Gas-particle interaction in detonation spraying systems

A model is developed to describe dynamic interaction of particles with the carrier gas during detonation spraying. Equations of mass, energy, and momentum conservation are integrated numerically for the two-phase particle-gas flow with the Hugoniot boundary conditions at the detonation wave front. Velocity and temperature of the sprayed powder and the gas parameters are calculated self-consistently, taking into account effects of friction and cooling of the gas in the vicinity of the gun barrel and effects of particle-gas interaction on the parameters of the gas phase. Calculations are performed for tungsten carbide particles of 30 μm diam and a 1.8 m long detonation gun using a stoichiometric mixture of oxygen and propane. Distributions of gas and particle parameters along the barrel are calculated for various moments of time. Tungsten carbide particles of 30 μm reach an exit velocity of 1278 m/s and a temperature of 1950 K. Exit particle velocity is a nonmonotonic function of the loading distance,L, with a distinct maximum atL = 75 cm. The proposed model can be applied to a broad range of problems related to detonation coating technology and allows evaluation of the effectiveness of various designs and optimization of operational parameters of detonation spraying systems.     

Analysis of gas flow evolution and shock wave decay in detonation thermal spraying systems

The reactive Euler equations with variable gas properties are solved in both axisymmetric and plane two-dimensional flows to analyze the gas flow evolution, shock wave decay, and shock reflections in pulsed detonation thermal spraying (PDTS) systems. The gas phase governing equations are numerically solved using a high-resolution shock capturing numerical method. Expansion-compression waves are formed upon external gas expansion and persist for a long time (on the time scale of a PDTS cycle) with wide fluctuations in the gas velocity and temperature. The results show that the reflected shock wave from the substrate dies out extremely fast that micron-sized particles used in PDTS do not encounter these transients. The external shock wave decay is also analyzed for different reactive mixtures and flow geometries and is related to the truncation of the computational domain and the implementation of numerical boundary conditions at the open end boundaries.     

Analysis of particle dynamics and heat transfer in detonation thermal spraying systems

A computational study of pulsed detonation thermal spraying is conducted using an axisymmetric two-dimensional transient gaseous detonation model. The variations of the particle velocity and temperature at impact on the target surface with the particle initial loading location are analyzed for different conditions. The geometry of the system and the loading locations of the particulate phase are key parameters in pulsed detonation thermal spraying. Since the process is extremely transient and the gas phase experiences a wide range of transient stages all on a timescale of a millisecond, the particle characteristics are strongly dependent on the instantaneous location in the gas stream. One cycle of detonation thermal spraying occurs on a time scale on the order of a millisecond due to the high gas velocities associated with detonation. Thus, a precise control of the process variable parameters is required to have a successful detonation coating process.     

爆炸喷涂Al-Cu-Cr准晶涂层的组织及硬度研究

为了研究热喷涂工艺对爆炸喷涂Al-Cu-Cr准晶涂层组织和硬度的影响规律，采用三种爆炸喷涂工艺参数在Q235A低碳钢基体上制备涂层，借助XRD、SEM和OM等技术手段对粉末和涂层的组织结构进行分析，并检测涂层横截面的孔隙率和显微硬度。结果表明，用于喷涂的A165Cu20Cr15准晶粉末中含有二十面体准晶相i-A165Cu24Cr11和极少量具有单斜结构的晶体相θ-Al13Cr2（即Al83Cu4Cr13）；而爆炸喷涂涂层中除i和θ两相外，生成了新相——体心立方结构的α-Al69Cu18Cr13（准晶i的晶体类似相）和Al2O3相。涂层呈典型层状结构，其它条件不变的情况下，涂层中各晶体相与准晶相i最强峰衍射强度的比值α／i、θ/i和Al2O3／i随爆炸喷涂工作气体流速的成比例提高而增加，同时涂层截面的孔隙率下降而显微硬度HV0.1升高。     

Gas detonation spray forming of Fe-Al coatings in the presence of interlayer

The present paper reports that gas detonation sprayed (GDS) NiAl and NiCr intermediate layers underneath of the intermetallic Fe-Al type coatings on plain carbon steel substrate form bilayer coating system interacting with external environment and/or metal elements. The interface layers are responsible for hardness, bond strength, thermal stability and adhesive strength of the whole GDS structure. The physical-chemical properties of the intermediate layers, combined with unique, very dense and pore free intermetallic Fe-Al coating obtained from self-decomposing powders resulted in new, beneficial features of the whole GDS structure which became more complex, enabled independent control of its functional properties and considerably reduced negative gradients of stress and temperature influencing the substrate and increasing adhesion strength.The achievement of homogenous and refined structure (comprising of small (< 1 µm) and equiaxed sized grains) creates a thermal barrier based on high-melting point intermetallic phases containing Al₂O₃ ceramics which is responsible for properties of the GDS bilayer coatings. The application properties were investigated and the specific multilayer structure of the GDS coating was analyzed such as the phase composition, the degree of order, grain morphology, the quality of substrate/interlayer/external coating bonds, and first of all the influence of hardness of the NiAl or NiCr intermediate layers on the hardness and thermal stability of the FeAl coating after gas detonation spraying and additional heating at 750 °C and 950 °C for 10 h.     

Control of structural and phase processes in detonation spraying of coatings

Issues of obtaining coatings with optinum structure and properties from nickel-clad powdered aluminium oxide are studied. A production method of metal–ceramic composition is offered, the ceramic component gradually increasing from the lower layer to the upper one, ensuring optimal properties of the coating.     

Tube expansion by gas detonation

The expansion of tubes by direct application of gas detonation waves is an alternative forming method for hollow section workpieces. In particular the process can be used for typical hydroforming parts, for example car body or exhaust elements in automotive industry. The gas charge of oxygen and hydrogen is both pressure medium and energy source and has the potential to cause high forming velocities. The introduced process belongs to the category of high speed forming methods and provides typical advantages such as higher achievable strains compared to quasi-static methods using high water pressure. Another advantage of this process is the avoidance of high press forces by application of an “inertia-locked tool” system due to the extremely short process time. To develop a controllable process, good knowledge of the interdependencies in the system “medium, workpiece and tool” is essential. This can be achieved using simulations in combination with experimental investigations. The results are topic of this paper, also including special investigations on the material behavior at high strain rates and temperature gradients. This paper is based on investigations within the scope of a research project in the Priority Program SPP 1180, which is kindly supported by the German Research Foundation (DFG). The investigations were carried out by cooperation of: University of Dortmund, Institute of Forming Technology and Light-weight Construction; RWTH Aachen University, Shock Wave Laboratory; University of Hanover, Institute of Materials Science.     

低温爆炸喷涂WC涂层的理论与试验研究

为解决爆炸喷涂过程中WC颗粒由于高温导致的脱碳问题,本文设计了一种带有拉瓦尔喷管的爆炸喷涂装置,利用该装置进行了爆炸喷涂WC涂层的理论和试验研究。基于等熵流的数值计算结果表明,喷管可以有效降低载气的温度,并可将WC颗粒加速至1000m/s以上。利用压力传感器测试了喷管入口和出口处的压力,计算了气流马赫数。在喷管出口处采用激光遮挡法测试了粒子的速度,测试结果与理论计算结果具有较好的一致性。采用扫描电镜、金相显微镜和XRD对碳化钨颗粒、涂层进行了表征,结果表明喷管的使用可以有效避免脱碳现象。该研究为解决爆炸喷涂碳化钨过程中的脱碳问题提供了一种新思路。     

WC-12Ni/NiCrBSi爆炸喷涂涂层制备及其滑动磨损特性

利用爆炸喷涂方法制备了不同比例的WC-12Ni/NiCrBSi涂层,研究了NiCrBSi含量对涂层的组织形貌、硬度与结合强度的影响,用扫描电镜和能谱仪分析了涂层的滑动磨损行为.结果表明:由于NiCrBSi的加入,改善了WC颗粒的熔融状态,提高了涂层的结合力,涂层中含15%的NiCrBSi时,可以明显的提高涂层的磨损抗力.     

爆炸喷涂Cr3C2-NiCr涂层的激光重熔工艺

借助半导体激光器对爆炸喷涂Cr3C2-NiCr涂层进行激光重熔处理。通过扫描电镜观察涂层激光重熔前后的形貌,测试涂层激光重熔前后的显微硬度,并对涂层进行盐雾试验和残余应力分析。结果表明,当激光功率为300 W,激光扫描速度为100 mm?min-1时,涂层的激光重熔效果最好。当涂层经过上述激光重熔工艺参数处理后,涂层中富Ni的NiCr合金相得到细化且均匀性提高,涂层的显微硬度、耐磨性和耐腐蚀性进一步提高,涂层中部的残余应力均转变为压应力。     

爆炸喷涂WC-Co/CoMoCrSi复合涂层的组织与性能

为了提高CoMoCrSi涂层的结合强度和力学性能,采用爆炸喷涂技术制备了WC-12Co涂层作为过渡层的WC-Co/CoMoCrSi复合涂层,借助SEM和EDS等手段分析了涂层截面组织形貌和化学元素组成,采用显微硬度计、万能拉伸机及销盘式摩擦磨损试验机等研究了涂层的力学及摩擦磨损性能。结果表明：在氧燃充枪比为60%的喷涂参数下,制备WC-Co/CoMoCrSi复合涂层平均结合强度高达66 MPa,涂层截面组织致密、均匀,孔隙率小于0.6%,平均显微硬度为667 HV0.1,复合涂层摩擦因数0.53~0.56,具有优异的耐磨性能。     

爆炸喷涂CoNiCrAlY-Al2O3涂层的组织及高温耐磨性研究

采用三种爆炸喷涂工艺在SCH19耐热钢基体上制备CoNiCrAlY-Al2O3高温耐磨涂层,借助XRD、SEM和EDS等技术对涂层的组织结构进行分析,并检测涂层的表面粗糙度、显微硬度和高温磨损失重。结果表明,涂层由γ-Ni（Co,Cr）、γ’-Ni3Al和α-Al2O3组成,涂层呈典型的层状结构,适当提高乙炔氧气流量比,可改善涂层的组织结构,并且使其在900℃的高温下具有更优异的耐磨性能。     

HVOF Combustion spraying of inconel powder

A major trend in the thermal spray industry has been to increase the gas jet velocity to obtain better coating attributes. One emerging technology now used in industry is the high-velocity oxygen fuel process (HVOF). High-velocity spray guns combine oxygen and a fuel gas to generate heat and extremely high particle velocities. In this study, Inconel 718 powder was deposited on steel substrates. The primary coating function was electrical resistivity for a heater application. Experiments were conducted using a Taguchi L8 statistical fractional/factorial design parametric study. The Taguchi experiment evaluated the effect of six HVOF processing variables on the measured responses. The parameters were oxygen flow, fuel flow, air envelope gas flow, powder feed rate, spray distance, and nozzle configuration. The coatings were characterized by hardness tests, surface profilometry, optical metallography, and image analysis. This article investigates coating hardness, porosity, surface roughness, deposition efficiency, and microstructure with respect to the influence of the processing parameters. Analytical studies were conducted to investigate gas, particle, and coating dynamics for two of the HVOF thermal spray experiments.     

Simulation of high velocity compaction of powder in a two dimension mould using lattice Boltzmann method

This work presents a numerical study on the dynamic high velocity compaction of the metal powder. The analysis of the process is based on a mesoscopic approach using multi-speed lattice Boltzmann method. The boundary condition and the relaxation time are tailored to the situation. The dynamic compaction process is vividly presented and the shock wave can be easily found in the simulation. The density is analyzed in order to explore the mechanism of the high velocity compaction.     

Development of multimaterial coatings by cold spray and gas detonation spraying

The basic objective is the development of multifunctional multimaterial protective coatings using cold spraying (CS) and computer controlled detonation spraying (CCDS).As far as CS is concerned, the separate injection of each powder into different zones of the carrier gas stream is applied. Cu-Al, Cu-SiC, Al-Al₂O₃, Cu-Al₂O₃, Al-SiC, Al-Ti and Ti-SiC coatings are successfully sprayed. As to CCDS, powders are sprayed with a recently developed apparatus that is characterized by a high-precision gas supply system and a fine-dosed twin powder feeding system. Computer control provides a flexible programmed readjustment of the detonation gases energy impact on powder thus allowing selecting the optimal for each component spraying parameters to form composite and multilayered coatings. Several powders are sprayed to obtain composite coatings, specifically, among others, WC-Co-Cr + Al₂O₃, Cu + Al₂O₃, and Al₂O₃ + ZrO₂.     

High temperature properties of thermal barrier coatings obtained by detonation spraying

NiCrAlY/YPSZ and NiCrAlY/NiAl/YPSZ thermal barrier coatings (TBCs) were successfully deposited by detonation spraying. The results indicated that the detonation sprayed TBCs included a uniform ceramic coat containing a few microcracks and a bond coat with a rough surface. The lamellar structure and the presence of cracks and impurities could reduce the thermal conductivity of the ceramic coat. Oxidation kinetics at 1000–1150 °C of detonation sprayed TBCs have been measured and discussed. The role of a Ni–Al intermediate layer in improving the oxidation resistance of duplex TBCs has also been studied.     

Dynamic hardness of detonation sprayed WC-Co coatings

The objective of the present work was to determine the dynamic hardness of WC-Co coatings from the dynamic hardness of the coating substrate system. It was also the purpose of this work to evaluate the influence of coating composition, coating thickness, and substrate materials on the dynamic hardness of the coating. To achieve the above-mentioned objectives, WC-12%Co and WC-17%Co coatings were deposited by detonation spraying on three different substrate materials: mild steel, commercially pure (CP) aluminum, and CP titanium. The dynamic hardness of the coating/substrate composite was evaluated by a drop weight system. The dynamic hardness of the coating independent of the substrate was determined from the dynamic hardness of the coating/substrate composite.     

Formation of molybdenum boride cermet coating by the detonation spray process

The effects of the powder particle size and the acetylene/oxygen gas flow ratio during the detonation spray process on the amount of molybdenum phase, porosity, and hardness of the coatings using MoB powder were investigated by x-ray diffraction (XRD), scanning electron microscopy (SEM), etc. The results show that the presence of metallic molybdenum in the coating results from decomposition of MoB powder during thermal spray. The compositions of the coatings are metallic Mo, MoB, and Mo₂B, which are different from the phases of the original powder. The amount of molybdenum phase increases monotonously with the oxygen/acetylene ratio, but the increasing rate for the fine powder is faster than that for the coarse powder. The porosity and hardness of the coating are related to the amount of molybdenum phase. The phase constitution of the coating is discussed.     

Flattening of composite powder particles during thermal spraying

An investigation is conducted of the time evolution of the splat thickness, radius, and rate characteristics in the process of flattening of composites (agglomerated) powder particles at smooth and rough surfaces during thermal spraying. Considerations include the splat solidification, droplet mass loss, solid volume fraction, and variations of splat- surface friction and splat solidification velocity due to the presence of the solid phase. Effective values of the droplet viscosity, impact velocity, and Reynolds number taking into account characteristic features of the flattening process are introduced, and analytical formulas describing the final values of the splat thickness, radius, and rate characteristics are established. Results can be used to predict the splat flattening parameters.     

抽真空截止点的位置对压铸件气孔的影响

在真空压铸中 ,抽真空截止点的确定对压铸件的品质影响很大 ,过早和过迟将影响压铸的连续生产和不利于减少铸件的气孔。抽真空截止点应设在冲头快压射前 15～ 2 0 mm处 ,真空压铸可以提高铸件的密度 1%以上Influence of V