In-process monitoring techniques for laser cleaning

Diverse in-process monitoring techniques based on acoustic, chromatic and intelligent approaches have been developed for laser cleaning not only to achieve the sound-cleaned surface but also to control the process in an automatic manner. The cleaning of various materials such as copper, marble, paper have also been carried out by using Q-switched Nd:YAG radiation. The process was successfully monitored by detecting the acoustic emission induced by laser–surface interactions during laser cleaning. Novel surface monitoring was achieved by chromatic modulation technique. The monitoring of laser fluence based on neural network logic was carried out by means of the recognition of acoustic spectrum patterns. The prediction system of surface damage has been also developed using fuzzy rule base in the same way as a human expert. These techniques may provide unique information for characterising the process as well as a promise of successful applications for laser cleaning techniques in real practical fields.     

On all-optical laser cleaning and inspection of contaminated concave metal surfaces

A technique for all-optical laser cleaning and surface quality monitoring of concave metal surfaces is suggested. Contaminated pharmaceutical punches from a tablet compression machine were cleaned using high-energy laser pulses. Image information obtained from a diffractive-optical-element-based sensor was used in inspection of the surface quality change of the concave punches due to laser cleaning. Alternative method for mechanical cleaning of punches is introduced, which is based on simultaneous laser cleaning and surface quality inspection of the punches.     

激光除锈过程的实时监测技术研究

对激光除锈过程中的实时监测技术进行了实验研究,结果表明:除锈过程中产生的声波强度会随着表面清洁度的变化而发生明显的变化,声波信号包含了除锈过程中的大量信息,可作为激光除锈实时监测的基本信号.     

Holographic visualization of laser-induced plume in pulsed laser welding

High-speed holographic interferometry was applied to the experimental study of a laser-induced plasma plume in pulsed laser welding. We adopted two kinds of holographic interferometers for visualizing and imaging the refractive index distribution of the plume and vaporized metal; a real-time holographic interferometer with a high-speed camera and a double-pulsed holographic interferometer with a dual-reference-beam module. The high-speed photographs of the weld plume were compared with the visualized images by holographic interferometer. The experimental results show the process of generation and propagation of the laser-induced plume and give the feasibility of quantitative measurement of the density distribution of the laser-induced plume and vaporized metal in laser welding.     

Measurement of laser beam transmittance through laser produced vapour plume

Laser produced plume, consisting of vapor front and ejected substrate on the workpiece surface, is an intermediate between the incident laser beam and the workpiece on which the beam is directed. It partially blocks, defocuses, absorbs, scatters, and deflects the incident beam and thereby reduces the laser energy reaching the workpiece. However, plume additionally acts as a heat source enhancing the machining. Consequently, laser induced plume plays an important role in laser machining process. The present study investigates the transmittance of a reference beam by the plume generated during the laser-workpiece interaction. To achieve the transmittance measurements, a Nd:YAG laser was used and four different materials were employed in the experiment. To obtain realistic values of the plume transmittance in relation to the laser machining process, the reference beam was sampled from the incident laser beam. The study was extended to include the effect of the laser pulse parameters on the transmission process. It is found that about 10–30% of the transmittance of the reference beam through a vapor plume, produced due to laser ablation, occurs close to the workpiece surface.     

Morphological and chemical modifications and plume ejection following UV laser ablation of doped polymers: Dependence on polymer molecular weight

This work investigates the effect of polymer molecular weight (M_W) on the morphological and chemical modifications and on the plume ejection of doped polymethyl methacrylate (PMMA), and polystyrene (PS) films following irradiation at 248 nm. Micro-bubbles develop in the irradiated films of the low absorbing PMMA-based substrates. The extent and kinetics of the observed morphological changes are respectively larger and last longer in the low M_W polymer, as evidenced by optical microscopy and real-time monitoring of transmission of a CW laser. The changes observed in the Raman spectra upon irradiation indicate that degradation occurs to a higher extent in larger M_W polymers. Laser induced fluorescence (LIF) probing of the plume reveals the presence of NapH and PhenH products from, respectively, NapI- and PhenI-doped films and a slower ejection process in the plume of low M_W polymer. For highly absorbing PS, a less dramatic dependence on M_W is observed. Results are discussed in the framework of the bulk photothermal model, according to which ejection requires a critical number of bonds to be broken.     

激光清洗实时监测技术研究

针对目前在研激光清洗的实时监测问题进行实验和分析。得出两个结论:激光清洗具有传统方法无法具有的优越性,可以节省大量人力物力成为新型清洗方法而受到重视,激光清洗在设备维修保养中显示了良好的应用前景。激光清洗过程中产生的声发射信号包含了清洗过程的大量信息,利用声发射检测系统可以对激光清洗过程实时监控。     

LIBS used as a diagnostic tool during the laser cleaning of ancient marble from Mediterranean areas

The use of laser ablation for cleaning stone is a tried-and-tested method for preserving outdoor artwork surfaces exposed to environmental stresses. However, it is of interest to spectroscopically characterize the sample surface before and during the laser ablation in order to implement automatic control of the cleaning process. To this aim, we have undertaken systematic LIBS analysis on various clean and dirty surfaces of marble fragments collected from ancient quarries in Mediterranean areas, without the characteristic patina that comes from the protective layers usually deposited on the final artwork. The effectiveness of the cleaning process was then monitored by following the disappearance from the LIBS spectra of the encrustation elements during successive laser shots. The LIBS analysis of the clean surfaces of the samples examined confirmed that main bulk composition is based on calcium and magnesium carbonates, with the addition of strontium and, to a minor extent, of manganese and copper substituents. On the other hand, the encrustations were rich in sodium, aluminum, iron, silicon, titanium, lithium, manganese, and chromium, probably coming from sand/soil deposition and, only to a minor extent, from atmospheric pollution. Although SEM imaging and SEM-EDX analyses performed on the same samples at different resolutions showed remarkable surface inhomogeneities from the crustal region deep into the bulk material, the work demonstrates the possibility of a quantitative, minimally invasive, LIBS stratigraphy. The results from the technique are suitable for monitoring cleaning processes by determining appropriate elemental markers present on the surface at trace level (of the order of 100 ppm). PACS 52.25.Kn; 7.60.Rd; 32.30.Jc; 42.62.Fi     

Real-time monitoring of laser welding by correlation analysis: The case of AA5083

In this study, we present an innovative real-time laser welding monitoring technique employing the correlation analysis of the plasma plume optical spectra generated during the process. In order to look for a relationship between optical signals and welds quality, the influence of the experimental conditions on the correlation plots are also investigated. The correlation analysis results allow to evaluate the quality of the welds, through an on-line detection of common defects, such as oxidation or lack of penetration, with an excellent spatial resolution.     

Research of laser evaporation of fast-moving target

Evaporation of a stainless steel target moving with high speed (～50 m/s) under action of laser radiation was investigated theoretically and experimentally. In our experiments we used an electroionization CO₂—laser, which generated pulses with duration up to 1 ms and energy up to 100 J. We carried out the microscopic research of laser beam trace on the target surface and investigated the dynamics of the laser plume luminescence. For theoretical research we used 3D numerical model, which took into account: heating, melting and evaporation of target by laser beam, and, thermal effect of oxidation reaction. The results of calculations can explain the experimental data quite good. In particular, it is possible to explain occurrence of interrupted trace on the target at 12–24 kW laser power, that corresponds to intensity in the focal spot of ～10⁷ W/cm². This power is a threshold of unstable mode of laser evaporation. The unstable mode is caused by lack of oxygen, which was pushed away with metal vapor. The lack of oxygen leads to shutting down the oxidation reaction on target surface. The reaction resumes when the vapors fly away and oxygen riches the surface. As the result pulsed mode of evaporation takes place. This phenomenon was observed as pulse mode of laser plume luminescence and was obtained by calculations.     

NATURAL CONVECTION IN ROTATING DRUMS WITH INNER SURFACE HEATING

Two flow visualisation techniques, microencapsulated liquid crystal and tracer methods, are employed to observe the thermal and fluid flow fields, respectively, in a liquid that is enclosed in a rotating drum with inner surface heating. The flow patterns and temperature distribution thus obtained are correlated to determine the conditions for formation of thermally stratified layers. A method is developed to construct a three-dimensional structure of a hot plume ascending from the heating surface by synthesizing three flow structures on three mutually perpendicular cross sections (x-y, y-z, and z-x planes) obtained by the flow visualization technique. The method may be extended to obtain the real-time imaging of a three-dimensional hot plume. A combination of the results from the two tests on the heat and fluid flow fields opens a new dimension in the study of natural convection in a rotating system. Additionally, the temperature-time history inside the Eckman boundary layers is monitored to aid in the understanding of transient thermal behavior and rotational effects on the Eckman boundary layers. The conditions for the incipience of thermally stratified layers are disclosed.     

Impact of open de-ionized water thin film laminar immersion on the liquid-immersed ablation threshold and ablation rate of features machined by KrF excimer laser ablation of bisphenol A polycarbonate

Debris control and surface quality are potential major benefits of sample liquid immersion when laser micromachining; however, the use of an immersion technique potentially modifies the ablation mechanism when compared to an ambient air interaction. To investigate the machining characteristics, bisphenol A polycarbonate has been laser machined in air and under a controllable open liquid film. To provide quantitative analysis, ablation threshold, ablation rate and the attenuation coefficient of the immersing de-ionized (DI) water fluid were measured. In ambient air the threshold fluence was measured to be 37 mJ cm⁻². Thin film immersion displayed two trends: threshold fluences of 58.6 and 83.9 mJ cm⁻². The attenuation of DI water was found to be negligible; thus, the change in ablation rate resulted from increased confinement of the vapour plume by the liquid medium, generating higher Bremsstrahlung attenuation of the beam, lowering the laser etch rate. Simultaneously, splashing motivated by the confined ablation plume allowed release of plume pressure before plume etching commenced. This contributed to the loss of total etching efficiency. Two interaction scenarios were obsereved as a result of splashing: (i) intermediate threshold fluence, where splashing occured after every pulse in a mode that interrupted the flow entirely, leaving an ambient air interaction for the following pulse; (ii) high threshold fluence, where splashing occured for every pulse in a mode that allowed the flow to recommence over the image before the next pulse causing every pulse to experience Bremsstrahlung attenuation. Since attenuation of the immersion liquid was negligible, it is the action of the constrained ablation plume within a thin flowing immersion liquid, the resultant Bremsstrahlung attenuation and splashing events that are the critical mechanisms that modify the primary ablation characteristics.     

脉冲激光诱导Cu靶产生发光羽的特性分析

通过在不同的环境气压下拍摄脉冲激光烧蚀金属Cu诱导产生的发光羽,获取了不同区域具有不同颜色特征的发光羽照片.结果发现:发光羽的颜色随环境气压的改变而变化.采用空间分辨光谱技术,测定了激光诱导金属Cu靶产生发光羽辐射强度的空间分布,以及不同烧蚀环境气压对发光羽辐射强度的影响.研究了脉冲激光烧蚀Cu表面诱导发光的动力学过程,建立了可能的发光羽分区模型,对发光羽的不同区域发光粒子的激发机理进行了探讨,并用之定性地解释了所观察的实验现象.结果分析表明:脉冲激光诱导Cu产生的发光羽可以分为三个区域,不同区域的发光机理不同,Cu原子和Cu离子的激发机理不完全相同.     

用光偏转法研究气体中激光等离子体羽的特性

采用光偏转法研究气体中准分子激光与石墨和铜相互作用产生等离子体羽的特性。用空间和时间分辨的光偏转信号分析了等离子体羽的漂移速度和膨胀速度，并从光偏转信号中测得了物质波前面的激波速度。     

Comparison of the laser ablation process on Zn and Ti using pulsed digital holographic interferometry

Pulsed digital holographic interferometry has been used to compare the laser ablation process of a Q-switched Nd-YAG laser pulse (wavelength 1064 nm, pulse duration 12 ns) on two different metals (Zn and Ti) under atmospheric air pressure. Digital holograms were recorded for different time delays using collimated laser light (532 nm) passed through the volume along the target. Numerical data of the integrated refractive index field were calculated and presented as phase maps. Intensity maps were calculated from the recorded digital holograms and are used to calculate the attenuation of the probing laser beam by the ablated plume. The different structures of the plume, namely streaks normal to the surface for Zn in contrast to absorbing regions for Ti, indicates that different mechanisms of laser ablation could happen for different metals for the same laser settings and surrounding gas. At a laser fluence of 5 J/cm², phase explosion appears to be the ablation mechanism in case of Zn, while for Ti normal vaporization seems to be the dominant mechanism.     

Application of the laser ablation for conservation of historical paper documents

Laser ablation was applied for surface cleaning and spectroscopic diagnostics of historical paper documents and model samples in the framework of the conservation projects. During cleaning the spectra of ablation products were recorded by means of the LIBS technique which allowed for nearly non-destructive identification of surface layers such as contaminants, substrate and pigments. For consecutive laser pulses a strong decrease of band intensities of the emission lines of Ca, Na, K, Al and Fe ascribed to contaminants were observed. The effect was used for monitoring of the cleaning progress of stained paper. For surface cleaning and spectra excitation the Q-switched Nd:YAG laser of 6 ns pulsewidth operating at wavelengths of 266, 355, 532, and 1064 nm and of fluence selected from the range 0.3-0.9 J/cm² was applied. The ablation parameters were optimized in agreement with the literature and the results were confirmed by surface studies and testing of the mechanical and chemical properties, and also by the response to the ageing process of the paper substrate. In case of the model paper irradiated in the UV range at 266 and 355 nm a visual inspection revealed local damages of the cellulose fibers accompanied by a decrease of the mechanical strength of the substrate. The effect was more pronounced after artificial ageing. The best results were obtained for samples irradiated at 532 nm and at laser fluence below the damage threshold of 0.6 J/cm², which is in agreement with literature.     

Investigation of surface laser treatment of ancient calcite: the case of the grave in Torricelle (Naples, Italy)

The laser-cleaning technique has been effectively employed in many areas. Recently it has led to a huge development in art restoration. However, this technique must be optimised to remove contaminations and encrustations from surfaces without generating any physical or chemical changes of the original material.Its use has been considered to clean a 4th century B.C. chamber gravesite in Torricelle, near Nola (Naples). The wall painting could not be seen owing to bioorganic surface layer and resinous material originated from surrounding pinewood. In order to evaluate the technique effectiveness we studied the interaction of the light from a Q-switched Nd: YAG equipped with SHG to work at wavelength of 532 nm.Several characterization techniques have been employed to verify the laser effect on the cleaning of the surface versus the operating parameter configuration and to evaluate the possible damages induced by the beam. In particular, micro X-Ray fluorescence (μXRF) provided information on the chemical composition of the clean surface; X-ray microdiffraction (μXRD) was used to identify the phases and their microstructures. These techniques were effective in assessing the quality of the cleaning process and for following the changes in the microstructure of the sample.Preliminary results suggest a possible influence of the laser on the aragonite–calcite transformation. PACS 61.10.Nz; 33.20.Rm; 42.62.-b; 61.66.Fn; 81.65.Cf     

Laser based diagnostics – from cultural heritage to human health

An overview of applied laser-based diagnostics as pursued at the Division of Atomic Physics, Lund University, is given. The fields of application range from environmental monitoring including cultural heritage assessment, to biomedical applications. General aspects of laser-based methods are non-intrusiveness, high spectral- and spatial resolution, and data production in real-time. Different applications are frequently generically very similar irrespective of the particular context, which, however, decides the spatial and temporal scales as well as the size of the optics employed. Thus, volcanic plume mapping by lidar, and optical mammography are two manifestations of the same principle, as is fluorescence imaging of a human bronchus by an endoscope, and the scanning of a cathedral using a fluorescence lidar system. Recent applications include remote laser-induced break-down spectroscopy (LIBS) and gas monitoring in scattering media (GASMAS). In particular, a powerful method for diagnostics of human sinus cavities was developed, where free oxygen and water molecules are monitored simultaneously. PACS  42.62.-b; 42.68.Wt; 42.62.Be     

Phase explosion and recoil-induced ejection in resonant-infrared laser ablation of polystyrene

We investigate the phenomenon of resonant-infrared laser ablation of polymers using polystyrene as a model material. Ablation is carried out using various mid-IR laser wavelengths that are resonant with vibrational modes of a polystyrene target. Time-resolved plume imaging coupled with etch-depth measurements and thermal calculations indicate that ablation begins after a superheated surface layer reaches a temperature of ∼1000°C and undergoes spinodal decomposition. The majority of the ablated material is then expelled by way of recoil-induced ejection as the pressure of the expanding vapor plume compresses a laser-melted area.     

盘型激光焊金属蒸汽特征色调-色饱和度-亮度分析法研究

对于大功率盘型激光焊,金属焊件表面在激光束辐射下强烈汽化并形成等离子体状的金属蒸汽羽状物. 该金属蒸汽羽状物可逆向激光束传输,对激光有明显的屏蔽作用, 降低激光辐射至焊件的能量密度,影响焊接效率和质量. 因此研究金属蒸汽特征变化规律及其与焊接质量之间的关联 ,可实现由金属蒸汽特征实时监测激光焊接状态. 以10 kW大功率盘型连续激光焊接304不锈钢钢板为试验对象, 应用高速摄像机摄取金属蒸汽动态图像,将其转换至色调-色饱和度-亮度空间, 提取金属蒸汽面积、激光束受影响路径长度等相关特征量, 以焊缝熔宽的变化作为衡量焊接状态稳定性的参数. 通过金属蒸汽特征值的均值统计和方差分析,试验证明根据金属蒸汽面积和激光束受影响路径长度等金属蒸汽特征可有效地反映熔宽质量, 从而对焊接状况做出动态评估.