超音速火焰喷涂的火焰速度特性

本文介绍了超音速火焰喷涂系统的火焰特性。试验结果表明在使用氧气和丙烷燃气时．可在较大的压力和流量范围内,获得超音速火焰。喷枪出口火焰的速度可达1500m／s以上。根据火焰射流照片中的马赫锥角来计算马赫数,并依据火焰的其它参数估算出火焰的速度。     

表面工程应用实例 [例12]超音速火焰喷涂用于冷轧张力辊的复合制造与再制造

超音速火焰喷涂(简称HVOF)是利用可燃气体在氧气助燃下释放的化学能为热源,燃气在特殊结构的喷涂枪中形成超音速焰流使喷涂粉末以熔融状态高速撞击到工件表面制备出喷涂层.超音速火焰喷涂的特点:具有高的喷涂粒子速度和相对较低的温度,特别适合于喷涂WC等金属陶瓷材料;喷涂层的压应力结构有利于制备较厚的涂层;喷涂效率高,燃气价格较低,经济性好.     

[例12]超音速火焰喷涂用于冷轧张力辊的复合制造与再制造

超音速火焰喷涂(简称HVOF)是利用可燃气体在氧气助燃下释放的化学能为热源,燃气在特殊结构的喷涂枪中形成超音速焰流使喷涂粉末以熔融状态高速撞击到工件表面制备出喷涂层.超音速火焰喷涂的特点:具有高的喷涂粒子速度和相对较低的温度,特别适合于喷涂WC等金属陶瓷材料;喷涂层的压应力结构有利于制备较厚的涂层;喷涂效率高,燃气价格较低,经济性好.     

对用于涡轮机械上的优化涂层的HVOF工艺条件的评论

在磨损和腐蚀非常严重的情况下，由超音速火焰喷涂（HVOF）工艺产生的涂层成功地延长了涡轮机械零件的使用寿命。在喷涂过程中，粉末粒子达到极高的冲击速度，从而形成具有高耐磨抗腐蚀能力的坚固耐用的涂层。本文介绍有关HVOF工艺、涂层以及HVOF在涡轮机械上的典型应用。     

对用于涡轮机械上的优化涂层的HVOF工艺条件的评论

在磨损和腐蚀非常严重的情况下，由超音速火焰喷涂(HVOF)工艺产生的涂层成功地延长了涡轮机械零件的使用寿命。在喷涂过程中，粉末粒子达到极高的冲击速度，从而形成具有高耐磨抗腐蚀能力的坚固耐用的涂层。本文介绍有关HVOF工艺、涂层以及HVOF在涡轮机械上的典型应用。     

C_3H_8+H_2混合气体超音速火焰喷涂燃烧过程数值模拟

采用FLUENT软件模拟氧-混合气体燃料超音速火焰喷涂(HVOF)过程。随着计算机技术的发展,对HVOF的燃烧状态和气体流场的模拟成为近年来的研究热点。但到目前为止,对于混合燃料组分的燃烧过程的研究还有待深入。以丙烷和氢气的混合气体为燃料,开展HVOF的燃烧过程数值模拟,并将模拟结果与纯丙烷气体燃烧过程的气体流场模拟结果进行对比。进一步对90 mm喷管和110mm喷管长度条件下的HVOF的燃烧状态和气体流场状态进行对比研究。结果表明,混合气体中氢气的加入使得燃烧体具有能量密度高、热量产生集中等优点,对燃烧过程和气体流场的集中有重大影响;加长喷管对于气体流场产生的约束力更大,燃烧室内燃烧更充分,同时在喷枪出口处产生明显的激波。     

气体流量对超音速火焰喷涂CoCrW涂层颗粒熔化程度的影响

探讨了超音速火焰喷涂(HVOF)过程中C3H8、O2和压缩空气流量对CoCrW涂层颗粒熔化程度的影响,结果表明:O2流量对涂层颗粒的熔化程度影响最大,其次是C3H8,压缩空气流量的影响最小:气体流量优化后超音速火焰喷涂CoCrW涂层的结合强度达到54Mpa以上,孔隙率低于2%。     

多功能超音速火焰喷涂粒子特性的数值模拟

通过建立多功能超音速火焰喷涂火焰流场的数值模型,对超音速火焰喷涂粒子的速度特性和温度特性进行数值模拟,其结果揭示了多功能超音速火焰喷涂粒子的速度和温度的变化规律.根据数值模拟结果分析了粉末粒度对火焰焰流速度和温度的影响,用不同粒度参数重复模拟计算,这对喷涂粉末的加速和加热性能具有重要意义.     

DZ9000E型移动式超音速火焰喷涂系统的研制

与氧煤油超音速火焰喷涂系统相比,DZ9000E型氧燃气超音速火焰喷涂系统的开发更侧重于降低涂层的成本以及适应不同工况中现场使用的可移动性.DZ9000E型手持式喷涂枪上配有双气阀手柄、送粉开关及压缩/冷却气阀,为用户提供轻便、简捷的操作模式.整机价格便宜及运行成本低使这一系统更适应国内中小客户喷涂中低价位HVOF涂层的需求.采用DZ9000E系统喷涂WC-12Co涂层具有良好的涂层性能,平均显微硬度可达1 000 HV0.2左右,涂层与基体间的结合强度超过53 MPa.     

激光重熔超音速火焰喷涂Cr3C2-25％CrNi层的数值分析及实验验证

利用ANSYS软件建立激光重熔超音速火焰喷涂(HVOF)层的有限元模型,进行温度场的仿真模拟,考察激光重熔工艺参数对温度场分布的影响,结果表明单向匀速扫描过程中引起的热量积聚,通过双向扫面、变速扫描等方式可以解决.通过实验验证了模型的有效性,分析发现激光重熔能有效改善超音速火焰喷涂涂层的质量,根据仿真结果和试验可确定合...     

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.     

Numerical Investigation of Combustion and Flow Dynamics in a High Velocity Oxygen-Fuel Thermal Spray Gun

The combustion and flow behavior within a high velocity oxygen-fuel (HVOF) thermal spray gun is very complex and involves multiphase flow, heat transfer, chemical reactions, and supersonic/subsonic transitions. Additionally, this behavior has a significant effect on the formation of a coating. Non-premixed combustion models have been developed and are able to provide insight into the underlying physics of the process. Therefore, this investigation employs a non-premixed combustion model and the SST \(k - \omega\) turbulence model to simulate the flow field of the JP5000 (Praxair-TAFA, US) HVOF thermal spray gun. The predicted temperature and velocity have a high level of agreement with experimental data when using the non-premixed combustion model. The results are focused on the fuel combustion, the subsequent gas dynamics within the HVOF gun, and the development of a supersonic free jet outside the gun. Furthermore, the oxygen/fuel inlet turbulence intensity, the fuel droplet size, and the oxygen/fuel ratio are investigated to determine their effect on the supersonic flow characteristics of the combustion gas.     

Numerical Investigation of a Hybrid HVOF-Plasma Spraying Process

This study deals with the numerical investigation of a hybrid thermal spray process that combines HVOF and thermal plasma technologies. In this process, a thermal plasma is used to assist the combustion process that proceeds in a quasi-conventional HVOF system. It is expected that this coupling makes the HVOF system more flexible in terms of working parameters and sprayed materials. Also, a rather low fuel gas consumption and high deposition rate compared to that of most of the conventional HVOF guns are sought. Modeling this process can help to understand the phenomena that control the operation of the spray system and, therefore, help to optimize it. The model involves the plasma formation, combustion process, and expansion of the supersonic jet in the ambient atmosphere. In this study, the system uses argon as plasma-forming gas and methane as fuel gas. Fuel and oxidant are not premixed before entering the combustion chamber. In the model, methane oxidation is represented by a single-step global reaction considering only a few chemical species (fuel, oxidant, and product species); the turbulent non-premixed combustion is modeled by a fast-chemistry combustion model that assumes that the rate of chemical reaction is controlled by turbulence. The model equations are solved using the CFD software Fluent 6.3. The main gas flow characteristics (velocity, temperature, and pressure) in presence and absence of the plasma source are compared and discussed, and the benefits of the plasma source are discussed in the light of predictions and fuel combustion mechanisms.     

Optimizing HVOF Spray Parameters to Maximize Bonding Strength of WC-CrC-Ni Coatings on AISI 304L Stainless Steel

High velocity oxygen fuel (HVOF)-sprayed cermet coatings are extensively used to combat erosion-corrosion in naval applications and in slurry environments. HVOF spray parameters such as oxygen flow rate, fuel flow rate, powder feed rate, carrier gas flow rate, and spray distance have significant influence on coating characteristics like adhesion bond strength and shear strength. This paper presents the use of statistical techniques in particular response surface methodology (RSM), analysis of variance, and regression analysis to develop empirical relationships to predict adhesion bond strength and lap shear bond strength of HVOF-sprayed WC-CrC-Ni coatings. The developed empirical relationships can be effectively used to predict adhesion bond strength and lap shear bond strength of HVOF-sprayed WC-CrC-Ni coatings at 95% confidence level. Response graphs and contour plots were constructed to identify the optimum HVOF spray parameters to attain maximum bond strength in WC-CrC-Ni coatings.     

Characterization of thermal sprayed nanostructured WC-Co coatings derived from nanocrystalline WC-18wt.%Co powders

Nanostructured WC-Co coatings were synthesized using high velocity oxygen fuel (HVOF) thermal spray. The nanocrystalline feedstock powder with a nominal composition of WC-18 wt.%Co was prepared using the novel integrated mechanical and thermal activation (IMTA) process. The effects of HVOF thermal spray conditions and powder characteristics on the microstructure and mechanical properties of the as-sprayed WC-Co coatings were studied. It was found that the ratio of oxygen-to-hydrogen flow rate (ROHFR) and the starting powder microstructures had strong effects on decarburization of the nano-coatings. Decarburization was significantly suppressed at low ROHFR and with the presence of free carbon in the powder. The level of porosity in the coatings was correlated with the powder microstructure and spray process conditions. The coating sprayed at ROHFR=0.5 exhibited the highest microhardness value (HV_300g=1077), which is comparable to that of conventional coarse-grained coatings.     

A comparative study of high velocity oxygen fuel,vacuum plasma spray,and axial plasma spray for the deposition of CoNiCrAlY bond coat alloy

In the aerospace field as well as in the stationary gas turbine field, thermally sprayed coatings are used to improve the surface properties of nickel-super-alloys materials. Coatings are commonly used as bond coat and antioxidation materials (mainly MCrAlY alloys) and as thermal barrier coatings (mainly yttria partially stabilized zirconia). The purpose of the current study was to assess the properties of thermally sprayed bond coat CoNiCrAlY alloys comparing the performance of three different techniques: vacuum plasma spray (VPS), high velocity oxygen fuel (HVOF), and axial plasma spray (AxPS). The quality of the deposited films has been assessed and compared from the point of view of microstructural (porosity, oxide concentration, unmelted particles presence) and mechanical (hardness) characteristics. The surface composition and morphology of the coatings were also determined. Specific efficiency tests were performed for the three examined technologies. The highest quality coatings are obtained by VPS, but also high velocity oxygen fuel and AxPS sprayed films have interesting properties, which can make their use interesting for some applications.     

Effect of Operating Parameters on a Dual-Stage High Velocity Oxygen Fuel Thermal Spray System

High velocity oxygen fuel (HVOF) thermal spray systems are being used to apply coatings to prevent surface degradation. The coatings of temperature sensitive materials such as titanium and copper, which have very low melting points, cannot be applied using a single-stage HVOF system. Therefore, a dual-stage HVOF system has been introduced and modeled computationally. The dual-spray system provides an easy control of particle oxidation by introducing a mixing chamber. In addition to the materials being sprayed, the thermal spray coating quality depends to a large extent on flow behavior of reacting gases and the particle dynamics. The present study investigates the influence of various operating parameters on the performance of a dual-stage thermal spray gun. The objective is to develop a predictive understanding of various parameters. The gas flow field and the free jet are modeled by considering the conservation of mass, momentum, and energy with the turbulence and the equilibrium combustion sub models. The particle phase is decoupled from the gas phase due to very low particle volume fractions. The results demonstrate the advantage of a dual-stage system over a single-stage system especially for the deposition of temperature sensitive materials.     

Effect of particle state on the adhesive strength of HVOF sprayed metallic coating

NiCrBSi and Ni-50Cr coatings were deposited using the high velocity oxygen fuel (HVOF) spray process under different spray parameters with two powders of different sizes to clarify the influence of the melting state of spray particles on the adhesive strength of the coating. The adhesive strength of the coating was estimated according to the American Society for Testing and Materials (ASTM) C633-79. The melting state of the spray droplet was examined from the coating microstructure. It was found that the melting state of spray particles had a significant effect on the adhesive strength of HVOF sprayed Ni-based coatings. The significant melting of the spray particle did not contribute to the increase in the adhesion of HVOF metallic coatings. On the other hand, the deposition of a partially melted large particle contributed to the substantial improvement of adhesive strength of the HVOF coating. The subsequent coating presented a dense microstructure and yielded an adhesive strength of more than 76 MPa, which was double that of the coating deposited with completely molten particles. It can be suggested that the good melting of the spray particle is mainly related to the mechanical interlocking effect, which reaches the limited and approximately defined adhesive strength up to 40–50 MPa.     

Effect of oxygen/fuel ratio on the in-flight particle parameters and properties of HVOF WC-CoCr coatings

High Velocity Oxy-Fuel (HVOF) spray techniques can produce high performance alloy and cermet coatings for applications that require wear resistant surfaces. In HVOF process, the particle velocity and temperature determine the resultant coating properties and in many cases enables a better understanding of the process.The aim of this study is to investigate influences of different oxygen/fuel ratios on velocity and temperature of flying particles as well as properties of the HVOF thermal sprayed WC-CoCr coatings. Particle parameters were recorded just prior to impact on the substrate using in-flight particle diagnostic tool Accuraspray-g3®. Detailed correlation of particle parameters and the coating properties are evaluated in order to deduce particle parameter ranges providing coatings with optimum properties.     

Effect of spray particle trajectory on the measurement signal of particle parameters based on thermal radiation

The influences of the dimensions of optical components and the trajectories of spray particles on the variations of the waveforms of the radiation signals from the spray particles were studied both theoretically and experimentally for correct simultaneous measurement of the particle parameters including particle velocity, surface temperature, size, and spatial distribution. Two types of filtering masks, including single-windowed and dual-windowed, were used as models in the current study. The evolution of the radiation pulse from a moving thermal spray particle was simulated through the change of the projected area of the particle image spot on the filtering mask window. The experimental detection of the thermal radiation pulses was performed for the high velocity oxygen fuel (HVOF) process using an optoelectronic measurement system. The theoretical simulation clearly showed that the characteristic waveforms of the thermal radiation signals from the spray particles are varied with the distance and orientation of the trajectories of thermal spray particles with respect to the ideal image plane of the filtering window plane. The typical variations of the characteristic waveforms obtained theoretically have been observed experimentally with HVOF spraying. The waveforms expected theoretically were correlated well with those observed experimentally. The characteristic waveforms of the radiation signals from the spray particles in a trapezoid shape with a saturated top platform contain the information for spray particle parameters including velocity, surface temperature, size, and spatial distribution. With the dual-windowed filtering mask, the particle velocity can be correctly measured with the bi-peak radiation signal in triangle-like shape, and the surface temperature may be estimated reasonably. However, the particle size cannot be estimated correctly. It was revealed that the characteristics of the waveforms were remarkably influenced by the image spot size. Therefore, the expansion of the image spot based on the relation between the image spot size of an in-flight particle and optical lens parameters obtained optically was discussed. The influence of the image spot size on the waveform characteristics was examined.