symmetric computations of the flow through the nozzle were performed using the Reynolds averaged Navier-Stokes code in a computational fluid dynamics platform. 1D, isentropic, gas-dynamic equations were solved for the same nozzle geometry and initial conditions. Finally, the velocities of particles exiting a nozzle of the same geometry and operated at the same initial conditions were measured by a dual-slit velocimeter. Exit plume particle velocities as determined by the three methods compared reasonably well, and differences could be attributed to frictional and particle distribution effects.     

Particle Acceleration in a High Enthalpy Nozzle Flow with a Modified Detonation Gun

The quality of thermal sprayed coatings depends on many factors which have been investigated and are still in scientific focus. Mostly, the coating material is inserted into the spray device as solid powder. The particle condition during the spray process has a strong effect on coating quality. In some cases, higher particle impact energy leads to improved coating quality. Therefore, a computer-controlled detonation gun based spraying device has been designed and tested to obtain particle velocities over 1200 m/s. The device is able to be operated in two modes based on different flow-physical principles. In one mode, the device functions like a conventional detonation gun in which the powder is accelerated in a blast wave. In the other mode, an extension with a nozzle transforms the detonation gun process into an intermittent shock tunnel process in which the particles are accelerated in a high enthalpy nozzle flow with high reservoir conditions. Presented are experimental results of the operation with nozzle in which the device generates very high particle velocities up to a frequency of 5 Hz. A variable particle injection system allows injection of the powder at any point along the nozzle axis to control particle temperature and velocity. A hydrogen/oxygen mixture is used in the experiments. Operation performance and nozzle outflow are characterized by time resolved pressure measurements. The particle conditions inside the nozzle and in the nozzle exit plane are calculated with a quasi-one-dimensional WENO-code of high order. For the experiments, particle velocity is obtained by particle image velocimetry, and particle concentration is qualitatively determined by a laser extinction method. The powders used are WC-Co(88/12), NiCr(80/20), Al₂O₃, and Cu. Different substrate/powder combinations for varying particle injection positions have been investigated by light microscopy and measurements of microhardness.     

Effect of Nozzle Material on Downstream Lateral Injection Cold Spray Performance

In cold gas dynamic spraying, the gas nature, process stagnation pressure and temperature, and the standoff distance are known to be important parameters that affect the deposition efficiency and coating quality. This investigation attempts to elucidate the effect of nozzle material on coatings produced using a downstream lateral injection cold spray system. Through experimentation, it is shown that the nozzle material has a substantial effect on deposition efficiency and particle velocity. It is proposed that the effects are related to complex interaction between the particles and the internal nozzle walls. The results obtained lead to the conclusion that during the particle/nozzle wall contact, a nozzle with higher thermal diffusivity transfers more heat to the particles. This heat transfer results in lower critical velocities and therefore higher deposition efficiencies, despite a noticeable reduction of particle velocities which is also attributed to particle-nozzle interactions.     

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.     

Crystallization Evolution of Cold-Sprayed Pure Ni Coatings

Cold spraying is a coating technology on the basis of aerodynamics and high-speed impact dynamics. Spray particles (usually 1-50 μm in diameter) are accelerated to high velocity (typically 300-1200 m/s) by a high-speed gas (preheated air, nitrogen, or helium) flow that is generated through a convergent-divergent de Laval type nozzle. The coating forms through the intensive plastic deformation of particles impacting on the substrate at temperatures well below the melting point of the spray material. In the present paper, the main processing parameters affecting the crystallization behavior of pure Ni cold spray deposits on IN718 alloy are described. Various experimental conditions have been analyzed: gas temperature and pressure, nozzle to substrate distance. In particular, the study deals with those conditions leading to a strong grain refinement, with an acceptable level of the deposits mechanical properties. In precise spray conditions, a shift toward amorphous phases has been observed and studied. A systematic analysis of microstructural evolution, performed through TEM observations, as a function of processing parameters is presented.     

Investigation on in-flight particle velocity in supersonic plasma spraying

0Introduction Asakindofsurfaceengineeringtechnology,thermal sprayingcanprovideprotectiveorfunctionalcoatings whicharewidelyusedinmanyindustrieslikechemistryin dustry,papermaking,electricengineering,powerplant, aviation,automobileproducing,steelmill,glass…     

Utilization of Titanium Particle Impact Location to Validate a 3D Multicomponent Model for Cold Spray Additive Manufacturing

Cold spray is a solid-state rapid deposition technology in which metal powder is accelerated to supersonic speeds within a de Laval nozzle and then impacts onto the surface of a substrate. It is possible for cold spray to build thick structures, thus providing an opportunity for melt-less additive manufacturing. Image analysis of particle impact location and focused ion beam dissection of individual particles were utilized to validate a 3D multicomponent model of cold spray. Impact locations obtained using the 3D model were found to be in close agreement with the empirical data. Moreover, the 3D model revealed the particles’ velocity and temperature just before impact—parameters which are paramount for developing a full understanding of the deposition process. Further, it was found that the temperature and velocity variations in large-size particles before impact were far less than for the small-size particles. Therefore, an optimal particle temperature and velocity were identified, which gave the highest deformation after impact. The trajectory of the particles from the injection point to the moment of deposition in relation to propellant gas is visualized. This detailed information is expected to assist with the optimization of the deposition process, contributing to improved mechanical properties for additively manufactured cold spray titanium parts.     

Effect of Ceramic Particle Velocity on Cold Spray Deposition of Metal-Ceramic Coatings

In this paper, metal-ceramic coatings are cold sprayed taking into account the spray parameters of both metal and ceramic particles. The effect of the ceramic particle velocity on the process of metal-ceramic coating formation and the coating properties is analyzed. Copper and aluminum powders are used as metal components. Two fractions of aluminum oxide and silicon carbide are sprayed in the tests. The ceramic particle velocity is varied by the particle injection into different zones of the gas flow: the subsonic and supersonic parts of the nozzle and the free jet after the nozzle exit. The experiments demonstrated the importance of the ceramic particle velocity for the stability of the process: Ceramic particles accelerated to a high enough velocity penetrate into the coating, while low-velocity ceramic particles rebound from its surface.     

Atmospheric plasma spraying of yttria-stabilized zirconia coatings with specific porosity

The interdependence between plasma spray process parameters and porosity of YSZ coating microstructures was investigated with simultaneous consideration of the deposition efficiency. Based on a factorial experimental plan, the argon plasma gas flow, the current, the interaction of argon flow and current, and the spray distance for the Triplex II plasma gun were found to yield the main contributions to porosity as well as to deposition efficiency.Each of these three process parameters has a significant individual effect on the in-flight particle velocities and temperatures. The contribution to the effects on porosity arises almost exclusively from the particle temperature. Regarding the deposition efficiency, the larger contribution originates from the particle velocity.To achieve a targeted high porosity at reasonable deposition efficiency a simple linear regression model was applied yielding an argon flow of 50 slpm and a current of 470 A at a spray distance of 200 mm as the optimum parameter set. The average particle temperature estimated for this optimum is just above the melting temperature. At this setting, a porosity of 17.7% and a deposition efficiency of 32.5% may be expected.At a greater spray distance and lower power density (lower current and/or higher argon plasma gas flow) the deposition efficiency was observed to drop considerably. The cooling of the particles here becomes critical, i.e. the particles are only partly molten. This was verified by an analysis of the density distributions of measured in-flight particle temperatures.     

反极性等离子切割的流场及温度场数值模拟

通过建立反极性等离子切割的数值模型,模拟了不同参数下的等离子体热力学和动力学特征。结果表明,等离子体在枪体内被加热加速并在压缩孔道内达到峰值,而在扩散区域和工件切割腔内等离子体的温度和速度基本保持恒定;等离子枪体的几何尺寸(喷嘴直径和压缩孔道长度)和工艺参数(电流、离子气流量和喷嘴高度)对等离子体的温度和速度具有重要的影响。     

喷嘴形状对Al2O3-3TiO2粒子扁平化及其涂层性能的影响*

采用SprayWatch在线监测系统测量了F6大气等离子喷枪在不同喷嘴条件下产生的等离子射流中Al2O3-3TiO2粒子的温度和速度。利用201不锈钢和Q235钢作为基体,分别用来收集粒子和制备涂层。分析了不同喷嘴对飞行粒子温度和速度的影响,并通过扫描电镜(SEM)对扁平粒子的铺展程度和涂层显微组织进行了分析,并对比了涂层的结合强度、显微硬度和磨损失重量的差异。结果表明:在相同的测量位置,圆柱形喷嘴喷出粒子的速度比Laval喷嘴条件下的高出一倍,但是温度比Laval喷嘴条件下略低。圆柱形喷嘴获得的扁平粒子比Laval喷嘴获得的扁平粒子铺展程度要大;圆柱形喷嘴获得的涂层的孔隙率及磨损失重量比Laval喷嘴制备的小,其涂层的结合强度、显微硬度均高于Laval喷嘴制备的涂层。