铝合金焊前激光清洗的等离子体光谱在线检测

铝合金焊接技术在工业生产、制造和维修等领域有广泛的应用,焊缝内存在气孔导致焊接质量降低是铝合金焊接技术的常见问题。由于铝合金表面金属氧化物是导致气孔生成的主要来源,对激光清洗过程进行在线检测,不但可以实时分析表面氧化物的清洗状态,而且可以避免基体表面因为过度清洗造成损伤或二次氧化。提出采用激光诱导等离子体光谱(LIBS)在线检测铝合金焊前激光清洗过程,表征清洗后铝合金基体的表面状态。LIBS技术可以对多元素成分同时检测,拥有较低的检出限和较高的准确性。搭建基于Andor Mechelle 5000光谱仪的铝合金焊前激光清洗在线检测系统,剔除空气环境对实验结果的影响,测试6061铝合金表面氧化物和铝合金基体的LIBS光谱,分析两者独特的元素特征谱线,采用X射线能谱(EDS)测试结果验证元素特征谱线的准确性,并探讨激光清洗过程LIBS技术在线检测的可行性。实验测试等离子体光谱谱线强度与激光能量密度之间的关系,获得单次脉冲激光去除铝合金表面氧化物的损伤阈值,结合X射线能谱的检测结果研究激光损伤阈值的成因及影响。研究激光清洗过程等离子体光谱特征谱线与脉冲次数之间的关系,提出基于O/Al特征谱线强度比值作为在线检测清洗效果及二次氧化损伤的评判依据。为验证该评判依据的准确性,将O/Al特征谱线强度比值随清洗次数的变化趋势与X射线能谱测试获得的氧元素原子百分比变化趋势进行对比。实验结果表明:采用200~700 nm范围内激光诱导等离子体谱线特征分析激光清洗状态,可以剔除空气环境的影响;氧元素和铝元素特征谱线准确反映出表面氧化膜与铝合金基体的成分差异;X射线能谱检测元素成分和含量表明氧元素含量随着激光清洗能量密度先减后增,单次清洗铝合金的二次氧化损伤的激光能量阈值为11.46 J·cm-2,小于损伤阈值的激光能量密度对铝合金基体的多次清洗未造成损伤,等离子体光谱特征谱线强度与表面清洗状态相关, 656.5 nm(OⅡ)/396.2 nm(AlⅠ)谱线强度比值≤1.5%为激光清洗干净的依据。研究结果有利于铝合金的激光清洗实时控制技术和焊接装置集成化。

On-Line Plasma Spectrum Detection of Laser Cleaning of Aluminum Alloy Before Welding

The technology of aluminum alloy welding is widely used in industrial production, manufacturing and maintenance. Porosity in welding seam leads to the decrease of welding quality, which is a common problem in aluminum alloy welding technology. The metal oxide on the surface of aluminum alloy is the main source of pore formation, therefore, on-line detection of laser cleaning process can analyze the cleaning status of surface oxides in real time and avoid damage or secondary oxidation of matrix surface caused by excessive cleaning at the same time. In this paper, laser-induced breakdown spectroscopy(LIBS) was used to detect the laser cleaning process of aluminum alloy before welding and to characterize the surface state of aluminum alloy after cleaning. LIBS technology can simultaneously detect multi-element components, with a lower detection limit and higher accuracy. In this paper, an on-line detection system for laser cleaning of aluminum alloy before welding based on AdorMechelle 5000 spectrometer was built, eliminated the influence of air environment on the experimental results, the LIBS spectra of 6061 aluminum alloy surface oxide and aluminum alloy matrix were measured, analyzed their elemental characteristic spectra, verified the accuracy of elemental characteristic spectra by using EDS test results, and explored the feasibility of LIBS technology in on-line detection of laser cleaning process. The relationship between spectral line intensity of plasma and laser energy density was tested experimentally. The damage threshold of single pulse laser for removing oxide on aluminum alloy surface was obtained. The cause and effect of laser damage threshold were researched by combining the results of X-ray energy spectrum. The relationship between the characteristic spectral lines of plasma spectra and the number of pulses in laser cleaning process was researched. Based on the intensity ratio of O/Al characteristic spectral lines, a criterion for on-line detection of cleaning effect and secondary oxidation damage was proposed. To verify the accuracy of the criterion, the trend of the intensity ratio of O/Al characteristic lines with cleaning times was compared with that of oxygen atom percentage obtained by X-ray energy spectra. The experimental results showed that the influence of air atmosphere can be eliminated by analyzing the laser cleaning state with the spectral characteristics of laser-induced plasma in the range of 200~700 nm;The characteristic spectra of oxygen and aluminum elements accurately reflect the composition difference between the oxide film on the surface and the aluminum alloy substrate;The element composition and content detected by X-ray energy spectrum showed that the oxygen content first decreased and then increased with the laser cleaning energy density, and the laser energy threshold of secondary oxidation damage of single cleaning aluminum alloy is 11.46 J·cm-2. The laser energy density less than the damage threshold does not cause damage to the aluminum alloy matrix after multiple cleaning, and the intensity of plasma spectral characteristic lines is correlated with surface states;The ratio of 656.5 nm(O Ⅱ)/396.2 nm(Al Ⅰ) spectral line intensity(≤1.5%) is the criterion of laser cleaning. The research results are beneficial to the real-time control technology of laser cleaning of aluminum alloy and the integration of welding devices.