Modeling of Thermal Spraying Heat Transfer Processes by Exodus Stochastic Method

This article deals with the application of the stochastic Exodus method for modelling of thermal spraying heat transfer processes and for solving direct and indirect problems. The Exodus stochastic method has an advantage in straightforward solving of the transient inverse heat transfer multi-dimensional problems over other methods based on iterative fittings procedures used for example by finite element methods (FEM). Theoretical background of the method is introduced. Application capabilities of the method are shown on the example of high velocity oxygen fuel thermal spraying heat transfer process analysis. Comparisons with results of FEM computational method application are presented.     

FEA of Residual Stress During HVOF Thermal Spraying

Due to the recent advances in thermal spraying technology, considerable research emphasis has been placed on the development of models capable of predicting deposition mechanisms at various stages during the process. In order to gain a deeper knowledge of the mechanisms involved in thermal spraying, it is necessary to isolate the factors affecting these constitutive properties (e.g., residual stress generation) and in doing so quantify the effect of the individual factors. Finite element analysis (FEA) is used in the present research to predict the residual stress generated in a WC-Co deposit produced via the HVOF process. The model is compared to an analytical technique and validated experimentally, the result of which provides a thermo-mechanical modeling procedure with an accuracy greater than 80% of that found experimentally. Combining FEA techniques with analytical and experimental results will enhance the understanding of residual stress in thermal spray techniques.     

AE Monitoring and Analysis of HVOF Thermal Spraying Process

This work presents an in situ monitoring of HVOF thermal spraying process through an acoustic emission (AE) technique in an industrial coating chamber. Single layer thermal spraying on substrate was carried out through slits. Continuous multilayer thermal spraying onto the sample without slit was also conducted. The AE was measured using a broadband piezoelectric AE sensor positioned on the back of the substrate. A mathematical model has been developed to determine the total kinetic energy of particles impacting the substrate through slits. Results of this work demonstrate that AE associated with particle impacts can be used for in situ monitoring of coating process. Results also show that the amplitude and AE energy is related to the spray gun transverse speed and the oxy-fuel pressure. The measured AE energy was found to vary with the number of particles impacting the substrate, determined using the mathematical model.     

Microstructure and Wear Resistance of Fe-Based Amorphous Metallic Coatings Prepared by HVOF Thermal Spraying

Amorphous metallic coatings with a composition of Fe₄₈Cr₁₅Mo₁₄C₁₅B₆Y₂ were fabricated by means of high velocity oxygen fuel (HVOF) thermal spraying process. The microstructure and wear performance of the coatings were characterized simultaneously in this article. It is found that the coatings present a dense layered structure with the porosity below 1.5%. The coatings primarily consist of amorphous matrix and some precipitated nanocrystals, though a fraction of Fe-rich phases and oxide stringers also formed during deposited process. High thermal stability enables the amorphous coatings to work below 920 K temperature without crystallization. Depending on the structural advantage, the amorphous coatings exhibit high average microhardness of 997.3 HV_0.2, and excellent wear resistance during dry frictional wear process. The dominant wear mechanism of amorphous coating under this condition is fatigue wear, leading to partial or entire flaking off of the lamellae. In addition, the appearance of oxidative wear accelerates the failure of fatigue wear.     

Effect of Substrate and Process Parameters on the Gas-Substrate Convective Heat Transfer Coefficient During Cold Spraying

The final quality of cold-sprayed coatings can be significantly influenced by gas-substrate heat exchange, due to the dependence of the deposition efficiency of the particles on the substrate temperature distribution. In this study, the effect of the air temperature and pressure, as process parameters, and surface roughness and thickness, as substrate parameters, on the convective heat transfer coefficient of the impinging air jet was investigated. A low-pressure cold spraying unit was used to generate a compressed air jet that impinged on a flat substrate. A comprehensive mathematical model was developed and coupled with experimental data to estimate the heat transfer coefficient and the surface temperature of the substrate. The effect of the air total temperature and pressure on the heat transfer coefficient was studied. It was found that increasing the total pressure would increase the Nusselt number of the impinging air jet, while total temperature of the air jet had negligible effect on the Nusslet number. It was further found that increasing the roughness of the substrate enhanced the heat exchange between the impinging air jet and the substrate. As a result, higher surface temperatures on the rough substrate were measured. The study of the effect of the substrate thickness on the heat transfer coefficient showed that the Nusselt number that was predicted by the model was independent of the thickness of the substrate. The surface temperature profile, however, decreased in increasing radial distances from the stagnation point of the impinging jet as the thickness of the substrate increased. The results of the current study were aimed to inform on the influence and effect of substrate and process parameters on the gas-substrate heat exchange and the surface temperature of the substrate on the final quality of cold-sprayed coatings.     

Coating of High-Alloyed, Ledeburitic Cold Work Tool Steel Applied by HVOF Spraying

This study demonstrates the processing of a cold work tool steel (X220CrVMo13-4) coating using HVOF spraying. The coating formation was analyzed based on microstructure, phase, hardness, porosity, oxidation, and adhesion characteristics. An online diagnostic tool was utilized to find out the in-flight characteristics of powder such as temperature and velocity during the coating process to identify the influencing parameters to achieve dense cold work tool steel coatings with low oxidation. The influence of powder size, process parameters, and in-flight characteristics on the formation of cold work tool steel coatings was demonstrated. The results indicated that thick and dense cold work tool steel coatings with low oxidation can be obtained by the selection of appropriate powder size and process parameters.     

Modeling and Simulation of Heat Transfer in Loaded Continuous Heat Treatment Furnace

HEAT UP processing is the basic step for the workloadin heat treatment for further processing.It is also anenergy-intensive process.Thus,correct prediction ofthe temperature variation and distribution in theworkload is of significance to ensure the final qualityof the parts and to reduce energy consumption and timeas well.There are some studies about the optimizationof heat treating process in continuous furnace.FurnXpert'11is developed to optimize furnace designand operation for many types…     

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.     

超音速火焰喷涂低碳WC/12Co涂层的组织及性能研究

采用离心雾化干燥法制得团聚颗粒,经连续高温烧结成两种不同松装密度的热喷涂粉末。采用以C3H8/O2为燃料的超音速火焰喷涂（HVOF）工艺制备了WC/12Co涂层。对粉末及涂层做了显微组织观察和XRD分析,测定了涂层的厚度、显微硬度和粉末沉积效率。结果表明,在1120℃、1180℃烧结的粉末中主要有WC和W6Co6C,但无Co相;涂层有脱碳,有Co6W6C相,但未出现单质Co。涂层组织均匀致密,沉积效率可达65%。     

Coupled Modeling of Electromagnetic Fleld,Fluid Flow,Heat Transfer and Solidification During Conventional DC Casting and Low Ftequency Electrmagnetic Casting of 7xxx Aluminum Alloys

A comprehensive mathematical model has been developed to describe the interaction of the multiple physics fields during the conventional DC casting and LFEC (low frequency electromagnetic casting) process. The model is based on a combination of the commercial finite element package ANSYS and the commercial finite volume package FLUENT, with the former for the calculation of the electromagnetic field and the latter for the calculation of the magnetic driven fluid flow, heat transfer and solidification. Moreo...     

电渣重熔磁流体传热传质过程的数值模拟

建立了电渣重熔三维非稳态耦合数学模型。通过求解麦克斯韦方程组和质量、动量、能量以及溶质传输方程,得到电渣炉中渣金两相流动、温度分布、凝固和溶质分布情况。计算结果与测量值吻合的较好。     

Modeling of Two-Phase Flow and Heat Transfer in Low-Temperature Oxygen-Fuel Spray Process

The low-temperature oxygen-fuel (LTOF) spray is a modification of high velocity oxygen fuel spray. In this process, the high-temperature gas is accelerated to supersonic speed through a Laval nozzle followed by a straight barrel. By injecting room temperature gas into the mixing chamber, the temperature of the gas can be controlled in a range of about 1000-2500 K, so that some oxygen and temperature-sensitive materials, such as titanium and copper, can avoid oxidation or decomposition during the spraying process. The purpose of this paper is to establish a 2-D mathematical model to simulate the supersonic gas dynamics and particles behavior in LTOF process. The temperature and velocity of the flow fields, and the trajectory and heating of in-flight particles are predicted for different operating parameters. The model is validated by experimental data in the literature. Effects of the mixing gas flow rates, particle sizes, and injection conditions on this process were investigated as well.     

基于ANSYS铸件充型与凝固数值模拟的研究

根据流体力学和砂型铸造的特点建立了该条件下的控制方程、求解方法、初始条件及边界条件 ,并利用所建立的数学模型模拟了一试验件 ,在模拟的过程中分别用 2种方法对流场进行了模拟 ,并与实测结果进行比较 ,结果表明 ,采用充型过程的方法更接近实测值。     

热喷涂纳米结构涂层研究现状与展望

综述了国内外在利用热喷涂技术制备纳米结构涂层方面的研究进展。介绍了热喷涂用纳米结构喂料的制备、热喷涂纳米结构涂层的制备方法及涂层性能。与传统材料的热喷涂涂层相比,纳米结构涂层在力学、摩擦学以及耐磨防腐蚀性能方面均有较大提高。     

黏土类陶瓷原型在高温干燥中传质传热的有限元模拟

在某大学开发的直接原型熔射制模技术中,陶瓷材料由于其硬脆特性,加工时容易发生崩裂,因此切削量不能过大,导致加工大型模具时效率较低。研究通过工业机器人直接铣削陶瓷毛坯得到熔射陶瓷原型,但铣削后的黏土类陶瓷生胚原型在高温干燥固结中会发生较大的收缩变形。建立了黏土类原型材料在高温干燥过程中传质传热的数学模型,并在ABAQUS中进行了有限元模拟,得到黏土类原型材料的收缩变形规律,为后续工作打下基础。     

Microstructure and Sliding Wear Behaviour of Ni(Cr)-TiB_2 Coatings Deposited by HVOF Spraying of SHS Powders

THERMAL SPRAYING provides a large range ofcoatings,which increase the wear resistance ofsubstrates[1].One of the major coating families is thecermet,composed of hard ceramic particles with ametallic binder.The most commonly used cermetcoatings in industrial applications are based on eitherthe WC-Co or the Cr3C2-Ni(Cr)systems with WC-17wt%Co and Cr3C2-25wt%Ni(Cr)being typicalcompositions[2,3].Although WC-Co deposits are hardand wear resistant at ambient temperatures their rangeof ap…     

Deposition and Characteristics of Submicrometer-Structured Thermal Barrier Coatings by Suspension Plasma Spraying

In the field of thermal barrier coatings (TBCs) for gas turbines, suspension plasma sprayed (SPS) submicrometer-structured coatings often show unique mechanical, thermal, and optical properties compared to conventional atmospheric plasma sprayed ones. They have thus the potential of providing increased TBC performances under severe thermo-mechanical loading. Experimental results showed the capability of SPS to obtain yttria stabilized zirconia coatings with very fine porosity and high density of vertical segmentation cracks, yielding high strain tolerance, and low Young??s modulus. The evolution of the coating microstructure and properties during thermal cycling test at very high surface temperature (1400?°C) in our burner rigs and under isothermal annealing was investigated. Results showed that, while segmentation cracks survive, sintering occurs quickly during the first hours of exposure, leading to pore coarsening and stiffening of the coating. In-situ measurements at 1400?°C of the elastic modulus were performed to investigate in more detail the sintering-related stiffening.     

Temperature-Dependent Thermal and Chemical Stabilities as well as Mechanical Properties of Electrodeposited Nanocrystalline Ni

Nanocrystalline (NC) Ni electrodeposits (EDs) with a mean grain size of 34?±?12 nm has been investigated, from room temperature to 800 °C under a purge gas of argon, by both non-isothermal and isothermal differential scanning calorimetry measurements, in combination with characterization of temperature-dependent microstructural evolution. A significant exothermic peak resulting from superimposition of recrystallization and surface oxidation occurs between 340 and 745 °C at a heating rate of 10 °C/min for the NC Ni EDs. The temperatures for recrystallization and oxidation increase with increasing the heating rate. In addition, recrystallization leads to a profound brittle–ductile transition of the Ni EDs in a narrow range around the peak temperature for the recrystallization.     

激光熔覆与热喷涂镍基合金涂层的微观组织和性能

利用热喷涂和激光熔覆技术分别制备了镍基涂层,采用显微组织和显微硬度法对两种涂层进行了对比研究。结果表明:两种工艺得到的涂层组织基本相同,主要包括-γN i固溶体、硼化物、碳化物+镍γ-固溶体共晶以及合金渗碳体等金属间化合物,热喷涂涂层组织中未形成共晶体;两种涂层的平均硬度均高于基体,均达到了表面强化的目的。     

40Cr钢喷雾淬火换热系数的计算及数值模拟

为计算ф25 mm×100 mm 40Cr圆柱试件喷雾淬火冷却过程的换热系数,采用四通道采样系统测定了喷雾淬火过程的冷却曲线,并用反传热法中的非线性估算法计算出换热系数。计算结果表明,喷雾淬火过程分3个阶段:膜沸腾阶段、核沸腾阶段和对流换热阶段,并在冷却到120℃时,换热系数达到峰值9800 W·m-2·℃-1。采用此换热系数作边界条件,对40Cr钢的喷雾淬火过程进行了数值模拟,得到淬火过程中不同时刻的温度场、组织场、硬度场和应力场。