bluuebunny/crossref_metadata_2025 huggingface dataset:
Development of simulated pigmented lesions in an optical skin-tissue phantom: Experimental measurements in the visible and near infrared
Authors: Lualdi Manuela , Colombo Ambrogio , Mari Alberto , Tomatis Stefano , Marchesini Renato
Published: June 2006

Realistic, pigmented skin simulants (“phantoms”) can be useful in optimizing dermal laser procedures and for laser surgical training. Diffuse reflectance of skin pigmented lesions depends on the amount and distribution of the absorbing/diffusing chromophores, mainly hemoglobin and melanin, embedded in the skin layers. The reason why diffuse reflectance of benign melanocytic lesions is different from that of melanoma is still unclear. In an attempt to explain such a difference, phantoms able to simulate the diffuse reflectance of both types of pigmented skin lesions have been developed. The basic material for lesion- and skin-like phantoms is a transparent silicone rubber in which Al2O3 particles, melanin, and different types of cosmetic powders were added as diffusers and absorbers. Slabs with a minimal thickness of 100 μm were prepared to mimic lesions with different thicknesses and the skin layered structures. The optical properties of tissue phantom components, i.e., total attenuation, absorption, and scattering coefficients and phase function, have been determined in the visible and near-infrared spectral range by using direct and indirect techniques. Finally, reflectance measurements of the phantoms were performed. By varying the concentration of the scattering and absorbing particles, tissue-like layers were produced with predictable optical properties. In particular, when mixing Al2O3 particles, melanin, and cosmetic powders at suitable concentrations with the silicone rubber, diffuse reflectance of our phantoms reasonably agreed with that measured in vivo of benign and malignant pigmented lesions over a range of wavelengths from 400 to 920 nm. The proposed technique allows reproduction of skin moles. The possibility of reproducing the optical features of in vivo skin pigmented lesion measurements by means of lesion-like structures is expected to be a valuable tool to try to explain why melanoma usually appears different from benign naevi. Finally, the phantom presented in this work could be a useful tool to verify the reliability of simulation models, e.g., Monte Carlo methods, in the description of light interaction with biological tissues and subsequent rise in temperature, especially when heterogeneous structures are concerned.
Penetration control in laser welding of sheet metal
Authors: Postma S. , Aarts R. G. K. M. , Meijer Johan , Jonker J. B.
Published: June 2006

For economical reasons it is desirable to apply the highest possible speed during laser welding. Increasing the welding speed at a certain laser power might result in insufficient penetration of the weld. This work describes the design of a feedback controller, which is able to maintain full penetration in mild steel sheets. An optical detector, which is placed inside the Nd:Yttrium–aluminium–garnet laser source, measures the intensity of the weld-pool radiation through the optical fiber. This sensor signal is used as input of a feedback control system. A model, which describes the dynamic response of the welding process including the sensor and laser source dynamics, has been obtained, using system identification techniques. The input of this model is the laser power and the output is the modeled sensor signal. Based on this dynamic model a feedback controller is designed and implemented. The laser power is used as an actuator. The controller maintains full penetration during welding of tracks with disturbances like sudden artificial power fluctuations and sudden speed changes. This feedback controller opens the possibility to optimize the welding speed without risking lack of penetration.
Prevention of welding defect by side gas flow and its monitoring method in continuous wave Nd:YAG laser welding
Authors: Kamimuki Kenichi , Inoue Takashi , Yasuda Kouzou , Muro Mikio , Nakabayashi Tokuhiro , Matsunawa Akira
Published: June 2006

In the Nd:yttrium–aluminum–garnet (YAG) laser welding of thick plates, reduction of porosity and monitoring the keyhole/molten metal behavior are very important issues to assure the high quality welding. The authors applied a side gas flow to prevent the porosity in the bead on plate welding with 6 kW Nd:YAG laser equipment. At the same time, a reflected Ar+ laser in the same axis as the Nd:YAG laser beam and the light emission from the weld were measured as monitoring signals. Under the optimum side gas condition, pores in the weld metal could significantly decrease, the penetration depth increased slightly, and the bead width became narrower. Under that condition, moreover, the generation of spatters was quite few. An acceptable limit of the transverse misalignment of the side gas nozzle position was about 1 mm. The misalignment could be detected by using the above mentioned monitoring signals.
Laser-assisted chemical etching for embedded microchannels and overhanging microstructures on Si/SiO2 substrates
Authors: Vakanas George P. , Tseng Ampere A. , Winer Paul
Published: June 2006

The feasibility of fabricating buried (embedded or subsurface) microstructures between silicon and silicon oxide using laser-assisted chemical etching (LACE) in chlorine atmosphere is demonstrated in this article. An associated LACE ablation model and sample calculations based on one-dimensional thermal analysis and chemical kinetics are presented. A strategy for optimizing the laser process for 3D micromachining is outlined. Potential applications of such fabricated microstructures include microfluidic MEMS for transducers and electronic cooling.
Application of experimental-based modeling to laser cladding
Authors: Toyserkani Ehsan , Khajepour Amir , Corbin Steve
Published: June 2006

This article addresses experimental-based modeling of the powder injection laser cladding process. An accurate model for the process is essential for controller design and process automation. There are few dynamic and static models for the process, however, due to the limitations and intensive numerical calculations, their application to real-time process control are not practical. Two model structures, the Hammerstein–Wiener model and Elman recurrent neural network are implemented to identify the laser cladding dynamic model. The type of experiments and collected data are explained, and model predictions for different and unseen data are compared with experimental results. It is shown that the Hammerstein–Wiener approach can more accurately describe the transient response of the process. Results are promising and show that experimental-based models can effectively be used in laser cladding processes.
The development and characteristics of a hand-held high power diode laser-based industrial tile grout removal and single-stage sealing system
Authors: Lawrence J. , Minami K. , Schmidt M. J. J. , Li L. , Edwards R. E. , Gale A. W.
Published: June 2006

As the field of laser materials processing becomes ever more diverse, the high power diode laser (HPDL) is now being regarded by many as the most applicable tool. The commercialization of an industrial epoxy grout removal and single-stage ceramic tile grout sealing process is examined through the development of a hand-held HPDL device in this work. Further, an appraisal of the potential hazards associated with the use of the HPDL in an industrial environment and the solutions implemented to ensure that the system complies with the relevant safety standards are given. The article describes the characteristics and feasibility of the industrial epoxy grout removal process. A minimum power density of approximately 3 kW/cm2 was found to exist, while the minimum interaction time, below which there was no removal of epoxy tile grout, was found to be approximately 0.5 s. The maximum theoretical removal rate that may be achievable was calculated as being 65.98 mm2/s for a circular 2 mm diameter beam with a power density of 3 kW/cm2 and a traverse speed of 42 mm/s. In addition, the characteristics of the single-stage ceramic tile grout sealing are outlined. The single-stage ceramic tile grout sealing process yielded crack and porosity free seals which were produced in normal atmospheric conditions. Tiles were successfully sealed with power densities as low as 550 W/cm2 and at rates of up to 420 mm/min. In terms of mechanical, physical, and chemical characteristics, the single-stage ceramic tile grout was found to be far superior to the conventional epoxy tile grout and, in many instances, matched and occasionally surpassed that of the ceramic tiles themselves.
Appropriate regimes of laser drilling models containing melt eject mechanisms
Authors: Kudesia S. S. , Solana P. , Rodden W. S. O. , Hand D. P. , Jones J. D. C.
Published: June 2006

The significance of melt ejection during various drilling regimes of laser drilling is investigated. The predictions of two analytical models of the drilling process, with and without the melt ejection mechanism, are compared with experimental measurements made using a free-running Nd:YAG laser over a wide range of drilling conditions in aluminum and titanium. The importance of modeling the melt expulsion process is discussed in various drilling regimes. The significance of obtaining reliable model predictions is considered by investigating the dependence on the temporal structure of the drilling pulse. With titanium, discrepancies between the experimental measurements and model predictions are observed and the reactive nature of this material is considered as a source of discrepancy.
Evaluation of nozzle geometries for laser surface alloying
Authors: Tramel Terri L. , McCay Mary H. , Hopkins John A. , McCay T. Dwayne
Published: June 2006

Five general types of nozzles and nine configurations were tested in a water tunnel and using gas pressure analysis to determine their effectiveness at reducing entrained air from the outside environment during laser surface alloying. Two orientations with regard to the work piece were employed: The optics and nozzle being normal, the optics and nozzle being at a 15° oblique angle. Both argon and nitrogen were studied. In the impingement region, they produced roughly the same results regardless of the nozzle configuration. The change in orientation of the nozzle, however, had an effect. For 0° orientation the 7° simple nozzle with shroud is best and at 15° orientation the conic nozzle is the best when used in the reverse processing direction. A shroud surrounding the nozzles had minimal influence on the reduction of oxygen (approximately 1%) at normal and oblique incidence. The impingement region of the gas was found to be adequate in size for all of the cases.
Assessment of laser Doppler perfusion imager’s in vivo reliability: Can it be used for a prospective analysis?
Authors: Caselli Antonella , Bedi Damanpreet Singh , O’Connor Colleen , Shah Chirag , Veves Aristidis
Published: June 2006

The aim of this study was to assess the reliability of the laser Doppler perfusion imager (Lisca PIM 1.0; Lisca development, Linkoping, Sweden) for longitudinal analysis. We measured the skin blood flow under a biological zero condition at the forearm level in 84 patients enrolled in a 12 week prospective interventional trial. Since the biological zero is the background reading registered by the laser when blood flow has been occluded, any fluctuation of the measurements obtained under this condition can be attributable to the laser scanner’s variability rather than biological variation. After the blood flow of the arm was occluded and arrested by a pressure cuff, blood perfusion readings were taken before and after the iontophoresis of a 1% acetylcholine chloride solution and a 1% sodium nitroprusside solution at the beginning of the study and during the two follow-up visits. A total of 927 biological zero readings were recorded throughout the study. The coefficient of variation of these measurements was 15.7%±8.5%. We conclude that laser Doppler perfusion imaging is a reliable and reproducible technique to assess the skin blood flow for prospective study since the variability of the measurements depending on the instrument itself is low.
Interaction of the keyhole and weld pool in laser keyhole welding
Authors: Dowden John
Published: June 2006

There are several components to the motion of the liquid material in the weld pool in laser keyhole welding. Observations have shown not only that it moves in the plane perpendicular to the axis of the laser, but also that it moves parallel to the laser beam. It has been recognized for many years that the motion in the direction perpendicular to the surface of the work piece is important in connection with arc welding; it has, however, received less detailed study in connection with keyhole welding. The possible mechanisms are reviewed from a mathematical standpoint and Marangoni convection and viscous drag associated with vapor motion in the keyhole are identified as the two most probable causes of the vigorous motion observed in practice in the weld pool. The former is fairly well understood but the latter is not and it is this which is studied here. Simple mathematical models already available for motion in the two regions separately are linked in this article to demonstrate the ways in which the regions interact as a result of the vigorous axial motion in the keyhole. The significance of the role of viscosity is shown, and the sensitivity of the consequences of this linking to external conditions is demonstrated.
Effect of wire feed position on laser welding with filler wire
Authors: Salminen A. S. , Kujanpää V. P.
Published: June 2006

The utilization of the laser welding process with filler wire addition is often considered a complicated and difficult process which has too high accuracy demands for a work shop floor. This study shows the effect of some of the most important welding variables to the quality and efficiency of laser welding when welding with filler wire. The material used was a common structural steel St52/37 of 6 mm in thickness. The joint type used was a butt joint. Acceptable weld quality was achieved with most of the tested parameter combinations. The effect of the wire feeding parameters for the acceptable weld quality was smaller than expected. The variations of weld quality caused by the nonoptimized aiming of filler wire can be compensated by the adjustment of the filler wire feed rate and the heat input. The quality of the welds was compared with developed weld quality index. The increase in heat input will make it possible to accept wire feeding to a lower position or from backwards compared to the welding direction. The misalignment of wire in the transverse direction to welding direction and laser beam optical axis causes problems that may lower the quality of the weld. The comparison of the welds with variable air gap widths showed that the weld width is more dependent on the heat input that on the original gap volume.
Process study for laser-induced surface coloration
Authors: Zheng H. Y. , Lim G. C. , Wang X. C. , Tan J. L. , Hilfiker James
Published: June 2006

A KrF excimer laser beam is used to induce controllable and high-contrast colors on a stainless-steel surface under different processing conditions. The laser beam acts as a localized heating source that promotes the formation of semitransparent oxide films of varying thicknesses and morphologies on the sample surface. Ellipsometry and atomic force microscopy were employed to understand the laser-induced colors by studying the surface morphology and optical properties of the oxide films and their thickness. It is found that the laser-induced colors vary with the oxide layer thickness. The laser-treated areas have granular structures, which affect oxide growth and light scattering.
Visible and invisible laser radiation—problems in laser safety terminology
Authors: Sliney David H. , Lund David J. , Marshall Wesley J.
Published: June 2006

The spectral band between 400 and 700 nm is termed “visible” in laser safety standards, this is not the full extent of the visible spectrum. Light, or visible radiation, is that which is perceived by the human eye, and the spectral bandwidth for vision is really larger, extending into what some refer to as ultraviolet and infrared. This is important to recognize when speaking of visual effects such as after-images and glare. Several important questions arise with regard to this issue. The 400–700 nm spectral band was initially established as the extent of the spectral range for class 2 lasers, but should this spectral band be reconsidered? Indeed what laser wavelengths should be considered visible and what should be considered “invisible?”
Surface characteristics and corrosion behavior of laser surface nitrided NiTi shape memory alloy for biomedical applications
Authors: Man H. C. , Cui Z. D. , Yue T. M.
Published: June 2006

Laser gas nitriding (LGN) has been applied to fabricate a TiN coating on the surface of NiTi alloy for the purpose of improving the surface properties. The microstructure, surface characteristics, and corrosion properties of the laser gas nitrided layer on NiTi alloy surface were investigated. The nickel release rate of the LGN specimen in Hanks’ solution was also investigated. The results of x-ray diffraction, x-ray photoelectron spectroscopy, and scanning electron microscopy confirmed that a continuous TiN layer which is nearly free of nickel has formed on the outermost surface of the substrate. Good metallurgical bonding between the nitride layer and the substrate has been achieved. The corrosion resistance of the LGN alloy in Hanks’ solution at 37 °C showed significant improvement. The nickel release rate in Hanks’ solution at 37 °C of the surface nitrided alloy as determined by atomic absorption spectrometry has been decreased significantly. The surface TiN layer was very inert, and no obvious changes in chemical composition and surface appearance were observed after immersion test in Hanks’ solution at 37 °C for 15 days.
Melting characteristics of metals by combined laser beams with different wavelengths
Authors: Moon Jong Hyun , Mizutani Masami , Katayama Seiji , Matsunawa Akira
Published: June 2006

Infrared CO2 and yttritium–aluminum–garnet (YAG) lasers have great difficulty in melting metals such as pure aluminum and copper possessing high reflectivity and high thermal and electrical conductivity. On the other hand, the laser absorptance of such metals generally increases with the decrease in the wavelength, and thus the application of laser with higher power and shorter wavelength is required for welding highly conductive materials. Recently, a Q-switched second harmonic generation (SHG) YAG laser of the maximum power of 50 W, and the maximum repetition rate of 50 kHz has become commercially available. However, this laser has not as yet been employed as a welding heat source. Therefore, in this study, melting tendencies of various metallic materials were compared using a pulsed YAG laser only, a SHG YAG laser alone, and combined lasers of different wavelengths. Coaxial optical focusing system combining pulsed YAG laser and SHG YAG laser was constructed. A deep penetration of needle-like weld bead could be produced with the SHG laser itself. But it was found that the penetration depths of the weld fusion zones made with the combined beams of different wavelengths were 2–4 times deeper than those with a pulsed YAG laser only and with a SHG laser alone. The beam absorption efficiency of metals with the normal pulsed YAG laser was increased greatly by combining the output of the Q-switched SHG YAG laser.
Use of carbon dioxide (CO2) laser radiation in exposure of implants (stage II): A metallography evaluation in dogs
Authors: Guimarães Márcia Vieira Marcondes , Niccoli-Filho Walter , Bottino Marco Antonio , Schwab Carlos
Published: June 2006

In this study we analyzed possible damages that vaporization from laser radiation could cause to implant material. Fifteen standard titanium implants, measuring 3.75 mm in diameter by 7 mm in length, were placed into the upper and lower jaws of three dogs according to Brȧnemark’s system. After osseointegration, all implants were exposed. In group I (control) conventional exposure with a punch was used; in group II, a CO2 laser with 2 W (power density: 256 W/cm2; fluency: 0.077 J/cm2, and a pulse mode of 0.30 ms) was used, and in group III 4 W (power density: 512 W/cm2, fluency: 0.154 J/cm2, and a pulse mode of 0.30 ms) was used. After vaporization, the cover screws were removed and sent for metallographic examination. The results showed that cover screws irradiated with 2 and 4 W power caused no superficial or microstructural alteration. The results also showed that the prescribed power densities, fluencies, and the use of the pulse mode were suitable for exposing implants without damage to tissue or implant material.
Laser forming of thin foil by a newly developed sample holding method
Authors: Yoshioka Shunro , Miyazaki Toshiyuki , Misu Tadashi , Oba Ryugo , Saito Masatoshi
Published: June 2006

A sample holding method is newly developed for laser forming, in which the target material is held between two transparent plates. During laser beam scanning on the target, the target foil is kept flat. After the scanning is finished, the foil is detached from the holder; the foil bends toward the beam scanned surface. Experiments are carried out with a Nd:yttrium–aluminum–garnet laser beam for a spot size of 25μm and for a traverse speed of 10 mm/s. A bending angle of about 30° is obtained for stainless steel (AISI 304) foil of 10 μm in thickness with a power of 0.3 W and a scanning repetition number of 20. On the other hand, if the laser beam is focused and scanned on the convex side along the ridge line of a curved foil, it bends toward the concave side and the bending angle increases. The increment angle of about 20° for a prebent angle of 45° is obtained for stainless steel foil 20 μm in thickness under the conditions of the power of 1.3 W and scanning repetition number of 10. If the sample is rotated counterclockwise with the increase in bending angle, a total angle of 90° or more is easily obtained. A microcube, of which side length is about 500 μm, and other objects of complicated shapes are made on the basis of the bending characteristics obtained experimentally.
Specific energy for pulsed laser rock drilling
Authors: Xu Z. , Reed C. B. , Konercki G. , Parker R. A. , Gahan B. C. , Batarseh S. , Graves R. M. , Figueroa H. , Skinner N.
Published: June 2006

Application of advanced high power laser technology to oil and gas well drilling has been attracting significant research interests recently among research institutes, petroleum industries, and universities. Potential laser or laser-aided oil and gas well drilling has many advantages over the conventional rotary drilling, such as high penetration rate, reduction or elimination of tripping, casing, and bit costs, and enhanced well control, perforating and side-tracking capabilities. The energy required to remove a unit volume of rock, namely the specific energy (SE), is a critical rock property data that can be used to determine both the technical and economic feasibility of laser oil and gas well drilling. When a high power laser beam is applied on a rock, it can remove the rock by thermal spallation, melting, or vaporization depending on the applied laser energy and the way the energy is applied. The most efficient rock removal mechanism would be the one that requires the minimum energy to remove a unit volume of rock. Samples of sandstone, shale, and limestone were prepared for laser beam interaction with a 1.6 kW pulsed Nd:yttrium–aluminum–garnet laser beam to determine how the beam size, power, repetition rate, pulse width, exposure time and energy can affect the amount of energy transferred to the rock for the purposes of spallation, melting, and vaporization. The purpose of the laser rock interaction experiment was to determine the optimal parameters required to remove a maximum rock volume from the samples while minimizing energy input. Absorption of radiant energy from the laser beam gives rise to the thermal energy transfer required for the destruction and removal of the rock matrix. Results from the tests indicate that each rock type has a set of optimal laser parameters to minimize specific energy (SE) values as observed in a set of linear track and spot tests. As absorbed energy outpaces heat diffusion by the rock matrix, local temperatures can rise to the melting points of the minerals and quickly increase observed SE values. Tests also clearly identified the spallation and melting zones for shale samples while changing the laser power. The lowest SE values are obtained in the spalling zone just prior to the onset of mineral melt. The laser thermally spalled and saw mechanically cut rocks show similarity of surface microstructure. The study also found that increasing beam repetition rate within the same material removal mechanism would increase the material removal rate, which is believed due to an increase of maximum temperature, thermal cycling frequency, and intensity of laser-driven shock wave within the rock.
Characteristics of high-power diode–laser welds for industrial assembly
Authors: Walsh C. A. , Bhadeshia H. K. D. H. , Lau A. , Matthias B. , Oesterlein R. , Drechsel J.
Published: June 2006

Recent progress in available beam power and beam quality has meant that high power diode lasers may soon become prime tools for welding in many assembly applications in industrial production. Their compact size and low weight makes them particularly suitable for use in conjunction with robotic control. The advantages of laser welding over conventional welding techniques include more reproducible welds, smaller heat affected zones, and lower distortion at high welding speeds. The implementation of diode laser systems will require an understanding of the effect of operating parameters on weld performance. This article reports on the characteristics of diode–laser spot and seam welds in low carbon steel sheets. The size, shape, hardness, strength, and microstructure of the welds have been measured and analyzed as a function of parameters such as laser power, welding time, and welding speed.
Critical parameters of light emitting diodes and their implications on the optical radiation safety limits according to IEC 60825-1 +Amendment 2
Authors: Horak W. , Neuhaus R.
Published: June 2006

Radiation power and image size of the source on the viewer's retina (apparent source) determines the hazard for the eye. Following IEC 60825-1, the effective emission limits are dependent on the tabulated accessible emission limits and the assigned measurement conditions—both depending on the source size. In the case of a light emitting diode (LED), this emitting area is not only determined by the chip size but also by the housing—due to built-in lenses, reflectors, and scattering materials. Also the virtual source position will be distorted by the integrated lenses. Since the measurement distance is related to the apparent source, the source's location must be known for the proper classification and hazard assessments of LEDs. As the accompanying measurement instructions of the standard are developed for coherent sources, they do not fully match the peculiarities of LEDs and lead to different interpretations. A comparison of several approaches will be presented. In this context, the sometimes surprising implication of the new “Amendment 2” on the safety limits of LEDs will also be discussed.
Plasma assisted laser surface alloying
Authors: McCay M. H. , Sharp C. M. , Hopkins J. A. , Szapiro B. , McCay T. D.
Published: June 2006

The combination of plasma arc with a Nd:YAG laser during surface alloying produces a synergistic effect which significantly increases the depth of the melt pool beyond that expected by a summation of the individual depths. Incremental increases in laser power produce larger depth changes than incremental increases in torch amperage. Displacing the sources emphasizes the effect. This could be attributed to an increase in interaction time. The depth of the heat affected zone experiences a lesser effect, suggesting that the synergism produces more efficient energy transfer into the liquid, therefore reducing energy transfer into the solid or HAZ. This would be accomplished by changes in fluid flow distribution.
Deposition of WC/Ni clad layers with a pulsed Nd:YAG laser
Authors: Brandt M. , Huang S. W. , Samandi M.
Published: June 2006

A series of experiments was performed to investigate the production of WC/Ni clad layers on H13 tool steel using a pulsed Nd:YAG laser and optical fibers. The effects of laser parameters, such as laser pulse energy and pulse frequency as well as material parameters such as powder composition and shape on the clad layer geometry and microstructure, were investigated. The microstructure of the clad layers was assessed by optical and scanning electron microscopy in conjunction with x-ray diffraction. The results indicate that up to 1 mm thick, fully dense clad layers can be formed in a single pass from WC/Ni powder containing up to 70% by weight pure WC with an average laser power of 440 W delivered through a 600 μm diam, step-index glass fiber. The results further show that the WC powder composition and shape can affect the structure and composition of the coating. The coatings produced with pure, angular, monocarbide exhibited the highest retained volume fraction of WC particles.
Corrosion resistance of laser-fabricated metal–matrix composite layer on stainless steel 316L
Authors: Lo K. H. , Kwok C. T. , Cheng F. T. , Man H. C.
Published: June 2006

Laser fabrication of ceramic-reinforced metal–matrix composite (MMC) layer on stainless steel 316L was achieved by a two-step process. Metal–ceramic powders (Ni–WC, Co–WC, Ni–Al2O3, Ni–Cr3C2) and ceramic powders (WC, SiC, CrB2) were preplaced on 316L specimens by flame spraying and pasting, respectively. The preplaced coatings were then irradiated by a high power laser to form MMCs with low (up to 28%) and high (over 80%) volume fractions of ceramic particles. The microstructure and composition of the MMC layers were studied by optical microscopy, scanning electron microscopy, x-ray diffractometry, and image analysis. The corrosion behavior of the laser-fabricated layers in 3.5% NaCl solution at 23 °C was studied by the cyclic potentiodynamic polarization technique. The corrosion resistance of the laser-surfaced specimens varied depending on the type and volume fraction of the reinforcement incorporated. In general, the corrosion potential shifted in the noble direction relative to 316L, but the pitting corrosion resistance deteriorated. Moreover, the results of the present study indicate that a high volume fraction of ceramic particles in an MMC layer was unfavorable for pitting corrosion resistance due to the difficulty in the formation of intact passive films and the presence of vulnerable spots at the ceramic/matrix interface.
Direct write of resistive lines on SiC
Authors: Plew T. Y. , Leong K. H. , Redwing J. M.
Published: June 2006

The optimal parameters of a nanosecond pulsed laser for writing of tracks of controlled resistance onto high resistivity silicon carbide were investigated. A controlled energy deposition approach was carried out to minimize changes to the surface topography. The effect of fluence and irradiance on the ability to write narrow lines with controlled resistance were the explored parameters. Two focal length lenses and a large range of fluences were chosen to determine the best parameters to achieve controlled resistance. The experimental results indicate that both the beam fluence and irradiance have a significant effect on achieving an optimal resistance range.
Laser welding closed-loop power control
Authors: Bagger Claus , Olsen Flemming O.
Published: June 2006

A closed-loop control system is developed to maintain an even seam width on the root side of a laser weld by continually controlling the output laser power of a 1500 W CO2 laser. Quality control is done by a photodiode monitoring the root-side light emission from the process. The control system is successfully demonstrated to work in the bead-on-plate configuration on 2 mm sheets, both at constant speeds and at speeds ranging from 0.32 to more than 0.72 m/min in one welding trial. The power control system is able to deal with sheets of variable thickness in one welding trial, with the following thickness changes made as partly lap welds: 1.25 to 1.25 mm, 0.5 to 2 mm, and 1.25 to 2 mm. Sheets of 0.5 mm lap joined to 2 mm sheets displayed even root seams along the entire seam, except at the immediate edges of thickness change.
Microstructure and corrosion behavior of high power diode laser deposited Inconel 625 coatings
Authors: Tuominen J. , Vuoristo P. , Mäntylä T. , Latokartano J. , Vihinen J. , Andersson P. H.
Published: June 2006

A series of experiments were performed to investigate the one-step laser cladding of Inconel 625 powder, injected off-axially onto mild steel substrates. The experiments were carried out using a 6 kW high power diode laser mounted to a six axis robot system. The rectangular shape of the delivering beam was focused to a spot size of 22 mm×5 mm on the work piece. The powder feeding head, which consisted of a cyclone and flat nozzle, spread the powder stream to a spot size used. The coating samples were produced using different levels of powder feed rate (77–113 g/min), constant traveling speed of 400 mm/min, and laser power of 6 kW. Powder and laser-clad coating microstructures were studied by x-ray diffraction, scanning electron, and optical microscopy. The coating microstructure was found to consist of a directionally solidified single phase (highly alloyed Ni), face-centered cubic structure with a lattice parameter of 0.3596 nm. The corrosion resistance of the one-step laser-clad coatings was tested in 3.5 wt % NaCl using electrochemical methods. Laser-clad coating was compared with wrought Inconel 625 alloy, high-velocity oxy-fuel sprayed, and plasma transferred arc welded coatings. The corrosion resistance of laser-clad coatings was found to be equivalent to corresponding bulk material and superior to sprayed and welded coatings.
Condition monitoring of laser systems: Monitoring the cover slide
Authors: Tönshoff H. K. , Ostendorf A. , Kral V.
Published: June 2006

Laser systems are high investment production machines that require a high level of availability in order to be amortized. Laser condition monitoring methods can be used to avoid breakdowns and maintain a consistent quality. In particular elements such as the cover slide, which protect the focusing optics from getting splashed through the welding process, need to be monitored. The effects of dirt on the cover slide are detrimental to the beam quality reaching the workpiece and can cause a spontaneous failure of the cover slide. By using a laser based calorimetric test the absorption and thereby the state of the deterioration of the cover slide can be measured. In order to make this test practical in an industrial situation the method must be simplified and the indirect cooling through the head accounted for. With this simplified absorption test an objective measurement of the state of the cover slide can be done between each weld. By monitoring the deterioration of the cover slide spontaneous breaks and low quality processes can be avoided.
Optimization and modeling of laser forming of stainless steel circular sectors
Authors: Casalino G. , Ludovico A. D.
Published: June 2006

Mold-less forming produces precise metal parts by thermal variation generated used high power beam irradiation. Some technologies like the laser and the plasma arc have proved to fit the requirements of this new forming technique [M. Geiger and F. Vollersten, Ann. CIRP 42, 301–304 (1993); P. J. Li et al., J. Eng. Manuf. Proc. Part B 214, 0954–4054 (2000)], which can be applied both for rapid prototyping and rapid manufacturing. In particular, a complex shape can be generated using curved path for the irradiating strategy of a circular plate. The forming mechanism induced by this strategy is similar to that of linear irradiation, which is called bending, but its effects are on average lower [T. Hennie, J. Mater. Process. Technol. 102–108 (2000)]. In this article, the forming process of austenitic stainless steels AISI 304 carbon spray precoated sheets has been investigated by numerical and experimental analysis. The experiments were designed and the results optimized using design of experiments analysis techniques [D. C. Montgomery, Design and Analysis of Experiments (Wiley, New York, 1997)]. A CO2 laser, operating in continuous wave regime, was used for the experiments. Numerical analysis was performed in order to predict the final shape and residual stress of the formed or bent sheet. In view of the use of lasers for mold-less forming in the mechanical engineering field this investigation has provided useful knowledge of the process.
Steady-state laser cutting modeling
Authors: Mas Cédric , Fabbro Rémy , Gouédard Yannick
Published: June 2006

We present a global self-consistent model of the laser cutting process where we have taken into account the main parameters of the process that are the laser beam intensity distribution, the thermal phenomena controlling the melt parameters, the fluid, both cutting gas and melt hydrodynamics, and the local equilibrium of the geometry of the cutting front. This steady state approach provides us with different fluid velocities, surface temperature, and molten layer thickness along the front profile, and as a matter of fact available cutting thickness for a set of given input operating parameters. In a second step, data provided by these simulations are compared with experiments and a good correlation is observed.
Do in vivo terahertz imaging systems comply with safety guidelines?
Authors: Berry Elizabeth , Walker Gillian C. , Fitzgerald Anthony J. , Zinov’ev N. N. , Chamberlain Martyn , Smye Stephen W. , Miles Robert E. , Smith Michael A.
Published: June 2006

Techniques for the coherent generation and detection of electromagnetic radiation in the far infrared, or terahertz, region of the electromagnetic spectrum have recently developed rapidly and may soon be applied for in vivo medical imaging. Both continuous wave and pulsed imaging systems are under development, with terahertz pulsed imaging being the more common method. Typically a pump and probe technique is used, with picosecond pulses of terahertz radiation generated from femtosecond infrared laser pulses, using an antenna or nonlinear crystal. After interaction with the subject either by transmission or reflection, coherent detection is achieved when the terahertz beam is combined with the probe laser beam. Raster scanning of the subject leads to an image data set comprising a time series representing the pulse at each pixel. A set of parametric images may be calculated, mapping the values of various parameters calculated from the shape of the pulses. A safety analysis has been performed, based on current guidelines for skin exposure to radiation of wavelengths 2.6 μm–20 mm (15 GHz–115 THz), to determine the maximum permissible exposure (MPE) for such a terahertz imaging system. The international guidelines for this range of wavelengths are drawn from two U.S. standards documents. The method for this analysis was taken from the American National Standard for the Safe Use of Lasers (ANSI Z136.1), and to ensure a conservative analysis, parameters were drawn from both this standard and from the IEEE Standard for Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields (C95.1). The calculated maximum permissible average beam power was 3 mW, indicating that typical terahertz imaging systems are safe according to the current guidelines. Further developments may however result in systems that will exceed the calculated limit. Furthermore, the published MPEs for pulsed exposures are based on measurements at shorter wavelengths and with pulses of longer duration than those used in terahertz pulsed imaging systems, so the results should be treated with caution.
Process characteristics and effects of gas- and water-atomized stainless steel powders in laser-based rapid tooling
Authors: Pinkerton Andrew J. , Li Lin
Published: June 2006

In rapid tooling using the direct laser deposition process and stainless steel materials, only powders prepared using the gas-atomization method have been used because of their presumed superior flow characteristics. In this article, the use of water-atomized powder is investigated by comparing the behavior of the two types of 316 L powders, deposited using a CO2 laser and coaxial head. Results show little difference in the pneumatic conveyance of the powders, but considerably greater absorption of laser energy by the water–atomized powder between the nozzle and material deposition point. Walls produced using the water-atomized powder exhibited a more textured microstructure and smoother sidewall surfaces, with less surface metallic oxides, but the deposition rate was considerably lower than when using gas-atomized powder.
Continuous wave laser ignition thresholds of coal dust clouds
Authors: Dubaniewicz Thomas H. , Cashdollar Kenneth L. , Green Gregory M.
Published: June 2006

Laser-based instruments are used in areas where coal dust ignition presents a safety hazard. The National Institute for Occupational Safety and Health, Pittsburgh Research Laboratory (NIOSH, PRL) is conducting a study to help determine when an optical beam may be considered a potential ignition source in underground coal mines or coal storage facilities. Researchers conducted experiments to determine threshold igniting powers for Pittsburgh seam bituminous coal dust clouds using an 803 nm continuous wave laser. For fine sized coal dust dispersed in air, concentrations ranging from 600 to 2000 g/m3 were the most easily ignited. A heated layer of coal dust that deposited on the fiber tip during dispersion ignited the dust clouds. Minimum observed igniting powers for laser beams delivered by 200, 400, and 800 μm core fiber optic cables and directed into coal dust clouds were 2.0, 3.0, and 5.0 W, respectively. Threshold igniting power was proportional to beam diameter, and threshold igniting power density decreased with larger fiber diameters. Ignition delay times averaged 0.6 s and did not vary significantly with laser power under initially turbulent test conditions and with flammable targets. Comparisons are made with the results of other researchers.
Surface treatment of granite by high power diode laser
Authors: Pou J. , Trillo C. , Soto R. , Doval A. F. , Boutinguiza M. , Lusquiños F. , Quintero F. , Pérez-Amor M.
Published: June 2006

Granite stones have been used from times immemorial as structural element for public works and construction of every kind of building. At present, granite stones are used widely for façades, paving and flooring; polished being the most commonly used surface finish type. Recent years have seen a dramatic increase in the demand of granite plates having a rougher, more “rustic” surface finish. In this article we present the results of an exploratory study on the capabilities of the high power diode laser to treat the surface of granite plates. The results allow us to affirm that the laser presents several advantages in comparison to the conventional mechanical method: mechanical stresses are not induced in the granite plate, waste powder is reduced, and hazardous noise is not a byproduct.
Control of chaos in laser-induced vapor capillaries
Authors: Cho M. H. , Farson D. F. , Kim J. I.
Published: June 2006

Vapor capillaries associated with deep penetration laser welding (keyholes) are inherently unstable and it is well known that the bounded evolution of unstable modes of nonlinear dynamic systems can lead to complex fluctuations of state and/or output variables. In the case of keyholes, the magnitude of self-induced fluctuations is sometimes large enough to cause occasional closure (“collapse”) and trapping of voids in the solidifying weld metal. Since voids are serious and frequent defects in partial penetration laser welds, methods to improve the stability of the keyhole, and thereby reduce or eliminate their incidence are of considerable practical interest. One technique that is known to simplify the complicated behavior of chaotic systems involves the application of periodic perturbations to accessible system inputs or boundaries. Consequently, this work investigated the effect of power modulation on porosity formation in laser welds. Frozen glycerin was used as the medium for welding since its transparency allowed visualization of the keyhole, facilitating control parameter development. Statistical and spectral analysis of penetration depth signals for the uncontrolled system was performed to characterize the nonlinear keyhole dynamics. In the control studies, it was found that the optimum frequency of power modulation at 400 Hz was very effective for reducing porosity. A 15° leading beam incidence angle was found to provide further improvements. Three-dimensional phase portraits of the keyhole depth for uncontrolled and controlled signals showed that the control did not convert depth fluctuations to a completely predictable limit cycle but that the remaining fluctuations were more predictable than those of the uncontrolled system.
Three-dimensional finite element modeling of laser cladding by powder injection: Effects of powder feedrate and travel speed on the process
Authors: Toyserkani Ehsan , Khajepour Amir , Corbin Steve
Published: June 2006

This article addresses a novel three-dimensional transient finite element model of the laser cladding by powder injection process. The proposed model can predict clad geometry as a function of time and process parameters including beam velocity, laser power, powder jet geometry, laser pulse shaping, and material properties. In the proposed method, the interaction between powder and melt pool are assumed to be decoupled and as a result, the melt pool boundary is first obtained in the absence of powder spray. Once the melt pool boundary is calculated, it is assumed that a layer of coating material based on powder feedrate and elapsed time is deposited on the intersection of the melt pool and powder stream in the absence of laser beam. The new melt pool boundary is then calculated by thermal analysis of the deposited powder layer, substrate and laser heat flux. The results of numerical modeling for different process velocities and different powder feedrates are presented and compared with experimental results. The comparisons show a good agreement between the modeling and experimental geometry of the clads.
Hole drilling of Inconel 718 by high intensity pulsed ultraviolet laser
Authors: Wu Qihong , Ma Yurong , Jie Jiansheng , Yu Qingxuan , Liao Yuan , Fang Rongchuan , Chen Xiangli , Wang Kelvin
Published: June 2006

Holes were percussion drilled in Inconel 718 plates by a high intensity pulsed ultraviolet laser (266 and 355 nm) with 8 ns pulse width. It showed that shortening the laser wavelength resulted in thinner recast layers, less microcracks, and faster machining speed. In order to get high quality and fast speed in laser hole drilling, a two-step processing method was proposed and tested.