铝合金微弧氧化工艺研究/Investigation on Technology of Micro-arc Oxidation on the Surface of

2018-11-28 18:04:32

film ceramic 氧化 oxide 电解液



铝及其合金在工业上的应用越来越广泛,但铝及其合金的表面硬度低,耐磨损性能差,对卤化物溶液和无机酸等溶液的耐腐蚀性差,制约了铝合金的应用。可以通过表面涂装、热喷涂、气相沉积、电镀、化学转化膜、电化学氧化等表面处理方法对铝合金表面进行处理,提高铝合金表面性能,其中较常用的是电化学氧化方法中的阳极氧化工艺。阳极氧化工艺基本原理是,将铝或铝合金在碱性或酸性电解液中作为阳极进行电化学氧化,获得具有良好机械性能及耐腐蚀性能的氧化膜。氧化膜主要由无定形氧化物组成。
微弧氧化工艺又称为微等离子体氧化工艺或阳极火花沉积工艺,是在阳极氧化工艺基础上发展而来的新兴表面处理工艺。这是一种在轻合金表面通过微等离子体放电,进行复杂的电化学、等离子化学和热化学过程原位生长氧化物陶瓷膜层的新工艺。微弧氧化工艺生成的陶瓷氧化膜厚度可达200~300μm,显微硬度最高可达HV2000以上,极大提高了铝合金的应用范围。
本文通过一系列以交流电为电源的铝合金微弧氧化工艺实验,研究微弧氧化过程中陶瓷氧化膜层的生长规律,不同电解质组成和浓度对陶瓷氧化膜生长的影响,以及微弧氧化过程中电极间电压、试样表面电流密度及微弧氧化时间等工艺参数对陶瓷氧化膜生长的影响。采用扫描电镜(SEM)及X射线衍射相结构分析(XRD)对陶瓷氧化膜微观形貌及膜层结构进行分析,对陶瓷氧化膜的厚度、显微硬度、耐热冲击性、耐腐蚀性及抗外力冲击能力进行了测试。
SEM分析表明,陶瓷氧化膜层由疏松层和致密层构成,致密层约为整个膜层厚度的2/3。XRD分析表明陶瓷氧化膜中含有γ-Al2O3和α-Al2O3晶相成分,其中α-Al2O3相随氧化时间增多。试样经微弧氧化处理后,再进行500℃×60min的热处理,可以使陶瓷氧化膜中非晶相向γ-Al2O3相转变。
陶瓷氧化膜性能测试表明氧化膜具有良好的抗热冲击性,经过10次600℃-25℃激热-激冷冲击,氧化膜表面无裂纹;氧化膜耐腐蚀性也很好,在20℃的10%NaCl溶液,10%HCl溶液,10%NaOH溶液中浸蚀72小时,试样在分析天平测量范围内无失重。陶瓷氧化膜上的孔隙降低表面防腐蚀能力,对孔隙封闭处理后可大幅提高腐蚀防护能力。其抗外力冲击能力也较强,但铝合金基体的机械性能制约了氧化膜的抗外力冲击能力。陶瓷氧化膜的显微硬度达HV1800。
实验表明,陶瓷氧化膜层的生长过程可分为六个步骤:1.表面阻挡层的生成,即微弧氧化初始阶段试样表面会很快形成一层致密的无定形氧化膜;2.阻挡层的电击穿。升高电极间电压,使阻挡层电击穿,产生火花放电;3.无定型氧化膜层的生长。电极间电压及试样表面电流密度较低时,火花放电较弱,陶瓷氧化物形成较少,主要是无定形氧化膜的生长;4.局部陶瓷氧化膜层的形成。即无定形氧化膜在微等离子体放电产生的微区高温高压作用下瞬间融化,又在电解液作用下迅速凝固,晶化为含有γ-Al2O3和α-Al2O3晶相成分的晶型陶瓷氧化物颗粒,陶瓷氧化物颗粒不断长大,与邻近的陶瓷氧化物颗粒互相烧溶连接,形成局部陶瓷氧化膜;5.陶瓷氧化膜的电击。即电极间电压及试样表面电流密度增高,使陶瓷氧化膜电击穿;6.陶瓷氧化膜层的生长,陶瓷氧化膜增厚,等离子体放电发生在氧化膜的放电孔隙中,熔融氧化物从孔隙喷出瞬间凝固,陶瓷氧化膜不断生长。
实验表明,电解液的性质是微弧氧化工艺中关键的因素。电解液中电解质的组成和浓度对微弧氧化的电参数有直接影响。不同电解质组成的电解液,微弧氧化的起弧电压不同,对试样表面电流密度的影响也不同。电解液浓度升高,起弧电压下降,电极表面电流密度上升。
实验表明,电解液的温度、电极表面积以及电解液在电解槽中的截面积都对微弧氧化过程电参数有影响。电解液温度上升,试样表面电流密度增大,电解液对氧化膜的溶解速度也增大,电解液温度升高总的效果是对氧化膜的生长和膜层的厚度与性能造成不利影响;电极表面积增大会使试样表面电流密度减小,起弧电压升高;电解液在电解槽中的截面积增大相对提高了电解液中的传质效率,使试样表面电流密度增大。试样的外形对氧化膜膜层的均匀性有影响,圆柱形试样上氧化膜的均匀性比方形试样好。
实验表明,氧化膜的溶解速度对陶瓷氧化膜的生长有重要影响。微弧氧化陶瓷膜的形成过程是:氧化膜的生成与溶解交替发生,生成速率大于溶解速率的过程。在一定条件下,适当增大膜层的溶解速率可以增快膜层的生长。不同电解质组成电解液对氧化膜的溶解能力不同,配制电解液时,可以通过加入具有不同溶解速度的电解质来控制电解液对氧化膜的溶解速度,使氧化膜的生长速度提高,膜层厚度增大,以获得最佳成膜效率。实验中在3g/l的Na2SiO3加4g/l的KOH加10g/l的H3BO3电解液中进行的微弧氧化实验取得了较好结果,在较短的氧化时间内获得了较厚的陶瓷氧化膜层。
实验表明,提高电解液的浓度可提高陶瓷氧化膜的成膜速度,增加氧化膜的厚度。当电解液浓度增高到一定值时,浓度的增加对氧化膜生长的影响力减弱,氧化膜的成膜速度提高不大,膜层厚度也趋近一极限值,而且电解液的浓度的升高使起弧电压也趋近一极限值,但试样表面电流密度却增大,增加了能量消耗,氧化膜的性能反而有所降低。
实验表明,试样表面电流密度对氧化膜的生长有较大影响。电流密度较低,试样表面积聚的能量低,无法击穿氧化膜;增大电流密度可提高氧化膜的生长速度,增加氧化膜膜厚;电流密度增加太大会导致弧光放电,破坏氧化膜。
本文通过大量实验,较系统地对铝合金表面微弧氧化工艺进行了研究,并找出了一些工艺参数对氧化膜生长的影响规律。文章最后总结了作者的研究工作,并探讨了今后应当进一步研究和解决的问题。



Aluminum and its alloy are widely used in industry. But their application is limited by their low surface hardness, poor wear-resistance and poor anti-corrosion to the solution of halide and inorganic acid. To improve the surface capability of aluminum alloy, we can treat its surface by using surface painting, thermal spraying, PVD, electroplating, chemical transform film, electrochemistry oxidation and so on surface treatments. Among them, the technology of anodic oxidation is often used. The basic principle of anodic oxide technology is that aluminum or its alloy is electrochemistry oxidized as positive in the alkaline or acid electrolyte; then the oxidation film can be formed, which has excellent machinery and corrosion-resistance properties. Oxide-film is mainly composed of amorphous oxides.
Micro-arc oxidation is also called micro-plasma oxidation or positive spark deposit technology. It is a new surface treatment technique, which is developed from anodic oxidation. By micro-plasma discharging, through complex electrochemistry, plasma-chemistry and thermal-chemistry processes, ceramic oxide-film is created on the surface. The thickness of ceramic oxide-film that created by micro-arc technology can be up to 200~300μm, the micro-hardness can be over HV2000, which extends the application range of aluminum alloy.
By a series of micro-arc oxidation experiments, I have studied the law of the growth of ceramic oxide-film, such as the action of different composition and concentration of electrolyte influence on the growth of ceramic oxide-film, and the influence by the voltage between electrodes, current density and oxidizing time. Using SEM and XRD, I have analyzed the micro-shape and fabric of ceramic oxide-film, and tested the capability of anti-heat-impact, anti-corrosion, and anti-external strike of ceramic oxide-film.
The photos of SEM show that ceramic oxide-film is made up of two layers, the external porous layer and the internal denser one. The denser layer is as about 2/3 thick as the whole layer. The analysis using XRD points out that there are γ-Al2O3 and α-Al2O3 crystal in the ceramic oxide-film. The more oxidizing time the moreα-Al2O3. If we heat treat the ceramic oxide-film by 500℃×60min treatment, γ-Al2O3 will increase.
The capability tests show that oxide-film has excellent anti-heat strike ability. The oxide-film surface still no slight crack after ten times of 600℃-25℃ rapid cooling-rapid heating impact. The anti-corrosion of oxide layer is very good. At 20℃, when it is placed in 10%NaCl solution, 10%Hcl solution, 10%NaOH solution, the samples don’t lose their weight within the measure range of the scale. The small opening on the ceramic film will decrease the surface anti-corrosion ability, but which will be greatly improved after being sealed. Its ability of anti-external strike is strengthened too, but the mechanical property of the aluminum alloy matrix restricts the anti-external strike ability of the oxide-film. The micro-hardness of ceramic oxide-film can be up to HV1800 at most.
Experimentations indicate that the process of growth of ceramic oxide-film has six phases: 1. Creating of surface obstruction film. The surface of sample quickly forms a dense amorphous oxide-film. 2. Electro-puncture of obstruction film. Through increasing the voltage between electrodes, obstruction film is punctured by electricity, and spark discharge happen. 3. Growth of amorphous oxide-film. Because the voltage between electrodes and current density is low, spark discharge is weak; so less ceramic oxide is found, mainly more amorphous oxide-film grows. 4. Creating of local ceramic oxide-film. By the action of high temperature and high-pressure that produced by micro-plasma discharge, amorphous oxide-film melts quickly, then freezes quickly by the action of cool electrolyte. Amorphous oxide melts into ceramic oxide-grain, which contains γ-Al2O3 and α-Al2O3 crystal. Oxide-grain grow successively, and it joins the neighborhood Oxide-grains. The local ceramic oxide-film creates. 5. Electro-puncture of ceramic oxide-film. With the increasing of voltage and current density, ceramic oxide-film is punctured by electricity. 6. Growth of ceramic oxide-film. Ceramic oxide-film become thick, micro-plasma discharge happens in the discharge hole of oxide-film. The melt oxide ejects from the hole, and then freezes quickly. So ceramic oxide-film is growing continuously.
Experimentations indicate that the quality of electrolyte is the key factor of technology of micro-arc oxidation. The composition and concentration of electrolyte have direct influence on the electricity parameters of micro-arc oxidation. In different composition of electrolyte, the arc-voltage is different, and the influence on current density on sample surface is different. With increasing of concentration, the arc-voltage declines, and the current density rise.
Experimentations indicate that the temperature of electrolyte, the surface area of electrode and the section area of electrolyte in the electrolysis cell has influence on the electricity parameters of micro-arc oxidation. When the temperature of electrolyte rise, and the current density increases, the dissolving speed of oxide-film in the electrolyte increases too. High temperature of electrolyte is harmful to the growth and capability of oxide-film. Enlarge surface area of electrode can decrease current density, and increase arc-voltage. Enlarge the section area of electrolyte in the electrolysis cell can raise the efficiency of transmission, hence increase current density, the shape of the sample also has influence on the homogeneity of the oxide-film. The homogeneity of oxide-film in shape of cylinder is better than that of square shape.
Experimentations indicate that the dissolving speed of oxide-film in the electrolyte has important influence on the growth of ceramic oxide-film. Different electrolyte has different capability of dissolving to the oxide-film. Compounding different electrolytes can control the dissolving speed of oxide-film. It can increase the growth speed and thickness of oxide-film. In a relatively short time, thicker ceramic film was obtained by micro-arc experimentation in the electrolyte, which is composed of 3g/l Na2SiO3 and 4g/l KOH and 10g/l H3BO3.
Experimentations indicate that increasing the concentration of electrolyte can increase the growth speed and thickness of oxide-film. When the concentration goes up at to a certain value, the action that influenced on the growth of oxide-film by the increasing of concentration weakens. With increasing of concentration, thickness of oxide-film tends to a limit value, and the capability of oxide-film depress.
Experimentations indicate that the change of current density on the sample surface has quite big influence on the growth of oxide-film. If current density is small, the energy gathering on the sample surface is low, and it can’t puncture the oxide-film. Increasing the current density can increase the growth speed and thickness of oxide-film. But too big current density can lead to arc-discharge, and oxide-film will be destroyed.
Through lots of experimentations, I have made a systematic study to the technology of surface micro-arc oxidation on the Aluminum alloy, and found some law of technological parameters influencing on the growth of oxide-film. Finally, I’d like to sum up my studying work, and discussed some problems, which need further study in the future.