本科生毕业设计（论文）专业外文翻译原文：Magnesium alloy electric wheel hub
micro-arc oxidation production research 译文：镁合金电动车轮毂微弧氧化生产研究
指导教师：张清郁职称：讲师
学生姓名：陈孟丽学号：1002130301
专业：机械设计制造及其自动化
院（系）：机电工程学院
2015年4月10日
Magnesium alloy electric wheel hub micro-arc oxidation production research
Most electric vehicles at home and abroad is configured to aluminum alloy wheel hub, its quality, energy saving, shock absorption, noise reduction and vehicle dynamics characteristics index is much lower than magnesium alloys. Magnesium alloy is 30% lighter than aluminum alloy, the damping effect is 30 times that of aluminum alloy. Replace the aluminum alloy with magnesium alloy wheel hub, driving the development of magnesium alloy material development and deep processing technology, to reduce electric vehicle weight and power consumption, energy conservation and environmental protection; To reduce vibration and noise; Improve ride comfort and electric vehicle dynamic characteristics such as objective (transportation quality each reduce 10%, energy consumption will be reduced 8% ~ 10%). But its corrosion resistance is poor, seriously limits the application. Commonly used chemical oxidation and anode oxidation formation of oxide film on magnesium alloy has certain protective effect, but its corrosion resistance, environmental friendliness, appearance is not satisfactory, be badly in need of the development of new surface treatment. In recent years, people trying to develop a variety of new technologies, such as micro arc oxidation technology, the better
One Micro-arc oxidation mechanism
Micro-arc oxidation technology is a new surface treatment technology of green environmental protection, can grow in light metal surface in situ ceramic layer directly. Its technological characteristics, surface treatment, as well as the performance of the since the technology was invented by the favour of people, its mechanism is to light metals such as aluminum, magnesium, titanium and its alloy put in electrolyte aqueous solution as anode, using the method of electrochemical spark discharge spots on the surface of the material, the thermal chemistry, plasma chemistry and electrochemistry, under the joint action of metal oxide ceramic layers of a surface modification technology
Two research methods and technology
This topic in the research on magnesium alloy electric wheel hub, higher requirements on the toughness of the alloy, so choose AM60B, melt and initial temperature of 468 ℃, the melting end temperature is 596 ℃, the liquidus temperature range of 165 ℃. The chemical composition as shown in table 1.
Because of the magnesium alloy electric wheel hub surface area is larger, general above 0.4 m2, require micro-arc oxidation power supply is bigger, this subject adopts the lanzhou university of technology institute of materials and development of MAO - 300 type nc micro-arc oxidation production device (figure 1) micro-arc oxidation on magnesium alloy wheel casting processing, its similar to ordinary anodic oxidation equipment, including special high-voltage power supply, micro-arc oxidation alkaline solution of electrolytic tank, mixing system, cooling system, workpiece with stainless steel plate for peer electrode.
With micro-arc oxidation method in sodium silicate and sodium hydroxide electrolyte fluid system in the preparation of magnesium alloy wheel casting oxide ceramic membrane, the concrete technological process first set oxidation process parameters and the alkaline tank sodium silicate solution, the cleaning after micro-arc oxidation of magnesium alloy wheel casting into cell 15 ~ 20 min, clean with clear water tank 2 ~ 4 min, add hot water in hot water (80 ℃, 10 ~ 15 min), closed, then cool in the cold water tank 2 min, hoisted out drainage, drying, examine the hub. After micro-arc oxidation treatment must be closed by hot water, formed by micro-arc oxidation discharge holes so the distribution of the channel and the surrounding a large number of micro cracks will be closed, prevent oxygen to cause oxidation. After completion of micro-arc oxidation, from after micro-arc oxidation on magnesium alloy wheel casting intercept film sample were analyzed, and to facilitate test analysis, request samples made of circular plate, so the sample interception location choice among wheels, most is shown in figure 2. Using scanning electron microscope analysis of oxide film
Figure 1 MAO - 300 type nc micro-arc oxidation power supply
Figure 2 after micro-arc oxidation magnesium alloy wheel hub casting and
interception of membrane layer analysis sample
Three micro-arc oxidation process parameters on the quality of the film Based on the research of the sample and analysis of micro-arc oxidation technology is, in fact, the substrate magnesium magnesium oxide. Figure 3 for the dimension of samples before and after oxidation appearance schematic simulation, which is suitable for ceramic oxide film a outward growth, namely the increase of size part, b is the depth of the internal oxidation to the matrix, a and b interface for initial sample surface position, h for the total
thickness of oxide film.
Figure 3 samples dimension changes before and after micro-arc oxidation diagram Larger influence on test has a positive voltage, frequency, duty cycle, current density and oxidation time on the process parameters. Due to the electric casting of magnesium alloy surface area is larger, micro-arc oxidation micro-arc discharge must be formed in the surface can occur after a certain thickness of oxide film, so the formation of the oxide film is needed for the voltage doesn't need much, the current is larger, the oxide film formation and the process of thickening, often accompanied by current and voltage mutation. When the oxide film thickness reaches a certain degree, the need to increase the voltage on both ends of the workpiece, usually at around 150 V in the micro arc discharge between the workpiece and the electrolyte. Increased with the increase of voltage, current, micro-arc density is more and more close, more and more bright, and micro-arc constantly beating, basically, the current and voltage, linear increase about 180 V voltage, the density of micro-arc basically meet the technological requirements, the current growth slowly. When the thickness of oxide film reaches a certain electricity
From electric casting magnesium alloys is not hard to find in the micro-arc oxidation test result analysis, micro-arc oxidation in the process can be divided into two steps, namely the oxide film formation stage and the stage of micro-arc oxidation film discharge, the formation of oxide film phase as the initial stage, the stage of the supply voltage is small, and after the film to produce micro-arc discharge requires high voltage, for magnesium alloy electric casting the large workpiece with micro-arc oxidation processing surface area is larger, the film for a long time, to a large extent affected the production efficiency.
Experimental results also found that the dc power of oxide film faster than pulse power, in the absence of micro arc discharge, oxide film layer is not dense, it can be seen
from appearance, need again with pulse power supply for micro-arc oxidation discharge, the oxide film become more dense. In order to improve the production efficiency, to meet the needs of industrial production, suggest early low voltage adjustable dc constant voltage power supply are available to set up the initial oxidation film, forming a complete insulation film in place to ensure that the first phase, and the oxide film in the late discharge can use digital pulse type adjustable power supply, it can shorten the artifacts of micro-arc oxidation time.
The size of the current density in a certain extent reflects the intensity of micro-arc oxidation, strongly affect the resulting performance of the micro arc oxidation ceramic layer. The duration of oxidation also seriously affects the coating corrosion resistance: oxidation time is too short, although generated mainly the dense layer, but the film is too thin, don't have good corrosion resistance; After oxidation time is too long, at some time, with the increase of time, although the overall film thickness increases, but the increase is a loose layer, layer density and thinning trend, does not favor the coating corrosion resistance, also not economic. The density of micro arc also related with the pulse frequency, when the pulse frequency increases, the density of micro arc also gradually increased. Will have the electric field set up suddenly, can produce micro arc. In the basic process parameters such as electrolyte concentration, duty ratio and pulse number of uncertain, the arc voltage is constant commonly, so when the frequency increases, the sustain micro-arc voltage frequency increases, the micro-arc density will increase
Four micro-arc oxide film layer structure characteristics
After micro-arc oxidation of magnesium alloy wheel hub interception by Mef3 large metallurgical microscope observation of the sample, the micro-arc oxide film surface morphology as shown in figure 4. Can be seen from the figure in the wheel hub surface layer is made up of many tiny "small volcanic cone" (figure protuberant part around the holes) in dendritic combination, constitute the mesh structure. "Small volcanic cone" center has a small hole, this is the electrolyte reaction with matrix micro-arc discharge channel, namely when the micro-arc spewed molten oxide channel. In addition, because the current micro area local plasma channel is different that differ by the size of the hole, big hole are also distributed around a large number of micro cracks, the generation of micro cracks often related to the stress that exist in the film. With SSM Analysis Analysis software [6] to
analyze the surface density, including 25 m film for sample, the hole surface area ratio of 18%, that of micro-arc oxidation film density is better.
Figure 4 magnesium alloy wheel hub micro-arc oxide film layer surface morphology
Figure 5 AM60B magnesium alloy micro-arc oxidation film section morphology by SEM Figure 5 is through JMS - 6700 - f field emission scanning electron microscopy (sem) observed the micro-arc oxide film layer section morphology photos. Figure 5 shows the average film thickness of about 22 (including m, the oxide film and substrate with good, decomposition of a distinct, density on the interface is good, no big holes. By figure 5 can also see, micro-arc oxide film by the outermost layer of loose layer, the inside of the transition layer and layer in between density of three parts, the transitional layer is the interface film layer and substrate, holes and other defects existing in the loose layer, dense layer is the key to improve its corrosion resistance.
Figure 6 is obtained by Phlip X 'pert X-ray diffractometer AM60B magnesium alloy wheel hub of micro-arc oxidation film XRD spectrum, according to the intensity of diffraction peak accumulation analysis shows that the matrix of Mg peak relatively obvious, the main phase of micro-arc oxidation coating is cubic structure of MgO style, surface with Mg2Si2O4 and MgAl2O4 spinel phase, according to the test conditions that may also contain SiO2, MgF2 and small amounts of Mg (OH) 2, and the oxide of Al, K and Na. Studies have shown that MgAl2O4 and Mg2Si2O4 can improve the wear resistance of ceramic layer and MgO style the corrosion resistance of ceramic layer play a very important role. This is the micro-arc oxide film performance is higher than the root cause of the anode oxidation membrane performance. In addition, micro-arc oxidation ceramic layers of low porosity, and to improve the corrosion resistance of the coatings; Ceramic layer from the substrate on the growth, combined with matrix closely, therefore, is not easy to fall off. In addition, the technology can generate uniform film both inside and outside the material surface layer, expand the scope of application of micro-arc oxidation.
Figure 6 AM60B magnesium alloy micro-arc oxidation film XRD spectrum
Five To detect the corrosion resistance of the micro-arc oxide film layer In order to meet the requirements of the use of electric cars, micro-arc oxidation on magnesium alloy electric wheel hub on the corrosion resistance test, salt spray testing machine mainly USES the WJ - 90 after micro-arc oxidation treatment of the surface of the wheel hub for salt spray test. After testing found that did not use hot water seal processing of the surface of the wheel hub 48 h corrosion rate was 0.108%, while only 0.073%, after
hot water hole sealing hubs such as chromium than other chemical surface treatment processing of low corrosion rate (0.6%). [9], that magnesium alloy after micro-arc oxidation electric wheel hub surface corrosion resistance is superior. To evaluate a rough check the appearance of the film, feel is very good, membrane layer uniform light show that membrane surface appearance level is higher. Practice shows that without the micro-arc oxidation of the surface of the magnesium alloy wheel casting coating, its poor corrosion resistance, abrasion resistance, in a very short period of time, began to appear on the surface of parts oxidation falls off phenomenon, it is difficult to sell in the market; After micro-arc oxidation treatment, its corrosion resistance, wear-resisting performance is significant
Six The conclusion
(1) quality of micro-arc oxidation on magnesium alloy electric wheel hub surface influence factor has a positive voltage, frequency, duty cycle, current density and oxidation time on the process parameters. Optimum process parameters for 150 ~ 180 V voltage, current density of 1.1 A/dm2, oxidation time to 20 min, 400 Hz frequency, duty cycle of 20%.
(2) the oxide film is divided into two layers of loose layer and dense layer structure, the dense layer is the main body, the film formed by cubic structure of MgO style, the surface is MgO style and MgA12O4, spinel phase mixture, and combined with matrix and closely for hard ceramic layer and played a key role of the magnesium alloy surface anticorrosion
(3) the micro-arc oxidation technology for new surface treatment technology of environmental protection, but its large area needed for the magnesium alloy casting film for a long time, the production efficiency is low, the mass production to meet the large area of magnesium alloy castings, micro-arc oxidation power supply can be established by using dc power first initial oxidation film layer, then use pulse power arc discharge strengthening oxide film layer, the ways which are already so dense and hard ceramic oxide film layer can be obtained, also can greatly improve production efficiency.
镁合金电动车轮毂微弧氧化生产研究
国内外大多数电动车车辆配置为铝合金轮毂，其在质量、节能、减震、降噪和车
辆动力学特性等指标大大低于镁合金。镁合金比铝合金轻30％，减振效果是铝合金
的30倍。以镁合金轮毂代替铝合金轮毂，带动镁合金材料开发和深加工技术发展，
达到降低电动车自重及用电消耗，节能环保；减少振动和噪声；改善驾乘舒适度，提
高电动车辆动力学特性等目的（交通工具质量每减轻10％，能耗将减少8％～10％）。
但它的耐蚀性差，严重限制了其应用。常用的化学氧化和阳极氧化形成的氧化膜对镁
合金有一定的保护作用，但是其耐蚀性、环境友好性、外观等方面还不能令人满意，
急需开发新的表面处理方法。近年来，人们尝试开发了各种新技术，如微弧氧化技术，
较好满足了上述要求。微弧氧化又称为微等离子体氧化，是近年来在普通阳极氧化基
础上开发的一种新技术。它采用较高的能量密度，通过热化学、等离子体化学和电化
学的共同作用，镁合金表面原位形成陶瓷质氧化膜，极大地提高了镁合金铸件表面的
耐蚀性能，具有广阔的应用前景。
一、微弧氧化机理
微弧氧化技术是一种绿色环保的新兴表面处理工艺技术，可直接在轻金属表面原
位生长陶瓷层。其工艺特点明显，表面处理的性能优势突出，自该技术被发明以来，
备受人们的青睐，其机理是将铝、镁、钛等轻金属或其合金置于电解质水溶液中作为
阳极，利用电化学方法在该材料的表面产生火花放电斑点，在热化学、等离子体化学
和电化学的共同作用下，获得金属氧化物陶瓷层的一种表面改性技术。
二、研究方法及工艺
本课题的研究对象是镁合金电动车轮毂，对合金的韧性要求较高，故选用AM60B，
其熔化初始温度为468 ℃，熔化结束温度为596 ℃，固液相线温度区间为165 ℃。
其化学成分如表1所示。
/％)
表1 AM60B合金化学成分(W
B
由于镁合金电动车轮毂表面积比较大，一般在0.4 m2以上，要求微弧氧化电源
较大，本课题采用兰州理工大学材料学院开发的MAO-300型数控微弧氧化生产装置
（图1）对镁合金轮毂铸件进行微弧氧化处理，其装置类似普通阳极氧化设备，包括微弧氧化专用高压电源、碱性溶液电解槽、搅拌系统、冷却系统，工件与不锈钢板为对等电极。
用微弧氧化方法在硅酸钠-氢氧化钠电解液体系中制备镁合金轮毂铸件氧化陶瓷膜，具体工艺流程为先设定氧化工艺参数及配制碱性槽硅酸钠溶液，将清洗后的镁合金轮毂铸件吊入电解槽微弧氧化15～20 min，用清水槽清洗2～4 min，再放入热水中进行热水封闭（80 ℃，10～15 min），然后在冷水槽冷却2 min，吊出轮毂排水、烘干、清理检查。微弧氧化处理完后一定要通过热水封闭，这样因微弧氧化放电形成的小孔通道及周围分布的大量微裂纹会封闭，防止氧气进入引起氧化。微弧氧化完成后，从微弧氧化后的镁合金轮毂铸件上截取膜层试样进行分析，为便于试验分析，要求试样做成圆形片状，故试样截取位置选择轮毂中间较大部分处，如图2所示。利用扫描电镜观察分析氧化膜的表面和对试样进行测量。通过改变电解液的组成、设备工艺参数调整等形式来改变生成膜层的组织和性能，从而得到适合镁合金电动车轮毂需要的膜层和表面质量要求。
图1 MAO-300型数控微弧氧化电源
图2 微弧氧化后镁合金轮毂铸件及截取膜层分析试样处
三、微弧氧化工艺参数对膜层质量的影响
通过对试样的研究和分析，微弧氧化技术实际上是把基体镁变为氧化镁。图3为氧化前后样品外形尺寸的变化示意图的模拟：其中a为陶瓷氧化膜向外生长即尺寸增加部分，b为向基体内部氧化的深度，a、b分界面为样品初始表面位置，h为氧化膜总厚度。
图3 微弧氧化前后样品外形尺寸的变化示意图
对试验影响较大的有正电压、电流密度、频率、占空比以及氧化时间等工艺参数。由于镁合金电动车铸件的表面积比较大，而微弧氧化的微弧放电必须在工件表面形成一定厚度的氧化膜后才能发生，故氧化膜的形成所需的正电压不需要很大，所需的电流则较大，在氧化膜形成并增厚的过程中，往往伴随着电流和正电压的突变。当氧化膜厚度达到一定程度后，则需要增加工件两端电压，通常在150 V左右就会在工件与电解液间产生微弧放电。随着电压的增加，电流不断增大，微弧密度越来越密，越来越明亮，且微弧在不停跳动，此时电流与电压基本上呈线性增加，电压约为180 V，微弧密度基本上达到了工艺的要求，此时电流的增长速度比较缓慢。当氧化膜达到一定厚度时，电压即使增加，电流的增加也很小，基本上趋于恒定，这时在工件上的微弧也逐渐稀薄，微弧氧化过程也就可以停止。如果继续升高电压，微弧就会明显变大，
工件与电极之间会出现电弧光，导致氧化膜被坑蚀，甚至在氧化膜表面出现小裂纹，使工件表面质量变差。微弧是在一定的电场激发下，且在氧化膜与内层金属之间产生的一种放电现象。
从镁合金电动车铸件微弧氧化试验结果分析中不难发现，微弧氧化在工艺过程中可分两步进行，即氧化膜生成阶段和微弧放电氧化膜阶段，氧化膜的形成阶段为初始阶段，该阶段所需的电源电压较小，而成膜后若要产生微弧放电则需要较高电压，对于镁合金电动车铸件这种大型工件来说，由于微弧氧化处理表面积较大，所需的成膜时间很长，很大程度上影响了生产效率。
试验还发现直流电源建立氧化膜的速度要快于脉冲电源，由于没有微弧放电，氧化膜层不致密，这点从外观可以看出来，还需要用脉冲电源来进行再次的微弧放电氧化，使氧化膜变得更加致密。为提高生产效率，适应工业化大生产需求，建议前期可用低压直流可调的恒压电源来建立初始氧化膜，以保证第一阶段形成完整的绝缘膜，而后期的氧化膜放电可采用数字化的脉冲式可调电源来实现，这样可以缩短整个工件的微弧氧化时间。
电流密度的大小在一定程度上反映微弧氧化的剧烈程度，强烈影响着最终得到的微弧氧化陶瓷层的性能。氧化时间的长短也严重影响着膜层的耐蚀性：氧化时间过短，虽然此时生成的主要是致密层，但膜层太薄，不具有很好的耐腐蚀性；氧化时间过长，到了一定时间后，随着时间的增加，虽然整体膜层的厚度在增加，但是增加的是疏松层，致密层还有减薄的趋势，不利于膜层的耐腐蚀性，也不经济。另外微弧的密度还与脉冲频率有关，当脉冲频率增加时，微弧密度也逐渐增加。这时一定电场的突然建立，就会产生微弧。在基本工艺参数如电解液浓度、占空比、脉冲个数确定的情况下，起弧电压一般为恒定，所以当频率增加时，维持微弧的电压频率增加，微弧密度就会增加；当频率减小时，单位时间内出现零电压的时间就会增加，使微弧熄灭的时间变长，所以表现在微弧密度的降低。最终经过大量的生产试验得出最佳参数为电流密度
1.1 A/dm2，氧化时间20 min，频率400 Hz，占空比20％。
四、微弧氧化膜层结构特征
微弧氧化后的镁合金轮毂截取试样后通过Mef3大型金相显微镜进行观察，其微弧氧化膜表面形貌如图4所示。从图中可以看出轮毂表面层是由无数微小的“小火山锥”（图中小孔周围的突起部位）呈枝状结合而构成的网状结构。“小火山锥”中心有小孔，这是电解液与基体微弧放电反应留下的通道，即微弧产生时熔融态氧化物喷
出的通道。此外，由于微区局部等离子体通道的电流不同致使孔的尺寸有差别，大孔的周围还分布有大量的微裂纹，微裂纹的产生往往与膜层中存在的应力有关。用SSM Analysis分析软件[6]对表面致密性进行分析，得到膜层为25 μm的试样，孔洞所占表面积比率为18％，说明生成的微弧氧化膜致密性较好。
图4 镁合金轮毂微弧氧化膜层表面形貌
图5 AM60B镁合金微弧氧化膜SEM截面形貌
图5是通过JSM-6700F场发射扫描电镜观察到的微弧氧化膜层截面形貌照片。图5显示平均膜层厚度约为22 μm，氧化膜与基体结合良好，分解面较清晰明显，界面上致密性较好，无大的孔洞。由图5也可看出，微弧氧化膜层由最外层的疏松层、最里面的过渡层和介于两者之间的致密层三部分组成，过渡层是膜层与基体的交界面，疏松层中存在孔洞和其他缺陷，致密层是提高其耐蚀性的关键所在。
图6是通过Phlip X’pert X射线衍射仪得到的AM60B镁合金轮毂微弧氧化膜XRD图谱，根据衍射峰值累积强度分析表明，基体Mg的峰值较为明显，微弧氧化膜
主体相为立方结构的MgO，表层含有MgAl
2O
4
和Mg
2
Si
2
O
4
尖晶石相，根据试验条件推测，
可能还含有SiO
2、MgF
2
和少量Mg(OH)
2
，以及Al、K、Na的氧化物。研究显示MgAl
2
O
4
和Mg
2Si
2
O
4
能提高陶瓷层的耐磨性，MgO对陶瓷层的抗腐蚀性起到很重要的作用。这
是微弧氧化膜性能高于阳极氧化膜性能的根本原因。另外，微弧氧化陶瓷层的孔隙率低，也提高了膜层的耐腐蚀性能；陶瓷层从基体上生长，因此与基体结合紧密，不易脱落。此外，该技术能在材料内外表面生成均匀膜层，扩大了微弧氧化的适用范围。
图6 AM60B镁合金微弧氧化膜XRD图谱
五、微弧氧化膜层耐腐蚀性检测
为了满足电动车的使用要求，对微弧氧化后的镁合金电动车轮毂进行了耐腐蚀性检测，主要采用WJ-90盐水喷雾试验机对微弧氧化处理后的轮毂表面进行盐雾试验。检测后发现，未采用热水封闭处理的轮毂表面48 h腐蚀率为0.108％，而热水封孔后仅为0.073％，比其他化学表面处理的轮毂如铬化处理的腐蚀率（0.6％）低的多，说明微弧氧化后镁合金电动车轮毂表面耐蚀性能优越。对膜层的外观进行粗略检查评价，手感极好、膜层均匀发亮表明膜层表面外观等级较高。实践表明，未经微弧氧化的镁合金轮毂铸件表面涂装后，其抗腐蚀性、耐磨性较差，在很短一段时间内，零件表面就开始出现氧化脱落现象，很难在市场上销售；经微弧氧化处理后，其抗腐蚀性、耐磨性能得到显著的提高，零件表面几乎没有出现氧化脱落现象，从而大大提高了轮毂的使用寿命，满足了电动车的日常使用要求。综上所述，笔者认为微弧氧化技术在目前对镁合金铸件来说是较理想的表面处理技术。
六、结论
（1）对镁合金电动车轮毂表面微弧氧化质量影响因素有正电压、电流密度、频率、占空比以及氧化时间等工艺参数。最佳工艺参数为电压150～180 V，电流密度1.1 A/dm2，氧化时间20 min，频率400 Hz，占空比20％。
（2）氧化膜分为表面疏松层和致密层两层结构，致密层是膜的主体，由立方结
构的MgO构成，表层则是MgO和MgA1
2O
4
、尖晶石相混合体，与基体结合紧密且为坚
硬的陶瓷层，对镁合金表面防腐起到关键作用。
（3）微弧氧化技术为新型环保表面处理技术，但其大面积镁合金铸件所需的成膜时间较长，生产效率低下，为满足大面积镁合金铸件批量生产，微弧氧化电源可先采用直流电源建立初始氧化膜层，再用脉冲电源起弧放电强化氧化膜层的方式分别进行，这样既可得到致密且坚硬陶瓷氧化膜层，也可大大提高生产效率。