钛合金的蚀刻机加工工艺Etching machining technology of titanium alloy

一、钛和钛合金的概述：

　　Ti在地壳中的丰度为0.56%(质量分数,下同),在所有按元素中居第9位,而在可作为结构材料的金属中居第4位,仅次于Al、Fe、Mg,其储量比常见金属Cu,Pb,Zn储量的总和还多。我国钛资源丰富,储量为世界第一。钛合金的密度小,比强度、比刚度高,抗腐蚀性能、高温力学性能、抗疲劳和蠕变性能都很好,具有优良的综合能,是一种新型的、很有发展潜力和应用前景的结构材料。近年来,世界钛工业和钛材加工技术得到了飞速发展,海绵钛、变形钛合金和钛合金加工材的生产和消费都达到了很高的水平,在航空航天领域、舰艇及兵器等军品制造中的应用日益广泛,在汽车、化学和能源等行业也有着巨大的应用潜力。

　　2001年以来钛得到了更加广泛的应用，在一些新兴市场上应用的迹象得到了增长，如汽车、石油和天然气。从长远来看，汽车上的用钛量将不断增加，成为钛应用的一个更有保障的竞争领域。在高端消费品领域钛合金的应用也已经基本成熟，如钛合金在高尔夫球杆和网球拍上的应用。当今社会老龄化问题越来越严重，医药市场对钛需求量不断增加，每年正以5%～7%的速度增长，在生物医药部门钛已成为另一个热门且具有广阔的应用发展前景。以上新兴市场钛的消费量每年都呈增长趋势，2001年钛消费量为6000吨，2006年增加到10000吨。2007年以后随着经济的发展和国防事业的需要，国际市场对钛合金产品的需求大幅增加，特别是航空航天领域将成为未来几年国际市场的主要消费和应用领域。

　　钛及钛合金具有许多优良特性,主要体现在如下几个方面:

　　(1)比强度高。钛合金具有很高的强度,其抗拉强度为686~1 176 MPa,而密度仅为钢的60%左右,所以比强度很高。

　　(2)硬度较高。钛合金(退火态)的硬度HRC为32~38。

　　(3)弹性模量低。钛合金(退火态)的弹性模量为1.078@105~1.176@105MPa,约为钢和不锈钢的一半。

　　(4)高温和低温性能优良。在高温下,钛合金仍能保持良好的机械性能,其耐热性远高于铝合金,且工作温度范围较宽,目前新型耐热钛合金的工作温度可达550~600e;在低温下,钛合金的强度反而比在常温时增加,且具有良好的韧性,低温钛合金在-253e时还能保持良好的韧性。

　　(5)钛的抗腐蚀性强。钛在550e以下的空气中,表面会迅速形成薄而致密的氧化钛膜,故在大气、海水、硝酸和硫酸等氧化性介质及强碱中,其耐蚀性优于大多数不锈钢。

　　二、钛合金等离子刻蚀机加工的研究进展：

　　蚀刻机加工作为一种化学切削加工技术，其历史可以追溯到很久远的年代。在欧洲，化学蚀刻机加工到15世纪变得流行起来，当时主要是用于铠甲的加工及艺术品的蚀刻机加工。最早的欧洲文字记载所有的蚀刻液配方是用盐、活性炭和醋配制而成。记载最早的防蚀材料是用亚麻油涂料做保护层，稍晚一些的记载也有用石蜡作为防蚀剂。

　　17 世纪是一个医药化学和试验酸、碱对各种不同材料影响的时代。约翰·格莱伯在这方面取得了不少的进展，其中包括对盐酸制造方法的改进。在这一时期化学腐蚀技术在未来航空工业上的应用也显露出来。当时人们采用酸和铁屑反应的方法来制取氢气球进行升空。这就是说，在人们企图飞行的最初阶段，已开始利用酸的腐蚀能力来帮助航空事业的发展。到 20 世纪 60 年代中期，由于蚀刻机加工及防蚀技术的发展，已经使蚀刻机加工成为一种非常有实用价值的生产加工方法，在航空航天工业中得到广泛应有。

　　蚀刻机加工技术和其它技术相比，它能使许多零件更简单、更容易、更便宜地生产出来，为一些零件的加工提供了机械加工方法难于实现的一种方法。同时，蚀刻机加工作为一种精密而科学的化学加工技术，在多种金属材料上被大量用于腐蚀各种不同的图文及外形加工。近几十年来，随着经济的发展，国防事业的需要，蚀刻机加工技术的应用越来越被人们重视。例如：旅游纪念品的开发，铭牌、奖牌、编码盘和显示屏电极的制作、印花辊筒和模版、精细零件等，都离不开蚀刻机加工技术。

　　三、钛合金化学蚀刻机加工液配方：

　　钛合金蚀刻机加工液中主要含有蚀刻液、氧化剂和添加剂： (1)蚀刻液

　　钛合金蚀刻机加工液中 HF是超过其它任何酸的强蚀刻液。在室温的 HF 溶液中，钛就能被腐蚀，随着酸浓度的增加，腐蚀速度明显增大。

　　(2)氧化剂

　　H2CrO4和 HNO3为蚀刻机加工液中常用的氧化剂，早期钛的蚀刻机加工，在美国广泛采用的蚀刻液是以 HF 和 H2CrO4为基础的混酸型蚀刻机加工液，随后英国人采用 HF 和 HNO3为基础的混酸型蚀刻机加工液。但 H2CrO4成本高、对环境的污染严重、反应后试样易氢脆;而 HNO3作为强氧化剂，有强烈的氧化性，使氢气随即被氧化成水，抑制了吸氢反应，使钛表面吸附的氢减少。

　　(3)添加剂

　　添加剂具有改善试样表面质量的作用，钛合金蚀刻机加工时要防止试样表面产生气沟、蚀沟、波纹、垄状物、麻点等缺陷，同时还要注意减少吸氢量，防止材料发生氢脆，优良的表面活性剂要具有降低表面张力、润湿等多种作用，一种添加剂很难同时具备这些作用，所以需要同时加入几种添加剂通过协同作用在不影响材料机械性能的前提下达到蚀刻机加工精度要求。钛合金蚀刻机加工选择加入添加剂种类时要考虑以下几个方面：

　　① 反应过程中是否可以减少试样表面的吸氢量，防止氢脆; ② 对试样加工前后各项力学性能无任何影响; ③ 试样腐蚀溶解速度均匀，提高试样表面质量;

　　④ 不影响蚀刻机加工液各成分发挥效用，使蚀刻机加工液寿命延长。 四、蚀刻机加工的优点和局限性： 蚀刻机加工具有诸多优点，对于某些用机械方法难于加工的零件确实有其独到之处：

　　① 能加工高硬度高韧性的金属材料;

　　② 能加工复杂形状的工件，如单双曲度薄壁工件等;

　　③ 加工过程中无机械力，无加工应力，对工件几乎无刚度要求; ④ 蚀刻机加工时工装设备简单、价廉;

　　⑤ 能加工沿着整个长度方向上截面均匀变化的，长而薄的锥形工件; ⑥ 蚀刻机加工能不间断的进行工件加工且能很快加工出想要的各种形状。 但蚀刻机加工也不是一种万能的加工方法，它也会受到很多因素的限制。其中最主要的限制为：

　　a. 蚀刻机加工只能以零件原有的表面状态为基准，累进式进行切削。因此经蚀刻机加工后的零件形状及表面质量，与零件原始的形状及表面质量有直接关系。更多的情况是经蚀刻机加工后的加工表面完全与原来的初始基准表面状态保持平行。从这些限制可以看出，蚀刻机加工不能用表面粗糙的板材、棒材等来加工形状复杂的零件;

　　b. 蚀刻机加工不可用于加工窄而深的凹槽，这是因为在蚀刻机加工反应过程中产生的气泡会集聚在防蚀层边缘的下面，这些堵在防蚀层下面的气泡把金属表面与蚀刻液隔开，造成一种非常不规则的腐蚀，形成很不整齐的边缘，这对于深度大的零件加工非常不利;

　　c. 不能用蚀刻机加工方法进行钻孔，蚀刻机加工钻孔和机械方法及电解方法钻孔都不相同，它不能加工出后面两者所能加工出来的孔形。选择合适的工艺方法可以钻出孔壁平直的孔来，而蚀刻机加工钻孔只能钻出不规则的锥形孔。对于深度蚀刻机加工钻孔由于腐蚀时间长，而使公差增大，所以蚀刻机加工钻孔一般不采用;

　　d. 蚀刻机加工过程中各种酸液挥发，易造成环境污染，影响人身健康。 五、钛合金精密蚀刻机加工：

　　蚀刻机加工的尺寸精度受两个方面的影响：a、蚀刻机加工本身在加工过程中所产生的偏差，这是由各个被加工表面上不同腐蚀速度造成的，这就使得腐蚀去掉的金属厚度或腐蚀深度有了差异。b、被腐蚀材料的精度，这是指零件在进行化学腐蚀之前，原材料本身即已存在的偏差，也称为毛胚偏差。蚀刻机加工中的公差是这两个精度偏差的总和，所以在进行蚀刻机加工时必须要把这两个偏差都计算在内，否则经蚀刻机加工的零件会有很大程度的尺寸偏差。蚀刻机加工的加工公差主要受以下几个因素的影响：

　　① 零件材料成分的影响：

　　同一蚀刻机加工液对不同材料的蚀刻机加工速度和侧蚀率不同，从而使不同材料腐蚀后得到不同的表面精度。然而，即使同一种材料也可能因为材料型号和批次的不同而产生不同的腐蚀速度及效果，在同一腐蚀液中也会产生不同的腐蚀公差。

　　② 零件尺寸大小的影响： 不同于小型零件的蚀刻机加工，对于一个大型的板材零件如果是垂直放于腐蚀槽中，经腐蚀后由于蚀刻机加工速度的不同会出现下薄上厚的锥形，所以在计算公差时也应把这种由于蚀刻机加工本身造成的公差计入总公差。对于小型零件这种现象可以忽略不计。

　　③ 蚀刻机加工深度的影响：

　　蚀刻机加工深度的大小与零件在腐蚀液中浸泡时间的长短有直接关系，蚀刻机加工深度越深，零件在腐蚀液中浸泡时间越长，蚀刻机加工液中发生腐蚀溶解反应，其中各种化学成分的组分和浓度变化也越来越大，进而使蚀刻机加工速度及腐蚀效果变化增大。同时过深的蚀刻机加工，当深度达到一定值后，由于浓差极化及腐蚀残渣的沉积，其腐蚀溶解行为及机理行为也会发生变化。

　　④ 蚀刻液的影响：

　　因蚀刻机加工溶液主体成分和浓度的不同，对零件的蚀刻机加工速度和表面质量有一定的影响。随着溶液中蚀刻液和氧化剂浓度的改变，蚀刻机加工速度也会随之改变，这时对零件表面质量也会产生一定的影响，进而影响零件总公差的改变。

　　六、蚀刻机加工对材料力学性能的影响:

　　金属经蚀刻机加工后，原有力学性能会受到一定的影响，这种影响随着腐蚀程度的增加而加大

      1、 Overview of titanium and titanium alloy: 

        The abundance of Ti in the earth's crust is 0.56% (mass fraction, the same below), ranking 9th in all elements, and 4th in metals that can be used as structural materials, next to Al, Fe, Mg. Its reserves are more than the sum of common metals Cu, Pb, Zn. China is rich in titanium resources, with the largest reserves in the world. Titanium alloy is a new type of structural material with great development potential and application prospect because of its low density, high specific strength, high specific stiffness, good corrosion resistance, high temperature mechanical properties, fatigue resistance and creep resistance. In recent years, the world titanium industry and titanium material processing technology have been developed rapidly. The production and consumption of sponge titanium, deformed titanium alloy and titanium alloy processing materials have reached a very high level. They are widely used in the field of aerospace, naval ships, weapons and other military products manufacturing. They also have great application potential in automobile, chemistry, energy and other industries. 

    Since 2001, titanium has been used more widely, and its application in some emerging markets, such as automobiles, oil and natural gas, has increased. In the long run, the amount of titanium used in automobiles will continue to increase and become a more secure competitive field for titanium applications. The application of titanium alloy in the field of high-end consumer goods has also been basically mature, such as the application of titanium alloy in golf clubs and tennis rackets. Nowadays, the aging problem is becoming more and more serious. The demand for titanium in the pharmaceutical market is increasing at a rate of 5% - 7% every year. Titanium in the biomedical sector has become another hot and broad application prospect. The consumption of titanium in the above emerging markets is increasing every year. In 2001, the consumption of titanium was 6000 tons, and in 2006, it increased to 10000 tons. After 2007, with the development of economy and the need of national defense, the demand for titanium alloy products in the international market has greatly increased, especially in the field of aerospace, which will become the main consumption and application field in the international market in the next few years. 

    Titanium and its alloys have many excellent properties, mainly in the following aspects: 

(1) high specific strength. Titanium alloy has very high strength, its tensile strength is 686 ~ 1176mpa, and its density is only about 60% of steel, so its specific strength is very high. 

(2) High hardness. The hardness HRC of titanium alloy (as annealed) is 32-38. 

(3) Low modulus of elasticity. The elastic modulus of titanium alloy (annealed) is 1.078 @ 105 ~ 1.176 @ 105Mpa, about half of that of steel and stainless steel. 

(4) High and low temperature performance. At high temperature, titanium alloy can still maintain good mechanical properties, its heat resistance is far higher than aluminum alloy, and the working temperature range is wide. At present, the working temperature of the new heat-resistant titanium alloy can reach 550-600e; at low temperature, the strength of titanium alloy is higher than that at normal temperature, and it has good toughness. At - 253e, the low temperature titanium alloy can still maintain good toughness. 

(5) Titanium has strong corrosion resistance. When titanium is in the air below 550E, a thin and compact titanium oxide film will be formed on the surface of titanium rapidly. Therefore, in the oxidizing media such as atmosphere, sea water, nitric acid and sulfuric acid and strong alkali, its corrosion resistance is better than that of most stainless steels. 

2、 Research progress of plasma etching machining of titanium alloy:

   As a chemical cutting technology, etching machining has a long history. In Europe, chemical etching became popular in the 15th century, when it was mainly used for the processing of armor and the etching of artwork. The earliest European writing recorded that all the etchant formulations were made of salt, activated carbon and vinegar. The earliest anti-corrosion material recorded is flax oil coating as the protective layer, and later some records also use paraffin as the anti-corrosion agent. 

  The 17th century is an era of medicinal chemistry and the experiment of the influence of acid and alkali on various materials. John Graber has made a lot of progress in this field, including the improvement of hydrochloric acid manufacturing method. In this period, the application of chemical corrosion technology in the future aviation industry is also revealed. At that time, people used acid and iron chips to react to make hydrogen balloons for launching. That is to say, in the initial stage of people's attempt to fly, acid corrosion ability has been used to help the development of aviation industry. By the mid-1960s, due to the development of etching and anti-corrosion technology, etching has become a very practical production and processing method, which is widely used in aerospace industry. 

  Compared with other technologies, etching machining technology can make many parts more simple, easier and cheaper to produce, which provides a method that is difficult to realize for machining some parts. At the same time, as a kind of precise and scientific chemical processing technology, etch machining is widely used to corrode a variety of different graphics and shapes on a variety of metal materials. In recent decades, with the development of economy and the need of national defense, people pay more and more attention to the application of etching machining technology. For example, the development of tourist souvenirs, the production of nameplates, medals, coding plates and display electrodes, printing rollers and templates, fine parts, etc., are all inseparable from the etching machining technology.

      3、 Formula of chemical etching machining fluid for titanium alloy: 

     The etching machining fluid for titanium alloy mainly contains etching fluid, oxidant and additive: 

(1) HF in etching machining fluid for titanium alloy is stronger than any other acid. In HF solution at room temperature, titanium can be corroded. With the increase of acid concentration, the corrosion rate increases obviously. 

(2) The oxidants H2CrO4 and HNO3 are commonly used in the etching machining fluid. In the early stage of titanium etching machining, the etching fluid widely used in the United States was a mixed acid etching machining fluid based on HF and H2CrO4. Later, the British adopted the mixed acid etching machining fluid based on HF and HNO3. However, the cost of H2CrO4 is high, the pollution to the environment is serious, and the sample is easy to hydrogen embrittlement after reaction. As a strong oxidant, HNO3 has strong oxidability, which immediately oxidizes hydrogen to water, inhibits the hydrogen absorption reaction, and reduces the hydrogen adsorbed on the titanium surface. 

(3) Additives Additives have the function of improving the surface quality of the sample. During the etching and machining of titanium alloy, defects such as air groove, erosion groove, corrugation, ridge and pockmarks should be prevented. At the same time, hydrogen absorption should be reduced to prevent hydrogen embrittlement of the material. Excellent surfactants should have many functions such as reducing surface tension and wetting. It is difficult for an additive to have these functions at the same time Therefore, it is necessary to add several additives at the same time to achieve the requirements of etching accuracy without affecting the mechanical properties of materials. The following aspects should be considered in the selection of additives for titanium alloy etching machining: 

① in the reaction process, whether the hydrogen absorption on the sample surface can be reduced to prevent hydrogen embrittlement;

② there is no effect on the mechanical properties of the sample before and after processing; 

③ the corrosion dissolution speed of the sample is uniform to improve the surface quality of the sample; 

④ It does not affect the effectiveness of the components of the etching fluid, so as to prolong the service life of the etching fluid. 4、 Advantages and limitations of etch machining: 

      Etch machining has many advantages, for some parts that are difficult to be machined by mechanical methods, it has its own unique features: 

① it can process metal materials with high hardness and toughness; 

② it can process workpieces with complex shapes, such as single hyperbolic thin-walled workpieces, etc; 

③ In the process of machining, there is no mechanical force, no machining stress, and there is almost no rigidity requirement for the workpiece; 

④ the tooling equipment is simple and cheap during etching machining; 

⑤ the long and thin tapered workpiece with uniform section change along the whole length direction can be processed; 

⑥ the etching machining can continuously process the workpiece and quickly process the desired shapes. But etching machining is not a universal processing method, it will also be limited by many factors. The most important limitation is: 

a. the etching machining can only be based on the original surface state of the parts, and the progressive cutting. Therefore, the shape and surface quality of parts machined by etching are directly related to the original shape and surface quality of parts. More often, the etched machined surface is completely parallel to the original initial reference surface. From these limitations, it can be seen that etching machining can not use rough surface plates, bars and other parts to process complex shapes; 

b. Etching machining cannot be used to process narrow and deep grooves, because the bubbles generated in the process of etching machining reaction will gather under the edge of the anti-corrosion layer. These bubbles blocked under the anti-corrosion layer will separate the metal surface from the etching solution, resulting in a very irregular corrosion, forming a very irregular edge, which is very unfavorable for the processing of deep parts; 

c. It can't be drilled by etching machining. Etching machining drilling is different from mechanical drilling and electrolytic drilling. It can't process the hole shape that the latter two can process. Choosing appropriate technology can drill holes with flat wall, while etching machining can only drill holes with irregular cone shape. Because of the long time of corrosion, the tolerance of deep etching machining drilling increases, so the etching machining drilling is generally not used; 

d. in the process of etching machining, all kinds of acid volatilization is easy to cause environmental pollution and affect personal health. 

5、 Precision etching machining of titanium alloy: the dimensional accuracy of etching machining is affected by two aspects:

 a. the deviation of etching machining itself in the process of machining is caused by the different corrosion speed on each machined surface, which makes the thickness or corrosion depth of metal removed by corrosion different. 

b. The accuracy of the corroded material refers to the deviation existing in the raw material itself before the chemical corrosion of the parts, also known as the rough embryo deviation. The tolerance in the etching machining is the sum of these two precision deviations, so both deviations must be calculated in the etching machining, otherwise the parts processed by the etching machining will have a large degree of dimensional deviation. The machining tolerance of etching machining is mainly affected by the following factors:

       ① Influence of material composition of parts: the same etching fluid has different etching machining speed and side etching rate for different materials, so that different materials can get different surface precision after corrosion. However, even if the same material may have different corrosion rate and effect due to different material model and batch, different corrosion tolerance will be produced in the same corrosive solution. 

       ② Influence of part size: different from the etching machining of small parts, if a large plate part is placed vertically in the corrosion groove, there will be a taper with thin bottom and thick top due to different etching machining speed after corrosion, so the tolerance caused by the etching machining itself should also be included in the total tolerance when calculating the tolerance. This phenomenon can be ignored for small parts. 

       ③ Effect of etch machining depth: There is a direct relationship between the etching depth and the immersion time of the parts in the etching solution. The deeper the etching depth is, the longer the parts are immersed in the etching solution, and the corrosion dissolution reaction takes place in the etching solution, in which the composition and concentration of various chemical components change more and more, thus increasing the etching processing speed and corrosion effect. At the same time, when the depth reaches a certain value, due to concentration polarization and corrosion residue deposition, the corrosion dissolution behavior and mechanism behavior will also change. 

      ④ Effect of etching solution: due to the difference of the main composition and concentration of etching solution, it has a certain impact on the etching speed and surface quality of parts. With the change of the concentration of etching solution and oxidant in the solution, the machining speed of etching will also change, which will have a certain impact on the surface quality of parts, and then affect the change of the total tolerance of parts. 

      6、 Effect of etching machining on mechanical properties of materials: after etching machining, the original mechanical properties of metals will be affected to some extent, which will increase with the increase of corrosion degree.