Heat Transfer during the Solidification of Hot Dip Aluminizing Coating

Hot dip aluminizing is one of the most effective methods of surface protection for steels and is gradually gaining popularity.Although the pulling speed is one of the most important parameters to control the coating thickness of aluminizing products,however,there are few publications on the mathematical modeling of pulling speed during the hot dip process.In order to describe the correlation among the pulling speed,coating thickness and solidification time,the principle of mass and heat transfer during the aluminizing process is investigated in this paper.The mathematical models are based on Navier-Stokes equation and heat transfer analysis.Experiments using the self-designed equipment are carried out to validate the mathematical models.Specifically,aluminum melt is purified at 730 ℃.The Cook-Norteman method is used for the pretreatment of Q235 steel plates.The temperature of hot dip aluminizing is set to 690 ℃ and thedipping time is set to 3 min.A direct current motor with stepless speed variation is used to adjust the pulling speed.The temperature change of the coating is recorded by an infrared thermometer,and the coating thickness is measured by using image analysis.The validate experiment results indicate that the coating thickness is proportional to the square root of pulling speed for the Q235 steel plate,and that there is a linear relationship between coating thickness and solidification time when the pulling speed is lower than 0.11 m/s.The prediction of the proposed model fits well with the experimental observations of the coating thickness.

Structures of ceramics coatings on steel fabricated by hot-dipping and micro-arc oxidation

Firstly, an aluminum coating was produced metallurgically on mild steel by hot-dipping, then an aluminum oxide coating was formed self-growingly from the aluminum coating by micro-arc oxidation treatment. The structures of the composite coatings were investigated by means of SEM, TEM and XRD. The results show that the composite coating consists of three layers which are Fe-Al alloy, aluminum coating and aluminum oxide orderly outward from the steel substrate. There are amorphous phases, k-Al2O3 and θ-Al2O3 mainly in the aluminum oxide.

Hot-Dip Coating of Lead-free Aluminum on Steel Substrates with Ultrasonic Vibration

Hot-dip coating has been practically employed in manufacturing zinc alloy coated steel sheets.However,it is difficult to coat aluminum alloy on a bulky steel substrate without sufficient preheating,because a rapidly solidified layer containing gas babbles is formed on a substrate surface.A variety of iron-aluminides are also formed at the interface of a steel and aluminum hot-dip coating system,which is the main difficulty in joining of steel with aluminum.Ultrasonic vibration was applied to a steel substrate during hot-dip coating of aluminum alloy to control a rapidly solidified layer and a brittle reaction layer.Hot dipping of columnar steel substrates into molten aluminum alloy （Al-2.7 mass fraction Si-4.6 mass fraction Sn） was carried out through the use of a Langevin oscillator with resonant frequency of 19.5 kHz.The application of ultrasonic vibration is quite effective to control a rapidly solidified layer and a surface oxide layer from a substrate surface by the sonocapillary effect based on a cavitation phenomenon,so that the intimate contact is achieved at the beginning of hot-dip coating.The application of ultrasonic vibration to hot-dipping is effective to control a reaction layer with less than 5 #m in thickness.An impact test exhibits that the good adhesive strength is approved in hot-dipped aluminum coatings with a thin reaction layer of approximately 5μm.

Analysis of Coating Microstructure of Hot-Dip Aluminum of Deformed Low-Carbon Steel Containing Rare Earth

The coating microstructure of hot-dip aluminum (HDA) of deformed low-carbon steel containing RE was analyzed by metallography microscopy, TEM and XRD, and the forming mechanism was also discussed. The results show that, the Fe_2Al_5 phase, on whose subcrystal boundaries, Al particles with the size of 7～30 μm existing on parallel linear are, grows a strong orientation. And the spread activation energy of Al is 155.22 kJ·mol -1. In addition, the effects of deformation on coating microstructure of hot-dip aluminum and the function of RE were preliminarily analyzed.

ELECTROCHEMICAL CORROSION BEHAVIOR OF HOT DIP COATING IN ARTIFICIAL SEAWATER

The corrosion behavior of a zinc-baased alloy (compared with that of pure zinc) coating was studied in this research on the electrochemical corrosion behavior of hot dip coating in artificial seawater. The electrochemical parameters of these two hot dip coatings were measured by the galvanostatic method and accelerated corrosion test. The two coatings showed uniform corrosion, but the corrosion of the pure zinc coating was actively sustained continuously. The corrosion of the zinc-based alloy coating was very slow because of the formation of a stable anti-corrosion membrane. The corrosion mechanisms of these two coatings were also compared by the weak polarization curve fitting method. The corrosion of the zinc-based alloy coating is typical of that in natural passive corrosion system.

Effect of Mischmetal on Oxidation Resistance of 55Al－Zn Alloy Hot Dip Coating

The effect of mischmetal addition on high temperature oxidation resistance of 55wt% Al43.4 wt% Zn-1 .6wt% Si alloy hot-dip coatings has been investigated. It is found that rare earth addition improves high temperature oxidation resistance of the coatings. The oxidation tests at 800℃, 100 h and 1000℃, 50 h show that the coating with addition of 0. 1 % RE has the best properties. The morphology of oxide scale and element distribution of coating section were analysed by SEM, EPMA and XRD. It is indicated that mischmetal addition improves the adhesion between oxide scale and coating substrate, and spalling resistance of the scale is also improved with the addition of RE. Additionally, RE controls the degeneracy speed of Al-content in the coatings and inhibits the growth of Fe-Al intermetallic compound. For this reason, higher Al-content is kept in all the coatings with RE addition.

Effect of Pb content on the surface topography and corrosion resistance of hot dip Zn-Al-Mg coatings

In this study,the effect of Pb content on the surface morphology and salt spray corrosion resistance of hot dip Zn-AI-Mg coatings was investigated. The results showed that the coating surface easily formed small grains of zinc spangle structures and that the salt spray con＇osion resistance of the coating decreased when Pb content was greater than 0.01%. The microstructure and energy dispersive spectrum analysis of surface and cross-sectional areas was performed by scanning electron microscopy. Pb content present in the coating was analyzed by glow discharge spectrum. The results showed that the distribution of Pb in the coating was not uniform. The Pb content was segregated on the surface and at the cross-section of the Zn-A1-MgZn2 ternary eutectic structure,especially,on the surface of the Zn-A1-MgZn2 ternary eutectic structure.

Formability of aluminum-silicon coated boron steel in hot stamping process

Flow behavior of the Al-Si coated boron steel was investigated with Gleeble-3500,in comparison with the uncoated one.Effect of deformation conditions on the coating integrity was characterized by optical microscopy.Facture surfaces of the coated steels were inspected under SEM.Experimental results indicate that the ultimate tensile strength and ductility of the Al-Si coated boron steel are lower than those of the uncoated steel under test conditions.Extensive cracks occur in the coating after tensile tests;the width and density of cracks are sensitive to the deformation temperatures and strain rates.The bare substrate exposed between the separate coating segments is oxidized.Appearance of the oxide degrades the Al-Si coating adhesion.Remarkable difference between formability of the coating layer and the substrate is confirmed.The formability of the Al-Si coating could be optimized by controlling the phase transformation of the ductile Fe-rich intermetallic compounds within it during the austenization.

Formation and structure of composite coating of HDA and micro-plasma oxidation on A3 steel

Composite coatings were obtained on A3 steel by hot dipping aluminum(HDA) at 720 ℃ for 6 min and micro-plasma oxidation(MPO) in alkali electrolyte. The surface morphology, element distribution and interface structure of composite coatings were studied by means of XRD, SEM and EDS. The results show that the composite coatings obtained through HDA/MPO on A3 steel consist of four layers. From the surface to the substrate, the layer is loose Al2O3 ceramic, compact Al2O3 ceramic, Al and FeAl intermetallic compound layer in turn. The adhesions among all the layers are strengthened because the ceramic layer formed at the Al surface originally, FeAl intermetallic compound layer and substrate are combined in metallurgical form through mutual diffusion during HDA process.Initial experiment results disclose that the anti-corrosion performance and wear resistance of composite coating are obviously improved through HDA/MPO treatment.

Evolution of appearance profile of hot-dip galvanized car-body steel sheet during deformation process

In recent years, the waterborne free intermediate coating process has been widely used in the automotive industry. Because the baking times and coating thickness are decreased, the surface covering capability of the painting process is reduced, which directly affects the appearance quality（ long-and short-wave values） of the body paint. Thus, there are correspondingly higher requirements for the white body surface profile prior to painting. The surface profile of the white body is mainly affected by the plate material, the surface profile, and the deformation process. So,the change rule for the surface profile during deformation of the steel plate is a key factor in coating appearance optimization. In this paper, we first analyze the typical deformation of the outer cover of a car body. Then ,we examine the change tendency of the surface profile of steel plates with respect to different deformation rates, specifically for a steel plate comprising a hot-dip galvanized bake-hardened steel sheet. Based on our analysis of the influence of the deformation on the coating appearance,we selected 3% ,5% ,and 8% deformation rates in this research. We found the roughness （Ra） value in the typical deformation range （3% -8% ） of the car body to exhibit a decreasing trend at first and then an increasing trend. The Ra value of the 8% deformation is not more than the original plate test value. When the Pc value of the original plate is in the lower range （ about 60）, it exhibits a slight increasing trend in the deformation process （3 % -8 % ）. And when the Pc value of the original plate is in the higher range （ about 120 ）, it exhibits no increasing trend in the deformation process （ 3% -8% ）. In contrast,the waviness （WCA） value in the car body＇s typical deformation range （3%-8%） shows a significant growth trend.

A_3钢热镀Al-Si层的微弧氧化及其耐蚀性

用低碳钢表面热浸镀 Al- Si合金后进行微弧氧化的方法获得复合膜层。分析了热镀层的表面质量对形成陶瓷层的影响 ,对复合膜层进行了断面形貌观察和 X射线衍射分析。测定了复合膜层在不同浓度 Na Cl溶液及海水中的耐蚀性。结果表明 :微弧氧化要求热镀层厚度均匀 :陶瓷层由 α- Al2 O3 ,k- Al2 O3 及单质 Al,Si组成 ;复合层由三大部分组成 :最外层为陶瓷层 ,次层为 Al- Si合金层 ,再向内至基体为 Fe- Al- Si,Fe- Al,Fe- Si合金层。陶瓷层的耐蚀性优于热镀层 ,更好于低碳钢。

Enhancing ductility of the Al-Si coating on hot stamping steel by controlling the Fe-Al phase transformation during austenitization

In this study, austenitizing heat treatment before hot stamping of Al-10% Si coated boron steel is first investigated through en- vironment scanning electron microscopy （ESEM） equipped with energy dispersive x-ray analysis （EDAX）. The cracking be- havior of the coating was evaluated using Gleeble 3500, a thermo-mechanical simulator under uniaxial plastic deformation at elevated temperatures. The extent and number of cracks developed in the coating were carefully assessed through an optical microscope. The coating layer under hot-dipped condition consists of an Al-Si eutectic matrix, Fe2Al7Si, Fe3Al2Si3 and Fe2Al5, from the coating surface to the steel substrate. The coating layer remains dense, continuous and smooth. During austenitization, the Al-rich Fe-Al intermetallics in the coating transform to more Fe-rich intermetallics, promoted by the Fe diffusion process. The coating finally shows the coexistence of two types of Fe-Al intermetallics, namely, FeAl2 and FeAl. Microcracks and Kirkendall voids occur in the coating layer and diffusion zone, respectively. The coating is heavily cracked and broken into segments during the hot tensile tests. Bare steel exposed between the separate segments of the coating is oxidized and covered with a thin FeOx layer. The appearance of the oxide decreases the adhesion of the Al-Si coating. It is found that the ductile FeAl is preferred as a coating microstructure instead of the brittle FeAl2. Therefore, the ductility of the Al-Si coating on hot stamping boron steer could be enhanced by controlling the ductile Fe-rich intermetallic phase transformations within it during austenitization. Experiments indicate that a higher austenitizing temperature or longer dwell time facilitate the Fe-rich inter- metallics transformation, increasing the volume fraction of FeAl. This phase transformation also contributes to reducing the crack density and depth.

Influence of Air-Knife Wiping on Coating Thickness in Hot-Dip Galvanizing

In hot-dip galvanizing process, air jet wiping control is so crucial to decide the coating thickness and uni- formity of the zinc layer on the steel strip. The mathematical models developed predict the zinc coating thickness as a function of pressure and shear stress. The required pressure and shear stress profile on the strip surface were calcu- lated using regression analysis, and carried out using numerical simulation as FLUENT, a finite element analysis software. The influences of the outlet pressure, the nozzle to strip distance, the slot opening, the edge baffle plate, as well as the tilting angle of air knife were discussed. Combining with these results and regression analysis on the practical data, four first-order polynomial multi-parameter models were established for different targeted coating thicknesses with better regression coefficients. The validated model was used to carry out sensitivity analysis to de- termine the favorable controlling regime for the air jet wiping process.

Manufacturing process and microstructure of copper-coated aluminum wires

Copper-coated aluminum wires exhibit good electrical conductivity, high thermal conductivity, low contact resistance of copper and low density, and provide economic advantages over aluminum. However, there are some problems in the manufacring processes of hot-dip copper-coated aluminum wires, such as the difficulties in controlling coating process. In this work, the hot-dip copper-coating method of aluminum wires was investigated for producing copper-coated aluminum wire composites. The interface microstructure between the aluminum wire and the copper coating layer was analyzed by scanning electron microscopy （SEM） and energy-dispersive X-ray spec- trometry （EDS）. Five different fluxing agents were tested. Experimental results show that appropriate conditions for the hot-dip process are determined as the liquid copper temperature of 1085℃ and the treatment time less than 1 s. A success in hot-dip copper-coated aluminum wires is achieved by hot-dipping a low-melting-point metal into a high-melting-point metal liquid, which is significant for the further devel- opment and application of copper-coated aluminum wire composites.

Mechanism for colouring hot-dip zinc alloy coating

Hot-dip galvanizing is a widely used technique to prevent steel from oxidizing in air.However, only silver-grey zinc coating can no longer meet the need of people. At theWorld’s Fair in July 1989, Japanese first exhibited blue, red, yellow green andbrown hot-dip zinc coating sheets. These patent coatings are more incorruptible thantraditiona ones. This note tries to reveal the colouring mechanism of these coatings.

涂镀铝+微弧氧化工艺制备复合涂层研究进展

采用涂镀铝+微弧氧化的复合工艺可在钢材、镁合金、钛合金、陶瓷甚至高分子等多种材料表面制备以氧化铝为主的复合涂层,从而提高材料的绝缘、耐磨、耐腐蚀等性能。本文概述了热浸镀、喷涂、物理气相沉积、熔盐电镀以及包埋渗等涂镀铝工艺与微弧氧化工艺相结合制备复合涂层的国内外研究进展,重点分析了各种涂镀铝+微弧氧化工艺制备复合涂层的结构特点、影响因素及工艺的优缺点,最后对涂镀铝+微弧氧化复合工艺进行展望,并提出该领域亟待解决的问题。

热浸镀Zn-Al-Mg镀层在模拟湿热海洋大气环境中的腐蚀行为研究

目的 为详细研究热浸镀Zn-Al-Mg镀层在模拟湿热海洋大气下的腐蚀行为及作用机理,同时为热浸镀Zn-Al-Mg镀层在湿热海洋大气环境中服役提供数据参考。方法 采用腐蚀失重、XRD、SEM、电化学等测试方法对热浸镀Zn-Al-Mg镀层在模拟湿热海洋大气环境下的腐蚀行为进行研究。结果 腐蚀产物主要由Zn_(5)(OH)_(8)Cl_(2)·H_(2)O组成,腐蚀一段时间后,发现少量ZnO、Zn_(5)(OH)_(6)CO_(3),腐蚀产物具有与锌腐蚀类似的层状结构,1 848 h呈“三明治”型,相比于上下两层暗色物质,中层亮色腐蚀产物富集更多的Cl元素。热浸镀Zn-Al-Mg镀层腐蚀速率大体随时间延长呈上升趋势,只在672~840 h腐蚀速率下降,对比镀锌在模拟环境和锌在湿热大气环境中的腐蚀,热浸镀Zn-Al-Mg镀层在模拟湿热海洋大气中表现出较好的耐蚀性。结论热浸镀Zn-Al-Mg镀层在模拟湿热海洋大气下腐蚀产物演变与腐蚀过程中Mg的参与有关。腐蚀672~840h阶段腐蚀速率下降的原因与腐蚀产物中ZnO的减少和Zn_(5)(OH)_(8)Cl_(2)·H_(2)O占比增加有关。对比镀锌在模拟环境和锌在湿热大气环境中的腐蚀,预测热浸镀Zn-Al-Mg镀层在严酷湿热海洋大气中仍具有较高的耐蚀性,可以优先考虑作为湿热海洋环境的建设用材。

改良森吉米尔法热镀锌带钢针状漏镀缺陷原因分析及控制

[目的]探究改良森吉米尔法连续热镀锌生产线上高强支架钢表面针状漏镀缺陷的产生机理,提出相应的控制措施。[方法]通过激光共聚焦显微镜、扫描电镜等对针状漏镀缺陷的形貌特征和元素分布进行了分析。[结果]漏镀缺陷在带钢表面随机分布,呈黑色针眼状。在漏镀缺陷的亚表面存在Cr、Mn等合金元素富集现象,这些合金元素在带钢表面发生外氧化,使得表面可镀性下降,从而产生针状漏镀缺陷。[结论]在明火加热段对带钢预氧化,以及在退火段促进合金元素发生内氧化,可以有效预防针状漏镀缺陷的产生。

Sr元素含量对Zn-2.0%Al-1.2%Mg镀层的微观组织与耐蚀性的影响

为了进一步研究微量合金元素对镀层微观组织的调控以及耐蚀性能的影响机理,采用双镀法在Q235钢板上制备Zn-2.0%Al-1.2%Mg-x%Sr(x=0、0.03%、0.06%、0.09%、0.12%,质量分数)镀层,通过扫描电子显微镜(SEM)、X射线衍射仪(XRD)、5%NaCl浸泡实验等方法探究Sr元素含量对Zn-2.0%Al-1.2%Mg镀层的微观组织与耐蚀性的影响。结果表明:添加适量的Sr元素,可以有效细化Zn-2.0%Al-1.2%Mg镀层表面凝固组织中的初生η-Zn相,引导生成Zn/Al/MgZn_(2)三元共晶组织,使中间合金层中FeAl_(3)-Zn_(x)柱状晶变得更加致密。当Sr的添加量为0.09%时,镀层表面凝固组织的细化效果最为明显,中间合金层最薄且致密,此时镀层的浸泡腐蚀速率最低,耐蚀性最好。

稀土元素对桥梁缆索用热浸镀Zn-10Al镀层钢丝耐蚀性能的影响

通过双镀法制备Zn-10Al-xRE(x=0.05,0.08,0.11,0.14,0.17)五种热浸镀锌铝镀层,并利用中性加速盐雾试验、电化学实验及扫描电子显微镜(SEM)分析其腐蚀形貌及耐蚀性能。结果表明,五种镀层在中性加速盐雾试验下,可保证基体在120d以内不被腐蚀,而纯锌镀层在30d时已经出现红锈,证明Zn-10Al-xRE的耐蚀性能可达到纯锌镀层4~5倍。Zn-10Al-xRE镀层钢丝的耐蚀性能随稀土含量的上升先升高再降低,添加0.08%RE的Zn-10Al镀层耐蚀性最好。用SEM对120d盐雾试验下的镀层钢丝进行观察,可以看出纯锌镀层的腐蚀产物较为疏松,Zn-10Al-xRE的腐蚀产物较为致密,Zn-10Al-xRE中腐蚀产物大约分为韧窝状、球状及针片状,耐蚀性能最好的Zn-10Al-0.08RE镀层中含有大量韧窝状腐蚀产物。