脉冲激光清洗铝合金表面漆层的技术研究 
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Objective A paint layer can be applied to metals to enhance their surface characteristics. However, in many cases, paint often needs to be removed from the metal surface because of its potential damage to the environment. Paint removal using laser provides several advantages over the conventional techniques such as mechanical or chemical cleaning. Specifically, an accurate removal area, minimal detrimental effects to the substrate, reduction in contaminated waste, and fast cleaning rate are the key favorable factors in paint removal using laser. Several studies have been published in the literature that dealt with the effect of different process parameters for paint removal including the change of the temperature. Other processes that affect the relationship between the laser beam and paint have not been determined. In the present study, we report a novel type of research methods to understand the detailed micro process of paint removal, such as the plasma effect near the paint surface and the microscopic destruction process in the paint. We expect that our basic strategy and findings can help in understanding the characteristics and mechanisms of paint removal.
Methods In this work, 2024 aluminum alloy and polyacrylate resin-based paint were employed. A laser paint-cleaning test was carried out using pulsed laser with a wavelength of 1064nm and a pulse width of 1μs. In the experiment, the focal spot diameter of the Gaussian beam was approximately 78μm. The whole apparatus was completely automatic, that is, a computer controlled the laser power, repetition rate, and scanning speed. The cleaning residues were deposited on a silicon wafer, which was located 17 mm from the surface of the sample, as shown in Fig.1. The effects of scanning speed, pulse frequency, and laser power on the laser-cleaning quality were investigated. According to the morphology and element-valence changes in the cleaned surface and by combining the morphology of the cross section of the paint and particles generated during the cleaning process, the underlying process and mechanisms of the paint removal using pulsed laser were thoroughly investigated. Simultaneously, the temperature and stress-field distributions of the finite-element simulation using COMSOL Multiphysics software were also used for the auxiliary analysis.
Results and Discussions The paint in the experiments could be removed using pulsed laser. The laser-cleaning quality first increased and then decreased (Fig.3, Fig.4) and the surface roughness first decreased and then increased (Table 2, Table 3) with the increase in the scanning speed and pulse frequency. Furthermore, the laser-cleaning quality increased (Fig.5) and the surface roughness first decreased and then increased (Table.4) when the laser power increased. The morphologies and elements of the cleaned-surface study illustrate that the laser plasma and thermal combustion were affected by the absorption of laser energy by the paint during the laser-cleaning process (Fig.6). In addition, the X-ray photoelectron spectroscopy analysis indicates that C—H, C—C, O—H, C=O, C—O, and other covalent bonds in the polymer molecular chain of the paint were broken under the action of the pulsed laser (Fig.7). During the cleaning process, a layered structure was formed in the paint. Obvious cracks appeared that were parallel to the surface of the paint at the fracture section, which extended inside the paint. This result indicates the presence of a mechanical effect perpendicular to the surface of the paint. The cohesion of the lacquer was destroyed, which damaged the paint between the layers, and the paint layer was ejected (Fig.8). The study of the collected particles illustrates that the presence of mechanical mechanisms in the paint-damage process, such as vibration and impact, and the vaporized paint nucleated and grew in the high-energy limited area formed by the pulsed laser, which resulted in the formation of nanoparticles (Fig.9).
Conclusions In the present study, three different process parameters, namely, scanning speed, pulse frequency, and laser power, influence the laser paint-cleaning quality at different levels. The laser-cleaning quality first increases and then decreases with the increase in the scanning speed and pulse frequency and increases as the laser power increases. The laser-cleaning quality is good when the process parameters are as follows: laser power=16.5W, scanning speed=600mm/s, and pulse frequency=30kHz. Under different process parameters, the main mechanism of the laser paint removal is different. With regard to the analysis characterization, we conclude that the effect of the cohesive-failure and crack-propagation-fracture mechanisms is more efficient than the chemical bond-fracture combustion.
1 引言
在工业领域中,当金属表面的涂层出现脱落,或对底层的金属基底进行检修时,需要对原有的漆层进行去除[1-3]。传统的除漆方法主要包括机械除漆法、超声波除漆法和化学除漆法[4-5],但这些方法都存在着不同程度的弊端。如:机械除漆法的劳动强度大,噪声大,除漆效果差,易对基体造成不可逆转的损伤;超声波除漆法的能耗大,水资源浪费严重且除漆槽的尺寸限制了该方法的应用;化学除漆过程中使用的是有机清洗剂,会严重污染环境。激光除漆是激光在清洗领域的重要应用,覆盖了航空航天、船舶、高铁、汽车等领域,尤其是在飞机蒙皮的维修或再制造方面具有广阔的应用前景[6-7]。与传统的除漆方法相比,激光除漆技术具有以下优势[8-10]:高效、经济、快捷、便于自动控制,除漆过程中的产物可在清洗的同时进行收集,无污染。
激光除漆过程中激光参量的合理选择是提高漆层去除效率和质量的关键因素之一,但激光清洗过程与作用机制相当复杂,激光参量的选择对于研究者来说是一个巨大挑战。Schweizer等[11]认为,CO2激光器去除漆层的关键参量是激光功率密度,同时,激光束扫描线重叠率对除漆效率也有一定影响。田彬等[12]通过研究发现,激光脉冲宽度、激光能量密度以及基底的性质对除漆效果具有重要影响。Jasim等[13]采用纳秒脉冲光纤激光器对白色聚合物漆和铝合金基片进行了激光除漆研究,并分析了激光能量、脉冲频率和搭接率等工艺参数对烧蚀尺寸、烧蚀深度以及除漆后基底表面形貌和表面粗糙度的影响。Mateo等[14]指出,合理配置激光能量和脉冲频率能够对黄铜表面的漆层实现很好的清洗效果。施曙东等[15]指出,在保证激光功率密度和扫描搭接率适宜的同时,提高激光器的输出功率、脉冲重复频率或增加光斑直径,可以获得更好的除漆效果和更高的清洗效率,漆层清洗的主要机制为热振动效应和烧蚀效应。Brygo等[16]研究后指出,激光除漆过程极为复杂,在热分解和熔化烧蚀起关键作用的除漆过程中,激光能量密度对除漆效果的影响极大。章恒等[17]采用低频YAG脉冲激光器对FV520B基体的表面漆层进行了激光除漆试验,并对激光除漆的机制进行了研究,结果表明,激光除漆机制会因试验条件的改变而不同。罗红心等[18]采用大功率连续CO2激光器对飞机蒙皮表面漆层进行了去除试验,他们对试验结果进行分析后认为,激光除漆并不是单独一种机制在起作用,何种机制占主导作用与漆层的物理化学性质、基底材料、清洗用激光束的物理参数及清洗环境等诸多因素有关。上述研究主要针对工艺参数的影响规律和脉冲激光与漆层相互作用的温度变化(燃烧烧蚀、热应力)进行了研究,并未涉及脉冲激光与漆层的其他过程,而漆层表面附近的等离子体效应以及漆层内聚合物的微观破坏过程对漆层清洗的影响也值得关注。
为此,本论文针对飞机蒙皮漆层清洗的重大需求,利用脉冲光纤激光对2024铝合金表面的聚丙烯酸酯树脂基漆层进行激光清洗试验研究,分析扫描速度、脉冲频率、激光功率对激光除漆的影响规律。本文采用三维形貌仪和扫描电子显微镜(SEM)观察激光清洗漆层后铝合金的表面形貌以及清洗过程中收集的颗粒的形貌,采用X射线能谱(EDS)技术和光电子能谱(XPS)技术分析激光作用前后漆层表面元素成分及元素价态的变化,并结合COMSOL Multiphysics模拟清洗过程中的温度场和应力场分布,深入探究激光清洗漆层的作用机制,以期为脉冲激光清洗漆层的精确控制工艺提供参考依据。
2 试验与仿真
2.1 试验设备和材料
所用试验样品由石家庄飞机工业有限责任公司提供,为真实飞机蒙皮对标件。样品基底为2024铝合金,样品尺寸为15mm×15 mm×3mm。漆层为黄色聚丙烯酸酯树脂基漆层,厚度约为50μm,漆层内含有TiO2、SiO2、Al2O3等功能性粒子。
采用光纤激光清洗设备进行激光清洗。清洗设备主要由光纤脉冲激光器、扫描振镜、控制卡等组成,如
图 1. 激光清洗装置和清洗方法示意图。(a)激光清洗装置示意图; (b)漆层清洗方法示意图
Fig. 1. Schematics of laser cleaning device and cleaning method. (a) Schematic of laser cleaning device; (b) cleaning method of paint removal
表 1. 光纤激光器的主要参数
Table 1. Main parameters of optical fiber laser
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2.2 试验方法
铝合金表面漆层的激光清洗方法如
2.3 有限元模拟
激光清洗的三维有限元模型如
2.4 清洗过程产物分析
铝合金表面漆层清洗完成后,采用OLYMPUS LEXT-QLS4000三维形貌仪测试激光清洗表面的三维形貌及表面粗糙度;采用Nova Nano SEM50扫描电子显微镜(SEM)附带的X射线能谱技术分析激光清洗表面元素的组成及含量;采用SIGMA300 场发射扫描电镜分析收集产物、漆层断裂横截面及清洗表面的形貌(测试前进行真空喷金处理);采用ESCALAB 250Xi X射线光电子能谱技术分析清洗表面元素价态的变化。
3 分析与讨论
3.1 工艺参数对清洗效果的影响
3.1.1 扫描速度对清洗效果的影响
为研究扫描速度v对铝合金表面漆层激光清洗质量的影响,选取扫描速度v为200~1200mm/s进行工艺试验,其中,激光功率P=16.5W,脉冲频率f=30kHz,试验结果如
图 3. 不同扫描速度下清洗表面的三维形貌。 (a) 200mm/s; (b) 400mm/s; (c) 600mm/s; (d) 800mm/s; (e) 1000mm/s; (f)1200mm/s
Fig. 3. Three-dimensional morphologies of the surface cleaned at different scanning speeds. (a) 200mm/s; (b) 400mm/s; (c) 600mm/s; (d) 800mm/s; (e) 1000mm/s; (f) 1200mm/s
表 2. 不同扫描速度下的光斑搭接率和清洗表面的表面粗糙度
Table 2. Spot overlap rate and surface roughness of the surface cleaned at different scanning speeds
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3.1.2 脉冲频率对清洗效果的影响
为研究脉冲频率f对铝合金表面漆层清洗质量的影响,选取脉冲频率f为20~45kHz进行工艺试验,其中,激光功率P=16.5W,扫描速度v=600mm/s,试验结果如
图 4. 不同脉冲频率下铝合金清洗表面的三维形貌。(a)20kHz; (b)25kHz; (c)30kHz; (d)35kHz; (e)40kHz; (f)45kHz
Fig. 4. Three-dimensional morphologies of the surface cleaned at different pulse frequencies. (a) 20kHz; (b) 25kHz; (c) 30kHz; (d) 35kHz; (e) 40kHz; (f) 45kHz
表 3. 不同脉冲频率下清洗表面的表面粗糙度
Table 3. Surface roughness of the surface cleaned at different pulse frequencies
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3.1.3 激光功率对清洗效果的影响
为研究激光功率P对铝合金表面漆层激光清洗质量的影响,选取激光功率P为10.5~25.5W进行工艺试验,其中,扫描速度v=600mm/s,脉冲频率f=30kHz,试验结果如
图 5. 不同激光功率下铝合金清洗表面的三维形貌。(a) 10.5W; (b) 13.5W; (c) 16.5W; (d) 19.5W; (e) 22.5W;(f) 25.5W
Fig. 5. Three-dimensional morphologies of the surface cleaned at different powers. (a) 10.5W; (b) 13.5W; (c) 16.5W;(d) 19.5W; (e) 22.5W; (f) 25.5W
铝合金表面漆层的激光清洗过程可以看作是点热源对半无限大物体进行加热的过程,其温升模型为[20-23]
式中:ΔT为温度增加量,℃;P为激光功率,W;a为材料的导温系数,cm2/s;τ为激光脉宽,s;K为材料的热导率,W/(cm·℃)。
由(2)式可知,铝合金表面漆层的温升与激光功率呈线性关系,激光功率越大,清洗时铝合金表面漆层的温升越大,即更多的漆层被清洗。
表 4. 不同激光功率下铝合金清洗表面的表面粗糙度
Table 4. Surface roughness of the surface cleaned at different laser powers
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由上述分析可以看出,脉冲激光清洗的最佳工艺参数为:激光功率P=16.5W,脉冲频率f=30kHz,扫描速度v=600mm/s。在此工艺参数下,清洗表面无漆层残留,除漆率达到了100%,基体表面有轻微熔融,这与文献[ 24-25]的结果相似。清洗过程对基体的影响后续将开展深入研究。
3.2 激光清洗表面的形貌与EDS分析
图 6. 不同工艺参数下清洗表面的形貌。(a) SEM形貌,v=200mm/s;(b) SEM形貌,f=25kHz; (c) SEM形貌, v=1000mm/s; (d) 三维形貌,v=1000mm/s; (e)(f) SEM形貌,f=40kHz
Fig. 6. Morphologies of the surface cleaned at different parameters. (a) SEM morphology, v=200mm/s; (b) SEM morphology, f=25kHz; (c) SEM morphology, v=1000mm/s; (d) three-dimensional morphology, v=1000mm/s; (e)(f) SEM morphology, f=40kHz
表 5. 图6 中不同区域的EDS结果
Table 5. EDS results of different areas in Fig. 6
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图 7. 漆层表面碳、氧元素的XPS结合能谱。(a)(b)原始漆层表面; (c)(d)清洗表面(f=40kHz)
Fig. 7. XPS C 1s and O 1s spectra of the paint surface. (a)(b) Original surface; (c)(d) surface after cleaning with the frequency of 40kHz
表 6. 漆层表面碳、氧元素的分峰结果及结合能
Table 6. Results of the peak separation of XPS C 1s, O 1s spectra, together with the binding energy
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3.3 清洗表面残留漆层的形貌分析
3.3.1 残留漆层的横截面分析
图 8. 残留漆层横截面的SEM图像(f=40kHz)。(a) 2000×;(b) 10000×;(c) 50000×;(d) 20000×
Fig. 8. Cross-section SEM images of residual paint(f=40kHz). (a) 2000×; (b) 10000×; (c) 50000×; (d) 20000×
3.3.2 除漆过程收集颗粒的分析
激光清洗漆层过程中收集的颗粒表面不平整,存在明显的凹坑和裂纹,说明漆层清洗过程中存在力学机制,如振动、冲击。颗粒侧面存在漆层层间裂纹[如
图 9. 清洗过程中收集的颗粒的SEM图像。(a)层间裂纹; (b)层内未断裂痕迹; (c)球形颗粒及网状结构; (d)纳米颗粒的分布
Fig. 9. SEM images of collected particles during cleaning process. (a) Interlaminar crack; (b) no fracture trace in the layer; (c) spherical particles and network structure; (d) nanoparticles distribution
3.4 激光清洗漆层的过程及作用机制
脉冲激光的能量密度极高,脉冲激光照射漆层表面,将能量传递到漆层并转变成热能,激光作用表面的温度急剧升高,导致漆层聚合物分子链中的部分化学键断裂(不同元素、不同链长断裂所需要消耗的化学键能量不同),从而导致漆层燃烧。
图 10. t=0.000268 s时,x轴方向不同深度处的温度
Fig. 10. Temperature at different positions in the x-axis direction at t=0.000268s
离脉冲光斑中心越近的漆层,对激光的作用响应得越快,漆层温度急剧升高,产生的热应力较大,而较远的漆层产生的热应力较小。
图 11. t=0.000268s时,x轴方向不同深度处的热应力
Fig. 11. Thermal stress at different positions in the x-axis direction at t=0.000268s
漆层聚合物共价键发生断裂重排,生成新的有机物,如(—CH2CHC(CH3)CH2—)n和(—CH2CH(C(O)OH)—)n等,仅使聚合物分子链的完整性被破坏,成为潜在的裂纹源[34]。结合
综上可知,脉冲激光去除漆层是多种机制耦合作用的复杂过程,既包含能量吸收、传递、转化的化学键断裂燃烧机制,也包括克服聚合物内聚力的内聚力破坏机制,如热应力振动破坏、等离子体冲击破坏,还包含裂纹扩展断裂机制。脉冲激光清洗漆层的机制如
4 结论
本文通过2024铝合金表面漆层的激光清洗试验,研究了扫描速度、脉冲频率和激光功率对漆层清洗的影响,分析了脉冲激光清洗试样表面的微观形貌、元素价态、清洗漆层的横截面形貌和清洗过程收集的颗粒的微观形貌,并利用有限元分析软件COMSOL Multiphysics建立了脉冲激光清洗2024铝合金表面漆层的有限元模型,计算了漆层内不同位置处的温度变化与热应力变化,从化学键断裂、内聚力破坏以及微观裂纹扩展的角度探讨了脉冲激光清洗飞机蒙皮漆层的过程与作用机制。本文主要得到以下结论:
1)过大或过小的光斑搭接率均会导致铝合金表面漆层激光清洗质量降低,表面粗糙度增大。扫描速度和脉冲频率对表面粗糙度的影响规律基本一致,即表面粗糙度随扫描速度、脉冲频率的增加而先降低后增大。
2)随着激光功率增大,表面漆层激光清洗质量逐渐变好,激光功率越高,激光清洗漆层的能力越强。
3)在脉冲激光作用下,漆层高聚物分子链中发生了C—H、C—C、C
4)激光清洗聚丙烯酸酯树脂基漆层是多种作用机制耦合的结果,包括化学键断裂燃烧机制、内聚力破坏机制和裂纹扩展断裂机制,其中内聚力破坏和裂纹扩展断裂机制是主要的作用机制。
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Article Outline
赵海朝, 乔玉林, 杜娴, 王思捷, 张庆, 臧艳. 脉冲激光清洗铝合金表面漆层的技术研究[J]. 中国激光, 2021, 48(3): 0302001. Haichao Zhao, Yulin Qiao, Xian Du, Sijie Wang, Qing Zhang, Yan Zang. Research on Paint Removal Technology for Aluminum Alloy Using Pulsed Laser[J]. Chinese Journal of Lasers, 2021, 48(3): 0302001.
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