A deep cavity called keyhole is formed in the laser weld pool due to the intense recoil pressure of evaporation. The formation of keyhole leads to a deep penetration weld with high aspect ratio. However, a hole drilled in a liquid pool is primarily unstable by its nature and the instability of keyhole also causes the formation of porosity in the weld metal. The porosity formation is one of the serious problems in the very high power laser welding, but its mechanism has not been well understood. The authors have conducted systematic studies on observation of keyhole as well as weld pool dynamics and their related phenomena to reveal the mechanism of porosity formation and its suppression methods. The paper describes the real time observation of keyhole and laser plasma/plume behaviors in the high power CW CO₂ laser welding by the high speed optical and X-ray transmission methods, cavity formation process and its suppression measures.

A plume consisting of vapor and ionized particles of the workpiece is usually formed during various types of laser materials processing. The process parameters such as the laser power, spot diameter, scanning speed, material properties and shielding gas affect the properties of this plume. A one- dimensional model is presented to understand the effects of the vapor-plasma plume in continuous wave (cw) laser materials processing. A model for pulsed laser materials processing is also discussed. These models are used to analyze the transmission of the laser beam through the plume and the deposition of energy on the melt pool at the substrate surface. An experimental technique described as the pinhole experiment is devised for pulsed laser operations to measure the partitioning of laser energy between the plume and workpiece and to identify the process parameter regime for efficient energy transfer from the laser beam to the workpiece. The attenuation coefficient of the vapor-plasma plume was measured during cw CO₂ laser-assisted metal deposition conditions by directing a CO₂ probe laser beam horizontally through the plume and determining the ratio of irradiance of the beam after and before the plume. Assuming an isotropic attenuation coefficient through the plume, the energy partitioning between the plume and workpiece was determined.

It is well known that porosity is easily formed in high power laser welding, which is quite a serious problem to be solved. At present, there are few reports studying interrelationship between keyhole and plasma behavior with the objective of understanding the effect of shielding gas on porosity formation. In this study, therefore, the relationship between keyhole and plasma behavior was observed directly by using two synchronized ultra high-speed cameras and X-ray transmission observation system. In the case of He gas, metallic plasma was continuously formed, and the keyhole was always open. It was observed that many large bubbles, which were formed from the tip of a keyhole, were trapped at the solidifying front in the rear part of the molten pool, and lead to the porosity formation. On the other hand, in the case of N₂ gas, big nitrogen plasma was formed above the weld bead periodically, and its absorption of laser caused the disappearance of metallic plasma and keyhole. This periodical interval and duty were different among materials used and affected the bubble and porosity suppression beneficially.

In this study, the laser edge-welding process is investigated as a mechanism for joining laminates and eliminating steps on the surfaces of tools fabricated by layered manufacturing techniques. An Nd:YAG laser (Lumonics JK706) was used to shape the edges and join the laminates together. The experiments were carried out on 1 mm thick mild steel. The effect of peak powers, peak power densities, welding speeds and beam-edge overlap percentages and the angle of the laser beam incidences were investigated. The work also shows the use of the optimum parameters in the production of hemispherical tool primitives. It also presents results of tests to ascertain the resistance of these tools to typical working environments.

Characterization of a blue plasma was conducted using a high- speed I.I. video camera, a color CCD video camera, and a spectrum multi-channel analyzer. A blight blue plasma, which contains Fe, N₂ and N₂⁺ was observed from a cutting point in laser cutting with either through- and non- through penetration when the cutting quality was not satisfactory. From the results, a use of the blue plasma signals can be effective for the in-process monitoring of a cut surface quality in laser cutting. Also, removal processes of molten material, can be divided into two types, (1) by the dynamic pressure of a coaxial gas jet, and (2) by the recoil force with an evaporation. An explosive evaporation of molten material from a point of laser irradiation was observed almost simultaneously with the plasma formation. At the cutting front, a small crater was seen at a point of the plasma formation. It was found that N₂⁺ and N₂ spectra are not emitted directly through a direct absorption of a laser beam by a gas jet, but presumably through the secondary collisions of N₂ in a gas jet with electrons and/or Fe atoms emitted and accelerated from the material surface by a strong radiant energy of the laser beam.

Transient action of molten metal during laser cutting process in a kerf using different assist gases was experimentally observed. In the experiments, we used a typical mild steel sheet with a thickness of 4.4 mm. A nearly dross-free condition with oxygen as an assist gas is at a scanning speed of 1000 mm/minute, CO₂ laser power of 600 W pressure of an assist gas 0.15 MPa, while with air a scanning speed of 300 mm/minute, CO₂ laser power of 800 W and assist gas pressure of 0.4 MPa were used. Motions of molten metal in a kerf were captured with a high-speed digital CCD camera at a maximum frame rate of 18000 frames per second. As results, we found very bright portion (presumably higher temperature portion) in an erosion front moving toward bottom of a kerf. For oxygen as an assist gas, the portion was moved with a periodic action with a time interval of every 2.22 ms. While air was used, the motion was observed to be every 18.6 ms corresponding to ten times as long as obtained with oxygen as an assist gas.

With the objectives of obtaining a fundamental knowledge of laser welding technology inside and outside the spacecraft in space, pulsed YAG laser spot welding was performed on the metal plates in Ar gas atmosphere or a vacuum in the falling microgravity apparatus equipped with the fiber-delivered laser focusing optics. The influence of gravity or microgravity on penetration and welding defect formation was further clarified by comparing the welds made in the normal flat and overhead positions. Almost all results of weld penetration and defect formation under microgravity were similar to those under normal gravity except the welding result of aluminum alloy A5083 subjected to the high power density laser, and were between normal gravity and overhead position welding results. Welding in a vacuum was characterized by the formation of a narrower and cone-shaped bottom in any alloy weld. Porosity was easily formed in any deeply penetrated weld metal under high power density welding with a rectangular pulse-shaped laser, and could be reduced by utilizing pulse-controlled laser even under microgravity.

We study the keyhole geometry as a function of the main operating parameters such as welding speed, laser incident intensity or sample material. This model is based on a drilling velocity whose combination with the welding velocity causes the inclination of the front keyhole wall. This front inclination is shown to be stationary and stable all along the front keyhole wall (FKW). The penetration depth results from the product of this drilling velocity and a characteristic time defined as the beam diameter divided by the welding speed. By using a ray-tracing procedure, the dynamics and the complete keyhole geometry can be determined by taking into account the multiple reflections inside the keyhole and a description of the closure process of the rear keyhole wall (RKW). We show that this RKW cannot be stationary all along its surface and only and adequate laser intensity distribution can make it stationary. The interest of elongated focal spots or twin-spots is then demonstrated. At high welding velocity the front wall is inclined and is composed of several layers resulting of the successive reflections. The rear wall fluctuates around an apparent equilibrium, and corresponding fluctuations occur on maximum penetration depth.

Ultrashort laser pulses provide high power at low energy which is very promising to obtain precise microstructures inside and on the rear side of the transparent materials. Decreasing the pulse duration from several ps into the sub-ps range, we noticed strong differences in the processing and non-linear behavior. We performed experimental investigations on laser beam propagation through wide-band gap materials at high peak powers. We present recent results on different types of bulk modifications generated inside several different transparent materials and focus our attention on pulse duration dependent observations in a range between 0.2 and 4 ps.

We demonstrate in this paper a fabrication of three- dimensional microstructures using photopolymerization of resin by two-photon absorption. When a tightly focused laser beam was scanned in a light-curling liquid, solidified rods were formed following the trace of the scanning. If the solidification was arranged to occur along the frame of a microstructure, a designed spatial pattern would be transformed into material object. Due to a quadratic dependence of photopolymerization rate on the laser pulse energy, the size of solidified voxels was controlled down to submicrometer order. Infrared transmission measurement exhibited pronounced band gap effects from thus-fabricated photonic crystal structures.

We propose and utilize ultrashort laser pulses to tailor three-dimensional microstructures and their optical properties. When an intense femtosecond pulse was tightly focused into some transparent materials, a laser-induced microexplosion occurred, generating void holes inside the medium. When the thus-fabricated holes or cylinders were regularly organized, a microstructure with a periodic refractive index distribution was accomplished, which was liable to act as a photonic crystal structure. One-, two, and three-dimensional photonic lattices have been acquired by using this technique. Significant photonic band gap effects were confirmed by transmission measurements. The unique feature of the ultrashort laser micromachining of photonic crystal structures was the availability of arbitrary spatial geometry.

'In-situ' weld-alloying/laser beam welding is utilized to join SiC particle reinforced aluminum alloy metal matrix composite (SiCp/Al MMC) with titanium as the alloying element. Microstructure of the weld is characterized as functions of alloying content and processing parameters. Formation of the needle-like Al₄C₃ phase is suppressed and a composite reinforced by TiC and Ti₅Si₃ phase is produced in the laser welded joint on SiCp/Al MMC by weld-alloying with titanium. It is demonstrated that 'in-situ' weld- alloying/laser beam welding is a promising processing method for joining of SiCp/Al MMCs.

Beam-deflection technique was used to monitor the surface treatment of fullerence films near the ablation threshold region. When the fullerence films were irradiated by UV excimer laser with fluence below the ablation threshold, an increase of the electrical conductivity of up to six orders of magnitude was observed. The products resulting from the laser irradiation have been investigated by Raman spectroscopy.

Shock waves induced by laser-plasma in Water Confinement Regime (WCR) are used in order to improve the metallurgical properties of metallic materials. In a first part of this paper we discuss the basic principles which limit the pressure generation in this regime: the effects of laser intensity, target material, laser pulse duration and laser wavelength are discussed. Depending on laser parameters, the peak pressure is saturated and its duration is reduced above a laser intensity threshold, due to laser-induced breakdown plasma in the confining water. The observation of the interaction zone shows that this parasitic breakdown occurs only at the water surface and limits the efficiency of the process. The relative influence of main physical mechanisms occurring during the generation of the laser breakdown at the surface of water have been discussed. According to the wavelength effect which tends to authorize higher pressure with longer wavelengths, the influence of multiphotoionic processes dominates the effect of avalanche ionization. The second part of the paper deals with the surface modifications induced by laser-shock processing. Depending on the operating parameters, Residual Stresses (RS) field can be optimized. Typical present or future applications such as the improvement of fatigue or corrosion properties, are then presented.

Laser cleaning is growing in importance with the introduction of the Montreal protocol which proposes the long term reduction on environmental and public health grounds in the use of organic solvents such as CFCs that are normally used in industrial cleaning. There is also significant interest in laser cleaning in the conservation of sculptures, paintings and museum objects where the process offers advantages in terms of time saving and the enhancement of the ability to conserve certain artefacts. To date there has been insufficient consideration of the mechanisms involved in laser cleaning and how their understanding could lead to improved control and efficiency of the laser cleaning process. This paper considers an overview of the processes involved and their relevance in the different cleaning situations encountered in practice, mainly in terms of the application short pulse length lasers. The mechanisms to be considered include, (1) photon pressure, (2) selective vaporization, (3) shock waves produced by rapid heating and cooling, (4) evaporation pressure, (5) plasma detonation (spallation), (6) ablation.

This article reports the surface hardness values, transverse and longitudinal distributions of three kinds of carbon steels treated by laser surface heat treatment. The surfaces of the steel samples hardened by this way are harder than those of steels hardened by normal way. The surface hardness value of low-carbon steel sample, especially, increases a lot after being hardened by laser. This has much significance in the practical use in the future.

Laser drilling is the oldest production technique using lasers. Nevertheless, the number of industrial applications is lying far behind of those of marking, cutting and welding. In cases when high accuracy is required, like for instance fuel injection nozzles, laser drilling could not fulfill the requirements, up to now. The major problem is the relatively large amount of molten material produced when using common drilling lasers with pulse durations in the order of one millisecond. Shortening the pulse length down to the femtosecond range is widely proposed as a recipe to avoid melt, now. The required laser systems, however, seem to need some years of ripening before they can be used in industrial environment. On the other hand, reliable and powerful diode pumped solid state lasers with pulse durations in the order of 10 nanoseconds are on the market now. Picosecond systems are available as laboratory setup. The paper presents drilling results obtained in both pulse regimes with fundamental and higher harmonic wavelengths. With a new drilling procedure, the so called helical drilling, an unprecedented level of accuracy could be achieved. A model allowing to explain experimental results will be offered.

While developments in the field of diode pumped solid state lasers provide a foundation for precision machining of parts with high accuracy and small feature sizes, this promise can not be realized without considering the interactions of individual processes, systems and material parameters. This paper presents our results on the precision machining of small features in various materials using diode pumped solid state lasers. The machined features are characterized geometrically by using optical inspection techniques and the tolerance data is analyzed statistically. Machining parameters relevant to motion system and tool path compensation are discussed along with their relevance to machined feature geometry. The effect of laser beam polarization on the machined kerf width, kerf surface and feature dimensions is reported.

Ultrafast lasers are a class of laser that produce pulse widths of picoseconds (10^-12 sec) and femtoseconds (10^-15 sec). They can achieve extremely high peak power with low pulse energies. The most important characteristics of ultrafast laser-matter interaction are precise ablation threshold and absence of heat diffusion into the material during laser irradiation, both due to the shortness of the laser pulse. One of the advantages of applying ultrafast lasers in materials processing is the versatility of these lasers in processes where material removal is required. As the laser pulse width decreases from milliseconds through microseconds to nanoseconds and picoseconds, the material removal mechanism transitions from melt expulsion to direct ablative removal. This process is similar in many different solid materials, regardless of the material composition. In this paper a number of ultrafast laser machining examples in a variety of materials will be presented to illustrate this point. Precise ablation threshold and little or no heat-affected zone combine to yield high quality drilling and cutting in these cases.

Visible and UV lasers with nanosecond pulse durations, diffraction-limited beam quality and high pulse repetition rates have demonstrated an ability to machine a wide variety of materials with sub-micron precision and sub-micron-sized heat-affected zones. Manufacturing applications for these lasers include orifice drilling in fuel injection components and inkjet printers, micro-milling of micromolds, via hole drilling in printed circuit boards and silicon machining. Narrow line width UV is used for the fabrication of fiber Bragg gratings. Results achieved with copper vapor lasers (CVL) and diode-pumped solid state lasers will be presented. Recent advances in higher power, compact CVLs are opening new possibilities for manufacturing with this class of laser.

Laser welding includes many complicated phenomena such as absorption of laser light on material surface, phase transition from solid to liquid and from liquid to gas, laser light absorption and refraction within plasma, and so on. Two- dimensional unified simulation model was developed for laser welding of thick plate, and verification test using 4kW-YAG laser was carried out. Thermal-hydraulic phenomena of welding pool and keyhole are solved numerically using CIP (Cubic- Interpolated Propagation) and C-CUP (CIP-Combined Unified Procedure) method based on conservative equations of the multi-phase and multi-component fluids. Multiple reflection of laser light on the keyhole surface, absorption and refraction of laser light within the plasma are treated by ray tracing method. The availability of the model was confirmed to compare the results of experiments with numerical analysis.

The use of high-power pulsed excimer lasers, working in the nanosecond (ranging from 20 to 200 ns) duration regime, allows the deposition of a large amount of energy in very short times into the near surface region of amorphous silicon films deposited on glass. Under suitable conditions, the laser irradiation leads to the rapid melting of the a-si layer and its regrowth into polysilicon. In order to optimize the final quality of the poly-Si film and the formation of a large grained material through a so-called super-lateral-growth phenomenon, it appears necessary to control extremely carefully the surface melt dynamics of the laser processing, by developing a numerical simulation based on the resolution of the one-dimensional heat flow equation. It is demonstrated that the melting threshold, melt duration, depth of fusion and solidification velocity, are strongly dependent on the laser pulse duration, substrate temperature, thickness of the silicon and oxide (or nitride) barrier layers. These numerical analyses are also shown to be consistent with the experimental results.

The laser forming process of metal sheet has been investigated and has proved to be adequate to rapid prototyping. In order to successfully apply this technique, a deep understanding of laser-material interaction is required. Since the AISI 304 stainless steel is widely used for deformation processes, the authors decided to investigate its laser forming behavior. The forming effect has been evaluated in terms of the bending of sheet-shaped specimens. The influence of the main technological and geometrical parameters of the process were investigated by a factorial design, i.e. ANOVA and Surface Response. The experimental results were used to built a neural network that is able to predict some process parameters once the designer has fixed the initial and final geometry of the specimen. The obtained results show that the laser bending of AISI 304 can be a valid alternative to conventional forming and prototyping techniques.

Some new approaches to hydrodynamic mechanisms of deep penetration laser channels [keyholes (KH)] and of melt surface instabilities are proposed. In particular, a thermocapillary mechanism of KH growth is considered for the case of low-intensity laser beam: it is shown that a number of melt surface micro-instability mechanisms typical for 10⁴ - 10⁷ W/cm² laser beam intensity range can be excited by the thermocapillary, evaporative-capillary, Kelvin- Helmholtz and Rayleigh-Taylor mechanisms. Besides, some macro instabilities of KH melt surface, namely, the capillary collapse, 'moving shelf' and 'melt squeezing out' are possible under certain conditions. Taking into consideration a possibility of melt surface micro- and macroinstabilities, some integral nonlinear nonstationary mechanisms of penetration laser welding can be imagined. In particular, qualitative estimations of several pulse-periodical relaxational mechanisms of CW laser welding are presented.

We have been developed a micromachining system which is composed of a pulsed ultraviolet laser and a five axis mechanical stage. Micro windmill which was composed of a windmill and a bearing and has been successfully fabricated as an example of the most simple mechanical element. The material was polyimide and the typical length was about 1 mm. Taking the advantages of the size effect, we have successfully obtained the maximum number of rotation of about 90,000 rpm by blowing Nitrogen gas from a small nozzle.

For the stereo-lithography, XeCl excimer laser has been adapted to improve the utilization efficiency of laser energy. Because of the beam uniformity was made better by a homogenizer, the quantity of cured resin per laser energy was increased compare than that without the homogenizer. With this method a simple 3D object was created, and its was proved that XeCl excimer laser was possible to apply for the stereo- lithography.

Microstructuring of soda lime glass with excimer laser radiation is examined to present its process capabilities for production of surface structures particularly for microreaction technology. Material removal rates, geometries of microstructures and transmission/reflexion of processed soda lime glass are investigated for ArF excimer laser radiation ((lambda) _L equals 193 nm, (tau) _L equals 20 ns) and KrF excimer laser radiation ((lambda) _L equals 248 nm, (tau) _L equals 25 ns). Processing of glass at atmospheric pressure down to vacuum at p equals .10 mbar (He as processing gas) is carried out to investigate the correlation between processing gas pressure and generation of debris (solidification of molten material and re-deposition from expanding vapor/plasma). The influence of processing gas atmospheres consisting of He/F₂ (50/50 vol.%) with a total pressure in the vacuum chamber of 10 mbar less than p less than 250 mbar on height/width of the debris is examined. Ultrasonic cleaning of glass after processing is investigated for reduction of debris. Optical microscopy, white light interferometry, optical spectroscopy and X-ray photoelectron spectroscopy (XPS) are used for analysis of microstructures and debris.

If production technology is divided into three groups -- namely processes with material removal, processes with material addition and processes without a change of workpiece mass, it points out immediately, that the first two groups have been conquered to a large extent by the laser, but the third group suffers from an essential lack of laser application. The reason is, that for these processes -- mainly forming -- rather strong mechanical forces are needed, that cannot be generated by a deliberation of optical energy. Nevertheless, the strong dependence of the yield strength, that must be reached for plastic deformations, on the temperature shows a way to reduce mechanical forces in forming and to obtain thus a beneficial effect of laser assistance. Moreover, hard and brittle materials, that cannot withstand deformations at room temperature without cracks or rupture, can be treated successfully with the help of lasers. Compared to conventional hot working, laser heating can be applied only to those regions of the workpiece, where strong deformations take place, thus beneficially reducing production time. A good example for the above solution is laser assisted deep drawing (LADD), that has been investigated by the author experimentally and theoretically. The latter studies, that will be presented in the actual paper, show clearly, that the above predictions are true, whereas a reduction of the drawing force up to 50% seems to be feasible. Nevertheless, relatively high beam powers up to several 10 Kilowatts must then be used, thus giving rise to a promising application of very high power lasers.

Because of the weak points of the SLS spot Scanning process, a new rapid prototyping process -- SLS line scan using linear array of high power laser diodes regarded as energy sources is researched in this paper. A linear array with requisite length is formed by some high power laser diodes that can be derived individually. Beams of the linear array are transferred to the workplace and imaged some short and light lines by the corresponding optical collimators. They are lined up in a linear laser beam without separation whose length is equal to that of the linear array diodes. When sintering powdered material, the linear laser beam scans in one direction along x axis only. Only if the maximum line length is less than the y axial size of the workpiece, it is necessary that linear laser beam is lapped for some times in the y axis. The Scanning mode of x-y simultaneous guideways are used in this new system which differs entirely from the vibrating mirror scan. The scanning trace of the latter is an arc that will influence processing quality. This new process has higher efficiency and better quality than the traditional spot scanning method.

The authors have developed a new process of laser-induced shock compression to introduce a residual compressive stress on material surface, which is effective for prevention of stress corrosion cracking (SCC) and enhancement of fatigue strength of metal materials. The process developed is unique and beneficial. It requires no pre-conditioning for the surface, whereas the conventional process requires that the so-called sacrificial layer is made to protect the surface from damage. The new process can be freely applied to water- immersed components, since it uses water-penetrable green light of a frequency-doubled Nd:YAG laser. The process developed has the potential to open up new high-power laser applications in manufacturing and maintenance technologies. The laser-induced shock compression process (LSP) can be used to improve a residual stress field from tensile to compressive. In order to understand the physics and optimize the process, the propagation of a shock wave generated by the impulse of laser irradiation and the dynamic response of the material were analyzed by time-dependent elasto-plastic calculations with a finite element program using laser-induced plasma pressure as an external load. The analysis shows that a permanent strain and a residual compressive stress remain after the passage of the shock wave with amplitude exceeding the yield strength of the material. A practical system materializing the LSP was designed, manufactured, and tested to confirm the applicability to core components of light water reactors (LWRs). The system accesses the target component and remotely irradiates laser pulses to the heat affected zone (HAZ) along weld lines. Various functional tests were conducted using a full-scale mockup facility, in which remote maintenance work in a reactor vessel could be simulated. The results showed that the system remotely accessed the target weld lines and successfully introduced a residual compressive stress. After sufficient training for operational personnel, the system was applied to the core shroud of an existing nuclear power plant.

This paper presents the experimental research and analyses on cracking behavior during high power laser cladding of Stellite alloy and NiCrSiB alloy. The experiments demonstrate that Stellite alloy is not sensitive to cracking. Large area stellite laser clad (length 200 mm, width 56 mm, depth 1.9 mm) was achieved by 18 multi-pass overlap. NiCrSiB alloy is very sensitive to cracking and many transverse macro cracks appear on the clad pass. Most cracks originate from the interface between the clad layer and the substrate and then develop up into the surface, which belongs to the cold crack domain. During multi-pass overlap, the as appeared cracks will develop into previous passes or later passes. The cracking phenomenon and behavior is mainly affected by the dynamic interaction between the stress (strain) induced and the thermal plasticity change of the clad material during the solidification, which is related to physical properties of the cladding material, the microstructure and the solidifaction characteristics of the process. The strength and toughness of the as-cladded layer sometimes is more critical to the crack formation than the residue tensile stress induced during the cladding process.

High-temperature self-lubricating wear-resistant metal matrix composite coatings are fabricated on substrate of a (gamma) - TiAl intermetallic alloy Ti-48Al-2Cr-2Nb by laser cladding. The hybrid (gamma) -NiCr metal matrix composite coating is mainly reinforced by rapidly solidified wear-resistant phase of hyper-eutectic M₇C₃ and self-lubricating particles of Ag and CaF₂ or CaAgF₄. Microstructure and hardness within the whole laser clad composite coating is homogeneous and the bonding to the substrate is purely metallurgical. Both hardness and dry sliding wear resistance of the (gamma) -TiAl intermetallic alloy are significantly enhanced after laser cladding treatment.

Laser alloying of CSiB+NiMoCo by a 45kW CO₂ laser was investigated to achieve sound large functional layer with smooth surface, little oxidation and free of cracks on cheap and broadly used common carbon steel XC38. The protection gas has obvious influence on the alloying results. Compared with Ar and N₂, He results in the best surface smooth, finest microstructure (second dendrite arm Ar:N₂:He equals 1:1:0.6), highest cooling rate (Ar:N₂:He equals 1:1.1:4),highest average hardness HV0.2(Ar:N₂:He equals 1:1.14:1.15), most resolvability of alloying elements (about Ar:N₂:He equals 1:1.1:1.5). These results can be properly explained by mainly the difference in physical properties of these three gases.

Laser cladding of AlSn alloy onto a mild steel have been carried out. The laser source was a continuous Nd-YAG. Due to similar boiling temperatures of feeding materials, cladded layers are free from porosity. Microstructural analyses show that aluminum and tin do not mix, except at the eutectic composition of 97% of tin. Eutectic compounds are shown. Direct cladding onto the steel produces intermetallic AlxFey layer. This layer is very hard. In order to avoid the formation of this layer, the steel can be pre-coated with nickel. Tribological behavior of the cladded layers is then analyzed, in comparison with an industrial material. Experimental Stribeck curves are plotted. It is shown that lubrication regimes do not change from industrial to laser cladded material. Running in behavior is tested. Results show that a tin layer on the surface of the AlSn laser alloy improve this running in behavior.

Laser surface alloying with both Si and SiC powder pre- coatings is utilized to improve the wear resistance of the Ti- 6Al-4V alloy. Rapidly solidified 'in-situ' composite coatings reinforced by TiC and Ti₅Si₃ phases are produced on substrate of the titanium alloy. Microstructure of the laser surface alloyed composite coatings is characterized and the wear resistance is evaluated under two-body abrasive wear condition. Results show that both the hardness and the wear resistance of the titanium alloy are considerably enhanced after laser surface alloying with Si and SiC powder pre- coatings.

The paper presents an investigation of deformation response of monocrystalline silicon surface to the action of short laser pulses in the air and in vacuum P approximately equals 10^-2 Torr. An anomalously continuous change of the surface relief was identified on irradiation in the air. The observable phenomenon is explained by oxidation of surface layer, enriched with defects.

A novel laser processing system for industrial surface engineering has been developed. It consists of wide-band laser scanner and wide-band powder feeder. It is used to laser cladding, alloying and surface hardening on the larger area, as well as welding. The system can provide single pass quenching or cladding widths with adjustable from 10 - 35 mm. The system was designed for delivering powder at precisely measured tolerance limits less than 2% and adjustable fed rate 0.5 - 200 g/min to produce the continuous, uniform and controlled laser cladding of 0.2 - 10 mm thickness, emphasis has been placed on repeatability, reliability as well as high powder utilization factor over 90%. All kinds of powders including not only Ni, Fe, Co and Cu base, but also ceramics powders ZrO₂, Al₂O₃, WC and TiC can be fed. Without the carrier gas the powder flow depends mainly on gravity. It is successfully used to some surface engineering for metallurgical, automobile, oil and mechanical industries.

In order to apply aluminum alloys to structural components, they should be joined with sufficient strength and quality as high welding speed as possible. High-power laser welding is expected to achieve much higher productivity than conventional joining techniques. Welding of aluminum alloys was performed using 2-kW and 3-kW continuous wave Nd:YAG lasers. Two beams were delivered by optical cables 0.6 mm in diameter and focused on the surface of the specimens as dual spots. Overlap joints of 2-mm-thick sheets were made at various welding parameters, including beam distance, beam arrangement and welding speed. The quality of the bead, including its appearance and macrostructure, and the tensile strength of the joints were investigated. At a shorter beam distance of 0.36 mm, the weld bead surface was humped, making it unacceptable in terms of quality. Sound weld beads were obtained at beam distances of 0.6 mm and 1.0 mm. As the beam distance was increased, the weld depth became shallower. At a beam distance of 1.0 mm, the weld area was too small to provide sufficient strength.

The present paper describes the welding characteristics with a 10 kW class Chemical Oxygen-Iodine Laser (COIL), whose wavelength is 1.32 micrometer. Bead-on-plate welding tests of 304 stainless steel plates were carried out at laser power 8.5 kW and 11 kW. Three different shielding gases (N₂, Ar and He) were used through a coaxial conical shape nozzle under the lens. In COIL welding, the interaction between the laser beam and the laser induced plasma is very small because the wavelength of COIL is shorter than that of CO₂ laser, so that the laser beam reaches on the workpiece without absorption by the plasma. As the result of the welding tests, the welded bead shapes did not depend on a type of the shielding gas. Radiograph and longitudinal section tests of the welded beads were carried out. When He and Ar gases were used as the shielding gas, there were several porosities. On the other hand, the use of N₂ gas made no porosity. The full penetration on 10 mm thick plate was achieved in the high aspect ratio without the welding defects under the condition of laser power 8.5 kW and welding speed 1.5 m/min.

Laser beam welding of aluminum alloys is rendered difficult by their specific material properties. On the other hand the productivity and quality of the laser process is very attractive for the manufacturing of light-weight constructions for e.g. all kinds of vehicles. In many years of research a fundamental understanding of the relevant technological and metallurgical mechanisms has been acquired. This allows to offer guidelines for a successful adaptation of the welding process to specific needs of applications. Factors relevant for deep penetration threshold, process efficiency, process stability and for minimizing metallurgical defects are discussed. Strength values of butt and overlap joints are presented. The results achieved allows us to recommend the laser technique for near-to-production investigations and series application, today.

Aluminum alloys are getting increasingly interesting not only for the classical application in the aircraft industry, but also in rail and road vehicles and the aggregate manufacturing as well as in many other fields of the metal processing industry. The laser is rarely found as a tool for joining aluminum in series production, up to now. Among others, this is due to the fact that only few instructions for the design of an appropriate joining geometry are available in the literature. In addition, neither hints concerning the laser's suitability for industrial application are provided nor essential issues for series production by lasers are to be found, such as e.g. acceptable tolerances regarding the joining geometry. In this paper, exemplary solutions to the problems mentioned above are presented, resulting from the application of a 3 kW fiber-guided Nd:YAG-Laser for tasks in the automotive industry. Taking the example of an overlap joint geometry on the one hand, and the connection of two extrusions forming a 'T-joint-geometry' on the other hand, there will be shown which tolerance fields exist and in which way and up to which extent a gap can be bridged, not only in a gravity position. In addition, results will be presented, demonstrating that the energy coupling as well as the melt pool dynamics can be influenced by varying the wire position or using a second wire in an adequate position. Concerning an overlap joint, gaps of more than the doubled size can be bridged with this technique.

State of the art of welding with carbon-dioxide laser radiation is to weld steel material up to a wall thickness of 20 mm in one pass utilizing beam powers up to 20 kW. Welding material exceeding 20 mm wall thickness requires the application of multiple pass techniques. Since the process behavior deviates from the conventional deep penetration effect measures due to beam handling and shaping have to be taken into account. Within the paper the effects of beam oscillation and focused beam shaping on the process will be discussed. Experimental results concerning the influence of beam characteristics, beam handling, and wire feed on the seam quality will be presented. The experiments were carried out by means of a modern beam source with a nominal output power of 20 kW which offer the opportunity of raw beam shaping by a telescope. The material applied was a fine grain structural steel. The application of multiple pass welding with laser radiation offers great opportunities in industries dealing with materials of high wall thickness.

Titanium aluminide intermetallic compound is studied to find out good welding conditions using CO₂ laser irradiation. In the experiment, we used the casting titanium aluminide containing iron, vanadium and boron with a thickness of 2 mm. We carried out bead-on-plate laser welding at various initial temperatures of specimens varied from room temperature to 873 [K] in inert gas environment filled with argon. We measured fused depth, bead width and Vickers hardness. As a result of experiments, welding speeds that allow full bead-on- plate welding to be possible were strongly by dependent on the initial temperature, 3000 [mm/min], initial temperature 873 [K], 2600 [mm/mm], initial temperature 673 [K], and 2000 [mm/min] with 300 [K]. Transverse crack-free welding was achieved, when initial temperature was at 873 [K].

Significant problems in deep laser spot welding are formation of welding defects, and particularly porosities in the case of tantalum joining. In this study, we investigate and model porosities forming and trapping. Two types of porosity are observed: (1) Small porosities are round shape bubbles of less than 250 micrometer diameter. These defects may come from gas bubbles generated following hydrogen rejection during solidification or they may come from bubbles induced by both an intense evaporation inside the keyhole and a turbulent flow in the molten pool. (2) Large porosities are voids generated in the bottom of the welded zone. They arise from a lack of matter inside the weld as if the molten metal have not had enough time to fill up the keyhole completely before it solidifies. We elaborated a numerical model for the comprehension of little porosities trapping by comparing the calculated spot weld solidification time to the rising time of bubbles in the liquid phase. We also studied the melt flow back into the keyhole in order to explain the formation of the observed large voids. These two models are in good agreement with experimental observations, in particular with X-ray radiographs achieved during the solidification of tantalum spot welds.

The purpose of this study was to develop a technology for preventing the occurrence of Intergranular Stress Corrosion Cracking (IGSCC) by irradiating a high power YAG laser beam onto the sensitized Heat Affected Zone (HAZ) surface of SUS304 stainless steel. By irradiating a laser beam of the appropriate power density, a laser de-sensitization heat treatment (LDT) process was realized that formed both a molten layer of approximately 0.2 mm depth and a solution heat treated layer. The results of a Creviced Bent Beam (CBB) test to evaluate IGSCC showed that no cracks had appeared on the surface of the LDT parts. Also, after LDT was applied at a width of 40 mm in the vicinity of welding joints in the inside surface of pipes (thickness: 8 mm), approximately 250 MPa of tensile stress was measured as a residual stress on this LDT- processed surface. On the other hand, the tensile stress on the outside surface of these pipes decreased to the compression stress.

A steam turbine housing (casing) for power plant is subject to thermal fatigue in the long service. Evaluation of the life time is required for the replacement of turbine housing. In the present work, the possibility of thermal fatigue test by laser to accelerate the thermal damage of the materials (heat resisting Cr-Mo steel) and estimate the life time of casing in short time has been investigated using a pulse YAG laser. The test specimen are taken from the turbine housing which have been used for 100,000 hours in service. The pulse YAG laser of 100 pps was irradiated on the specimen with different beam spot sizes for one sec. and interrupted for 9 sec. as a thermal fatigue cycle. Max. cycle in this laser thermal fatigue tests was 5400 cycles. The peak temperature of theram cycle was about 220 degrees Celsius after 5400 cycles in this laser thermal fatigue test. The fatigue crack was observed at the root of circular groove after 5400 cycles.

This paper deals with the opto-mechanical configuration of a newly developed high-speed initializing system for phase change recording media such as CD-RW, DVD-RAM, and DVD-RW. The mechanical layout of the system that ensured the mechanical alignment of each diode laser to be field replaceable. With the unique optical configuration implemented, up to six field- replaceable high-power diode lasers of wavelength 840 nm can be simultaneously installed onto this newly developed system. When all six diode lasers were installed, the internal anamorphic optical system can project the near-field intensity distribution of the high power diode lasers into a large 180 micrometer by 3 micrometer spot size while maintaining enough initialization laser power and providing large depth of focus. In this operational configuration, the system is capable of converting the standard outer diameter of 120 mm phase change optical disk from its originally sputtered amorphous state to initialized crystalline state in less than 20 seconds, which is approximate 2 to 3 times faster than today's commercially available system.

The modifications induced by an excimer laser irradiation at 193 nm or 248 nm on organic surfaces, below their ablation threshold, were investigated for different kinds of polymers such as: PEEK (Polyether Etherketone), PC (Polycarbonate), PET (Polyethylene terephtalate). Treatments were carried under argon or air at different laser fluences. Treated surfaces were characterized by XPS (X-rays Photoelectron Spectroscopy) and surface wettability measurements. For, all the studied polymers, the results shows that the surface modifications first depended on the laser wavelength. Surface oxidation occurred at 193 nm, leading to the formation of polar groups (carbonyls, carboxyls, hydroxyls) and inducing an increase of the surface energy. Treatments at 248 nm never induced any oxygen enrichment of the surface. This is due to the loss of oxygen by CO or CO₂ desorption at this wavelength.

TFT flat panel displays are rapidly increasing their share in the display market. Polycrystalline-silicon TFT technology is opening the door to highly reliable, high-resolution, high performance and large size Active Matrix Liquid Crystal Displays (AMLCD). For the formation of polycrystalline silicon, excimer laser annealing has shown itself to be superior to all other techniques as far as quality, reliability and economy is concerned. The pronounced non- linearity of the annealing process, the high quality requirements and the high process speeds in the production lines put high demands on the laser beam parameters such as energy stability, beam uniformity and laser output power. In addition, the industrial TFT annealing laser has to be extremely user friendly, reliable, easy to maintain and economical. The remarkable progress in excimer laser technology over the last few years has resulted in a new generation of annealing lasers. Furthermore, in industrial polycrystalline TFT annealing, the beam of the excimer laser has to be modified to achieve high quality and throughput. Consequently, a dedicated Line Beam Optics system has been developed by MICROLAS (Germany). Finally, 'The Japan Steel Works' (JSW, Yokohama, Japan) has developed a highly sophisticated handling system for the production process to form a complete system for optimum shop floor TFT panel fabrication.

Production of laser beam welded tailored blanks requires both, high quality processing as well as a quality assurance by reliable monitoring systems for each welded part. Actual quality monitoring systems for tailored blank applications make use of different sensors for seam tracking, seam shape detection and process control. The suitable process diagnostic device is a plasma sensor which detects the optical emission of the weld plasma. Experimental evidence show that the reliability of the seam quality prediction can significantly be improved by using a camera system with a two-dimensional spatial resolution instead of an integrating plasma detector. The improvement is achieved by exploiting the information provided by the spatially distributed intensity of the plasma emission. In particular, by coaxial arrangement of the camera with respect to the laser beam axis, the direction of observation allows to detect significant process characteristics. Based upon these results a coaxial process control system was developed that can be adapted for different laser materials processing applications like welding, cutting and surface treatment. The system consists of a high speed camera mounted directly at the welding head. The optical path of the camera goes coaxial with the laser beam path through the focusing optics. The camera images taken from the process are analyzed using image processing algorithms. The algorithms are chosen depending on the type of application to be monitored. In the case of welding tailored blanks the system can monitor a full penetration of the workpiece, deviations from the desired welding path, seam width, stability of the capillary shape and defects of the seam caused by spatter and ejection of molten material. The camera system offers the ability to perform simultaneously different quality monitoring tasks like determination of seam and capillary shape, seam tracking and process control. Thus the number of sensors required for quality monitoring is reduced to one single system.

In order to enhance the performance of our 5-axis laser robot machine to the full extent, we installed an adaptive measuring system with protection on the ending arm of the machine. This brings at least three virtues: (1) Achieving measure functions by employing the same numerical-controlled mechanical structure without extra cost; (2) Increasing the capacity, efficiency, and accuracy of the raw measured data into machining format [computer aided design (CAD)/computer aided machining (CAM)/computer aided processing (CAP) process]; and (3) Realizing 3-dimensional (3-D) intelligent measurement for large size workpiece (up to 3 X 2 X 1 cubic meters) with given measurement accuracy (to 12 micrometer). Discrete 3-D signal point data are acquired and processed automatically by a personal computer, and the program employing network predicting concept has adaptive function with which the single- or multi-step measuring is determined by the surrounding points. By means of intelligent measurement a fast and effective measurement can be carried out for even a complex surface.

We analysis the advantages of CO₂ laser in remote sensing than other detection system. Some applications of multiple- electrode-pair TEA CO₂ laser to remote sensing are presented, such as target identification, crosswind sensing, objective velocity test, and so on. Multiple-electrode-pair TEA CO₂ laser can generate high-intensity laser pulse series with adjustable time interval by using several sets of electrodes sharing the same optical cavity and firing the sets sequentially. In this paper, we put emphasis on describing the target reflection polarization characteristic and target identification system. Some relevant formulas and curves are presented. These works are of great moment to the development of remote sensing and multiple-electrode-pair TEA CO₂ laser in theory and practice.

The diffractive optical element (DOE) is a revolutionary technology for sophisticated optical systems. It has recently been launched within the optical industry, which is constantly seeking improvements over conventional optics. We have designed and fabricated three types of binary-phase DOE and a 4-level phase DOE for spot array generation. The surface relief of a ZnSe substrate was patterned and etched with each intended phase distribution by using photolithography and reactive ion etching (RIE) technologies. The optical properties of anti-reflection coated samples were then examined by measuring the intensity distribution of the converging beams and the results compared with the calculated beam propagation.

Material processing using CO₂ laser is one of the most highly industrialized laser technologies. Recently, there have been growing demands to improve the processing quality by flexibly controlling the beam properties. Development of the adaptive mirror with variable curvature function, which is one of the vital optics to realization of these demands, has been extensively pursued. We developed the AM. It maintains the same accuracy of surface figure (PV value: 1 micrometer or below) as the conventional mirrors even when the curvature is changed, and is capable of high speed curvature control (maximum frequency: 1 kHz) aiming at application to beam sawing. Further, we applied the AM to laser cutting of mild steel as an example of its practical use, and studied smoothing of the cut surface by beam sawing.

We have been engaged in research and development concerning high power Nd:YAG laser equipment and overall application technology for welding, cutting and drilling. Especially, development of the technology and the system are required for to establish stable welding process. Higher the laser power used, the more laser beam interacted with material, leading to increased vapor, plume and spatter ejection from molten metal. They contaminate and damage the optical systems that are constructed by lens and cover glass plate. In general, in order to protect the optical system, shielding gas flow rate is controlled. But if the gas flow rate exceeds the proper value, molten metal does not protect from oxidation. Therefore we developed a new type co-axial nozzle device. We welded various material (mild steel, stainless steel and aluminum alloy) using new type nozzle and 4 kW YAG laser (MW4000). As the results of experiment, it was cleared that we can weld, within the speed range from 25 mm/min to 2 m/min, stably and easily.

The laser, which was invented in 1960, has been developed using various substances of solids, liquids, gases and semiconductors as laser active media. Applications of laser utilizing the coherent properties of laser light and the high power density light abound in many industries and in heavy industries respectively. The full-scale use of lasers in the steel industry began nearly 23 years ago with their applications as controllable light sources. Its contribution to the increase in efficiency and quality of the steel making process has been important and brought us the saving of the energy, the resource and the labor. Laser applications in the steel making process generally require high input energy, so it is essential to consider the interaction between the laser beam and the irradiated material. In particular, the reflectivity of the laser beam on the surface of material and the quantity of the laser-induced plasma are critical parameters for high efficient processes with low energy losses. We have developed plenty of new laser systems for the steel making process with their considerations in mind. A review of the following high-power-laser applications is given in the present paper: (1) Use of plasma as a secondary heat source in CO₂ laser welding for connecting steel sheets of various grades. (2) Laser-assisted electric resistance welding of pipes. (3) New type all-laser-welded honeycomb panels for high-speed transport. (4) Laser flying welder for continuous hot rolling mill using two 45 kW CO₂ lasers.

The authors have put the YAG laser of the kW class to practical use for repair welding of nuclear power plant steam generator heat exchanger tubes, all-position welding of pipings, etc. This paper describes following developed methods and systems of high power YAG laser processing. First, we apply the 6 kW to 10 kW YAG lasers for welding and cutting in heavy components. The beam guide systems we have used are optical fibers which core diameter is 0.6 mm to 0.8 mm and its length is 200 m as standard one. Using these system, we can get the 1 pass penetration of 15 mm to 20 mm and multi pass welding for more thick plates. Cutting of 100 mm thickness plate data also described for dismantling of nuclear power plants. In these systems we carried out the in-process monitoring by using CCD camera image processing and monitoring fiber which placed coaxial to the YAG optical lens system. In- process monitoring by the monitoring fiber, we measured the light intensity from welding area. Further, we have developed new hybrid welding with the TIG electrode at the center of lens for high power. The hybrid welding with TIG-YAG system aims lightening of welding groove allowances and welding of high quality. Through these techniques we have applied 7 kW class YAG laser for welding in the components of nuclear power plants.

Industrial applications of highly repetitive laser may cause precise exposure problems. This paper reports findings of an experimental study on integral gain accretion during repetitive excitation of distributed feedback dye lasers. To estimate the effect 10 to 20 pulses of second harmonic of a passively Q-Switched and mode-locked Nd:YAG laser were used to excite a distributed feedback dye laser (DFDL). The signals of DFDL and Nd:YAG laser were recorded by Imacon 675-streak camera with no relative delay. It was found that the peak of DFDL output envelope of pulses was delayed from peak of the excitation Nd:YAG envelope of pulses by more than one inter- pulse period (2L/C) of excitation laser. Various types of cases such as different excitation energies and inter-pulse time periods were studied and an intensity-based model was developed. Time delay between the peaks of pulse envelopes of Nd:YAG and DFDL was found to depend upon the inter-pulse period (2L/C) of the excitation laser. A computer program was used to simulate the experimentally measured delay to estimate thermal decay constants and energy retained by the medium. It was found that for smaller inter-pulse periods the effect of gradual gain build-up becomes very significant to affect some of the sensitive applications in welding and communication. This effect was used to measure thermal diffusion time constant of dye solutions.

Laser beams have been noticed as new heat resources with high energy concentration, which are different from plasma and arc. Conventionally, the only kW class industrial laser has been a carbon dioxide (CO2) laser. However, recently, several new high power lasers other than CO2 laser have been developed so that new methods of laser material processing have come out. As for YAG lasers, formerly, cw or pulse YAG lasers of several hundreds W class were used for welding or cutting of electrical appliants or cutting of thin metal plates. Now, the power has been raised to 5 - 6 kW, which enables YAG lasers to apply wider applications of material processing in many industrial fields, such as automobile industries, heavy industries and so on. It is a flexible fiber delivery that is the most remarkable advantage of YAG laser, which can be applied to ordinary machinery tools and robotic systems and makes it possible to deliver laser power to remote locations. Moreover, a shorter wavelength (1.06 micrometer) of YAG lasers than that of CO2 lasers is appropriate to metal processing. Figure 1 shows an example of YAG laser processing system utilizing those advantages. Also in IHI, the processing with YAG lasers has been studied for their practical application which has already succeeded in some sections such as cladding, repair welding and subdividing of nuclear power plants making use of YAG lasers' properties of fiber delivery of beam. Moreover, underwater processing technique is studied for practical use. In this paper, the examples of YAG laser application technology were described.

High power lamp pumped YAG lasers of 6 kW and 10 kW average output power with good beam quality have been developed by using MOPA arrangement with multi-pump cavities for oscillator and amplifiers. For 6 kW laser system, efficient pump cavity for lessening thermally induced beam distortion effects has been designed, which realized output power greater than 700 W and M² value less than 70. CW 3.5 kW has been realized from 5 cavity laser oscillator and CW 6.7 kW output power has been obtained by adding 5 cavity amplifier to the oscillator. During development, much amount of power of scattered and deformed light surrounding a main laser beam was observed for increased number of amplifiers. By applying spatial apertures between amplifiers, laser output beam quality has been improved to as good as M² of 73 [beam parameter product (2 (omega) *2 (theta) ) :100 mm (DOT) mrad], enough for fiber (NA 0.15, (phi) 0.6 mm) delivery. Furthermore, 14-rod cavities, 10 kW Nd:YAG laser system, which can generate world-highest output power of 10 kW or more as a solid state laser, have been developed for thick plate processing with 137 mm (DOT) mrad (M² approximately 101) beam parameter product. This laser system has achieved the transmission to 9 kW or more for (phi) 800 micrometer/NA 0.2 fiber.

Scaling laws to determine the physical dimensions of the active medium and optical resonator parameters for designing convective cooled CO₂ lasers have been established. High power CW CO₂ lasers upto 5 kW output power and a high repetition rate TEA CO₂ laser of 500 Hz and 500 W average power incorporated with a novel scheme for uniform UV pre- ionization have been developed for material processing applications. Technical viability of laser processing of several engineering components, for example laser surface hardening of fine teeth of files, laser welding of martensitic steel shroud and titanium alloy under-strap of turbine, laser cladding of Ni super-alloy with stellite for refurbishing turbine blades were established using these lasers. Laser alloying of pre-placed SiC coating on different types of aluminum alloy, commercially pure titanium and Ti-6Al-4V alloy, and laser curing of thermosetting powder coating have been also studied. Development of these lasers and results of some of the processing studies are briefly presented here.

In the frame of a large project on new materials technologies for photovoltaic and microelectronic applications (FOTO), the process of amorphous silicon (a-Si) transformation into polycrystalline silicon (poly-Si) by means of laser irradiation has been tested with a long-pulse (160 ns), 8 J/p XeCl source. Following the positive results, a laser source, having design parameters of 10 J/p, 120 ns, 10 Hz, has been designed and built, with the aim of realizing a laboratory line for the production of thin film transistors (TFTs) devices.

The history of materials processing teaches that each time a new processing technology becomes available, science and technology both can take a step forward. The new generation of free electron lasers now coming on line offers such an opportunity. Their high average power enables the use of fs to ps, high peak power pulses for materials processing as well as fundamental studies, with the further advantage of wavelength tunability. Taking together what has been learned with low average power fs-ps pulses, an assessment of materials processing needs, and the emerging capabilities of the new FEL's suggests what are the most promising near-term opportunities for these facilities. Progress with corresponding experiments and how than can be carried into manufacturing are described.

We have previously shown that the material removal rate scales linearly with pulse rate up to 15 kHz for pulsed UV-laser ablation of polymers, giving the potential for substantial gains in processing speeds in ablative micromachining using high-pulse-rate UV sources such as frequency-doubled copper vapor lasers and frequency-quadrupled diode-pumped solid-state lasers. These rapid processing speeds can be effectively utilized in direct-write UV-laser micromachining including trepanning. In this paper we present studies of machining rates for trepanning of a strongly absorbing polymer (PETG), and a weakly absorbing polymer (PMMA), aimed at establishing optimum conditions of pulse rate, linear write speed (laser spot overlap) and laser fluence for maximum machining rates and high quality of the machined structure using a high-pulse- rate (5 kHz) UV-CVL. For fixed fluence and pulse rate, machining rates for PETG are found to be independent of write speed in trepanning, however for PMMA machining rates increase for decreasing write speed (increasing laser spot overlap) where cumulative heating leads to enhanced dynamic etch rates. In the latter case, while reduced machining times can be achieved for high spot overlaps, this is generally at the expense of significant degradation in finish quality of the machined structure.

High-speed etching of polyimide films used as insulators for multi-layered printed circuit boards has been investigated using a Q-switched CO₂ laser tuned at 9.3 micrometer. A mechanical chopper inserted in the cavity realizes the Q- switching operation. The pulse repetition rate of the laser is as high as several tens of kHz, so that continuous processing of certain width is possible without beam aiming to each hole. The laser beam is scanned by a galvano-mirror and then focused to the work by a telecentric lens. Metal layer on the top of the printed-circuit board is used as contact mask, and it works as a multi-reflector in conjunction with a reflector placed above. Multi-reflection increases the processing speed by a factor of 2.5. Overall processing speed is 1.5 m/min for 0.1 m width or 0.15 m²/min.

High precision laser direct writing of two types of micro- optical structure is demonstrated using a workstation built around a commercial CO₂ slab waveguide laser. Drift and fluctuations in beam pointing, beam shape and pulse energy are quantified, providing an understanding of the precision capabilities. Quartz glass ablation by waveguide laser pulses of width in the 20 to 60 microsecond range has been characterized and a refractive microlens machined, involving material removal to a depth of 70 micrometer. Surface structures in aluminum with 143 micrometer lattice spacing have been produced to act as two dimensional photonic bandgap devices, modifying the transmission properties of hollow planar waveguides in the mid-infrared.

One large advantage of Nd:YAG laser beam is easy beam-delivery with an optical fiber. By using this advantage, application area of Nd:YAG laser beam spreads widely. One area is the outside of workshops, for example, a construction spot where people cannot approach, a disaster spot and an operation under water. Therefore, the goal of this study is development of cutting technology of thick steel with fiber-delivered high power Nd:YAG laser beam of 3.8 kW or more in output power. Used materials were mild steel (SS400) and stainless steel (SUS304) of 50 mm in thickness. Oxygen gas, nitrogen gas and dry air were used as an assist gas. Obtained results were summarized as follows. (1) When oxygen gas was used as the assist gas, the kerf width at lower part of the plate was wider than one at higher part because of self-burning. However, the kerf width was almost uniform at both of the upper and the lower parts of the plate in case of dry air assist. (2) The kerf width decreased rapidly with increasing the cutting speed. (3) Maximum cutting speed decreased with increasing the plate thickness and the speed in case of using oxygen gas assist was higher than one using dry air or nitrogen gas assist. (4) When oxygen gas was used as assist gas, the maximum cutting speeds in mild steel and stainless steel of 50 mm thick plate were 200 mm/min and 50 mm/min respectively. We are now trying to cut thicker plate with higher output power of Nd:YAG laser beam.

Characterization of a supersonic impinging jet in a laser cut kerf was conducted, with a Schlieren method, a total pressure measurement, and a CFD (Computational Fluid Dynamics) analysis to investigate gas dynamic effects of the jet in cutting processes. From the measurements, a flow field extension along a kerf direction inside the kerf, with higher feed pressure, smaller nozzle-workpiece distance, smaller kerf width, or lager nozzle diameter, was corresponding to the extension of the total pressure distribution at the kerf bottom. Also, with higher feed pressure, smaller nozzle-workpiece distance, wider kerf width, or lager nozzle diameter, positions of flow separation lines at both kerf side walls and a cutting front become deeper, and all values of total pressure distributions were increased. From the CFD results, it was shown that a nozzle jet was abruptly compressed forming a normal shock above the workpiece surface, and behind the shock the flow was expanded and accelerated downward and to a kerf direction. A flow separation and an associated recirculation zone occurring at the bottom of a cutting front were found. Imbalanced flow separations and recirculation zones at both side walls, and flickers of the flow inside the kerf were also observed. From the cutting experiments, it was found that better cutting performances were achievable with the conditions in which wider and deeper penetration of the flow field inside the kerf was obtainable.

This paper is a continuation of my work in describing the applications to which the Reverse Thermal Wave Transform may be applied. As the title implies, this paper addresses the application of the Reverse Thermal Wave Transform to Transmissive optical components. A number of researchers have asked about the time temperature history of imposed laser beams in transmissive crystals. Very often the crystals are being used to shift the wavelength of the laser beam to a shorter wavelength. The problems that are described in this paper are quite generic to any transmissive material problem. The equations are not limited to lasers only, however. The same equations and concepts apply when used with Synchrotron and X-Ray radiation sources. Not to put too fine a point on it, the equations are also applicable to those problems that are associated with Microwave radiation sources as well. In this lecture we will also point out some additional equations and concepts which have not been described before relative to transient heat transfer in flat plates. Heretofore, when using the equations shown by Carslaw and Jaeger the expectation was that the temperature shown in the illuminated area was, in fact, the average temperature throughout the irradiated area. As I show in this paper this is only true for the case where the irradiating source flux density is spread over a radius that is equal to, or greater than, R_o equal to or greater than 6 (root)(alpha) (tau) . When the radius is R_o less than 6 (root)(alpha) (tau) there is a temperature gradient from the center of the component out to the edge of the laser beam, X- Ray beam, et al. There is, then, the gradient from the edge of the laser beam out to the edge of the component. These are two different gradients that are computed very differently. Since Carslaw and Jaeger were only dealing with the one dimensional semi-infinite plate models, this detail failed to materialize in their equations. We will use a set of examples complete with nodal maps and graphical representations to describe how the temperature gradient results from the Reverse Thermal Wave Transform.

Optimization and adaptation of 15 - 200 micrometer small dia and 2 - 20 mm deep holes drilling with the hole shape variation less than plus or minus 10% are discussed. For this purpose, the single-frequency laser system with self-phase- conjugation and passive Q-switching by a scanned gradiently colored LiF:F₂^- crystal is used. Steels, hard- facing and aluminum alloys, and ceramics are used as samples.

The laser cutting of color metals and alloys by a thickness more than 2 mm has significant difficulties due to high reflective ability and large thermal conduction. We made it possible to raise energy efficiency and quality of laser cutting by using a laser processing system (LPS) consisting both of the YAG:Nd laser with passive Q-switching on base of LiF:F₂^- crystals and the CO₂ laser. A distinctive feature of the LPS is that the radiation of different lasers incorporated in a coaxial beam has simultaneously high level of peak power (more than 400 kW in a TEM₀₀ mode) and significant level of average power (up to 800 W in a TEM₀₁ mode of the CO₂ laser). The application of combined radiation for cutting of an aluminum alloy of D16 type made it possible to decrease the cutting energy threshold in 1.7 times, to increase depth of treatment from 2 up to 4 mm, and velocity from 0.015 up to 0.7 m/min, and also to eliminate application of absorptive coatings. At cutting of steels the velocity of treatment was doubled, and also an oxygen flow was eliminated from the technological process and replaced by the air. The obtained raise of energy efficiency and quality of cutting is explained by an essential size reducing of a formed penetration channel and by the shifting of a thermal cutting mode from melting to evaporation. The evaluation of interaction efficiency of a combined radiation was produced on the basis of non-stationary thermal-hydrodynamic model of a heating source moving as in the cutting direction, and also into the depth of material.

Laser equipment for the perforation of documents and securities is presented. This laser perforator (LP) differs by extended precision of perforation, high processing velocity, perfected automatic control. LP's operation is based on the preliminary theoretical and experimental research of laser irradiation and paper or/and organic tissue interaction. The results of CO₂-laser irradiation action upon different materials and samples of documents allowed to determine system requirements to LP. Developed LP is destined for perforation of paper documents with jackets with total thickness from 0.5 to 4 mm. Processing document, LP makes more than 100 conical perforation holes that improve protection rate of document. LP guarantees perforation time less than 3 sec, document's blank positioning precision plus or minus 0.2 mm, laser beam positioning precision plus or minus 0.01 mm. Due to the system parameters optimization it became possible to eliminate a singeing of hole edge, that improved perforation quality. Developed LP consists of laser-module, technological module, laser cooling module and automatic control system. Laser module includes continuous Q-switched CO₂-laser, scanner, power supply, controller, chopper. Technological module has X- Y-table, conveyer for blanks of documents, pneumatic block. Automatic control system, which includes two video cameras, illuminators, controller, PC, gives a possibility to control holes disposition in a matrix and to identify perforated number.

In the International Thermonuclear Experimental Reactor (ITER), the blanket is categorized into the schedule maintenance component and has to be replaced by remote handling technology due to activation by the 14-MeV neutrons during DT operations. The blanket is segmented into a number of modules to facilitate remote operation and requires the welding and cutting of cooling pipes connected to each module from the inside of the pipe due to space constraints. A prototype tool fabricated for branch pipe welding/cutting demonstrates the required mobility for traveling the cooling pipe with a diameter of 102.3 mm and a bent radius of 400 mm, and for accessing to the branch pipe with a diameter of 54.5 mm. The welding and cutting performance has been also tested, including the dependency of laser power, processing speed, and gaps on weldability. In addition, a composite optical fiber composed of a number of thin fibers arranged around a core fiber is also tested for direct viewing of the edge preparation before welding and for monitoring during welding/cutting. This paper describes the test results of the prototype tool performance and of in-pipe access welding and cutting operations.

A keyhole enhances the absorption coefficient greatly and helps the laser energy to penetrate deeper into the material in laser welding on aluminum alloys. We set up a simple model to estimate the threshold conditions to form a keyhole and analyzed the effects of various parameters on laser welding theoretically and experimentally. By the measurement of the spectral signals the laser welding state can be monitored during processing.

By means of a transverse flow 5 kW CO₂ laser with low- order mode laser beam output, 1 - 4 mm thick aluminum alloy plates 6063 and LY12 were successfully butt welded. The result shows that the butt weldability and the weld quality of the aluminum alloy plates are mainly dependent on incident laser power density, laser beam defocused distance and shielding gas. The relationship between the weld quality of the aluminum alloy plates and the welding parameters is discussed. The macrostructure and microstructure of the welded seams are analyzed. The mechanical properties of the welded seams are discussed.

Metallic alloys with a low mass density can be considered to be basic materials in aeronautic and automotive industry. Magnesium alloys have better properties than aluminum alloys in respect of their low density and high resistance to traction. The main problems of magnesium alloy welding are the inflammability, the crack formation and the appearance of porosity during the solidification. The laser tool is efficient to overcome the difficulties of manufacturing by conventional processing. Besides, the laser processing mainly using shielding gases allows an effective protection of the metal against the action of oxygen and a small heat affected zone. In this paper, we present experimental results about 5 kW CO₂ laser welding of 4 mm-thick magnesium alloy plates provided by Eurocopter France. The focused laser beam has about 0.15 mm of diameter. We have investigated the following sample: WE43, alloy recommended in aeronautic and space applications, is constituted with Mg, Y, Zr, rare earth. More ductile, it can be used at high temperatures until 250 degrees Celsius for times longer than 5000 hours without effects on its mechanical properties. A sample of RZ5 (French Norm: GZ4TR, United States Norm ZE41) is composed of Mg, Zn, Zr, La, rare earth. This alloy has excellent properties of foundry and it allows to the realization of components with complex form. Also, it has a good resistance and important properties of tightness. The parameters of the process were optimized in the following fields: laser power: 2 to 5 kW, welding speed: 1 to 4.5 m/min, focal position: -3 mm to +3 mm below or on the top of the metal surface, shielding gas: helium with a flow of 10 to 60 l/min at 4 bars. Metallurgical analyses and mechanical control are made (macroscopic structure, microscopic structure, interpretations of the structures and localization of possible defects, analyse phases, chemical composition, hardness, tensile test etc.) to understand the parameters influence of welding on the obtained beads. For a given laser power, we considered that the welding speed as well as the focal position strongly influence the macroscopic and microscopic welding aspect, whereas the dependence with the flow of the protection gas is weak. For WE43, the bead appears correct in the macroscopic scale for a laser power of 2 kW, a speed of 2 m/min, a focal position on the metal surface or 1 mm under; and an output helium gas of 50 l/min. For RZ5, a correct weld is obtained with a 3 kW laser power, a welding speed of 2 m/min, a focal position of 1.5 mm under the surface and a 50 l/min output helium gas. The microscopic examination showed that the size of the grains has clearly reduced (reduction factor can be up to 35) without formation of porosities, neither cracks nor inclusions; indeed the measured Vickers microhardness of the weld bead is slightly higher than the basic metal. Experiments show that we obtained adequate parameters for high quality welding without using filler material. In future, we plan to weld at higher speed by optimizing the various parameters of the laser welding (power, focal position welding speed and gas flow, ...). Furthermore, we will try to weld samples with a thickness superior than 4 mm.

This paper deals with amorphous structures in the laser cladding. A kind of Ni-Cr-Al alloy was sprayed on the substrate, which was ZL111 alloy, to be the coating material. The coating was clad by 5 kW transverse flow CO₂ laser. The observation of SEM and TEM revealed that in the laser cladding there were amorphous structures which appeared two different morphologies: one was space curved flake-like, which existed in the white web-like structures; the other was fir leaf-like, which existed in the grain-like structures. Differential thermal analysis (DTA) was used to semi- quantitatively determine the content of the amorphous structures. A curve relation was obtained between the content of amorphous structures and the dimensionless laser cladding parameter C. The changes of the amorphous structures after annealing were also shown.

KrF excimer laser has been used for physical and chemical transformations of metallic materials and coated metal samples. Aluminum alloys, steels and chromium coated mild steel have been treated under excimer laser radiation in order to improve their mechanical properties and their corrosion and oxidation resistance. The laser surface treatment leads, after surface remelting process, to important changes in the topography, the microstructure, the phases and the chemical composition of the near-surface region resulting in different hardness, wear properties and corrosion/oxidation behavior. We focus this paper on aluminum alloys (2000 and 6000 type) and steels irradiated using a krypton fluoride laser (20 ns, 0.5 - 10 J/cm2, up to 200 Hz) in laboratory air. The analysis were carried out by means of scanning electron microscopy, energy dispersive spectroscopy, coupled and low incidence angle X-ray diffraction, microhardness tester and electrochemical test equipments.

An optical monitoring system for the welding process has been developed; it is based on the study of the optical emission of the welding plasma plume, created during the welding of stainless steels and other iron-based materials. In the first approach a continuous wave CO₂ laser of 2500-Watt maximum power, available at the INFM Research Unit labs in Bari University, has been used as welding source. A detailed spectroscopic study of the visible and UV welding plasma emission has been carried out; many transition lines corresponding to the elements composing the material to be welded have been found. By means of an appropriate selection of these lines and suitable algorithms, the electronic temperature of the plasma plume has been calculated and its evolution recorded as a function of several welding parameters. The behavior of the registered signal has resulted to be correlated to the welded joint quality. These findings have allowed to design and assemble a portable, non-intrusive and real-time welding quality optical sensor which has been successfully tested for laser welding of metals in different geometrical configurations; it has been capable of detecting a wide range of weld defects normally occurring during industrial laser metal-working. This sensor has also been tested in arc welding industrial processes (TIG) with promising results.

High performance pulsed AlGaAs/GaAs wide stripe diode laser has been developed for the automotive distance-measuring scanning radar sensor. The laser diode is required high output power of 15 W and a long time reliability in spite of being used in a harsh environment such as wide temperature range, mechanical vibrations at the front bumper and so on. The device is designed by employing a multiple quantum well structure as an active layer for high output power with low drive current and high temperature operations. Moreover we reduce catastrophic optical damage power level and control the beam divergence angle by introducing optimized optical waveguide layers. In the chips bonding part, we developed a new thin film Au-Sn-Ni solder system. The bonding temperature can be lowered by using this system, whereby the thermal damage to the laser diode can be reduced. Furthermore, highly stable bonding is carried out by improving wetting ability in this system. We have achieved more than 22 W light output power at 20A pulse current under room temperature and more than 16 W light output power under 90 degrees Celsius. High reliability over 10,000 hours is performed for automotive use under pulsed operation at 90 degrees Celsius, 50 ns pulse width, 8 kHz frequency and 15 W light output power.

A single—mode radiation of the Nd:YAG laser with optically connected cavities has been received. An active Q—switcher based on a Fabry—Perot interferometer and passive Q—switcher on a LiF:F2 crystal were simultaneously used. The amplitude of radiation pulses increased 1.5—fold and radiation coherence length was more than 0.7 m at the optimal condition of interferometer controlling.

A simple co-axially excited pulsed CO₂ laser has been developed for the application of micro-machining. The laser tube is made by a ceramic pipe with metal electrodes at both ends, and the laser tube works as a switch. The output energy of 1 mJ to 40 mJ were used for the micro-machining such as micro-drilling of Pori-Imid circuit board and marking on the glass substrate.

In this paper, we propose two experimental methods to understand keyhole formation observed during pulse Nd:YAG laser welding. The first one gives us the possibility to observe keyhole formation using flash X-ray radiography. The X flash is delay triggered from laser pulse beginning and so, it is possible to make X-ray radiographs at different times from the beginning of the interaction. The material tested is TA6V (Titanium alloy). The first results show that a keyhole appears less than one millisecond after the beginning of the interaction (laser pulse duration is 5 ms) and grows until the end of laser pulse. The keyhole has a conical shape. The second one allows us to determine laser-matter interaction efficiency. It is based on microcalorimetry and different materials behavior under laser irradiation are studied. With this method, we measure the incident laser beam energy and the part of energy absorbed by material. The results obtained on TA6V show that laser-matter efficiency vary from 32% to 80%. Laser-matter efficiency is lower in conduction mode (typically for peak power 0.5 kW) and grows with peak power, and therefore when the keyhole volume increases.

Research work was aimed at the welding of thin sheets using high power CO₂ laser. Process parameters, evaluation of the weld by micro, macro and mechanical are briefly given. Characteristics of the laser welding are described. Optimal proces parameters which are power, weld speed, gas flow, focal point, gap distance, were used. Microstructural evaluation by light microscope and transmission electron (TEM) microscope for substructural analysis was employed. Therefore general weld imperfections were observed and laser weld evaluation was made with EN ISO 13919-1. Mechanical performance of welded sheets was done by uniaxial tensile test, Erichsen test, and microhardness test. Uniaxial tensile test was employed transverse-weld oriented to the tensile direction and longitudinal-weld oriented to the tensile direction. Results were compared with base metal properties. Maximum tensile strength was obtained from the longitudinal-weld with reduced ductility. In transverse-weld direction fracture was far from the weld. Microhardness test was applied to the cross section of the welded sheets. Maximum hardness was obtained from the weld fusion zone (FZ) where hardness was increasing from HAZ to weld FZ center. Therefore hardness results were verified by empirical equations, which are proposed by various authors. Erichsen test was employed for the ductility evaluations of the welded sheets whereby two types of defect were observed from the Erichsen test. The First one was observed in the weakest sheet (lowest gauge or lowest strength). It occurs when the major strain direction is perpendicular to the weld seam. The second one occurred across the weld by the higher strength and lower elongation of the weld while major strain direction was parallel to the weld seam. Process parameters, microstructural and substructural analyses were compared with mechanical performance of the welded sheets. As a result the laser welded thin sheets were evaluated in many aspects.

High Power Diode Lasers with a power from a few ten watts up to several kilowatts are entering the laser materials processing area since a few years now. Because of their construction, based on a high quantity of single semiconductor laser components, which provide a rather low power and which are coupled together by special optical elements, they can provide a high power, but a rather poor beam quality, compared with conventional lasers. Therefore, they are attractive for those applications, where moderate beam quality is acceptable or even advantageous for the process. In this paper we will explain the technology of the high power diode lasers as well as give examples for materials processing applications, which are partially even already introduced in industrial manufacturing.

Super high power laser systems have been pursued for the nuclear fusion research in these 30 years. GEKKO XII glass laser, LEKKO VIII CO₂ laser, Diode pumped solid state laser and PW cpa Nd glass laser are developed at Osaka University. They have proved to be very effective to the laser fusion research and also to the various applications in the wide fields of science and technology.

I treasure the analogy between aging and climbing a mountain. Of course there are differences, mountain climbing is usually voluntary. As you climb a mountain you lose the ground detail but on a clear day broad views of the landscape are visible. The view from the mountain top is sometimes helpful to those working on the ground. I hope that the broad view of the technological landscape I present today will be of some use to those of you still lucky enough to be tilling that landscape. I will refer primarily to the U.S. scene but I believe that the technological landscape is as universal has human nature. To pursue a vision, to survive the trials and tribulations of innovation requires both individual courage and an encouraging environment. When hopes for living in a better world overshadow fears of departing from time tested practice, innovation thrives. I want to talk today about the enthusiasm for technological progress today and compare it with the situation 30 or 40 years ago. Then most of us looked forward eagerly to advances in technology and to the expansions of humanity's horizons these advances would make possible.

The features of modern, high-power semiconductor diode laser arrays as sources for pumping high power solid state lasers are reviewed. The status and prospects for high power, high- beam quality Nd:YAG and Yb:YAG DPSSLs are examined. Developing concepts for novel high power DPSSLs are also outlined.