等离子多层熔积混相瞬态流场与温度场的模拟

建立了一种二维等离子熔积成形瞬态模型,该模型描述了熔积成形过程中多层熔积层的自由表面发展,并模拟了熔池内流体流动和传热.通过水平集(Level-Set)方法处理熔积轨迹、液/气界面等因素,考虑了熔体流动的主要驱动力-表面张力梯度、表面曲率以及浮力等.并采用SIMPLEC算法求解控制方程.以K163合金为例,对多层熔积成形过程混相瞬态场进行了模拟,所得到的计算结果与试验结果基本一致;并分析了熔积工艺参数(熔积电流、熔积扫描速度和送粉速度)对熔积层表面形貌及熔积质量的作用规律.     

Numerical Study of In-flight Particle Parameters in Low-Pressure Cold Spray Process

A 2-D model of the low-pressure cold spray with a radial powder feeding was established using CFD software in this study. The flow field was simulated for both propellant gases of nitrogen and helium. To predict the in-flight particle velocity and temperature, discrete phase model was introduced to simulate the interaction of particle and the supersonic gas jet. The experimental velocity of copper powder with different sizes was used to validate the calculated one for low-pressure cold spray process. The results show that the computational model can provide a satisfactory prediction of the supersonic gas flow, which is consistent with the experimental Schlieren photos. It was found that similar velocity was obtained with the drag coefficient formula of Henderson and with that of Morsi and Alexander. As the shape factor was estimated, the reasonable prediction of velocity for non-spherical particle can be obtained, to compare with the experimental results.     

选区熔化工艺的细观力学模拟研究现状

选区熔化工艺具有物理过程复杂、时间空间分辨率极高的特征,借助模拟方法可深入理解该过程的物理机制并为工艺提供参考。区别于宏观尺度的有限元热传导模型,基于粉床的细观力学模型可跟踪描述表面形貌,分析成形缺陷。首先,回顾了近年来在粉床细观力学模拟方面的工作,包括用离散元方法模拟铺粉和计算流体力学方法模拟选区熔化、流动及凝固的过程。在此基础上,详细介绍了构建多物理模型的技术方法,比较多物理场对铺粉/成形的影响。最后,结合离散元与计算流模型实现了选区熔化单道、多道及多层的成形,并分析了工艺参数的影响。     

Columnar-Structured Thermal Barrier Coatings (TBCs) by Thin Film Low-Pressure Plasma Spraying (LPPS-TF)

The very low-pressure plasma Spray (VLPPS) process has been developed with the aim of depositing uniform and thin coatings with coverage of a large area by plasma spraying. At typical pressures of 100-200 Pa, the characteristics of the plasma jet change compared to conventional low-pressure plasma-spraying processes (LPPS) operating at 5-20 kPa. The combination of plasma spraying at low pressures with enhanced electrical input power has led to the development of the LPPS-TF process (TF = thin film). At appropriate parameters, it is possible to evaporate the powder feedstock material providing advanced microstructures of the deposits. This technique offers new possibilities for the manufacturing of thermal barrier coatings (TBCs). Besides the material composition, the microstructure is an important key to reduce thermal conductivity and to increase strain tolerance. In this regard, columnar microstructures deposited from the vapor phase show considerable advantages. Therefore, physical vapor deposition by electron beam evaporation (EB-PVD) is applied to achieve such columnar-structured TBCs. However, the deposition rate is low, and the line-of-sight nature of the process involves specific restrictions. In this article, the deposition of TBCs by the LPPS-TF process is shown. How the evaporation of the feedstock powder could be improved and to what extent the deposition rates could be increased were investigated.     

Plasma–Powder Feedstock Interaction During Plasma Spray–Physical Vapor Deposition

Plasma spray–physical vapor deposition is a new process developed to produce coatings from the vapor phase. To achieve deposition from the vapor phase, the plasma–feedstock interaction inside the plasma torch, i.e., from the powder injection point to the nozzle exit, is critical. In this work, the plasma characteristics and the momentum and heat transfer between the plasma and powder feedstock at different torch input power levels were investigated theoretically to optimize the net plasma torch power, among other important factors such as the plasma gas composition, powder feed rate, and carrier gas. The plasma characteristics were calculated using the CEA2 code, and the plasma–feedstock interaction was studied inside the torch nozzle at low-pressure (20-25 kPa) conditions. A particle dynamics model was introduced to compute the particle velocity, coupled with Xi Chen’s drag model for nonevaporating particles. The results show that the energy transferred to the particles and the coating morphology are greatly influenced by the plasma gas characteristics and the particle dynamics inside the nozzle. The heat transfer between the plasma gas and feedstock material increased with the net torch power up to an optimum at 64 kW, at which a maximum of ~3.4% of the available plasma energy was absorbed by the feedstock powder. Experimental results using agglomerated 7-8 wt.% yttria-stabilized zirconia (YSZ) powder as feedstock material confirmed the theoretical predictions.     

Effect of interface wetting on flattening of freely fallen metal droplet onto flat substrate surface

A free-falling experiment was conducted as a simulation of a thermal spray process. The flattening behavior of the freely fallen metal droplet impinged onto a flat substrate surface was investigated in a fundamental way. The substrates were kept at various temperatures, and the substrates were coated with gold by physical vapor deposition (PVD) and were prepared in order to investigate the effect of wetting at the splat-substrate interface on the flattening behavior of the droplet. A falling atmosphere was created with atmospheric pressure of nitrogen to prevent the oxidation of the melted droplet. Experiments under low-pressure conditions also were conducted. The different types of splat morphology were recognized in experiments conducted under a nitrogen atmosphere with atmospheric pressure. The splat morphology on a substrate at room temperature was of the splash type, whereas that on a substrate at high temperature was of the disk type. The microstructure observed on a cross-section of the splat obtained on the substrate at room temperature was an isotropic coarse grain, whereas that on the substrate at high temperature was a fine columnar grain. The grain size changed transitionally with increasing substrate temperature. The temperature of the transition on the gold-coated substrate was higher than that on the naked substrate. The microstructure of the cross-section of the splat obtained under low pressure was finely columnar even on the substrate at room temperature. The results indicate that the metal droplet wets better under the low-pressure condition than under the atmospheric pressure nitrogen condition and that wetting has a significant role in the flattening of the droplet.     

An Investigation on Powder Injection in the High-Pressure Cold Spray Process

High-pressure cold spray process is a relatively new coating process that uses high-velocity powder particles to form coatings. One of the requirements for this process is to inject the particles to be sprayed into a prenozzle chamber where both the particles and the powder feed gas are entrained into the primary gas stream. In this study, we investigated the effects of powder injection on coating formation through both experimental studies and computational simulations. Several issues related to powder injection will be examined, including the size of powder injector, the differential pressure, powder gas flow, and injector clogging. It is shown that an improved powder injector design not only enables the use of reduced amount of powder carrier gas flow but also maintains steady, clogging-free spraying conditions. Combining with properly selected injection conditions, it can also lead to enhanced coating deposition by kinetic spray process.     

高压水射流的CFD仿真及分析

利用计算流体动力学(CFD)的方法对高压水射流进行了多相流的数值模拟,侧重研究在稳态湍流下的两相流和三相流条件下的水射流的动态特性,比较分析了淹没射流与非淹没射流下磨料水射流与纯水射流的区别,并对可视化的图形图像和计算结果进行了分析研究,结果表明,非淹没条件下磨料水射流是高压水切割工具的有效形式。     

Parametric study on a coaxial multi-material powder flow in laser-based powder deposition process

The manner with which the composite powder particles injected into the laser formed molten pool decides the deposition quality in a typical laser-based powder deposition of composite material. Since, the morphology and physical properties of nickel (Ni) and tungsten carbide (WC) are different their powder flow characteristics such as the powder particles stream structure, maximum concentration at the converging spot, and the powder particles velocity are noticeably different. In the current study, a computational fluid dynamics (CFD) based powder flow model is established to characterize the coaxial powder flow behavior of Ni–WC composite powders. The key powder flow characteristics such as the stand-off distance, the diameter of the powder stream at the stand-off distance, and the velocity of the powder particles are measured using three different vision based techniques. Both the numerical and experimental results reveal the exact stand-off distance where the substrate needs to be placed, the diameter of the concentration spot of powder at the stand-off distance, and a combination of suitable nozzle angle, diameter, and carrier gas flow rate to obtain a maximum powder concentration at the stand-off distance with a stable composite powder flow.     

Multi-Response Optimization of Process Parameters for Low-Pressure Cold Spray Coating Process Using Taguchi and Utility Concept

Most of the existing multi-response optimization approaches focus on the subjective and practical know-how of the process. As a result, some confusion and uncertainty are introduced in the overall decision-making process. In this work, an approach based on a Utility theory and Taguchi quality loss function has been applied to the process parameters for low-pressure cold spray process deposition of copper coatings, for simultaneous optimization of more than one response characteristics. In the present paper, two potential response parameters, i.e., coating thickness and coating density, have been selected. Utility values based on these response parameters have been analyzed for optimization using the Taguchi approach. The selected input parameters of powder feeding arrangement, substrate material, air stagnation pressure, air stagnation temperature, and stand-off distance significantly improve the Utility function (raw data) comprising quality characteristics (coating thickness and coating density). The percentage contribution of the parameters to achieve a higher value of Utility function is substrate material (50.03%), stand-off distance (28.87%), air stagnation pressure (6.41%), powder feeding arrangement (4.68%), and air stagnation temperature (2.64%).     

激光熔覆载气式同轴送粉喷嘴关键参数的探究

以激光熔覆载气式同轴送粉喷嘴为研究对象,建立了三通道激光熔覆喷嘴的三维模型,运用计算流体动力学理论对激光熔覆送粉喷头关键参数进行了数值分析,基于商用计算流体力学(CFD)软件FLUENT对送粉通道不同倾角和送粉口不同直径条件下的粉末流场情况进行了数值模拟。分析与计算结果表明,在其他参数一定的条件下,选样送粉通道倾角角度70°和送粉口直径1.5 mm时,有利于改善激光熔覆效果,为CFD技术在流体机械分析中的应用提供了参考。     

Experimental design in the deposition of BN interface coatings on SiC fibers by chemical vapor deposition

Boron Nitride (BN) coatings deposited by chemical vapor deposition (CVD) have been increasingly used as an interface material for SiC/SiC composites. In this work, the CVD of BN was investigated using a statistical design of experiments (DOE) approach. In order to determine the most significant parameters for the process a two-level screening design (Plackett-Burman) was employed. The deposition pressure, gas mixture dilution factor, deposition time, and the reaction gas flow ratios were found to be the most significant factors that influenced coating thickness. To optimize the deposition process, a three-level surface response design (Box-Behnken) was used with the aim of producing a predictive mathematical model of the process. The generated response surface modeling (RSM) showed that deposition time had the greatest effect on coating thickness while, temperature-time and temperature-NH₃/BCl₃ interactions may be large at low/high NH₃/BCl₃ ratios and high deposition time, respectively. Tensile strength was strongly influenced by the deposition temperature and deposition time. The response model showed the dependence of tensile strength on coating thickness, NH₃/BCl₃ gas flow ratios and time. The model interaction plots suggested a dependence of temperature-gas flow ratio on tensile strengths of BN coated SiC fibers.     

Influence of process conditions in laser-assisted low-pressure cold spraying

A laser-assisted low-pressure cold spraying (LALPCS) is a one-step coating process in which the laser irradiation interacts simultaneously with the spraying spot on the substrate or deposited coating surface in order to improve coating properties. It is expected that the LALPCS could be an effective method to improve a low-pressure cold sprayed coating deposition efficiency and denseness. The purpose of the additional energy from the laser beam is to create denser and more adherent coatings, enhance deposition efficiency and increase the variety of coating materials.In this study copper and nickel powders with additions of alumina powder were laser-assisted low-pressure cold sprayed on carbon steel. Coatings were sprayed using air as process gas. A 6 kW continuous wave high power diode laser and a low-pressure cold spraying unit were used in the experiments. The influence of laser energy on coating microstructure, density and deposition efficiency was studied. The coatings were characterized by optical microscopy and SEM. The coating denseness was tested with open cell potential measurements. Results showed that laser irradiation improved the coating denseness and also enhanced deposition efficiency.     

梯度功能材料等离子熔积成形过程的Level-Set方法模拟

利用Level-Set方法与有限体积法(finite volume method,FVM)建立梯度功能材料(functionally graded material,FGM)等离子熔积制造(plasma deposition manufacturing,PDM)过程多相混态场统一模型;研究了在不同热输入功率下熔池形貌、温度场与流场以及溶质分布的变化规律.结果表明,热功率是影响梯度功能材料成分分布和性能的重要工艺参数.并用等离子熔积制造方法制备Al2O3-AISI316梯度功能材料,验证了模拟计算结果的正确性和可靠性.     

高压气淬均匀性研究

高压气淬具有纯对流传热、易于控制、工件淬火后无需清洗和对环境影响小等优点。然而,典型的气淬设备显示出淬火料盘与工件之间的气流不均匀,导致工件的最终性能产生差异。业已发现,料盘的逆向气流布置是决定局部气流状态和热量传递的关键因素。尽管气流的主要部分在料盘与炉壁之间流动,对淬火过程不起作用,但是由料盘引起的流动阻力所产生的压降决定了淬火冷却强度。采用一种能较快收敛的多尺度模型对工业用高压气淬炉的内部流场进行了模拟,并通过速度测量和气流可视化技术对试验用淬火炉的内部气流进行了试验研究。最后,对圆柱体工件的双室真空炉淬火进行了模拟结果的验证,揭示了不同的逆向流速分布对淬火结果的影响。报道了多尺度模拟方法和流动过程的研究结果,并概述了对气淬工艺优化的指导性建议。     

On ultrasonic assisted abrasive flow finishing of bevel gears

Finishing of bevel gears is an important requirement in many machining shop floors. Variants of abrasive flow machining (AFM) could be plausible solutions for finishing such parts with intricate geometries. In the present work, a relatively new variant of AFM called ultrasonically assisted abrasive flow machining (UAAFM) technique was employed to finish bevel gears made of EN8 steel. An analysis of the process has been presented with suitable illustrations. A finite element simulation of the behavior of the medium during finishing of bevel gears using the UAAFM process has been presented. A 3D model was constructed to simulate the flow of medium through the outer wall of the gear tooth surface using computational fluid dynamics (CFD) approach. The velocity, pressure and temperature values along the length of the workpiece were computed for both UAAFM and the conventional AFM processes. Further, the effectiveness of the process was investigated through experimental trials by conducting a comparison study between classical AFM and UAAFM. Ultrasonic frequency, extrusion pressure, processing time and the media flow rate were considered as the input variables while improvements in surface finish and material removal were considered as the monitored outputs. Results confirm that improvements in surface roughness and material removal are significantly higher than those obtained with conventional abrasive flow machining. The study further reveals that, the applied high frequency (ultrasonic) vibration to the workpiece has the maximum influence on the process responses among the variables considered.     

Numerical simulation on syphonage effect of laval nozzle for low pressure cold spray system

Instead of injected by high pressure powder feeder, powders can be drawn into the nozzle by syphonage effect generated by supersonic gas flow in low pressure cold spray. This characteristic makes low pressure cold spray conveniently for on-site operation. However, no data have ever been reported on the relationship between the nozzle structures and the gas flow in the powder feeder pipe. In this paper, a CFD software (STAR CCM+) was used to calculate the gas flow in nozzle of the DYMET 413 commercial low pressure cold spray system. Variation of structures and process parameters based on the commercial system were also investigated. The syphonage effect is strongly influenced by the powder feeding location, the temperature and pressure in prechamber has little effect on syphonage effect in powder feeder pipe. The syphonaged gas will decelerate the gas velocity and low down the gas temperature in nozzle, so it is best to control the mass flow rate of powder feeding gas by selecting the location. One of the disadvantages is that the particles will collide with the nozzle wall which makes the nozzle a short service life.     

Deposition of metallic coatings on polymer surfaces using cold spray

Current coating technologies such as plasma spray, High Velocity Oxygen Fuel (HVOF) or laser cladding involve the delivery of molten materials during the deposition process. However, such techniques are not well suited to the deposition of metallic coatings on polymers and composites. Cold spray (CS) has attracted much industrial interest over the past two decades. In this method, a material in powder form is accelerated on passage through a converging-diverging nozzle to high speeds via a high pressure coaxial carrier gas jet. The high impact kinetic energy deforms the particles, which creates effective bonding to the substrate.This paper presents the results of an initial study on the potential of the CS process to produce metallic coatings on non-metallic surfaces such as polymers and composites for engineering applications. Experimental and Computational Fluid Dynamics (CFD) results when spraying copper, aluminium and tin powder on a range of substrates such as PC/ABS, polyamide-6, polypropylene, polystyrene and a glass-fibre composite material are presented and analyzed.     

Homogenisation of heat treatment process in high pressure gas quenching

Abstract

High pressure gas quenching has the advantages of pure convective heat transfer, high levels of control, avoidance of cleaning the quenched parts and low environmental impact. However, typical gas quenching facilities exhibit inhomogeneous flow conditions through the quench load and the parts, resulting in scatter in final properties. The upstream flow profile of the load has been identified as a key factor determining local flow conditions and heat transfer. The intensity of the quenching process is determined by the pressure drop that results from the flow resistance of the quench load, although a significant part of the flow passes between the load and the chamber walls and does not contribute to the quenching process. A simulation of the flow inside a commercial high pressure gas quenching chamber was carried out, using a multiscale model to give faster convergence. An experimental analysis of the flow inside a model quenching chamber through velocity measurements and flow visualisation was also performed. Finally, a quenching run with cylindrical parts in a double-chamber vacuum furnace was used to validate the model results. Various upstream velocity profiles were applied to demonstrate their influence on the quenching result. The multiscale simulation approach and the results of the flow process investigation are reported. Guidelines for gas quenching process optimisation are outlined.     

Effect of Zr, Mo and Y Adding on Microstructure, Mechanical and Electrical Properties of Au-Pd, Pt-Ir and Pd-Ru Systems

将金属粉末注射成形充模过程视为粉末、粘结剂和空气的三相流动过程,给出了金属粉末注射成形充模多相流动的控制方程。根据钳口复杂模腔的铁粉注射成形实际参数确定了充模多相流动控制方程的初边界条件,基于CFX计算流体力学软件对钳口铁粉注射成形充模多相流动过程实现了三维数值模拟。给出了钳口铁粉注射成形充模多相流动中固相粉末的速度流线图,分析了模腔中温度场和压力场分布的瞬态情况,以及模腔中不同位置的温度和压力随时间变化的曲线;分析了钳口铁粉注射成形充模多相流动过程中不同时刻粉末体积分数的分布情况。数值模拟结果可用于分析注射成形过程中产生粉末粘结剂两相分离和成形坯密度分布不均的原因,数值模拟为研究注射成形产品缺陷产生的原因提供了直观分析方法。