bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of surface preparation and process parameters on the porosity generation in aluminum alloys
Authors: Haboudou A. , Peyre P. , Vannes A. B.
Published: June 2006

The 4 kW cw Nd:yttrium–aluminum–garnet laser welding of solid solution hardened A5083 (5% Mg) and precipitation-hardened A356 (7% Si) alloys, sensitive to porosity formation was carried out at different welding speeds, under He gas shielding, and for partial penetration conditions, on bead on plate configuration. The porosity formation was examined through observation and evaluation of weld beads, based on x-ray radiography inspection coupled with image analysis, and quantification of gas volume percentage by densitometry. The results show that both surface preparation and dual-beam configuration can have a beneficial effect on the porosity content: (1) After polishing, laser cleaning, sand blasting, or degreasing of the surface, hydrogen and oxygen sources are decreased or removed. The resulting effect is a reduction from 15% to less than 2% of the porosity rate on A356 alloy, with better results, in the case of laser cleaning. (2) The use of dual beam conditions ensures a larger weld pool, facilitates the porosity’s ejection, and stabilizes the keyhole as compared with single beam configurations. For instance, a longitudinal bi-spot configuration reduces the porosity rate from 15% to less than 0.5% on A356 alloy. In A5083 welds, most of the macroporosities remain in the beads, due to an unstable keyhole (Mg vapors) which can collapse and entrap millimeter-like blow holes.
Design and performance of multilevel phase fan-out diffractive optical elements for laser materials processing
Authors: Fuse Keiji , Hirai Takayuki , Ushiro Toshihiko , Okada Takeshi , Kurisu Kenichi , Ebata Keiji
Published: June 2006

A multilevel phase fan-out diffractive optical element (DOE) has been developed and introduced into various kinds of laser materials processing such as drilling, cutting, welding, and soldering. The larger the number of arrayed spots the DOE generates on the surface of the workpiece, the more sensitive the intensity uniformity of the spots becomes to fabrication errors, which are deviations between designed and fabricated surface microstructures. Errors in etch depth have, in particular, a significant effect on the intensity uniformity. A new design method has been developed for increasing the tolerance to the etch depth error, and applied to the design of a 16-level phase 7×7 fan-out element. The result indicates a uniformity less sensitive to etch depth error. The effect of a linewidth error due to the side etch introduced during a plasma etching process is also evaluated by computing high-resolution graphics data representing the phase of the DOE with the line width errors. Mask alignment errors, the slant of sidewalls, and other fabrication errors are discussed further. The diffraction efficiency and intensity uniformity are measured for fabricated DOE prototypes, and a comparison made between the calculated and measured properties shows good agreement.
Laser–material interaction and process sensing in underwater Nd:yttrium–aluminum–garnet laser welding
Authors: Zhang Xudong , Chen Wuzhu , Ashida Eiji , Matsuda Fukuhisa
Published: June 2006

Laser–material interaction and process sensing technology for local-dry underwater Nd:yttrium–aluminum–garnet laser beam welding were studied. The optical emissions induced by laser–water interaction were detected with an infrared (IR) optical sensor and observed with a charge coupled device camera. It was found that under laser irradiation, a kind of water–vapor plasma formed immediately above the water surface if the water depth was more than 3 mm. This plasma severely reduced the laser power reaching the workpiece. The water above the workpiece surface must be excluded by forming a local-dry cavity so as to perform underwater laser welding. In local-dry underwater laser welding, a coaxial gas-shielding nozzle was used to form a local-dry cavity. The IR optical signal and weld bead shape variations with various shielding conditions of the local-dry cavity were investigated. The experimental results showed that the IR signals could reflect well the status of the local-dry cavity and predict the weld quality. The optimal shielding condition could be determined by IR signal stability.
Laser-assisted etching of titanium foil in phosphoric acid for direct fabrication of microstructures
Authors: Shin Y. S. , Jeong S. H.
Published: June 2006

Characteristics of laser-induced thermochemical wet etching of titanium foil in phosphoric acid were investigated to examine the feasibility of this method for direct fabrication of microstructures. Variations of etch width, depth, and profile were examined for different combinations of laser power density, scanning speed of the beam focus, and etchant concentration which are the major process parameters governing temperature distribution on the sample surface and the reaction rate. Etch width increased almost linearly with laser power showing little dependence on scanning speed while etch depth showed wide variation with both laser power and scanning speed. A well-defined etch profile with good surface quality was obtained at high concentration condition but deteriorated as the concentration was lowered. Fabrication of microcantilever beam, microhole, and rectangular structure was demonstrated.
Plume attenuation under high power Nd:yttritium–aluminum–garnet laser welding
Authors: Greses J. , Hilton P. A. , Barlow C. Y. , Steen W. M.
Published: June 2006

During high-power continuous wave (cw) Nd:yttritium–aluminum–garnet (YAG) laser welding a vapor plume is formed containing vaporized material ejected from the keyhole. The gas used as a plume control mechanism affects the plume shape but not its temperature, which has been found to be less than 3000 K, independent of the atmosphere and plume control gases. In this study high-power (up to 8 kW) cw Nd:YAG laser welding has been performed under He, Ar, and N2 gas atmospheres, extending the power range previously studied. The plume was found to contain very small evaporated particles of diameter less than 50 nm. Rayleigh and Mie scattering theories were used to calculate the attenuation coefficient of the incident laser power by these small particles. In addition the attenuation of a 9 W Nd:YAG probe laser beam, horizontally incident across the plume generated by the high-power Nd:YAG laser, was measured at various positions with respect to the beam-material interaction point. Up to 40% attenuation of the probe laser power was measured at positions corresponding to zones of high concentration of vapor plume, shown by high-speed video measurements. These zones interact with the high-power Nd:YAG laser beam path and, can result in significant laser power attenuation.
Diode laser beam absorption in laser transformation hardening of low alloy steel
Authors: Pantsar Henrikki , Kujanpää Veli
Published: June 2006

Defining and controlling the absorption of the laser beam is important since all of the heating energy is brought to the material through absorption. Even small variations in the absorption change the laser power needed by hundreds of W. In this study the absorption of a diode laser beam to low alloy steel has been measured by a liquid calorimeter and the surface temperature has been measured with a dual wavelength pyrometer. The varied processing parameters were the power intensity of the beam, the interaction time, and the angle between the surface and the optical axis of the laser beam. Surface temperatures during hardening varied from the Ac1 temperature to the melting point. Tests were done with a 3 kW diode laser with a 12×5 mm hardening optic. The absorptivity of a machined clean steel surface ranged from 46% to 72% depending on the processing parameters. Aluminum oxide blasting of the surface increased the relative amount of energy absorbed to the work piece. The coupling rates for blasted surfaces varied from 66% to 81%. Best absorptivity was achieved by applying graphite coating on the surface. Absorptivity values in excess of 85% were measured.
Quality and costs analysis of laser welded all steel sandwich panels
Authors: Pantsar Henrikki , Salminen Antti , Jansson Anssi , Kujanpää Veli
Published: June 2006

Laser welding is a fast and flexible way to manufacture all steel sandwich panels. Economically the most feasible solution is often to weld as large panels as possible. In corrugated core all steel sandwich panel applications the effect of an air gap in welding is difficult to eliminate when welding large panels. A system for welding large panels with a laser is often considered to be complicated and expensive. The processibility of large panels with specified core types was estimated by welding large all steel sandwich panels with two different core types. Materials used were low-carbon steel DC01 and stainless steel ASTM 304 with sheet thickness from 0.5 to 1.5 mm. The panels were welded with a 6 kW CO2 laser. The manufacturing of large steel panels was possible with acceptable weld quality (EN-ISO 13919-1). The total flatness of the panels was better than 11.3 mm, depending on the manufacturing procedure, core type, and material. Total manufacturing costs of high power CO2, Nd:yttrium–aluminum–garnet and diode lasers were compared in welding of large weight optimized panels. Laser system, panel size, fixturing, and the welding parameters affect the total costs. The size and type of the panel core has together with the batch size a strong effect on the manufacturing costs. The economical evaluation of the cost of the panel depends considerably on the application.
Laser direct writing and doping of diamond-like carbon, polycrystalline diamond, and single crystal silicon carbide
Authors: Salama I. A. , Quick N. R. , Kar A.
Published: June 2006

Nanosecond pulsed Nd:yttrium–aluminum–garnet laser treatment is applied to alter the electric properties of free standing polycrystalline diamond substrates, diamond-like carbon (DLC), and 4H–SiC single crystal wafers. In the case of DLC samples, the laser irradiation, nitrogen doping, as well as the incorporation of cobalt into the DLC layer reduces its electric resistance. Laser fabrication of Schottky barrier diode at the DLC–cobalt contact is demonstrated, and its nonlinear 1/C2 vs V curve indicates a nonuniform dopant distribution. The nitrogen dopant profile in the laser-doped SiC wafer is obtained by secondary ion mass spectroscopy and the corresponding nitrogen diffusion coefficient under laser processing parameters is calculated. Laser doping enhances the nitrogen diffusivity in SiC and allows in situ fabrication of metal contacts with no additional materials. Scanning electron microscopy, wavelength dispersive spectroscopy, and x-ray photoelectron spectroscopy are used to study the surface composition and the binding states in the laser-treated samples.
In-process monitoring and feedback control during laser microspot lap welding of copper sheets
Authors: Kawahito Yousuke , Katayama Seiji
Published: June 2006

The establishment of laser welding of copper sheets, as an electronics manufacturing process is anticipated. This study was performed with an objective of stably and consistently producing fully penetrated lap welds in two copper sheets of 0.1 mm thickness. A new procedure for in-process monitoring and feedback control was developed for microspot lap welding with combined beams of a pulsed fundamental yttrium–aluminum–garnet (YAG) laser and an AO Q-switched second harmonic YAG laser. It was found that reflected laser power and heat radiated from the weld area were effective as process monitoring signals. A database of these values corresponding to the sizes of the bottom fusion zones of laser spot welds was prepared in terms of joint strength for feedback control utilizing a neural network model. A pulse width during the irradiation of a fundamental YAG laser was controlled at 0.15 ms intervals according to the neural network prediction based upon the monitoring signals. Upon investigation of 200 samples, incomplete melting was detected in seven samples under the laser irradiation conditions used conventionally. On the other hand, fully penetrated spot welds were repeatedly produced in 200 samples subjected to laser lap welding under the proposed in-process monitoring and feedback control.
The influence of the pulse length on the drilling of metals with an excimer laser
Authors: Schoonderbeek Aart , Biesheuvel Cornelis A. , Hofstra Ramon M. , Boller Klaus-J. , Meijer Johan
Published: June 2006

Laser parameters, which significantly influence laser-material interaction processes, are the wavelength, the energy, and the power density. Additionally, there are parameters, like the pulse length, which also strongly influence processing speed and quality. Studies where different types of lasers have been used indicate that long pulses are beneficial for processing speed. However, when different types of laser systems are used to study the effect of the pulse length, a direct comparison of the results is difficult because the use of different lasers involves a simultaneous variation of other parameters (e.g., wavelength) as well. In this study a technique of pulse length variation is used in which the pulse length is the only varied parameter and thus enables the desired direct comparison. Pulses with different lengths are sliced out of pulses of a long pulse XeCl excimer laser, keeping all other laser parameters unchanged. Results are shown of hole drilling experiments in 125 μm nickel, 25 μm aluminum, and 125 μm aluminum foil with pulse lengths between 9 and 150 ns. The influence of the pulse length on material processing is discussed in connection with energy and power of the pulses. The experiments show that both for pulses with the same energy and the same power long pulses remove more material than short pulses and, moreover, long pulses can yield higher quality of the drilled holes.
Laser synthesizing NiAl intermetallic and TiC reinforced NiAl intermetallic matrix composite
Authors: Zhong Minlin , Xu Xiangyang , Liu Wenjin , Sun Hongqing
Published: June 2006

The NiAl intermetallic layers and NiAl matrix composite layers with TiC particulate reinforcement were successfully synthesized by laser cladding with coaxial powder feeding of Ni/Al clad powder and Ni/Al+TiC powder mixture, respectively. With optimized processing parameters and powder mixture compositions, the synthesized layers were free of cracks and metallurgical bond with the substrate. The microstructure of the laser-synthesized layers was composed of β-NiAl phase and a few γ phases for NiAl intermetallic; unmelted TiC, dispersive fine precipitated TiC particles and refined β-NiAl phase matrix for TiC reinforced NiAl intermetallic composite. The average microhardness was 355 HV0.1 and 538 HV0.1, respectively. Laser synthesizing and direct metal depositing offer promising approaches for producing NiAl intermetallic and TiC-reinforced NiAl metal matrix composite coatings and for fabricating NiAl intermetallic bulk structure.
Solidification model of laser cladding with wire feeding technique
Authors: Hung Chien-Fu , Lin Jehnming
Published: June 2006

The object of this study is to investigate the solidification problem in laser cladding with the wire feeding technique. The solidification phenomenon of laser cladding was analyzed by numerical simulation and experimental visualization. Good agreement has been obtained for 304 stainless steel wires cladding on round tubes and it transpires that there were three basic modes of clad profiles: thinning, thickening, and wavering of the clad thickness at different velocity settings between the feeding wire and the substrate in laser wire cladding.
Methods for hazard assessment from viewing fiber optics with eye loupes
Authors: Marshall Wesley J. , Bell John , Sliney David H.
Published: June 2006

Current laser safety standards overestimate retinal hazards when viewing optical fiber tips with magnifying aids. Methods for assessing optical hazards are based on laser beam characteristics and assume worst-case exposure to both the unaided eye and when using a variety of optical aids. These standard methods provide overly conservative results when assessing the increased risk of viewing an emitting fiber with commonly available magnifiers. A more accurate and simpler method could be used to assess the risks of optically aided viewing when examining fiber tips.
Eye hazard characterization of an optical klystron free electron laser beam
Authors: Edwards Ben E. , Pinayev Igor V. , Wallace Patrick W.
Published: June 2006

A free electron laser (FEL) uses an accelerated electron beam to generate laser light. By adjusting the electron beam energy and other parameters, FELs can generate tunable, coherent light over a range from ultraviolet to millimeter wavelengths. Many FELs are building-sized systems that remain at the site of construction, and are not produced for sale or commercial distribution. Federal laser regulatory requirements for hazard characterization and laser classification do not apply to such noncommercial FELs, so much of the information normally provided to end users by laser manufacturers is not available. Nevertheless, some basic characterization of the FEL beam hazard is warranted to allow development of adequate and appropriate control measures. The FEL’s wide spectral range and variable pulse repetition frequency makes hazard characterization challenging. This article describes an approach for characterizing the eye hazard associated with the coherent beam of the Optical Klystron 4 (OK4/Duke) storage ring FEL located in the Free Electron Laser Laboratory on the campus of Duke University in Durham, North Carolina.
A procedure for the estimation of intrabeam hazard distances and optical density requirements under the ANSI Z136.1-2000 Standard
Authors: Thomas Robert J. , Rockwell Benjamin A. , Marshall Wesley J. , Aldrich Robert C. , Gorschboth Mary F. , Zimmerman Sheldon A. , Rockwell R. James
Published: June 2006

Over the past few years, a comprehensive rewrite of the American National Standard for Safe Use of Lasers (ANSI Z136.1) has been conducted [American National Standards Institute, Z136.1-2000 American National Standard for Safe Use of Lasers (American National Standards Institute, New York, 2000)]. Important parts of many laser safety evaluations are the determination of the maximum distance at which the laser presents a hazard and the computation of optical density requirements for eye protection. In the ANSI Z136.1-2000 Standard, two approaches for the computation of hazard distance are presented. We specifically address the derivation of these two methodologies including the effects of atmospheric attenuation and optically aided viewing. In addition, these methodologies may be applied to the computation of visual interference distance as described in the ANSI Z136.6-2000 Standard [American National Standards Institute, Z136.6-2000 American National Standard for Use of Lasers Outdoors (American National Standards Institute, New York, 2000)]. This publication represents the third in a series of tutorial articles designed to clarify laser hazard analysis procedures under these ANSI Standards [R. J. Thomas, B. A. Rockwell, W. J. Marshall, R. C. Aldrich, S. A. Zimmerman, and R. J. Rockwell, Jr., J. Laser Appl. 13, 134–140 (2001); 14, 57–66 (2002)].
Laser transmission welding of polymer and wood composites: Material and joint mechanism related studies
Authors: Haferkamp H. , von Busse A. , Barcikowski S. , Ostendorf A. , Bunte J.
Published: June 2006

Laser beam transmission welding is an innovative and promising technology for many industries such as the automotive, electronic, and medical industry. Extension to application of this technology to wood components makes it also interesting for the woodworking and furniture industry. Therefore, several investigations in the field of welding of fiber reinforced thermoplastic and wood composites with polymers have been carried out. Focus is given to the determination of the weldability and weld behavior of different thermoplastic and wooden materials in matrices relevant for recent industrial applications. Different laser sources with a variety of wavelengths are examined, e.g., high power diode lasers (810, 940 nm) and Nd Yttritium–aluminum–garnet (YAG) laser systems (1064 nm). New approaches using thermographic methods for an easy and fast evaluation of materials qualification regarding their weldability will also be presented. Different plastic and wood components taking into account properties like filling content are investigated. Results show that the thermographic transmission and heating rate are more or less different for Nd:YAG and diode laser radiation depending on the type of laser-transparent polymer material, production conditions, and on the presence of pigments and other filling materials. Investigating the absorbing materials, it has been shown clearly that a modification of the carbon content is observable very easily in the thermographic images recorded during the heating period. Since natural fiber composites have strong material inhomogenities and anisotropic structures, special process strategies for their application have been developed and are presented in this article.
Comprehensive assessment of the CO2 laser cut quality of ceramics with different assist gas injection systems
Authors: Quintero F. , Pou J. , Lusquiños F. , Boutinguiza M. , Soto R. , Pérez-Amor M. , Wagner F.
Published: June 2006

C O 2 laser cutting is an efficient and advantageous process for cutting of ceramics when the hardness of such materials makes the conventional machining methods unproductive. At the same time, the application of laser cutting to ceramics involves the assessment of the different process parameters to select the suitable conditions for every specific ceramic. In this work, a comprehensive analysis of the CO2 laser cutting of mullite-alumina is presented. The cut quality was assessed under the criterion of facilitate the comparison of the results obtained using different process parameters and two different assist gas injection systems. For this reason, some quantitative standard parameters were analyzed (kerf width, roughness, perpendicularity), besides of the preliminary survey of some features and the microscopic examination of the heat affected zone. The results demonstrate the improvement of the cut quality using an assist gas injection system based on an off-axis De Laval nozzle.
A finite element method approach for thermal analysis of laser cladding of magnesium alloy with preplaced Al–Si powder
Authors: Guo L. F. , Yue T. M. , Man H. C.
Published: June 2006

A numerical model based on the finite element method (FEM) has been developed to simulate the temperature profile of laser cladding with preplaced metal powders. The FEM model is capable of simulating the three-dimensional thermal field of the cladding zone under the condition of a moving heat source, taking into consideration the thermal contact resistance between the powder bed and the substrate. Verification experiments were conducted by the laser cladding of Al–12%Si alloy on ZM51 magnesium alloy, using a high-power Nd:yttrium–aluminum–garnet laser. The results show that the model can be employed to determine the appropriate processing parameters for achieving good interfacial bonding between the clad layer and the substrate. On the other hand, the model shows that under the normal cladding conditions, significant dilution would occur in laser cladding of magnesium alloys.
Microstructural aspects of laser dissimilar welds of hard metals to steels
Authors: Costa A. , Quintino L. , Miranda R. M.
Published: June 2006

Laser beam welding has been widely used in industry in the past decade. Recently, applications in dissimilar welds involving hard metals have been investigated. Hard metals are commonly used in tool manufacturing and this application requires joining to steel holders. The physical, structural, and mechanical properties of these materials pose several welding problems thus stimulating the application of laser welding. Research efforts have been mainly focused on developing procedures in order to achieve narrow fusion and heat affected zones with minimal distortions. Investigations on the weld microstructures and defect formation were also carried out. This paper presents an analysis of the influence of CO2 and Nd:yttrium–aluminum–garnet laser welding parameters on the microstructural aspects of the welds, discusses solidification microstructures and solid state transformations in dissimilar welds of carbon steels to hard metals, aiming at understanding the weldability of these materials.
Micromelting and its effects on surface topography and properties in laser polishing of stainless steel
Authors: Mai T. A. , Lim G. C.
Published: June 2006

This article presents the results of an investigation into using laser for polishing 304 stainless steel (SS) surfaces. The effects on surface topography, reflectivity, hardness, and corrosion resistance of 304 stainless steel were analyzed. An in-house developed finite difference heat conduction model was used to simulate the rapid melting and solidification process. The nonlinear problem of the solid/liquid moving boundary was solved by a novel hybrid numerical method. Melting depth was in the order of submicrons and polishing rate in a range of 5–15 cm2/min. The improvement of surface reflectance could be seen as the result of surface smoothing with roughness reduction. A decrease of surface roughness from 195 to 75 nm was measured using an atomic force microscope. Laser polishing increased the specular surface reflectance to 14%, while diffusive reflectance was reduced up to 70%. The heterogeneous microhardness distribution was transformed into a homogeneous one. Laser polishing could improve the pitting corrosion resistance of 304 SS due to microstructural changes caused by laser rapid melting and solidification. The melting effect was found also useful for sealing of micropores and -cracks and removing of surface scratches.
Dimensional and material characteristics of direct deposited H13 tool steel by CO2 laser
Authors: Choi J. , Hua Y.
Published: June 2006

Laser aided direct metal/material deposition process builds metal parts layer-by-layer directly from the computer aided design representation. Success of this technology in the die and tool industry depends on the parts quality to be achieved. To get designed geometric dimensions and material properties, delicate control of the process variables such as laser power, spot diameter, traverse speed, and powder mass flow rate is critical. With a closed loop optical height control system, the dimensional accuracy was achieved within designed tolerance. To establish the deposited material completeness, the effects of the process variables on macro- and microstructure and mechanical properties of H13 tool steel samples were investigated using a statistical technique, the design of experiments. The experimental data were analyzed to determine optimal process parameters and to verify dimensional and material characteristics of the part, and the results are summarized in the article.
Laser assisted cleaning of oxide films on SUS409 stainless steel
Authors: Guo H. , Martukanitz R. , DebRoy T.
Published: June 2006

Laser assisted cleaning of oxide films on SUS409 stainless steel plates was studied both experimentally and theoretically. The highly oxidized surfaces with oxide scales of about 25 μm thickness were irradiated with Q-switched pulsed Nd:yttritium–aluminum–garnet lasers with and without frequency doubling, i.e., at 1064 and 532 nm wavelengths. Both multiple irradiation on single spots and scanning with pulsed lasers were employed. The variables studied were laser wavelength, pulse duration, number of pulses, and scanning velocity. The results indicate that heavily oxidized stainless steel surfaces can be cleaned with short, high power density pulses. Shorter laser wavelength, higher power density, and irradiation with multiple pulses improved surface cleaning. Stress wave induced spallation played an important role in the cleaning process. No loss of the underlying metal layer was observed during treatment.
The role of electron–phonon coupling in ultrafast laser heating
Authors: Chen J. K. , Latham W. P. , Beraun J. E.
Published: June 2006

Most of the ultrafast laser heating analysis to date has been accomplished with a constant electron–phonon coupling factor (G). Due to the significant changes in the electron and lattice temperature caused by high-power laser heating, G could be temperature dependent. In this article a phenomenological temperature-dependent G is introduced to simulate ultrafast laser heating in metals. The electron temperature and the ablation depth computed with the temperature-dependent G compare well with experimental data.
Microstructural characterization of dissimilar laser weld between austenitic and ferritic stainless steels
Authors: Kaul Rakesh , Ganesh P. , Nath A. K.
Published: June 2006

The present work involved microstructural characterization of thin sheet dissimilar laser welds between type 304 austenitic stainless steel and stabilized 17% Cr ferritic stainless steel and their comparison with welds produced by autogenous gas tungsten arc welding (GTAW). Low heat input of laser welding (LW) effectively reduced the size of fusion zone (FZ) and heat affected zone (HAZ). LW with focused laser beam (LB), preferentially displaced towards austenitic stainless steel (ASS), produced FZ with uniform and highly refined microstructure. Using nitrogen as the shroud gas during LW with preferentially displaced LB towards ASS, brought about a significant microstructural change, i.e., the development of largely austenitic FZ. It marked a significant improvement over the coarse-grained FZ microstructure of the GTA weld that was comprised of large blocks of ferrite and martensite/retained austenite. Austenitic microstructure of laser weld will result in significant improvement in the ductility over that of dissimilar welds made by autogenous GTAW.
Fracture control of unsupported ceramics during laser machining using a simultaneous prescore
Authors: Segall A. E. , Cai G. , Akarapu R. , Romasco A. , Li B. Q.
Published: June 2006

While lasers offer many advantages when machining ceramics, costly premature fractures and related damage is the tradeoff often faced by manufacturers. This tradeoff is especially apparent for higher-speed machining of complex shapes where traditional “nail-bed” supports are not always practical. To help overcome these problems, research efforts have focused on a unique method of simultaneously scoring and cutting to help control fracture. Using a customized beam delivery system, a lower-power beam simultaneously created a shallow groove or prescore directly ahead of the higher-power cutting beam. To ensure that the prescore groove is completed before fracture is likely, beam spacing was set at half of the plate width. Results for 0.59 mm thick alumina plates under exaggerated mixed-mode conditions indicated that prescoring did indeed work with markedly improved fracture surfaces when compared to traditional single-beam cuts. In fact, a complete reduction of the mixed-mode fracture was achieved with net prescore groove as shallow as 14.6 μm. Modeling via a customized finite-element algorithm that includes melting/ablation indicates that the two beams can be used in relatively close proximity of 2–4 beam diameters without any significant overlaps of the independently developing stress fields.
Property enhancement of diffusion borided layers by laser treatment
Authors: Shariff S. M. , Jain Vipin , Sundararajan G. , Joshi S. V.
Published: June 2006

Tribological properties such as abrasion and erosion resistance of boride coatings on steels have been proven to be outstanding when compared with conventional hardening techniques, e.g., carburizing and nitriding. Pack boriding is a relatively simple and economical technique among all the methods available for boriding. A single phase Fe2B layer is desirable in the boride coating for its superior performance and this can be achieved in several ways. However, the boride coating is often associated with inherent porosity, especially near the surface and this is undesirable from the standpoint of structural integrity of the coating particularly for applications involving highly loaded parts. Laser-based postprocessing is an effective technique for eliminating these undesirable microstructural features without deteriorating the mechanical properties of the coating. In the present investigation, a low activity boronizing mixture was used for pack boriding of a medium carbon steel to obtain a single phase Fe2B layer. The coating was laser treated with a pulsed Nd–yttrium–aluminum–garnet laser at different laser powers to control the extent of melting of the boride layer. Characterization of both as-borided and laser treated coatings was carried out to examine changes in surface roughness, microstructural features, and microhardness profile and phases as a result of laser treatment. Testing of the coating under erosive and abrasive wear conditions was also carried out to ascertain the influence of laser treatment on tribological performance of the coating. It was found that, under controlled processing parameters, it is possible to improve the durability of coating properties through laser treatment.
An experimental and numerical study of the influence of diode laser beam shape on thin wall direct metal deposition
Authors: Pinkerton Andrew J. , Li Lin
Published: June 2006

Use of the high powered diode laser (HPDL) for direct metal deposition enables multiple layer cladding to be performed with a nominally rectangular beam, traversed at different angles to the fast axis, or with a fiber-coupled, circular beam. These factors may provide an advantage over the traditional beam profiles of CO2 and Nd:YAG lasers, so there is a need to examine the effect that they have on the process and final product characteristics. This article reports such an examination. A 1.5 kW HPDL and 316 L stainless steel powder are used to experimentally investigate the dimensions, microstructure, surface finish, and hardness of multiple layer walls produced using nominally rectangular and circular beams under a range of conditions. The diode beam profiles are modeled using a “Gaussian beam array” method for the nominally rectangular beam and a “concentric series” method for the fiber-coupled beam. Quasistationary state temperature profiles are produced for both beam shapes by superposition of Gaussian source solutions. The experimental results show little difference in microstructural properties, elemental composition, or hardness due to beam shape or traverse direction, but track dimensions are dependent on beam geometry. Wall width is particularly dependent on beam size orthogonal to the traverse direction. Modeling allows some of the results encountered to be explained in terms of temperature distributions and cooling rates. Fiber coupling seems to provide a benefit if even thin-wall dimensions are required for applications such as rapid prototyping, while using the rectangular beam provides advantages for bulk material addition for applications such as repair and refurbishment of worn components.
Experimental study of CO2 laser welding inside a groove— Application to high thickness laser welding
Authors: Hamadou M. , Fabbro R. , Coste F. , Slimani S.
Published: June 2006

Laser welding inside a groove has to be considered when a high thickness material is welded. The use of a groove shape allows the access of the laser beam to the lower part of the groove. The choice of the groove geometry (groove width, depth, or aperture angle), as well as the assist gas setup (leading, trailing or backside protection) are then significant operating parameters for the process control. In this study, U- and V-shaped groove welding with a CO2 laser was investigated. The U-shaped groove welding generates an increase in penetration depth, a very strong reduction of the process noise and of the plasma plume volume, compared to a bead on plate or a V-shaped groove welding. Also, the penetration depth for a U-groove welding is 20% greater than for a V groove. The groove geometry and the surface tension effects are determinant for these improvements. The influence of shielding gas shows that a trailing protection (gas flow emitted from the nozzle towards the welding direction) should be adopted in order to better control the oxidation of the weld bead seams.
Cladding of Stellite-6 and vanadium carbide on carbon steel using a yttrium–aluminum–garnet laser robot system
Authors: Xu Guojian , Kutsuna Muneharu , Rathod Manoj
Published: June 2006

In the present work, a laser robot system was setup for cladding and repairing machinery parts using pulsed 300 W Nd:yttrium–aluminum–garnet laser facility, six-axis robot, optical fiber, powder feeding system with a special nozzle for powder supply. A mild steel plate was deposited with powder mixtures of Stellite-6 alloy and vanadium carbide (VC). The shape of clad layer, microstructure, and its wear performance were investigated by changing the weight fraction of VC in the range of 0%–100%. The microstructure observation showed that as the weight fraction of VC in the powder mixture increases, the amount of primary solidified VC phase, penetration depth, and crack susceptibility increased. This also increased the hardness and wear resistance of the clad layer. Depending on the weight fraction of VC in the powder, the resultant microstructures can be divided into two types—hypoeutectic structure containing primary Co-rich γ phase with eutectic mixture and hypereutectic structure containing primary VC, complex carbides, and eutectic mixture.
High-speed scanning laser-induced breakdown spectroscopy at 1000 Hz with single pulse evaluation for the detection of inclusions in steel
Authors: Bette Holger , Noll Reinhard , Müller Gregor , Jansen Hans-Werner , Nazikkol Çetin , Mittelstädt Horst
Published: June 2006

Spatially resolved information about the distribution and chemical composition of inclusions in steel are gained by scanning methods, such as scanning electron microscopy with energy dispersive X-ray spectroscopy, electron probe microanalysis or capillary-X-ray flourescence. Scanning laser-induced breakdown spectroscopy (LIBS) offers distinct advantages in contrast to these conventional methods. Sample polishing or high vacuum conditions are not necessary, a simple grinding of the sample is sufficient for the preparation. Analysing times can be reduced significantly. At Fraunhofer ILT a measuring system was developed enabling for the first time high-speed scanning LIBS with measuring frequencies of up to 1000 Hz. Sample surfaces with dimensions up to 110×45 mm2 can be analyzed. Light elements such as C, N, O, P, and S are detected, which is demonstrated by the identification of nonmetallic inclusions in steel. Principle, features and results of high-speed scanning LIBS with a spatial resolution<20 μm are presented.
Investigation of the irradiance-dependent spectral transmittance of laser filters in the nanosecond- and femtosecond-pulse regime in the wavelength range between 700 and 800 nm
Authors: Schirmacher A. , Sutter E. , Werhahn O. , Siegner U. , Nevejina-Sturhan A.
Published: June 2006

Induced transmittance is observed with polycarbonate laser filters when they are irradiated by laser pulses with a width of 200 fs. These results are compared with measurements in the nanosecond time domain. Calculations by a simple absorption model show, that single photon absorption is dominating the light-matter interaction at the pulse widths used in this study, at least at radiant exposure levels at which the filter material is not damaged. The use of eyewear showing induced transmittance can lead to a risk for the user, if the filter is irradiated by a strong laser beam in an accident. This situation can be even worse when an additional temperature effect is taken into account, which shifts the transmittance characteristics to lower optical densities when the temperature of the laser filter is maintained at a lower level. An example for this effect is given for nanosecond-pulse laser interactions.
Effect of steel composition on the laser cutting behavior of 25-mm-thick plates
Authors: Manohar M. , Bodnar R. L. , Asfahani R. I. , Chen N. , Huang C.
Published: June 2006

Attempts were made to gain a better understanding of the effects of steel composition and thermophysical properties, such as solidus and liquidus temperatures, heat of oxidation, surface tension, viscosity and thermal conductivity, on laser-cutting behavior. Laboratory-produced heats containing varying levels of C, S, Si, Cu, Ni, Cr, and Mo were hot-rolled to 32-mm-thick plates. Laser cutting was carried out after the plates were blanchard-ground to 25 mm and shot-blasted to eliminate surface effects and roughness variability. Under these conditions, Cu and Ni were found to improve cutting performance, while Cr degraded cutting performance. C and Si were not found to affect cut quality. No direct correlation was obtained among steel composition, laser cut quality, and thermophysical properties. However, limited confocal microscopy and energy dispersive x-ray spectroscopic analysis of selected cut surfaces suggested that enrichment at the cutting front of alloying elements resistant to oxidation might play a major role in determining cut quality.
Temperature distributions due to annular laser beam heating
Authors: Zeng D. , Latham W. P. , Kar A.
Published: June 2006

Optical trepanning is a new laser drilling method using an annular beam. Since laser heating of the substrate occurs first due to heat conduction, this article investigates the temperature distribution in the workpiece due to pulsed annular laser beams by solving an axisymmetric transient heat conduction equation. The annular beams allow numerous irradiance profiles to supply laser energy to the workpiece and thus provide more flexibility in affecting the hole quality than a traditional circular laser beam. Such profiles include half Gaussian with maximum intensities at the inner and outer radii of the annulus, respectively, and full Gaussian with maximum intensity within the annulus. In addition to this spatial beam shaping, the temporal profile of the laser pulse can be shaped to improve the hole quality. The Hankel and Laplace transforms have been used to obtain an analytic solution for the temperature distribution in a semi-infinite workpiece. The effects of the temperature distribution on laser drilling are analyzed to understand the influence of different laser parameters on the drilling process.
Developments towards controlled three-dimensional laser forming of continuous surfaces
Authors: Edwardson S. P. , Abed E. , French P. , Dearden G. , Watkins K. G. , McBride R. , Hand D. P. , Jones J. D. C. , Moore A. J.
Published: June 2006

There has been a considerable amount of work carried out on two-dimensional laser forming, using multipass straight line scan strategies to produce a reasonably controlled bend angle in a number of materials, including aerospace alloys. However, in order to advance the process further for realistic forming applications and for straightening and aligning operations in a manufacturing industry it is necessary to consider larger scale controlled three-dimensional (3D) laser forming. The work presented in this article uses a predictive and adaptive approach to control the 3D laser forming of a 1.5 mm mild steel sheet into a desired continuous surface. The surface considered in the study was the pillow (or dome) shape. Key to the control of the process was the development of a predictive model to give scan strategies based on a required geometry. The forming rate and distribution of the magnitude of forming across the surface were controlled by the process speed. When the geometry is not formed within one pass, an incremental adaptive approach is used for subsequent passes, utilizing the error between the current and desired geometry to give a new scan strategy, thus any unwanted distortion due to material variability can be accounted for.
An investigation into delamination mechanisms in inclined laser drilling of thermal barrier coated aerospace superalloys
Authors: Sezer H. K. , Pinkerton A. J. , Li Lin , Byrd P.
Published: June 2006

Thermal barrier coatings are widely used in aircraft engine hot-end components to protect the component materials from direct exposure to the damaging high temperatures. Effusion cooling techniques are widely used in these situations and laser drilling is normally employed to produce holes of less than 1.5 mm diameter in tough aerospace alloys coated with heat-resistant ceramics. In order to provide the essential cooling effects, a large proportion of these holes must be drilled at acute angles to the surface. During this low-angle laser drilling, microstructural damage of the thermal barrier coating may occur; this is highly undesirable as propagation of the delamination may occur in service, leading to premature failure of the coating. In this article, the role of melt ejection and the coaxial assist gas jet in low angle laser drilling of a thermal barrier coated substrate is studied using a finite volume method. The work identifies the significant role of melt ejection in the formation of cracks and delamination at the coating/substrate interface on the leading edge of a laser-drilled cooling hole. The numerical model shows that the melt particles are accelerated towards the leading edge by the coaxial assist gas jet, and as a result of molten metal flowing across the edge the damage through mechanical stresses and subsequent erosion is found to be more serious here than at the trailing edge. The higher shear stress found at the leading edge also implies higher viscous forces acting parallel to the free stream direction to overcome the bonding strength of the coating interface.
Faster and damage-reduced laser cutting of thick ceramics using a simultaneous prescore approach
Authors: Pereles-Santiago V. , Washington M. , Brugan P. , Cai G. , Akarapu R. , Pulford S. , Segall A. E.
Published: June 2006

Laser machining of structural ceramics is increasingly gaining acceptance as an alternative to traditional machining methods. However, despite the great promise of lasers for a variety of cutting and drilling procedures, premature fractures and prohibitively low cutting speeds are still among the greatest obstacles encountered, particularly when thick cross sections are involved. While many factors contribute to the fractures encountered during laser machining, it is the inevitable and localized increase in temperature and the ensuing thermal stresses that usually cause the damage. As such, the minimization of heat buildup and the resulting thermal stresses often requires the slow and expensive practice of multiple pass or interrupted cutting or drilling. To help avoid the use of multiple-cut methods, while at the same time allowing for faster machining, a unique method of simultaneously scoring and cutting known as prescoring was explored using alumina plates. Using this technique and a design of experiment approach to systematically investigate the effects of various parameters, the use of prescoring was shown to significantly increase cutting rates for relatively thick alumina plates. In addition to improving cutting rates, the method was also shown to be capable of dictating the crack path when fracture could not be avoided.