Numerical Modelling of Ar-N<Subscript>2</Subscript> Plasma Jet Impinging on a Flat Substrate

Substrate heating in the plasma spray process is one of the important parameters, which affects the microstructure of coatings and bonding between coating and substrate. In this study, a three-dimensional numerical model is developed to study the thermal exchange between the plasma jet and the substrate. The plasma jet temperature and velocity distributions and thermal flux to the substrate surface are predicted. The effects of arc current, gas flow rate, and stand-off distance on the temperature and velocity fields of the impinging plasma jet and thermal flux to the substrate are clarified. Results indicate that the three-dimensional effect has a very weak influence on the substrate heating. The air entrainment is compared for different cases. The present model is validated by comparing the present results with previous predictions and measurements. The temperature distributions in the substrate for different stand-off distances are predicted.     

Gas-Substrate Heat Exchange During Cold-Gas Dynamic Spraying

In this study, the temperature distribution of the surfaces of several substrates under an impinging gas jet from a cold spray nozzle was determined. A low-pressure cold-gas dynamic spraying unit was used to generate a jet of hot compressed nitrogen that impinged upon flat substrates. Computer codes based on a finite differences method were used to solve a simplified 2D temperature distribution equation for the substrate to produce nondimensional relationships between the surface temperature and the radius of the impinging fluid jet, the axial velocity of the cold spray nozzle, the substrate thickness, and the heating time. It was found that a single profile of the transient nondimensional maximum surface temperature could be used to estimate the dimensional maximum surface temperature, regardless of the value of the compressed gas temperature. It was found further that, as the thermal conductance of the substrate increased, the maximum surface temperature of the substrate beneath the gas jet decreased. Heat exchange between the substrate and the compressed gas jet during motion of the nozzle to produce heat conduction within the substrate was characterized by the nondimensional Peclét number. It was found that lower Peclét numbers produced higher temperatures within the substrate. The close agreement of the numerical results with the experimental results suggests that the nondimensionalized results may be applied to a wide range of conditions and materials.     

工艺参数对铝合金强风冷过程界面换热效率的影响

通过一系列风冷淬火试验,研究了气体高速冲击金属热表面的换热过程,采用反传热法对界面热流密度 (q) 和界面传热系数 (h)进行了求解,探究了试样的表面粗糙度和淬火初始温度、试样表面的冷却介质流量密度对换热过程的影响。结果表明:试样淬火初始温度对风冷淬火界面换热有显著影响,当其从470 ℃增大到520 ℃时,q 和h的最大值增大约50%,淬火表面温度下降到200 ℃的平均冷却速率增大约43%。随试样表面介质流量密度增大,界面热交换呈现出先增大后减小的趋势,即存在一个与最高界面换热效率对应的临界试样表面介质流量密度,且喷射角度越接近90°,该临界值越小。随试样表面粗糙度增大,界面换热不断减小,这可能归因于越粗糙的表面对边界层内流体的钉扎作用越明显,越不利于提高界面换热效率。此外,在250~380 ℃区间,界面换热系数随表面温度变化曲线普遍存在一个凹陷区域,这可能与铝合金淬火冷却过程中二次相的析出有关。     

铝板在双面喷流加热和辊底摆动过程中的温度均匀性

基于辊底式铝板淬火炉加热工艺,针对板材双面多喷口气体喷流换热与内部导热的耦合问题,建立了板材喷流加热过程数理模型,研究了喷口排布、热风流量、铝板厚度和辊底摆动周期对板材表面换热及板温均匀性的影响。结果表明,喷口垂直喷流的换热效果优于倾斜喷流;垂直喷流时喷口以1/4间距错排布置时,铝板表面温度均匀性最好;喷流造成的铝板表面温度不均匀主要集中在加热前期,易造成板材的变形,且对于薄板更加显著;可采用较小的热风流量及较短的辊底摆动周期以改善铝板表面温度均匀性,减小材料的应力和应变。     

Effect of some parameters on microstructure and hardness of alumina coatings prepared by the air plasma spraying process

The spraying distance, substrate temperature, coating thickness and surface roughness of substrate during deposition play an important role on the plasma spray coating process and effect the final properties of the coatings. Al₂O₃ coatings on AISI 304 L stainless steel substrate were prepared to investigate the effects on the coating of these parameters. The results indicated that the parameters such as the spraying distance, substrate temperature, coating thickness and substrate roughness were fairly effected the hardness, porosity and surface roughness of Al₂O₃ coatings. The lowest surface roughness and the lowest porosity and the highest hardness values of Al₂O₃ coating were obtained for the spraying distance of 12 cm and the surface roughness of 3.28 μm and the substrate temperature of 500 °C. It also found that the increases of coating thickness were lowered the hardness and enhanced the porosity and the coating roughness.     

金属材料喷雾淬火过程的界面热交换分析

通过末端淬火试验,对铝合金试样喷雾淬火过程的界面热交换进行了研究。采用反热传导法求解了所有试验的界面热流密度(q)和界面传热系数(h),重点分析了喷雾压力、喷嘴直径和试样表面粗糙度对界面热交换的影响。结果表明:喷射压力对整个淬火界面换热过程均有影响,但对过渡沸腾阶段影响更大,且喷射压力越大,q及其峰值q_max越大,进入核沸腾阶段的时间越短;喷嘴直径越大,q、q_max越大,越早进入核沸腾阶段,但增大喷嘴直径对界面换热的影响存在上限;随表面粗糙度增大,q、q_max先减小后增大;在本试验条件下,上述喷射压力、喷嘴直径和表面粗糙度对界面热交换的影响规律均不受另外两个参数取值的影响。此外,由于喷射的微小液滴均匀覆盖了整个热表面,产生了剧烈的核沸腾,导致在部分试验中,q曲线在核沸腾阶段出现了二次升高现象。     

Comparison of plasma-sprayed coatings produced in argon or nitrogen atmosphere

When spraying is conducted at ambient atmosphere, the entrainment of air cools the plasma jet and affects its expansion. It can also cause oxidation or chemical decomposition of the sprayed materials. Inert plasma spraying (IPS), generally conducted in an argon atmosphere, prevents these phenomena. However, the main drawbacks of IPS in comparison with air plasma spraying are the capital and operating costs. This paper presents a study in which nitrogen is used as a substitute for conventional argon atmosphere, thus reducing costs by 25 to 30%. Titanium carbide and niobium powders were sprayed in both argon and nitrogen atmospheres. Cryogenic cooling of the substrate was used during the spray process. This helps to maintain a low temperature in the chamber, produce thick coatings, and allows the use of substrate materials that are sensitive to heat. The velocity, temperature, and composition fields of the argon-hydrogen plasma jet flowing in argon or nitrogen at atmospheric pressure are compared from numerical simulation. The adhesion, roughness, and microstructure of the niobium and TiC coatings produced in both atmospheres are discussed as well as their nitrogen content.     

On some aspects of gas dynamics of the cold spray process

This paper presents an overview of results of recent studies conducted at the Institute of Theoretical and Applied Mechanics of the Siberian Division of the Russian Academy of Science in the field of gas dynamics and heat transfer of the supersonic air jet under conditions typically used in the cold spray process. These studies are related to various aspects of the problem including a flow in the nozzle and the outflow of the jet, as well as effects of the interaction of the jet with a flat obstacle. They are conducted with a supersonic nozzle with a rectangular section at the exit with a Mach number M ₀ between 2 and 3.5. The gas flow in the nozzle is theoretically and experimentally studied. It is shown that the boundary layer on the walls of the nozzle affects significantly the flow parameters (for example, Mach number M, pressure p, temperature T, and density ρ of the gas). A method of calculation of the gas parameters in the flow core of the nozzle is suggested, and it is shown that they depend mainly on the ratio of the nozzle width to its length. The results of the investigation of the supersonic air jets with stagnation temperature ranging from 300–600 K flowing in the atmosphere are presented. The corresponding dimensions of the jets, profiles, and axial distributions of the gas parameters are obtained. The interactions of the supersonic jet with the flat obstacle are studied. Self-similarity of the distribution of the pressure and of the Mach number on the obstacle surface is shown for the jets with various values of the Mach number and the angle of impingement. The oscillation regimen of the jet impingement, as well as a compressed layer structure is observed with the aid of a Schliren visualization technique. Some problems of heat exchange of the jets with the obstacle are considered. Distributions of stagnation temperature and heat exchange coefficient in the near-wall jet are obtained. The temperature of the obstacle for the stationary case is calculated, and it is shown that for heat conductive materials the surface temperature is lower than the stagnation temperature due to the redistribution of heat inside of the substrate.     

Porosity of Electroless Nickel Coatings Investigated Using Different Porosity Tests and Their Application

The porosity of electroless nickel coatings on mild steel substrates having different substrate surface roughness and coating thickness was investigated using NSS (Neutral Salt Spray), ferroxyl, and SO₂ tests. In addition the influence of substrate surface roughness and coating thickness upon the properties of electroless nickel coatings was studied.

It was found that all porosity tests showed the same tendency for the porosity of electroless nickel coatings to decrease with decreasing substrate surface roughness and increasing coating thickness. The use of the SO² and ferroxyl test have the advantage over the NSS test as a routine quality control procedure in that they are both quicker to carry out and easier to interpret. A relationship between porosity, substrate surface roughness and coating thickness was established which might be used as an industrial quality control tool.     

Influence of Substrate Properties on the Formation of Suspension Plasma Sprayed Coatings

In this study, YSZ coatings were deposited on different substrate materials (stainless steel and aluminum) using suspension plasma spray technique. The effects of substrate properties (material, surface topology, temperature, and thickness) on the formation of coatings were investigated. The results showed that, with the identical spray parameters, the porosity is higher for the coatings deposited on aluminum than that on stainless steel due to the high thermal transfer ability of the former substrate material. The SEM results revealed that the microstructure of as-prepared coatings could be tailored from the vertical cracked structure to the columnar structure by increasing the substrate surface roughness and their formation mechanisms were discussed. The substrate preheating temperature has an influence on the microstructure of the coatings, especially in the interfacial region. Increasing the substrate temperature is an effective means for reducing the interface defects and for improving the adhesion of the coatings. With the increase in the substrate thickness, the quantity of the vertical cracks in the coatings is reduced and their width becomes narrower.     

射流倾斜角度对热轧无缝钢管冷却均匀性的影响

为了获得射流倾斜角度对热轧无缝钢管冷却效率及均匀性的影响规律,运用Fluent软件对单股倾斜射流冲击冷却热轧无缝钢管的流动特点及传热特性进行了有限元模拟。研究了不同射流倾斜角度下无缝钢管表面剪切力、湍流动能及压力分布特点,并获得冲击点处热流量及钢管1/2厚度处温度的变化曲线。结果表明,射流倾斜角度对钢管表面剪切力、湍流动能及温度分布具有显著的影响;随着射流倾斜角度的增加,钢管表面剪切力及湍流动能非对称性逐渐增加,且在顺流方向3°附近出现最大值;从钢管1/2厚度处温度分布发现,随着射流倾斜角度的增加,换热效率及温度均匀性得到了改善,先升高后降低;通过标准差计算,当入射角θ=10°时,冲击点两侧温度均匀性最佳。     

介质流量密度对铝合金喷射淬火界面热交换率的影响规律及机理

通过喷射淬火试验,研究了在高温铝合金表面进行水射流淬火、喷雾淬火和高速空气淬火的热交换过程,并对这3种在线淬火界面的热流密度 (q) 和传热系数 (h) 进行了反分析求解,重点探究了试样表面介质流量密度 (q_s) 对界面热交换率的影响规律及其机理。结果表明,随q_s增大,喷射淬火界面热交换率先增大后减小,即存在一个临界q_s,此时临界热流密度 (q_c) 取得最大值。当q_s小于其临界值时,喷射淬火界面热交换率随q_s增大而增大;当q_s大于临界值时,随q_s继续增大,喷射淬火界面热交换率反而减小。喷雾淬火的单位体积冷却介质最大吸热量 (Q_max) 最高,且淬火介质相同时,Q_max随q_s增大而减小。增大q_s对提高喷水淬火表面的热交换率效果最好。     

水性红外隐身涂料制备工艺优化

目的优化水性红外隐身涂层材料制备工艺,提高低发射率红外隐身涂料隐身性能。方法采用红外辐射率测量仪、红外光谱吸收仪等,研究涂层固化温度、涂层表面粗糙度和涂层厚度对低发射率红外隐身涂料隐身性能的影响。结果固化温度对涂层红外发射率和基体树脂红外吸收光谱影响不大,但随着固化温度升高,涂层固化时间明显缩短;随着涂层表面粗糙度的增加,涂层红外发射率增加;表面粗糙材料红外发射率受测试角度影响小于表面光滑材料;在基材上制备不同厚度的涂层,当涂层厚度小于30μm时,涂层红外发射率受基材表面红外发射率影响较大,当大于30μm时,影响较小。结论可以根据实际时间需求选择合适的涂层固化温度,宜选择刮涂方式使涂层表面保持一定的粗糙度,涂层厚度宜为30~40μm。     

Characterization of Low-Pressure Cold-Sprayed Aluminum Coatings

Aluminum alloys are widely used as materials for engineering components of automobiles and airplanes because of their light weight and high corrosion resistance. However, cracks may develop sometimes in aluminum components, which have to be repaired by welding. It is difficult to weld aluminum components due to its high specific thermal conductivity and high coefficient of thermal expansion. The low-pressure cold-spray technique can be used instead of welding for repairing cracks. However, the effects of surface conditions on particle deposition and the mechanical properties of cold-sprayed coatings have not been investigated thus far. In this study, the effect of surface conditions focusing on active newly formed surface on aluminum particle deposition is studied and the mechanical properties of low-pressure cold-sprayed aluminum coatings are investigated by four-point bending tests. It is found that for efficient particle deposition it was necessary to obtain active newly formed surface of the substrate and particle surfaces by several impingements because the existence of inactive native oxide films has an adverse effect on the deposition. Furthermore, the strength of a cold-sprayed specimen is found to be higher than that of a cold-rolled specimen under compressive loading.     

Experimental study of heat transfer coefficient on hot steel plate during water jet impingement cooling

Experiments were performed, under transient conditions, to investigate the heat transfer phenomena of stationary hot steel plate under multiple top circular jets on run-out table. Based on inverse heat conduction model, a two-dimensional finite difference program was developed to calculate the local surface convective heat transfer coefficients and corresponding temperatures. The cooling water jet flow rate was varied from 15 L/min to 35 L/min and its effect on the convective heat transfer coefficient and surface temperature was analyzed. The results show that heat transfer coefficients are nonlinear functions of surface temperature. The cooling flow rate has no effect on heat transfer coefficient and surface temperature at stagnation point. Within 70 mm distance from stagnation line, heat transfer coefficient ratio changes slightly from 0.87 to 0.97. Beyond surface temperature of 350 °C, heat transfer coefficient ratio decreases with increasing distance from stagnation line.     

模拟轧制过程的镁合金带材动态接触传热机理

改进接触传热测量装置，利用有限元模拟镁合金轧制过程辊缝界面的瞬态换热特性，以准确分析温度、压力和粗糙度对接触传热系数的耦合影响。结果表明，接触传热存在2个明显的临界阈值。当温度在150℃以下且界面压力低于22.1 MPa时，存在良好的线性规律。当超过第一阈值后，接触传热明显增强，呈现显著的非线性特征。另外，当界面压力超过50 MPa（第二阈值）且温度超过300℃时，接触传热很快趋于稳定。显然，此时的第二阈值与镁合金带材表面摩擦峰的弹塑性变形直接相关，通过增加微接触面积和摩擦峰的交互扩散，从而形成高压接触传热。基于这一现象的规律特征，有助于精确控制辊缝的接触界面温度，便于设计合适的轧制参数或优化轧制工艺。     

生物医用类金刚石薄膜的力学性能评价

用射频等离子体增强化学气相沉积在钛合金表面制备类金刚石薄膜,利用原子力显微镜、纳米力学探针、划痕仪、滑动摩擦和微动摩擦等研究了涂层的表面形貌和力学性能。结果表明：随涂层厚度的增加,表面粗糙度先增后降,最后趋于稳定,硬度和划痕临界载荷提高,室温空气条件下的滑动摩擦系数减小;对于确定的膜厚,随相对湿度增加,摩擦系数降低,这有利于其在体液环境中的应用。     

Solidification modeling of plasma sprayed TBC: Analysis of remelt and multiple length scales of rough substrates

A two-dimensional, finite-element model based on an enthalpy formulation, was developed to simulate a splat solidifying on a rough substrate (with an idealized, sinusoidal-shaped roughness) capturing the multiple-length scales seen in real coatings as well as different aspect ratios. The model was used to study the effects of substrate temperature, splat temperature, and roughness characteristics on the onset and extent of remelt. Remelt is studied since it is indicative of local heat transfer conditions and might explain observed coating properties. Multiple splats were simulated using the two-dimensional model for short-time cooling coupled to a one-dimensional model for long-time cooling to predict substrate temperature rise prior to subsequent splat impacts. The presence of roughness promoted substrate remelting at conditions under which no remelting was observed for a smooth surface, suggesting that substrate roughness is an important parameter to include in splat solidification studies. The effects of splat temperature and substrate temperature on remelt were consolidated into a single nondimensional parameter, which captured a number of critical phenomena including characterization of the onset of remelt with a nondimensional remelting point.     

Analysis of Thermal History and Residual Stress in Cold-Sprayed Coatings

Residual stress in coatings has significant effect on their performance. In cold-sprayed coatings, in which particles impact the substrate at high velocity in solid state, in-plane residual stresses are usually conceived to be compressive. In this research, analysis of residual stresses in cold-sprayed deposits is performed by analytical and numerical modeling. The influence of various parameters such as the dimensions and elastic properties of the coating and the substrate on the residual stress are analyzed. In addition, the amount of heat input as a key parameter in the build-up of the residual stress is examined. It has been found that the heat input and the associated thermal history have a major influence on the final distortion and the residual stress, to an extent that the in-plane stress can in some cases change from compressive to tensile. Based on these results, a simple model is put forward for the prediction of the final state of the stress and distortion in cold-sprayed flat components.     

Influence of pressure and surface roughness on the heat transfer efficiency during water spray quenching of 6082 aluminum alloy

The heat flux (q) and heat transfer coefficient (h) at the interface between hot aluminum surface and spray water were determined by using an inverse heat conduction method. Good agreements between numerically calculated temperatures with the inverse identified h and experimentally measurements demonstrate that the method is valid for solving the q and h of spray quenching process. The estimated heat flux consists of three main stages of transition boiling, nucleate boiling and single-phase cooling. The results show that both the heat flux and heat transfer coefficient increase with the increasing of spray pressure. When the surface temperature is lower than 170 °C, the q, h and the maximum heat transfer coefficient (h_max) decrease and then increase as surface roughness increases. However, when the surface temperature is higher than 170 °C, the influence of surface is insignificant. This phenomenon may be attributed to the variation of nucleation site density with surface roughness.