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.     

The potential use of plume imaging for real-time monitoring of laser ablation cleaning of stonework

In this work, the potential of plume imaging was investigated as a real-time monitoring technique for the control of the laser cleaning process in cultural heritage objects with emphasis on encrusted stonework. Optical pulses generated by Nd:YAG laser were used to remove a high absorbing layer of crust from the surface of a lower absorbing marble substrate. Two regimes of operation have been employed for laser cleaning test, Q-switched ns mode, and short free running μs mode. Laser induced breakdown spectroscopy (LIBS) was also employed to study the stratigraphy of the cleaned surfaces, and thus to confirm the validity of the suggested monitoring technique. The tests proved the prospective of introducing plume imaging techniques as a straightforward and inexpensive tool for the real-time monitoring of laser ablation and the determination of the critical point for a safe surface cleaning intervention.     

Acoustic substrate expansion in modelling dry laser cleaning of low absorbing substrates

Acoustic expressions have been derived for the thermal expansion of substrate surfaces due to irradiation by an exponential laser pulse. The result of acoustic effects on three substrates (silicon, glass and silica) with different absorptions has been calculated.It has been shown that for substrates having relatively low absorptions, like silica and glass, acoustic considerations substantially reduce thermal expansion of the substrate caused by irradiation by nanosecond laser pulses relative to a quasi-static expansion model. In particular, the expansion of the substrate occurs over a much longer time frame than when the quasi-static approximation holds. Consequently, acceleration of the substrate surface is greatly reduced and laser cleaning threshold fluences for particle removal are increased.The predictions of the model of Arnold et al. when developed for acoustic considerations give reasonable agreement with experimentally found threshold fluences for alumina particles on silica and glass substrates although it underestimates the ratio of the threshold cleaning fluences of silica and glass. This could be due to the model underestimating the contribution of surface expansion to the laser cleaning process. The influence of multiple reflections in the substrate and departure from one dimensionality in the heat conduction on the threshold fluence was found to be insignificant. Thermal contact between the particle and the substrate was also found to have little effect on laser cleaning threshold fluences. Another mechanism that may enhance surface expansion is the 3D focussing of radiation by the particles. PACS 42.62.Cf; 81.65.Cf; 42.55.Lt     

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。     

Acoustic emission monitoring during laser shock cleaning of silicon wafers

A laser shock cleaning is a new dry cleaning methodology for the effective removal of submicron sized particles from solid surfaces. This technique uses a plasma shock wave produced by laser-induced air breakdown, which has applied to remove nano-scale silica particles from silicon wafer surfaces in this work. In order to characterize the laser shock cleaning process, acoustic waves generated during the shock process are measured in real time by a wide-band microphone and analyzed in the change of process parameters such as laser power density and gas species. It was found that the acoustic intensity is closely correlated with the shock wave intensity. From acoustic analysis, it is seen that acoustic intensity became stronger as incident laser power density increased. In addition, Ar gas has been found to be more effective to enhance the acoustic intensity, which allows higher cleaning performance compared with air or N₂ gas.     

Laser cleaning of stone by different laser pulse duration and wavelength

The present work focuses on the main phenomenological features of stone cleaning by lasers. These are the removal rate, cleaning degree, and chromatic appearance of the treated surface associated with different conservation problems and laser parameters. A set of three different outdoor stone conservation problems were investigated here. The measurement of the ablation rates were carried out on encrusted stone artifacts and two sets of standards in order to derive general behaviors through repeatable measurements. The analysis of the irradiation tests provided quantification of the different efficiencies, cleaning degree, and chromatic appearances associated with the fundamental harmonic of Q-switching, long Q-switching, and short free-running Nd:YAG lasers (1064 nm), as well as with the second harmonic of Q-switching sources (532 nm).     

Acoustic field imaging by light reflection at the free surface of a liquid

In this paper we present a novel technique for acoustic field imaging. This technique is based on reflection of a collimated laser beam at the free surface of a liquid. The reflected beam becomes phase modulated by the acoustic wave as in acoustical holographic systems. We do not use a reference acoustical beam for holographic reconstruction but we observe this phase modulation using dark-field techniques. It gives a measurement of the acoustic field power as a function of the position. The authors have built an optical imaging system and carried out experiments with piezocomposite transducers. The technique presented in this work is able to give fast quantitative information about the performance of individual ceramic rods of the piezocomposite.     

Laser removal of copper particles from silicon wafers using UV, visible and IR radiation

Laser removal of small copper particles from silicon wafer surfaces was carried out using Nd:YAG laser radiation from near-infrared (1064 nm) through visible (532 nm) to ultraviolet (266 nm). It has been found that both 266 nm and 532 nm are successful in removing the particles from the surface whereas 1064 nm was shown to be ineffective in the removal of particles. The damage-threshold laser fluence at 266 nm was much higher than other wavelengths which provides a much wider regime for safe cleaning of the surface without causing any substrate damage. The cleaning efficiency was increased with a shorter wavelength. The effect of laser wavelength in the removal process is discussed by considering the adhesion force of the particle on the surface and the laser-induced cleaning forces for the three wavelengths. Received: 31 May 2000 / Accepted: 14 July 2000 / Published online: 20 June 2001     

光谱测量在激光加工在线监测上的应用研究进展

随着现代工业的发展,复杂加工环境和对象、大动态范围、高效率和高精度激光加工需求愈加迫切,在线监测并实时优化激光加工参数是一条重要的解决途径。与此同时,激光与物质相互作用时可产生与加工参数、加工过程和目标特性密切相关的光信号和表面光学特性变化,在线测量光信号光谱可分析加工过程和状态,故光谱测量有望成为一种重要的激光加工在线监测手段。实际上,光谱测量已应用于激光焊接、激光切割和钻孔、激光清洗打磨、微纳结构制备和增材制造等几乎所有激光加工工艺,具有分辨率高和光谱信息丰富等特点。分析和总结了用于激光加工在线监测的光谱测量技术,包括等离子体光谱、反射光光谱和非线性光信号光谱等。基于单脉冲和多脉冲激光加工激发等离子体信号的光谱测量,除实现化学成分定性和定量监测外,还可以根据特征谱线相对强度变化实时调焦,根据等离子体温度监测和调控激光加工过程中与热效应相关的物理过程;作为一种无损伤且工作距离较远的监测方法,反射光光谱监测可通过测量特定波段反射光信号光谱积分功率、特征谱线和波段位置和强度来有效监测材料表面清洁度、损伤、色度和成分变化等;而在特定条件下产生的谐波信号、荧光信号和拉曼信号等非线性光信号,尽管应用场景有限,但提供了一种实现成分、焦距和材料损伤等监测的新方法。进而,展望了光谱测量在激光加工在线监测上应用的未来发展趋势,包括多种光信号的光谱协同监测与光、声、温度及图像等多种信号测量的复合监测。同时,人工智能技术与在线监测和激光加工的深入结合将进一步推动激光加工技术的智能化发展。     

Process monitoring of powder pre-paste laser surface alloying

Instead of the continuous powder delivery method using a powder feeder for thick layer laser cladding, pre-pasting of the alloying powder on the substrate is a widely used method to supply the coating powders into the melt pool for LSA. A method to monitor the process of laser surface alloying based on the infrared emission from the melt pool using infrared photodiodes was developed. The technique is solely aimed at the process of laser surface alloying using pre-paste metal powder on the substrate surface prior to laser melting. This monitoring technique is able to distinguish the existence or the absence of the pre-paste powder and the consistency of the laser surface alloying process. The technique is of low cost and is simple to implement into the process.     

Laser ultrasonic surface wave inspection of alumina ceramics of varying density

In this paper, the surface acoustic wave velocity results acquired from the inspection of specially manufactured and characterised alumina ceramic materials are presented. Ultrasonic velocity data of alumina-based ceramics in the range 60-100% theoretical density was generated utilising non-contacting laser-ultrasonic measurements based on laser generation and detection of surface acoustic waves with the objective of creating a routine technique for industrial advanced alumina inspection. With linear fitting the surface acoustic wave velocity data serves as a calibration graph for using laser ultrasonics for routine monitoring of alumina. A second laser ultrasonic technique based on the laser generation and foil transducer detection of surface acoustic waves was used to validate the surface acoustic wave velocities measured by the laser generation/detection technique.     

Energetic study of ultrasonic wettability enhancement

Many industrial and biological interfacial processes, such as welding and breathing depend directly on wettability and surface tension phenomena. The most common methods to control the wettability are based on modifying the properties of the fluid or the substrate. The present work focuses on the use of high-frequency acoustic waves (ultrasound) for the same purpose. It is well known that ultrasound can effectively clean a surface by acoustic cavitation, hence ultrasonic cleaning technology. Besides the cleaning process itself, many authors have observed an important wettability enhancement when liquids are exposed to low and high (ultrasonic) frequency vibration. Ultrasound goes one step further as it can instantly adjust the contact angle by tuning the vibration amplitude, but there is still a lack of comprehension about the physical principles that explain this phenomenon. To shed light on it, a thermodynamic model describing how ultrasound decreases the contact angle in a three-phase wetting system has been developed. Moreover, an analytical and experimental research has been carried out in order to demonstrate that ultrasound is an important competitor to surfactants in terms of energy efficiency and environmental friendliness.     

Angular laser cleaning for effective removal of particles. from a solid surface

Laser removal of small particles from a metal surface is carried out by changing the incident angle of the laser beam. It has been found that a dramatic improvement of cleaning efficiency in terms of area and energy is observed when using the laser at glancing angle of incidence as compared to perpendicular. Furthermore substrate damage is greatly reduced and probably eliminated at glancing angles. The process mechanism is discussed by considering the adhesion and the laser-induced cleaning forces for different incident angles. It is shown that there are different laser–matter interactions operating. Received: 25 April 2000 / Accepted: 9 May 2000 / Published online: 5 October 2000     

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.     

Acoustic laser cleaning of silicon surfaces

We investigate the detachment of small particles from silicon surfaces by means of acoustic waves generated by laser-induced plasma formation at the back side of the sample. It is demonstrated that sufficiently high acoustic intensities can be reached to detach particles in the submicron regime. In order to study this “acoustic laser cleaning” in more detail, we have developed an interference technique which allows one to determine the elongation and acceleration of the surface with high temporal resolution, the basis for an analysis of the nanomechanical detachment process, which takes place on a temporal scale of nanoseconds. We find that the velocity of the detaching particles is significantly higher than the maximum velocity of the substrate surface. This indicates that not only inertial forces, but also elastic deformations of the particles, resulting from the acoustic pulse, play an important role for the cleaning process. PACS 81.65.Cf; 68.35.Np     

Optical focus control system for laser welding and direct casting

A non-intrusive optical sensor system has been developed for focus control of laser welding. This detects the light generated by the process through the laser delivery optics, and exploits the chromatic aberrations of these optics to detect any laser focal error at the workpiece. This system works for a wide range of materials and welding parameters, and example results are presented. The sensor has also been applied to laser ‘direct casting’, a process in which 3-D structures are built by flowing metal powder into a focused laser beam. In this case, melt pool temperature is also important, and so additional optics are incorporated into the sensor to provide a pyrometric temperature measurement which is used to control the laser power.     

Laser removal of TiN coatings from WC micro-tools and in-process monitoring

Current environmental challenges require sustainable and extended use and re-use of materials. For example, the service life of engineering tooling can be extended by using thin film coatings such as titanium nitride (TiN). However, when errors arise in the coating process or when the tooling needs to be re-used it is necessary to remove the coating. Decoating is also useful when a large batch of cutting tools needs to be re-directed for a different application, which requires a new generation of coating. Existing technology uses chemical methods which are not environmentally friendly or ideal for selective removal. In this work, excimer laser striping of TiN from coated tungsten carbide (WC) micro-tools has been demonstrated as a viable alternative to chemical methods. Also, in order to raise the integrity of the decoating process and to make the process more accurate and reliable, two online monitoring systems were developed exploiting probe beam reflection (PBR) and laser plume emission spectroscopy (PES). The online monitoring system facilitated a simultaneous prediction of surface elements as coating layers are progressively removed and ensures better control over the laser irradiation process so as to avoid under or over stripping of the coating.     

Study of the laser cleaning on plaster sculptures. The effect of laser irradiation on the surfaces

The focus of this paper is to study the effects caused by the laser irradiation on nineteenth and twentieth century plaster sculptures. Before applying the laser cleaning on the sculptures, it was tested on samples prepared in laboratory according to the results of the scientific investigation carried out on the selected works of art. The characterization of the surface finishing materials of the sculptures was performed by Fourier Transform Infrared spectrometry (FTIR), X-ray Fluorescence spectroscopy (XRF), UV fluorescence photography, and internal micro stratigraphic analysis. Regarding the finishing materials, shellac, zinc white, siccative oil and proteins were found on the surfaces. The results of the scientific investigation, together with the examination of the ancient technical manuals, were used to create the laboratory samples to carry out the irradiation tests with laser. The laser irradiation and cleaning tests were carried out with a Q-switched Nd:YAG system. The irradiated surfaces were analyzed before and after the laser tests with the aid of a video microscope and a reflectance spectrophotometer, in order to evaluate the color changes of the surfaces. The possible morphological modifications caused by laser irradiation were also investigated by Scanning Electron Microscopy (SEM) together with ancillary Energy Dispersive Spectroscopy (EDS) elemental analysis. Concerning the laser cleaning test on the samples, in general little color changes were observed both with the 532 and 1064 nm wavelength. Total color changes, expressed as ΔE*, are always small apart from the samples made of shellac and zinc white in linseed oil, as finishing layer. As regards these samples the surface irradiated with the laser greyed lightly, corresponding to a decrease of L* parameter (lightness). SEM imaging of the treated and not-treated samples, both at low and high magnification, does not show evidence of significant morphological differences due to the laser beam irradiation. Starting from these results, some laser cleaning tests were carried out on the original sculptures, putting them in comparison with an aqueous cleaning method. It was found that the laser cleaning removes effectively the dirt layer preserving the original finishing on the sculpures’ surface, whereas the aqueous cleaning system removes also the finishing.     

Surface cleaning of metals by pulsed-laser irradiation in air

The surface-cleaning effect of metals was investigated using KrF-excimer-laser irradiation of metal surfaces in air. The laser-induced cleaning of copper, stainless steel and aluminum surfaces was studied. It is found that laser cleaning is an effective cleaning process for metals even if the metal surfaces are heavily contaminated. It is also found that short wavelength and pulse duration are necessary for laser surface-cleaning. The energy density of the laser pulse is an important parameter in the cleaning process. Low energy density results in a cleaner surface but a larger pulse number is required, whereas high energy density can achieve higher cleaning efficiency but the temperature rise can cause surface oxidation and secondary contamination. In contrast to the KrF-excimer-laser, the pulsed CO₂ laser is not effective in surface-cleaning. The mechanisms of laser cleaning may include laser photodecomposition, laser ablation and surface vibration due to the impact of the laser pulse. Laser cleaning provides a new dry process to clean different substrate surfaces and can replace the conventional wet cleaning processes such as ultrasonic cleaning with CFC and other organic solvents.     

Excimer laser irradiation at 248 nm of wooden archaeological objects and polymeric consolidants used in conservation: a study of cone formation and optimum cleaning parameters

Samples of a prehistoric consolidated vegetable basketry, coming from an important pile building in the North-East of Italy (Fiavé Carrera, Trento), had been irradiated with excimer laser pulses to remove the consolidant layer and restore the artifact. Some problems, related to the nature of the consolidant resin, were found in the preliminary laser cleaning process. The two main obstacles were related to the unknown composition of the polymeric resin and to the irregular surface morphology of the archaeological find. We observed that large numbers of laser pulses, for selected laser parameters, strongly modify the resin surface morphology with formation of a large number of cones. After cone formation, the surface was so irregular that it was almost impossible to complete the laser cleaning procedure. To find a solution to the problem, we here try to understand the mechanisms of cone formation in the present polymeric material. Scanning electron microscopy (SEM) and Fourier transformation infrared spectroscopy (FTIR) have been used to study surface morphology and chemical modification after selected irradiation processes. To understand the influence of laser irradiation on surface modifications and to establish a cleaning procedure for this specific application, investigations have been carried out separately on models of resin consolidants and wood (both dried and buried wood sample). The consolidants were commercial resins, typically used in archaeological conservation (Plexisol and Paraloid), deposited on a silicon substrate. Silicon was used because it is an appropriate substrate in FT-IR analysis. Finally, we chose wood because of its wide use in prehistoric artefact (Viburnum lantana). PACS  81.65.Cf; 82.50.Hp; 87.64.Ee; 87.64.km