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2A12铝合金作为一种高强硬铝合金,常被作为飞机重要部件的材料,在现今的工业加工中,常利用硫酸将其阳极氧化后喷涂涂层的方式提高耐蚀性、耐磨性。此次实验的试样如图1所示,其中图1(a)为硫酸阳极氧化后得到氧化层的表面形貌,图1(b)为试样经TB06-9涂层喷涂后的表面形貌,图1(c)为试样的截面图。试样基材为2A12铝合金,然后进行硫酸阳极氧化、喷涂TB06-9涂层处理,最终得到如图1(c)所示的结构,其中最上层为30 μm厚的锌黄色涂层,中间层为3~5 μm厚的氧化层。涂层及氧化层的主要元素原子数占比如表1所示。
图 1 (a)样品氧化层的表面形貌图;(b)样品涂层的表面形貌图; (c)截面图
Figure 1. (a) Surface topography drawing of oxide layer of the sample; (b) Surface topography drawing of coating of the sample; (c) Cross section
表 1 涂层及氧化层的主要元素原子数占比
Table 1. Atomic fraction of the major elements in coating and oxide layer
Element O C Si Ti Mg Al S TB06-9 37.81% 49.88% 5.9% 4.93% 1.47% - - Oxide layer 67.07% - - - - 30.79% 2.14% -
采用YDFLP系列100 W脉冲激光器进行激光清洗实验,该激光器采用高斯光源,波长为1064 nm,焦点处的光斑直径为130 μm。采用X/Y轴双振镜控制系统对光斑运动路径进行控制,实验平台及激光清洗系统示意图如图2所示。
图 2 (a)实验平台和(b)激光清洗系统示意图
Figure 2. (a) Test platform and (b) Schematic diagram of laser cleaning system
激光清洗路径如图3所示,采用“之”字形的扫描方式,其中图3(a)为第一步激光清洗的扫描路径示意图,图3(b)为在第一步清洗的基础上进行第二步激光清洗的扫描路径示意图,图3中圆形区域为光斑位置,箭头方向为激光运行方向。激光清洗时设定清洗区域恒为14 mm×14 mm。
图 3 (a) 第一步激光清洗路径示意图;(b) 第二步激光清洗路径示意图
Figure 3. (a) Schematic diagram of the first step laser cleaning path; (b) Schematic diagram of the second step laser cleaning path
激光单光斑的平均能量密度 [30]为:
$$ E=\frac{P}{fs} $$ (1) 式中:
$ P $ 为激光平均功率;$ f $ 为激光的频率;$ s $ 为光斑的面积。激光在扫描过程中的搭接率包含光斑搭接率和路径重叠率,其示意图如图4所示。光斑搭接率如图4(a)所示,描述的是同一条直线路径上,前一个光斑与后一个光斑之间的搭接情况,与激光扫描速度、频率、光斑直径有关,其计算关系[30]为:$$ \eta =\frac{a}{b}=\left(1-\frac{v}{fd}\right)\times 100 {\text{%}} $$ (2) 式中:
$ v $ 为激光的扫描速度;$ d $ 为光斑直径。路径重叠率如图4(b)所示,描述的是两条激光轨道的重叠情况,与激光线间距、光斑直径有关,其计算关系[30]为:$$ \xi =\frac{c}{d}=\left( { 1-\frac{D}{d} } \right)\times 100 {\text{%}} $$ (3) 式中:
$ D $ 为激光路径线间距。图 4 (a)光斑搭接率示意图;(b)路径重叠率示意图
Figure 4. (a) Schematic diagram of spot overlap ratio; (b) Schematic diagram of path overlap ratio
由公式(1)与公式(2)可知,激光的脉冲频率是一个重要参数,与单光斑的能量密度成反比,与光斑搭接率成正比。因为在此次研究中,氧化膜的厚度仅有4 μm左右,单次清洗容易对氧化膜造成损伤,同时涂层也难以清洗干净,所以文中设计了一种两步法激光清洗工艺对试样进行激光清洗。第一步是在低频率的条件下,利用高光斑能量对试样进行初步清洗去除大部分涂层,第二步是在高频率的条件下,利用低光斑能量密度对试样进行多次清洗减少对氧化膜的损伤。
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首先以激光功率为唯一变量对试样进行第一步激光清洗。开展了多组实验研究,发现激光频率为20 kHz,光斑搭接率为60%时,能够达到较好的清洗效果,此时根据公式(2)和公式(3)分别计算得出扫描速度为1040 mm/s、线间距为0.052 mm。
图5为在不同激光功率清洗后试样表面形貌图。从图5(a)中可以看出,当激光功率为30 W时,此时激光能量不会对试样造成明显的影响,表面仅出现微小的凹坑,氧化层仍旧全部被涂层覆盖。从图5(b)与图5(c)可以看出,当激光功率为35 W和40 W时,试样表面部分涂层被清洗去除裸露出氧化层。对比图5(b)与图5(c)激光清洗后的试样表面可知,当激光功率为40 W,试样表面涂层有明显的减少,清洗的效果更好。从图5(a)~图5(c)可以看出,当激光功率小于等于40 W时,试样表面均未清洗干净,同时也未发现氧化层损伤区域,随着激光功率的提高,激光清洗涂层的效果逐渐变好。从图5(d)~图5(f)中可以看出,当激光功率增大到45 W时,由于激光的能量密度较大,试样表面涂层几乎被清洗去除干净,但是可以看到部分区域氧化层被高强激光束清洗去除裸露出基材,同时在裸露出的基材区域能够明显地看出金属重熔的迹象,这就说明基材在清洗过程中的瞬间温度已经达到基材的熔点,材料经历了融化后再凝固的过程。对比图5(d)~图5(f)可以看出,随着激光功率的逐渐增加,试样表面涂层逐渐减少,但是氧化膜的损伤也随着激光功率的增加而增多。
图 5 不同激光功率清洗后试样的扫描电镜图。 (a) 30 W;(b) 35 W; (c) 40 W; (d) 45 W; (e) 50 W; (f) 60 W
Figure 5. SEM of samples cleaned at different laser powers. (a) 30 W; (b) 35 W; (c) 40 W; (d) 45 W; (e) 50 W; (f) 60 W
图6为激光清洗后试样表面元素的含量图。由图可以看出,随着激光功率的增加,TB06-9涂层的特有元素(碳、钛、硅、镁)含量逐渐减少,氧化层及基材特有的铝元素含量逐渐增加,这说明随着激光功率的增大,试样表面的涂层逐渐减少,裸露的氧化层或基材逐渐增多。硫元素是氧化层特有的元素,可以反应激光清洗效率,从图6(a)中可以看出,硫元素含量呈先增加后降低的趋势,这是因为当激光功率较低时,脱落涂层的面积较小,随着激光功率的增加,越来越多的涂层被清洗去除从而裸露氧化层。当激光功率达到50 W时,氧化层受到严重破坏裸露出基材,硫含量出现明显的降低。
图 6 (a)激光清洗后试样钛、硅、镁、硫元素的含量;(b)激光清洗后试样铝、氧、碳元素的含量
Figure 6. (a) The contents of Ti, Si, Mg and S in the sample after laser cleaning; (b) The contents of Al, O and C in the sample after laser cleaning
激光功率小于35 W时,试样表面仅留下微小的凹坑,试样表面依旧有大量的涂层残留;激光功率大于45 W时,试样氧化层出现了明显的损伤;当激光功率为40 W时,试样表面有部分涂层未被清洗,观察试样表面氧化层未出现损伤。最终确定激光的频率为20 kHz,功率为40 W,光斑搭接率为60% (1040 mm/s,0.052 mm)时,可以保证在不损伤氧化层的前提下激光清洗的效果较好,并将此工艺标定为G1。
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涂层有一定的清洗阈值(T_1),氧化层同样有一定的清洗阈值(Y_1),由于涂层与氧化层的颜色、对激光的吸收率、密度、熔点、分子间结合力等参数存在较大的差距,所以T_1与Y_1必要存在一定的差值。图7为在激光频率为1000 kHz,功率为30 W,线速度为100 mm/s的相同参数下激光对涂层及氧化层进行单轨道清洗的效果图,其中图7(a)为激光照射氧化层后的效果图,可以看出在该工艺下氧化层表面未受影响。图7(b)为激光照射涂层后的效果图,图7(c)为图7(b)中AB两点之间利用VHX-600E型超景深三维显微镜测得的表面轮廓图,可以看出,在同样的参数下涂层出现了宽度为107 μm,深度为26.2 μm的凹痕,由此可推断出T_1<Y_1,即存在能够有效清洗涂层并且不会对氧化层造成损伤的工艺参数。
图 7 相同工艺参数下激光对(a)氧化层及(b)涂层的影响;(c)涂层上AB两点间的表面轮廓图
Figure 7. Effect of laser on (a) oxide layer and (b) coating under the same process parameters; (c) Profile between two points AB in (b)
以第一步工艺参数G1为前提,探究第二步激光的清洗工艺参数。根据公式(1) 可知,激光单脉冲能量密度随着激光频率的增加而减少,为避免单脉冲能量过高对氧化层造成的损伤,第二步清洗将频率设定为激光器的最大值1000 kHz进行多次清洗实验。开展了多组实验,发现当扫描速度为690 mm/s,线间距为0.0345 mm能够达到较好的清洗效果,通过改变激光功率、控制激光清洗次数探究最佳的清洗工艺。表2所示为不同激光功率下激光清洗的效果,图8为不同激光功率下试样的表面形貌图。从表2、图8可以看出,当激光功率为40 W时,经过10次的激光清洗仍未将涂层清洗干净,当激光功率为60 W、80 W时,激光清洗后表面未观察到涂层残留,可以看到试样表面经硫酸阳极氧化后的黑色斑点及条纹。激光功率为60 W时,需要激光清洗5次才能将涂层清洗干净,激光功率为80 W时,只需清洗2次。当激光功率为100 W时,在激光清洗的过程中涂层受热燃烧,致使试样表面的温度急剧升高,对试样造成损伤。最终得出,当扫描速度为690 mm/s,线间距为0.0345 mm,激光功率为80 W时,可以保证在经过多次清洗后,能够有效地去除表面涂层并且不会损伤基材,将该参数标定为T1。
表 2 不同激光功率下激光清洗效果
Table 2. Laser cleaning effect under different laser powers
Laser power/W 40 60 80 100 Number of cleaning 10 5 2 - Effect Dirty Clean Clean Fire 图 8 激光清洗后试样的表面形貌图。(a) 40 W;(b) 60 W;(c) 80 W;(d) 100 W
Figure 8. Surface topography of the sample after laser cleaning. (a) 40 W; (b) 60 W; (c) 80 W; (d) 100 W
以第一次清洗采用工艺G1,第二次及之后采用工艺T1进行清洗。在每一次清洗后利用高精度电子天平(精度0.0001 g)测量试样的质量,计算出每次清洗前后试样的质量差值,如图9所示。从图9中可以看出,第4次清洗仅仅去除0.0001 g左右的物质,同时在激光清洗过程中未观察到音爆和高亮现象,即可以认为未能再清洗掉涂层[24],清洗去除涂层的总质量为0.0091 g。经过4次激光清洗后,用酒精对试样进行擦拭后得到如图8(c)所示的试样表面,与图1(a)氧化层表面对比可知,试样表面形貌相似,氧化层保留完好。清洗后试样表面的扫描电镜图及能谱图如图10所示,结合表1可知,激光清洗后试样表面的主要元素及其占比与氧化层相近,并且没有发现涂层特有的元素(碳、钛、硅、镁)存在,即表明涂层已被完全清洗干净。
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试样清洗后的力学性能是判断清洗效果的重要参数之一,对试样的显微硬度、拉伸性能进行研究。利用维氏硬度计(精度0.1 HV)和万能实验机将清洗后试样与未进行喷涂涂层的原始试样进行对比研究。图11(a)为激光清洗后试样与原始试样的表面显微硬度,在激光清洗后,试样与原始氧化层表面以试样中心位置为5号点取点进行测试,计算得出氧化层的显微硬度平均为44.84 HV,激光清洗后试样的显微硬度平均为45.14 HV,硬度值为原始试样的100.66%;图11(b)为激光清洗后试样与原始试样通过拉伸实验得到的应力应变曲线,可以看出原始试样的抗拉强度为414.291 MPa,经激光清洗后试样的抗拉强度为413.957 MPa,试样的抗拉强度为原始试样的99.92%。激光清洗后试样与原始试样的表面显微硬度、抗拉强度几乎保持不变,这是因为文中研究是采用纳秒激光对涂层进行清洗,激光作用的时间很短,高能脉冲光束的能量大部分被涂层所吸收,通过控制功率、搭接率等参数能够较为精确地控制激光清洗效果,同时采用两步法的激光清洗工艺能够在一定程度上减少激光热效应对基材的影响。结果表明该两步法激光清洗工艺实现了对基材的无损清洗,较好地保证了原性能不变。
Study on nondestructive laser cleaning of aviation aluminum alloy surface coating
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摘要: 激光清洗以绿色、安全、便于控制等优点,在航空航天、电子、交通等领域有着重要的应用价值。采用纳秒脉冲激光清洗航空2A12铝合金表面TB06-9涂层,研发了一种新型的两步法无损激光清洗工艺。运用扫描电子显微镜、能谱仪,超景深三维显微镜和万能电子实验机等分析激光清洗涂层。结果表明,第一步采用单次激光清洗,随激光功率的增加,试样表面涂层逐渐减少裸露出氧化层及基材。激光功率为40 W时氧化层保留完好,功率为45 W时氧化层开始出现损伤,随着功率的增加,损伤逐渐增多。最终确定第一步优化参数为激光的频率为20 kHz,功率为40 W,扫描速度为1040 mm/s,线间距为0.052 mm。第二步在第一步的基础上进行多次清洗,获得的优化参数为激光频率为1000 kHz,功率为80 W,扫描速度为690 mm/s,线间距为0.034 5 mm。两步法激光清洗试样的表面与原始试样表面形貌相似,表面显微硬度及抗拉强度基本保持一致,较好地保留了材料的原有力学性能。Abstract: The laser cleaning has some advantages as non-pollution, safety and controllability, which has significant applications in aerospace, electronic field and transportation. The nanosecond pulsed laser was used to clean TB06-9 coating on the surface of aviation 2A12 aluminum alloy. A novel two-step nondestructive laser cleaning process was developed. Scanning electron microscope, energy dispersive spectrometer, ultra depth of field three-dimensional microscope and universal electronic testing machine are applied to investigate laser cleaning coating. The results show that, the first step is to use a laser cleaning, with the increase of the power, the coating on the sample surface decreases gradually, and the oxide layer and substrate are exposed. When the laser power is 40 W, the oxide layer is well preserved. When the laser power is increased to 45 W, the oxide layer begins to be damaged, with increase of the laser power, the damage of oxide layer gradually increased. The optimization parameters of the first step are the laser frequency of 20 kHz, power of 40 W, scanning speed of 1 040 mm/s and line spacing of 0.052 mm respectively. The second step is to carry out multiple cleaning based on the first step. The second parameters are laser frequency of 1000 kHz, power of 80 W, scanning speed of 690 mm/s, line spacing of 0.034 5 mm respectively. The surface morphology of sample by two-step laser cleanning samples is similar to that of the original samples, and the surface microhardness and tensile strength of the samples remain consistent. The mechanical properties of the material are well maintained.
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Key words:
- laser coating removing /
- aluminum alloy /
- coating /
- oxide layer /
- nondestructive
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表 1 涂层及氧化层的主要元素原子数占比
Table 1. Atomic fraction of the major elements in coating and oxide layer
Element O C Si Ti Mg Al S TB06-9 37.81% 49.88% 5.9% 4.93% 1.47% - - Oxide layer 67.07% - - - - 30.79% 2.14% 表 2 不同激光功率下激光清洗效果
Table 2. Laser cleaning effect under different laser powers
Laser power/W 40 60 80 100 Number of cleaning 10 5 2 - Effect Dirty Clean Clean Fire -
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