In general, an electrical to optical power conversion efficiency for a high power LD ranges from 40 to 50%. Therefore, 50 to 60% of the input electricity is transformed into heat. The generated heat degrades the LD's optical characteristics and their lifetimes dramatically. We must not only suppress the heat generation but also remove the heat effectively from the LDs. Several years ago we proposed "FUNRYU", a water cooled heat sink. The sink has great advantages, due to its simpler structure than a micro channel cooler, in terms of cost, productivity, and water pressure.

The application field of high power semiconductor lasers is growing rapidly and covers e.g. solid state laser pumping, metal and plastic welding, hard and soft soldering, suface treatment and others. Preferably those applications are attractive, which do not require extremely high beam quality. We have investigated high power diode-laser bars from 808 nm to 980 nm. The scope of this presentation is on focusability and beam quality. For better beam shaping structures with reduced fill factor of 25% to 30% were developed. They were operated in continuous wave operation at power levels of up to 55 W. Tests indicate extrapolated lifetimes of more than 100,000 hours at 40 W at 980 nm cw and about 10,000 hours at 45 W - 50 W at 940 nm and 808 nm. Monolithically stacked Nonostacks^R were investigated. Operation up to 100°C with excellent lifetimes could be demonstrated. New concepts and applications for low mode number high power diode lasers like tapered laser bars are presented. Examples for various current areas of interest in European research facilities will be given.

The German national research project "Modular Diode Laser Systems -- MDS" is aimed to the improvement of high power semiconductor laser elements, to investigation of mounting, cooling and beam forming technologies, the integration of these elements into high power diode laser systems and finally the investigation of novel applications with such systems. Consequently, the project shows a vertical organization of three groups: "Chip-Technology," "Laser- and Systems-Technology" and "Processes and Applications." The organization as well as newest results are the content of this contribution. Besides "conventional use" of the high power diode laser source, applications with a "modular" setup of the diode laser elements will be presented. The five year project with 22 partners from research and industry is now in its fourth year.

During the recent years the performance of high power diode lasers in terms of output power and lifetime has increased significantly. However, for many applications not only a high output power but also a good beam quality is necessary -- in other terms, high brightness is required. While the beam quality of classical broad area-type high power diode lasers is poor, special laser structures have been developed to achieve an improved beam quality. Examples are the tapered laser, the alpha-DFB-laser and -- the latest development -- the so-called "z-laser." The z-laser uses internal total reflection for the suppression of higher-order modes. The effectiveness of this working principle was first demonstrated by performing extensive numerical simulations. During the last year the first z-laser structures have been processed and characterized. The experimental results of these first test lasers are compared with the predictions from the numerical simulations and show a very good agreement. With these first lasers, approximately 500 mW output power at 6-times diffraction limited beam quality have been demonstrated. Nevertheless, there are also some not well understood features of the z-laser to be investigated, like a reduced conversion efficiency and untypical characteristic curves showing kinks. Understanding these features, demonstrating the reproducibility of the structure and further performance improvements are the goals of current rsearch.

The compactness of electronic equipments is leading the narrow pitch assembly of components on board or film by soldering. Diode-laser soldering system had already been realized as a high-density assembly process; however, the soldering quality became unstable when the system targeted to achieve short time, such as sub second, soldering, i.e. high productivity. A high speed soldering process for lead-free solder with diode-laser was developed. By measuring soldering temperature profiles of chips on film, it was found that the time period for soldering temperature reaching melting point at both sides of chip, which was over 0.6 sec, determined the soldering quality. According to SEM observation and EDS analysis, diffusion and inter-metallic layers, which were determined by plated materials of land, formed near the boundary of solder and land.

The requirements set for modern car exhaust purity is ever higher due to this the design of environmentally friendlier car is ever more difficult. The metallic catalytic converter consists of catalytic metal core and outer tube. It has advantages over to the ceramic convertes by the flexibility of the converter design, but suffers from the higher manufacturing cost. This study concentrates on the development of joint between the catalytic core of the converter and the outer tube structure. A comparative study was performed between CO₂-, Nd:YAG- and HPDL laser welding in respect of productivity, investment and weld quality. The results reached with each laser were of higher quality than arc welding. However the heat conduction limited welding turned out to be more suitable for welding this application than the keyhole welding mode. With diode laser welding the joining can be performed with the heat conduction mode welding. An analysis of advantages and restrictions was performed for each welding process.

Austenitic stainless steel sheets and ordinary cold-rolled carbon steel sheets with variable thickness were welded with 1 kW diode laser. Different weld joints were utilized. The optimal parameters for each case were determined. The joints were examined by metallography and mechanical testing. The results show that diode laser is an optimal tool for sheet metal welding, when a considerable narrow weld is aimed. The edges prepared by mechanical sheering are acceptable as the joint preparation. The tensile strength and ductility of all the joints were acceptable and on the same level or better than that of base metal. The shielding gas seems to play a much higher role than in conventional laser welding (CO₂ or Nd:YAG laser welding). When using the non-oxidizing shielding gas (nitrogen or argon), the welding speed to be reached is much slower than when welding without any shielding gas. This is probably due to the increase of absorption by oxygen.

The use of lasers for welding plastics was demonstrated in the early 1970s. However, it was not until late in the 1990s that production applications started to be considered widely. This followed the broad realization that by selection of a suitable combination of radiation wavelength and plastics additives, to control light transmission and absorption, heat could be generated at the joint of a pre-assembled part without melting its outer surfaces. It is of added benefit that the window of transmission for an unpigmented and unfilled plastic typically covers the wavelengths delivered by small and cost effective diode lasers. Recent developments in the transmission laser welding process for plastics are discussed, including methods for the generation of welds between two clear plastics, application of similar techniques to the joining of thermoplastic textiles and new equipment, able to heat a complete joint and assist in the sealing of assemblies where the joint surfaces are not particuarly smooth. An analytical heat flow model for the welding of clear plastics is shown in use for selecting process parameters.

Laser Beam Hardening with High Power Diode Lasers is presented as an excellent method for local heat treatment and minimum distortion. An overview is given about several strategies for local heat treatment and different industrial applications. Precise measuring and controlling of the surface temperature makes the process very reliable and is an essential tool for industrial users. To keep a constant penetration of the hardening zone at constant surface temperatures the feed rate can be adapted to local heat flow conditions. A former postprocessor of Fraunhofer IWS generates a CNC-program for the treatment and changes the feed rate in dependence of the surface shape. The new processor additionally considers local heat flow variations of a part caused by boreholes, grooves and changing local thickness. The processing is very fast and can be applied for solving daily problems of laser beam hardening. Some examples show the performance of the new postprocessor.

A series of exerpiments were performed to investigate the one-step laser cladding of Inconel 625 powder, injected off-axially onto Fe37 and 42CrMo4 substrates. The experiments were carried out using a 6 kW high power diode laser (HPDL) mounted to a 6 axis robot system. The rectangular shape of the delivering beam was focused to a spot size of 22 x 5 mm on the work piece. The coating samples were produced using different levels of powder feed rate (77 - 113 g/min), traveling speed (300 - 400 mm/min) and laser power (4.8 - 6 kW). Hot corrosion resistance of laser-clad Inconel 625 coatings were tested in Na₂SO₄ - V₂O₅ at 650°C for 1000 hours. Wet corrosion properties of the obtained coatings were tested in immersion tests in 3.5 wt.% NaCl solution. Diode laser power of 6 kW (808 and 940 nm) was high enough to produce 20 mm wide laser-clad tracks with a thickness of 2.5 mm in a single pass, when powder feed rate was more than 6 kg/h and traverse speed was 400 mm/min. Wet corrosion properties of laser-clad Inconel 625 coatings were found to be superior to sprayed and welded coatings. Hot corrosion resistance was even slightly better than corresponding wrought alloy. Finally, one-step HPDL cladding was demonstrated in coating of shaft for hydraulic cylinder with Inconel 625 powder. Due to high coating quality, high deposition rate and traverse speed HPDL devices are very promising for large area cladding applications.

The investigation of Laser cladding of Co-based super alloy on dual phase stainless steel have been curried out to repair the mechanical parts of power plant using 3 kW direct diode laser with high efficiency and a rectangular beam profile. In the present work, effects of process parameters such as laser power output, travel speed and defocusing distance were investigated to get a sound clad layer for repairing of the parts. Comparing the clad by conventional CO₂ laser and YAG laser, wide weld bead, less dilution and less distortion could be obtained in high power diode laser cladding. This repairing technique using diode laser with a small volume is desirable for practical application to the site maintenance on parts of various kinds of plants such as power plant, chemical plant, steel plant, paper plant, etc.

Laser soldering systems have been available for many years but the recent arrival of high power direct diode systems has given a boost to the acceptance of these systems by industry. Diode lasers are now being employed in industry for a range of soldering applications, most of these are the more challenging soldering applications where the controllability of laser processing is required and the diode laser brings added benefit to these. Very little work has been published on the quality of laser soldered joints and this paper addresses this shortfall by exploring laser parameters and joint quality using a number of techniques.

Laser welding of thermoplastic polymers is a non-contact process especially efficient for joining thermoplastic polymers. This innovative technology is already used for industrial series production in different sectors (automobile, packaging,...). The majority of the basic research concerns the weld strength depending on polymer nature, optical properties, butt design and process parameters. Nevertheless, a lack of knowledge concerning the influence of thermal history of the weld seam on morphology of semicrystalline polymer still exists, when this parameter strongly influences the strength of the weld. Actual results of diode laser transmission welding (LTW) experiments on polypropylene, a semicrystalline polymer widely used in industry, could contribute to a better understanding of the process itself and to success in practical applications.

From process considerations and economical reasons, an ideal welding laser should provide the necessary power at short wavelength and the highest possible beam quality at a high overall efficiency. The concept of a diode-pumped Yb:YAG thin disc laser, in principle, fulfills these requirements at the same time. Its characteristic features are discussed and results of fundamental theoretical and experimental investigations are presented. Performance data of first industrial devices underline the advantages that can be expected from this design: a power of 1.5 kW is delivered through a 0.15 mm fiber at an optical-to-electrical efficiency of 27%. Some preliminary results of welding studies with up to four of such devices demonstrate its great potential for thin metal sheet welding and many other applications where the achievement of well-defined slender weld seams is essential.

The Japanese national R&D project "Advanced Photon Processing and Measurement Technology" started at the end of August, 1997 within the framework of the Industrial Science and Technology Frontier Program of MITI (now METI, Ministry of Economy, Trade and Industry) terminated its 5 year duration at the end of March, 2002, and is going to enter the stage of final evaluation. R&D activities of the project were promoted energetically by the 14 RIPE members, and various excellent results of world top level were obtained. This paper summarizes the outline of the several latest results obtained in the development of high-power, high-efficiency diode-pumped solid-state lasers within the project.

As a tool for high speed and high precision material processing such as cutting and welding, we developed a rod-type all-solid-state laser with an average power of more than 10 kW, an electrical-optical efficiency of more than 20%, and a laser head volume of less than 0.05 m³. We developed a highly efficient diode pumped module, and successfully obtained electrical-optical efficiencies of 22% in CW operation and 26% in QCW operation at multi-kW output powers. We also succeeded to reduce the laser head volume, and obtained the output power of 12 kW with an efficiency of 23%, and laser head volume of 0.045 m³. We transferred the technology to SHIBAURA mechatronics corp., who started to provide the LD pumped Nd:YAG laser system with output power up to 4.5 kW. We are now continuing development for further high power laser equipment.

We have achieved a high-quality beam generation with highly efficient quasi-cw Nd:YAG laser over 1-kW with a novel side-pumping configuration using micro-lens free diode-stacks as a part of "Advanced Photon Processing and Measurement Technologies" program. We have demonstrated a power scaling of Nd:YAG rod laser over 1-kW while maintaining high-beam-quality and high-efficiency by cascaded-coupling of two identical bifocusing compensation resonators. Laser power of 1050W was obtained with the beam quality of M² = 8 at the electric-to-optical efficiency of 23%. In this work, we also demonstrated the focusing ability of less than 50 μm diameter on the focal plane by using a f50 mm lens (N.A. 0.2).

Laser Materials Processing is no longer a scientific curiosity but a common day Industrial reality. Lasers in manufacturing sector are currently used in welding, cutting, drilling, cladding, marking, cleaning, micro-machining and forming. Recently, high power laser diode, LD pumped YAG laser, 10 kW lamp pumped YAG laser, 700 W fiber laser and excimer laser have been developed in the industrialized countries, which had their own big national projects. As a result of large numbers of research and developments, the modern laser materials processing has been realized and used in all kinds of industries now. At the beginning of 20th century, the price of laser system has become lower competitive to the conventional lasers. Lower the price of laser diode systems is, wider is the field of their applications. In the present paper, the researches and developments in laser materials processing in US, EU and Japan are outlined and metallurgical aspects are described on the base of laboratory researches.

Research on change of Rayleigh range Z_R of output beams and effect of focusing characteristics caused by the dynamic process of resonator deformation especially for the HR coupler in high power CO₂ Laser is given in this paper. An adaptive optics system which can compensates the change of Rayleigh range Z_R was used. The real-time control of focus position was achieved.

A folded resonator for pulsed reactor-pumped lasers (RPL) is presented, which has a zigzag propagation inside and high coupling efficiency with the nuclear reactor. The equivalent length of gain cell in this device is multiplied. Thus higher optical intensity may be coupled out when high reflectors are available. For a ³He/Ar/Xe system, the energy loading is relatively uniform and the appropriate size for resonator can be easily achieved by calculations. A method for optimizing of outcoupling is discussed in the work as well.

Yb:YAG is an attractive material for high-power and ultra-short pulse laser because of its high quantum efficiency exceeding 90% and wide gain spectrum larger than about 10 nm. Our research is development of high-average power Kerr-lens mode-locked Yb:YAG laser. A Kerr-lens mode-locked laser has been designed with thin-rod Yb:YAG for high average power oscillator. Thin and long Yb:YAG crystal is used for gain medium and it was pumped directly by stacked LD bar. The Z-folded cavity was designed by simple ABCD matrix law. An additive Kerr-medium is used for stable mode-locked operation instead of semiconductor saturable absorber mirrors (SESAM), because Kerr-effect of the Yb:YAG crystal is weak. SF57 glass is used to stabilize the mode-locked oscillation to decrease the intra-cavity loss to increase the oscillation bandwidth for transform limited pulse generation. The Group Delay Dispersion (GDD) of the optical element in the laser cavity was compensated by SF10 prism pair. The output is expected to be the transform limited ultrashort pulses of sub 200 fs width with a high average output power around 60 W with 200 W pump power.

We report characteristics of laser induced gas breakdown by changing wavelengths, gas media as well as controlling energies. Pulsed YAG laser beam was focused into the gas chamber in order to study breakdown thresholds and laser transmission through produced plasmas. Laser transmission indicates a lot of information about breakdown threshold and transmission properties. Experimental data indicate that combination of shorter wavelength, shorter focal length, Ar gas of higher pressure is suggested for plasma generation.

The Yb (ytterbium) laser material is promising for high-power all-solid-state ultrashort pulse lasers because of its high quantum efficiency and wide gain spectrum. A didoe end-pumped technology is developed for high-average-power and efficient all-solid state ultrashort pulse lasers with high beam quality output. Two 100 W CW-LDs are used for pumping and the beams are focused on the end surfaces of a thin and long Yb:YAG rod. A pair of parallel side surfaces of the rod is placed in contact with cupper heat sinks to remove heat. A one-dimensional distribution of thermal stress induced birefringence was observed in the rod by this simple heat flow. The loss due to thermal birefringence was measured to be smaller than 3%, when the probe beam was linearly polarized in parallel or perpendicular to the direction of cooling. It can be expected theoretically that a moderate single-path gain of 4 for efficient amplification and the optical conversion efficiency of 50% are realized for multimode operation with a pump power of 200 W. In the experiment, output power was measured to be 55 W with beam quality factor of M_x² x M_y² = 5.5 x 6.0 and 27 W with M_x² x M_y² = 1.3 x 1.4 for simple plane-plane linear cavity configuration with no thermal lens compensations.

The performance of an industrial laser is very much depending upon the characteristics of the laser beam. The ISO standards 11146 and 11154 describing test methods for laser beam parameters have been approved. To implement these methods in industry is difficult and especially for the infrared laser sources, such as the CO₂-laser, the availabl analyzing systems are slow, difficult to apply and having limited reliability due to the nature of the detection methods. In an EUREKA-project the goal was defined to develop a laser beam analyzing system dedicated to high power CO₂-lasers, which could fulfill the demands for an entire analyzing system, automating the time consuming pre-alignment and beam conditioning work required before a beam mode analyses, automating the analyzing sequences and data analysis required to determine the laser beam caustics and last but not least to deliver reliable close to real time data to the operator. The results of this project work will be described in this paper. The research project has led to the development of the Modematic laser beam analyzer, which is ready for the market.

The focusing characteristics of higher-order mode Gaussian beams in flying optics were investigated in detail. On the basis, a novel adaptive laser processing system for flying optics was developed. In the system, a lens with long focal length and two adaptive deformable mirrors controlled by hydraulic were employed. The lens was near the laser source for some distance and the two adaptive mirrors were integrated together with parabolic focusing mirror. The system has the advantages of compact structure and easy control. The system can keep the focus position and the focus radius constant for long distance laser processing.

Laser processing is now being used increasingly in the area of electronics, especially, for drilling micro holes in printed circuit. But the intensity distribution of laser beam mainly based on Gaussian is not uniform. Therefore, the demand for uniform intensity distribution is rising rapidly in the field of heat processing. To obtain higher uniformity, attempts must be made to convert non-uniform Gaussian distribution into top-hat shaped uniform intensity distribution for smoothly bending laser beams. In this study the authors propose an aspheric beam homogenizer made from ZnSe that can convert non-uniform Gaussian distribution into top-hat shaped uniform intensity distribution. The ZnSe beam homogenizer consists of two aspheric lenses. First one converts Gaussian profile to uniform irradiation, and second one performs phase matching. The authors design this optical component with a special method based on wave optics. In the design, the authors define the target intensity distribution as the super-Gaussian shaped one instead of completely uniform top-hat shaped one. Compared with the homogenizers of traditional design, the newly designed homogenizer achieved 70% increase in the uniformity of signal intensity even after propagation. The paper reports the measured intensity distribution after propagation from the beam homogenizer with high power CO₂ laser.

We have applied Fast Fourier Transform analysis to optical and acoustic emissions during bead-on-plate welding with CO₂ and YAG lasers in our previous study. Some characteristic peaks in their frequency spectra were correlated with the bead profile. In this study, we have done butt welding of mild steel and stainless steel with 4 kW CW YAG laser. The optical and acoustic emissions during the welding were measured and the frequency spectra were analyzed. The major frequency peaks of the acoustic emission appeared in the range of 1 - 4 kHz and 6 - 7 kHz, although there were some additional peaks up to 40 kHz. These strong peaks appeared always in deep penetration mode welding. However, these peaks became small when the penetration depth became shallow by increasing the welding speed. We have shown that the acoustic spectrum of deep penetration mode welding is different from that of shallow penetration mode welding. The deference of the frequency spectrum should be useful in the development of a feedback system for control of weld quality.

Automatic or semi-automatic welding methods are required for improving efficiency and safety, then various seam tracking technologies have been developed as an important part of them. For laser welding, although industrial application is magnifying widely, traditional seam-tracking technologies are difficult to apply. We have developed new monitoring system of weld zone and its around at the same time under high luminosity of laser welding, using the 2nd harmonics generator of pulsed YAG laser for illuminating the situation of weld. Using this system, we can observe the welding situations clearly in monitors, even for especially high luminous welding phenomenon, such a laser welding. Using this system, we have developed the seam tracking control system of laser welding by analyzing the images in computer in order to decide the position of the weld line and beam point and conducting tbe feedback coontrol of laser torch position. For examining this system, we conducted the I-bat welding on stainless steel plate changing welding track and welding speed. Through the experiment, followings were understood. This system can use high speed welding and works sufficiently in laser welding.

We have studied in-process monitoring technique in Nd:YAG laser welding. We used a CCD camera and a photodiode as the monitoring sensor, and observed laser processing coaxially with the laser beam. There were differences in the image of the CCD camera between full and partial penetration welding and the detection for full and partial welding was achieved by the image processing of the detected image data. And, it was suggested that a change in the focal position could be detected because a change in the luminescence intensity could be caught with the photodiode when the deviation of the focal position occurred.

High precision, high productivity and high quality are the three absolute requirements in today's laser welding production. Automated laser welding places extreme demands on tool position accuracy. Accurate real-time tracking and inspection systems for laser materials processing make use of high-performance laser sensors. The reliability of the monitored signal can be significantly increased by using high resolution, digital CMOS sensors and high speed real-time image processing technologies. This paper presents the latest developments in high-performance optical joint tracking systems and optical inspection systems based on these technologies. Optical joint tracking systems allow for precise control of part fit-up, machine self-alignment, and adaptive process control; optical inspection systems allow for automated in-line verification, insuring laser welds meet quality standards and customer's specification. Geometric features of welds can be precisely measured and compared to allowable tolerances while undesirable attributes like surface porosities and external defects can be accurately detected.

Some of Japan's oldest nuclear generation plants are now entering their 30th year, which is close to their designed lifespan. The rules on fitness-for-service for nuclear power plants undertaken by utilities are to extend the lifetime of plants to 60 years. Upon lifetime extension new rules must be applied and JSME (Japan Society of Mechanical Engineers) issued new rules in May 2000. These rules define allowable flaw sizes in operating plant components. In these circumstances, in-situ flaw sizing and repairs are urgently needed. Accordingly, laser inspections such as laser UT and laser holography are being developed.

A new method to shape the geometry and to enhance the quality of weld seams is presented in this paper. Hereby, electromagnetic volume forces are produced by utilizing various origins of the magnetic field and the electric current. Acting in the liquid metal they directly affect the flow field and lead to favorable conditions for the melt dynamics and energy coupling. Experimentally conducted full and partial penetration welds in aluminum alloys with CO₂ and Nd:YAG lasers demonstrate that this method can be used to influence the seam geometry and top-bead topography, as well as the penetration depth and the evolution of process pores. In the case of full penetration, it is also possible to lift the weld pool against gravitational force.

Mathematical models and the associated numerical techniques have been developed to study the following cases: (1) the formation and collapse of a keyhole, (2) the formation of porosity and its control strategies, (3) laser welding with filler metals, and (4) the escape of zinc vapor in laser welding of galvanized steel. The simulation results show that the formation of porosity in the weld is caused by two competing mechanisms: one is the solidification rate of the molten metal and the other is the speed that molten metal backfills the keyhole after laser energy is terminated. The models have demonstrated that porosity can be reduced or eliminated by adding filler metals, controlling laser tailing power, or applying an electromagnetic force during keyhole collapse process. It is found that a uniform composition of weld pool is difficult to achieve by filler metals due to very rapid solidification of the weld pool in laser welding, as compared to that in gas metal arc welding.

Theoretical and experimental studies have been carried out in order to improve the understanding of the mechanism of pore formation in keyhole laser spot welding in a qualitative and also quantitative manner. A semi-analytical mathematical model of the keyhole collapse illustrates the different characteristic time scales of the contributing physical processes: post-vaporization (order of magnitude: 100 ns typically), excess keyhole vapor relaxation flow 10 μs), inertia driven collapse (100 μs), followed by bubble contraction, re-condensation and rising (10 ms), and re-solidification (10 ms). The conditions of the keyhole just before switching off the laser beam, observed by X-ray imaging, are essential for the subsequent collapse mechanism. In case of a bottleneck-shaped keyhole, which can easily form due to the paradox of vapor flow inversion, bubble formation is likely to occur due to necking. When the thermally contracting bubble is trapped by the re-solidification front, a pore is formed. The model is complementary to high speed X-ray observations of the keyhole shape, particularly in liquid Zn that enables investigation of keyhole and bubble formation not constrained by surrounding solid.

The focused laser beam, alongside the electron beam, is one of the highest power density sources available to industry today. Laser welding of titanium alloys has been of interest especially in the aerospace industries, where quality of its products is very important. The application of the laser beam, however, causes a thermal-mechanical response in the workpiece and thus a complicated pattern of residual stresses is set up. It is if interest to characterize and analyze the development of residual stresses, which has a profound effect in component strength and fatigue life. In this paper, a process model suitable for the prediction of residual stresses induced during the laser welding of Ti-6Al-4V is described. For the purpose of model validation, processing trials have been carried out. The heat transfer during welding has been characterized and the fusion zone has been observed, which have enabled a suitable thermal model to be developed. The residual stresses have been measured using the synchrotron x-ray diffraction technique, and it is shown that the predictions are in reasonable agreement with the observations. The modeling is then used to rationalize the state of residual stresses induced.

This study was aimed at establishment of a model that can predict tensile shear strength and fracture portion laser-welded lap joints in the tensile test. To clear influence of the bead length and width on them, the joints employed steel sheets with a thickness in the range of 0.8 mm to 1.2 mm were evaluated. It was found that the tensile shear strength increased with the bead size, and the fracture occurred at base metal (BM), weld metal (WM) or portion between them with a curvature (referred to as portion R). Also to clarify rotational deformation process around WM during the tensile test, joint cross-sections were observed at some applied load levels in the test. This observation derived the relationship between the radius, Ri, at the inner plane of portion R and the rotational angle, θ, of the center of sheet thickness, and the relationship between Ri and applied load. A plastic analysis based on these functions and assumptions that the joint consists of BM, WM and R, which are under simplified stress mode respectively, could estimate the tensile shear strength and the fracture portion of the joints. This estimation made good accord with experimental results.

We have been developing numerical simulation technique for laser welding. This study present some results obtained recently on the improvements of simulation models and numerical procedures, experimental data for code validation and the results of comparison of experiments with numerical analysis. We added the surface evaporation model under boiling temperature and introduced the level-set method as accurate and low numerical diffusion tracking procedure. We extend our gas phase model as enable to treat multi species of gas and mutual diffusion process. Two kinds of experiment were carried out in this study for code validation. One is the experiment to observe the surface tension driven convective flow and the other is to confirm the threshold of laser power density to form the keyhole. In these experiments, 6 kW YAG laser is used in CW mode and test pieces of aluminum alloy A1050P are irradiated in an inert atmospheric box. We calculated the heat conduction type of welding using improved code based on the actual welding conditions including the surface tension and surface evaporating models. And we simulated a series of transient behavior of molten pool irradiated by high laser power density.

In order to understand laser keyhole welding phenomena, pulsed or continuous wave laser was irradiated on a molten metal of Sn or Zn, and spattering and keyhole evolution were observed by high-seed video camera and X-ray transmission method, respectively. It was confirmed from the observation of the surface that a keyhole was initiated to form by far earlier in the molten metal than in the solid metal. According to the X-ray transmission real-time observation result in Zn liquid metal, bubbles were predominantly generated from the tip of a keyhole, which is the same formation mechanism as we revealed in general laser welding. Furthermore, simplified numerical calculation demonstrated that surface tension should affect the formation of such bubbles.

Solidification mode in laser welds sometimes exhibits the primary austenite, which enhances hot cracking suceptibility, because of an austenitizing effect of nitrogen and the high solidification rate. Therefore, laser welds of high nitrogen stainless steels likely occurs hot cracking. This investigation was conducted to clarify effects of nitrogen and solidification rate on hot cracking susceptibility in laser welds of austenitic stainless steels varied with nitrogen contents. Hot cracking susceptibility in laser welds was markedly increased with increase in the solidification rate and nitrogen contents. On the other hand, solidification mode in weld metals was changed from primary ferrite to primary austenite, as raising the solidification rate and the nitrogen content in laser welds. Experimental results indicated that the abrupt increase in hot cracking susceptibility is consistent with transition in the solidification mode from primary ferrite to primary austenite in the weld metal. The transition of solidification mode from primary ferrite to primary austenite in laser welds could be estimated by calculation using the modified KGT model considering the effect of nitrogen. These results suggest that hot cracking susceptibility in laser welds of the steels can be also predicted by using the same analytical method of solidification mode in the welds.

Crack testing concerning small and fast solidifying laser welds in austenitic stainless steel has been studied. A set of methods has been applied to investigate alloy properties, including (1) Application of known information to predict solidification phases, (2) Weld metal solidification rate measurements for prediction of phases, (3) Various crack tests to assess the crack susceptibility of alloys and (4) A combination of the above for selection of suitable, weldable alloys. The possibility of using such specific methods for alloys and applications has been investigated and recommendations are given. Results from the solidification rate measurements had high variations. They do not show an expected correlation between the crack resistance and the solidification rate. The employment of pulsed seam welds is assessed not to be usable in the present measurement method. From evaluation of several crack tests, the Weeter spot weld test has been chosen to form a basis for the development of a practicable method to select specific alloys for welding applications. A new test, the Groove weld test was developed, which has reduced the time consumption and lightened the analysis effort considerably. The Groove test showed results in good compliance with the Weeter test.

Laser welding process of aluminum alloys are recognized as an important producting system increasingly recently in automobile and electronics industries. However, even if the alloy is welded in the proper welding condition, depending on an alloy, weld defects, such as a solidification crack and porosity, occur, and it has been a problem. In the present work, the Fan-shaped solidification crack test method was investigated to evaluate the crack susceptibility of the alloy and to solve the problem. The solidification crack, which occurs in bead-on-plate, welding at an angle to the edge of the specimen, and spread in various aluminum alloy welds from the start of bead evaluation of solidification crack susceptibility was tried by measuring the crack length. Moreover, the same experiment was conducted using the adaptive mirror, which the focal point is changed by 0 to 1000 Hz in the beam direction. By oscillating the focal point of laser beam, weld pool and keyhole were stabilized and it result in reducing solidification crack susceptibility.

The purpose of this study is to develop thermal and dynamic three dimensional strain analysis method for the laser weldment in order to obtain the plastic strain at elevated temperature in HAZ of the laser weldment, and to develop the prediction method of liquation crack initiation in HAZ of laser weldment. The U-type hot cracking test was performed as an experimental evaluation method for liquation cracking susceptibility of laser weldments of Inconel 718. At the same time, thermal and dynamic three dimensional elastplastic strain analyses were performed by FEM for U-type hot cracking test specimens. Heat transfer analysis and elastplastic strain analysis for laser weldments were confirmed by experiments. From this analysis, it becomes clear that the plastic strain at elevated temperature affects liquation crack initiation in HAZ, and the critical strain at elevated temperature, which controlled liquation crack initiation, can be calculated precisely by using this proposed analysis method.

Laser was employed to anneal a thin surface layer of aged Inconel 718 by dissolving the strengthening phase, γ". The HE (Hydrogen Embrittlement) resistance of the alloy was improved via such laser surface annealing (LSA) processes. To establish a general LSA technique for engineer applications, experimental LSA processes were conducted to study the effects of the laser process parameters on the formation of the annealed surface layers, and applicable process parameter ranges were obtained. Next, a numerical method was developed for predicting the formation of the laser annealed surface layers in the following steps. Because only the γ" phase was dissolved in the LSA process, the dissolution kinetics of this phase was studied via thermal cycling experiments, and it was proved to follow an Avrami equation. FEM (Finite Element Method) simulations were conducted to calculate the thermal distribution in each laser annealed surface layer, and thermal history data were extracted every certain depth. The volume fractions of the γ" phase at these depths were calculated using these thermal history data based on the deduced Avrami equation. Using a developed relationship between the hardness variation of the alloy and the volume fraction variation of the γ" phase, the hardness distribution in the annealed surface layer and this layer's thickness were calculated. The predicted applicable laser process parameter ranges were obtained. These calculated results were compared with their corresponding experimental results. The good agreements between the calculated and measured results suggested that this numerical prediction approach is feasible for engineer applications.

A face welding technology of a titanium sheet on a steel plate for adding corrosion resistance was investigated. In the welding tests, 1800 W YAG laser beam was irradiated at different locations near a touching line of both plates through f:θ lens and was scanned within 13 mm width along the line at the frequency of 10 Hz, 20 Hz or 40 Hz. Twin rolls were set to load forces on both plates subjected to welding and simultaneously for pulling them out. The tensile shear test was carried out on each piece. Consequently, the highest strength was obtained at the frequency of 40 Hz. SEM observation and EDX analyses revealed that the formation of intermetallic compounds was smallest in this weld.

Dynamic keyhole behavior has been observed to elucidate the formation and suppression mechanism of the porosity in 20 kW CO₂ laser welding with the depth of 20 mm. The results indicate that the bubble is formed by capillary instability of the cylindrical keyhole. The tip of the keyhole is broken up by instability during rapid decrease in the depth, so called spiking phenomenon. Spontaneous fluctuation in the keyhole depth and spontaneous keyhole perturbation during welding promotes the bubble formation. Pulse modulation of the laser power is effective in stabilizing the keyhole and hereby suppressing the porosity if the frequency coincides with the eigenfrequency of the molten pool oscillation. The suppression effect is enhanced if the waveform is controlled appropriately.

When laser welding mild steel under high power densities, vaporized material is ejected from the keyhole and forms a plume/plasma above the weld pool. In previous studies on plume formation and extent of ionization, the influence of the laser wavelength and the gas environment has been observed. In this study a comparison between CO₂ and Nd:YAG laser welding has been performed using the same energy density (approximately 1.24 MW/cm², produced using 3.5 kW of power and a focal spot size of 0.6 mm) in He, Ar and N₂ gas environments and in vacuum. Plume/plasma evolution was recorded with high-speed video at 9000 frames/second and these images have been correlated with the characteristics of the weld cross-section. The fusion and heat-affected zone profiles have been measured to analyze the melting efficiency at different processing speeds. The temperatures and electron densities in the plume/plasma have also been calculated by spectroscopic methods to estimate the losses caused by the plume/plasma development. By analyzing the differences in the weld shape profiles and the plume/plasma behavior, the temporal evolution of the laser welding process efficiency was also obtained.

The range of laser welding applications is widening from applications in car manufacturing to normal machine building. Laser welding has suffered from the tight demands for component and joint manufacture. This investigation studies the effect of various welding and filler wire feed variables on the weld quality and efficiency of the laser welding process. Welding was found to be possible with several parameter combinations and the width of air gap used was 1 mm, when the material thickness was 6 mm. The utilization of filler wire feed introduces some new parameters to the laser welding process. There is a noticeable effect from the wire feed position and feed angle on the welding process. The variations, like lack of penetration, of weld quality, was caused by inaccurate positioning of filler wire and can be compensated by the adjustment of the filler wire feed rate and the energy input to some extent. The efficiency of laser welding with filler wire is equal to that of autogenous welding, but the overall energy input must be increased according to the air gap volume. Filler wire feed provides the process with less stringent demands, but requires additional energy input to the workpiece.

The use of laser welding as a high quality low distortion welding process is well known. However, as a thermal welding process some distortion is still introduced which can be problematic for high precision welding of critical components. Lwo Stress No Distortion techniques (LSND) have been shown to eliminate buckling distortion in arc welding. In this paper the LSND technique has been extended to laser welding. The thermal tensioning forces are introduced via a cryogenic cooling medium that is sprayed immediately behind the weld pool. This technique has been successfully applied using direct laser diode and CO₂ laser welding to virtually eliminate buckling distortion when welding a wide range of materials including stainless steels and aluminum alloys. The results presented will include an assessment of the control of distortion as well as the effects of this technique on the weld microstructure and residual stresses in the weld zone. In-process thermal imaging of the process has also been used to show the efficiency of the process to control the thermal field in the weld zone.

It is difficult for heat resisting alloys to be welded together, and a few data on welding of Inconel that represents Ni-base heat resisting alloys have been reported, although there is an active demand for the complicated form processing of these alloys. In this study, each pair of Inconel 713C and Inconel 718 plates was butted and was welded with YAG laser beams through a quartz glass in the atmospheres of the air, of a vacuum and argon. Multi-pulse laser beams were irradiated to the material with different laser outputs. Four-point bending tests were performed to evaluate the welding characteristics. By considering the experimental results, appropriate conditions for welding Inconel were found as follows: It is possible to weld Inconel materials under the condition of a short pulse width on the side of biggest pulse energy without occurring the blow in the welding surface. It is unable to weld Inconel 713C in the atmosphere of the air or of a vacuum, although it is possible to weld it in argon. It is possible to weld Inconel 718 in the air or in argon, however it is unable to weld it in a vacuum because of the blow of welding surface.

High power CO₂, YAG and LD-pumped solid-state lasers have been developed to produce a deep penetration type of high-quality, high-performance and high-speed weld joints. However, porosity is easily formed in such deep keyhole-type weld beads. The authors have developed microfocused X-ray transmission imaging system, and revealed keyhole behavior and porosity formation mechanism in high power laser welding. This paper will describe a summary of porosity formation mechanism and prevention procedures during cw laser welding of aluminum alloys. Especially, many bubbles were formed by the evaporation of the metals from the bottom tip of the keyhole and flowed upwards according to the liquid flow near the solid-liquid interface inside the molten pool. The majority of them were trapped and captured at the solidifying front of the weld beads, leading to the formation of porosity. Moreover, it was revealed that the shielding gas was chiefly included in the porosity. Main melt flows were observed as a function of welding speed. As the speed was increased, vapor plume was ejected from the keyhole inlet more and more normal to the plate surface, and consequently induced the upward flow of the keyhole-surrounding liquid. On the basis of the above knowledge, full penetration welding, properly pulse-modulated laser welding, vacuum or low pressure welding, welding using the tornado nozzle, very low or high speed welding, and so on were investigated, and it was consequently confirmed that these procedures were beneficial to the reduction in porosity.

Double focus welding is a proven technique to reduce or even to avoid blowholes and porosity in the welding process of aluminum. The application of two focal spots arranged close to each other offers advantages concerning process stability and quality, which is a result of a more stable keyhole geometry respectively of modified energy coupling and fluid dynamic conditions. The opportunity of designing a task-adapted intensity distribution opens possibilities for the bridging of gaps and the shaping of the weld seam geometry which means an increase of flexibility. This contribution will discuss the joining process and will show the connection between spot distance and intensity distribution on one hand and the welding result -- depth, efficiency and quality -- on the other hand.

Studies have been made to state the influence of dual beam configuration on porosity formation. Welding experiments were carried out on two aluminum alloys, extremely sensitive to porosity formation under laser beam. The results show that the bispot configuration reduces drastically the porosity rate. Indeed, the use of dual beam conditions ensures a larger and more stable weld pool that allows the porosity's ejection. This has been observed by a high speed CMOS camera, which has followed the fluctuations of the keyhole, and has given the approximate melting pool dimensions in single and dual beam configurations, for several welding speed and beam distances. We have already observed an evolution of the weld pool geometry, which tends to stabilize the melt pool flow, providing free of porosity beads with a stable bead aspect. Indeed, the bead surface is smoothed, and a regular chevron welded structure is built during solidification. After all, we tried to set up a correspondence between the oscillations of the keyhole, the melt flow examined, the geometry of the weld pool achieved upon the bead aspects obtained, and the good metallurgical quality reached in dual beam arrangement.

In high power YAG laser welding of steels, a rectangularly modulated beam with high peak power is usually used to get deep penetration. On the other hand, many spatters and solidification cracks are generated when some aluminum alloys are welded with a rectangularly modulated beam because of its high heat conductivity, high reflectivity, low surface tension, large contraction, wide solidification temperature range, etc. Therefore, a properly modulated beam or a continuous beam is usually used in aluminum alloy welding, although the penetration depth is shallow. In this research, sinusoidal wave or rectangularly modulated wave of YAG laser combined with TIG arc was tried to improve the weldability of A6061 aluminum alloy. As a result, when TIG arc was superimposed behind the YAG laser beam, deeply penetrated weld beads with good surface appearances were produced without spatter losses and cracks.

Numerous advantages of hybrid welding, in which laser beam and arc has combined, over autogenous laser welding has been reported. Especially in case of inaccurate joint preparation or fixturing of the plates to be welded because of the filler metal added to the process through MIG-welding. Also additional heat, coming from the arc to the process, enables higher welding speed and deeper penetration. Aluminum alloy (AlMg3) was used in the experiments. Welding was carried out by using the hybrid process (combination of Nd:YAG- and MIG-welding) in the flat position. The joint preparation was carried out as shear cut and different gap widths were used. Welding experiments were made systematically using a statistical experiment procedure called TAGUCHI-method. Parameters, for example alignment of point of arc and laser, varied in experiments. Also characteristic parameters of both welding methods were changed according to the experimental procedure. In this paper results of welding experiments are reported as well as parameters used. A phenomenona of the hybrid process with aluminum is discussed and also reasons for weld defects occurred are pointed out.

Laser beam welding is attractive for joining age-hardenable aluminum alloys, because its low over-all heat input results in a narrow weld heat affected zone (HAZ), where softening caused by dissolution of age precipitates occurs. In the present work, 1mm-thick 6061-T6 aluminum alloy plates were welded using a 2.5 kW CO₂ laser and it was experimentally proved that the width of the softened region in the laser beam weld was less than 1/7 that of a TIG weld. Moreover the hardness in the softened region of the laser beam weld was found to be almost fully recovered to the base metal hardness by applying a post-weld aging treatment at 443 K for 28.8 ks without solution annealing unlike the TIG weld. These results characterize the advantage of laser beam welding in joining of the age-hardenable aluminum alloy as compared with the conventional arc welding. The hardness distributions in the HAZ were theoretically evaluated based on kinetic equations describing the dissolution of hardening β' (Mg₂Si) precipitates and the precipitation of non-hardening β' (Mg₂Si) precipitates during the weld thermal cycles to quantitatively prove above mentioned advantageous characteristics of laser beam welding.

CO₂ laser welding of thin aluminum sheets was performed at welding speeds of up to 20 m/min to investigate the weldability, weld pool dynamics and mechanical properties of the weld bead of aluminum alloys. High-speed camera observation of weld areas showed that the thickness of the keyhole-front-face decreased to 100 μm under high-speed welding conditions and the weld pool became unstable. The focal length was optimized to increase the spot power density and thereby easily melt the aluminum sheets. Using a 76-mm focal length lens, which corresponds to 11 MW/cm² power density, we obtained a keyhole mode weld bead with a depth of 1.3 mm at 20 m/min welding speed at 2 kW laser power. It was also possible to reduce the heat affected zone (HAZ) width to only 1.6 mm when the welding speed was 20 m/min. The HAZ width decreased as welding speed was increased. The tensile strength test of A6N01 weld beads showed that the fracture strength increased as the welding speed was increased up to 16 m/min, probably because the soft region of weld specimens was decreased. On the other hand, solidification cracks formed in the weld bead center at higher speeds, resulting in decreased strength.

In this study, a statistical analysis was carried out about influences of energy variance, pre-pulse and post-pulse setting to the main pulse on the melt zone. In the experiment, laser beams were irradiated on the plates of aluminum alloy by two spots, and then, depth and width of melt were measured. Thus, the fluctuation in the dimensions measured was analyzed using statistical methods.

Using a high power pulse transverse flow CO₂ laser developed in our lab, a series of thin Al alloy plates were successfully welded. Effects of processing parameters (beam quality, laser power, welding speed and assisting gas et al) on weldability of Al alloy plates were given. A key technique, artificial keyhole caused by the gap between two Al alloy butt plates is successfully used, which helped to break through their high reflectivity at 10.6 μm wavelength and enhanced the energy coupling efficiency. The weld thickness of Al alloy plates reached 4 mm with 3 kW CO₂ laser average output power. Microhardness and tensile tests showed that for some Al alloys, mechanical properties of the welds could be near or equal to base material with the artificial keyhole technique and suitable processing parameters.

Nd:YAG laser welding of an aluminum alloy assisted by high-peak pulsed lasers such as excimer and Q-switched Nd:YAG laseers was investigated. In this novel laser hybrid welding, a plasma induced by a high-peak power laser absorbs the Nd:YAG laser energy and transmits it to a work piece. In the laser welding of the aluminum alloy using the Nd:YAG laser of which maximu power was 6 kW, the heat input and the penetration depth increased by 20% and 8%, respectively, by using simultaneous irradiation of the excimer laser of which average power was only 16 W. According to the optical emission of the plasmas, not only KrF excimer laser but also 2nd harmonic of a Q-switched Nd:YAG laser is applicable for the laser hybrid welding as the high-peak pulsed laser.

Researches and developments of laser and arc hybrid welding has been curried out since in 1978. Especially, CO₂ laser and TIG hybrid welding has been studied for increasing the penetration depth and welding speed. Recently laser and MIG/MAG/Plasma hybrid welding processes have been developed and applied to industries. It was recognized as a new welding process that promote the flexibility of the process for increasing the penetration depth, welding speed and allowable joint gap and improving the quality of the welds. In the present work, CO₂ Laser-MAG hybrid welding of carbon steel (SM490) was investigated to make clear the phenomenon and characteristics of hybrid welding process comparing with laser welding and MAG process. The effects of many process parameters such as welding current, arc voltage, welding speed, defocusing distance, laser-to-arc distance on penetration depth, bead shape, spatter, arc stability and plasma formation were investigated in the present work. Especially, the interaction of laser plasma and MAG arc plasma was considered by changing the laser to arc distance (=D_LA).

Laser welding is capable of high-efficiency low-strain welding, and so its applications are started to various products. We have also put the high-power YAG laser of up to 10 kW to practical welding use for various products. On the other hand the weakest point of this laser welding is considered to be strict in the welding gap aiming allowance. In order to solve this problem, we have developed hybrid welding of TIG, MIG arc and YAG laser, taking the most advantages of both the laser and arc welding. Since the electrode is coaxial to the optical axis of the YAG laser in this process, it can be applied to welding of various objects. In the coaxial MIG, TIG-YAG welding, in order to make irradiation positions of the YAG laser beams having been guided in a wire or an electrode focused to the same position, the beam transmitted in fibers is separated to form a space between the separated beams, in which the laser is guided. With this method the beam-irradiating area can be brought near or to the arc-generating point. This enables welding of all directions even for the member of a three-dimensional shape. This time we carried out welding for various materials and have made their welding of up to 1 mm or more in welding groove gap possible. We have realized high-speed 1-pass butt welding of 4m/min in welding speed with the laser power of 3 kW for an aluminum alloy plate of approximately 4 mm thick. For a mild steel plate also we have realized butt welding of 1m/min with 5 kW for 6 mm thick. Further, in welding of stainless steel we have shown its welding possibility, by stabilizing the arc with the YAG laser in the welding atmosphere of pure argon, and shown that this welding is effective in high-efficiency welding of various materials. Here we will report the fundamental welding performances and applications to various objects for the coaxial MIG, TIG-YAG welding we have developed.

Modern lifting equipment is normally constructed using hollow section beams in a telescopic arrangement. Telescopic lifters are used in a variety number of applications including e.g. construction and building maintenance. Also rescue sector is one large application field. It is very important in such applications to use a lightweight and stable beam construction, which gives a high degree of flexibility in working high and width. To ensure a high weld quality of hollow section beams, high efficiency and minimal distortion, a welding process with a high power density is needed. The alternatives, in practice, which fulfill these requirements, are laser welding and hybrid welding. In this paper, the use of hybrid welding process (combination of CO₂ laser welding and GMAW) in welding of hollow section beam structure is presented. Compared to laser welding, hybrid welding allows wider joint tolerances, which enables joints to be prepared and fit-up less accurately, aving time and manufacturing costs. A prerequisite for quality and effective use of hybrid welding is, however, a complete understanding of the process and its capabilities, which must be taken into account during both product design and manufacture.

Hybrid welding was carried out on Type 304 stainless steel plate under various conditions using YAG laser combined with TIG arc. During arc and laser-arc hybrid welding, arc voltage variation was measured, and arc plasma, laser-induced plume and evaporation spots as well as keyhole behavior and liquid flow in the molten pool were observed through CCD camera and X-ray real-time transmission apparatus. It was consequently found that hybrid welding possessed many features in comparison with YAG laser welding. The deepest weld bead could be produced when the YAG laser beam of high power density was shot on the molten pool made beforehand stably with TIG arc. A keyhole was long and narrow, and its behavior was rather stable inside the molten pool. It was also confirmed that porosity was reduced by the suppression of bubble formation in hybrid welding utilizing a laser of a moderate power density.

Mid 80^th the steel industry discovered the laser as a tool to develop new products made from steel -- the tailored blanks. That means welding single blanks together, which are of different gauge or grades and coating. In the meantime this product is one of the key solutions for light weight vehicles with increasing performances. The market development world wide confirms this statement. But the development of this product is still going on. New high power lasers and new laser generations as Nd:YAG lasers are the basis. Today welded blanks with almost any seam/blank configuration are in high volume production. These blanks offer an additional potential for the optimization of the final product. To produce flat blank is only one possibility. New developments are the tailored tubes as a prematerial for the hydroforming process. This product becomes more and more important for optimized body in white solutions. But this design elements need new solutions in the assembly shops. So the laser is going to get more importance in the 3D welding process as well. This was shown for example in the ULSAB(-AVC)-project. Future vehicles more and more contain different materials. For example the joining of steel and aluminum to Hybrid Blanks can be done successfully by the use of laser. So the laser is one of the most important tools in the future.

Laser-arc hybrid welding combines the laser and arc welding processes to provide advantages not found in either. This process can weld lapped steel sheets that have a larger gap than is possible with laser welding. Blowholes form when lap-welding zinc-coated steel sheets because of the zinc that is vaporized. The laser-arc hybrid welding process can lap-weld zinc-coated steel sheets without causing blowholes. The welding speed of laser-arc hybrid welding is nearly equivalent to that of laser welding. Laser-arc hybrid welding produces high-quality lap joints and is ideal for assembly welding of automotive parts.

Laser welding for automotive bodies has been introduced mainly by European car manufacturers since more than 10 years ago. Their purposes of laser welding introduction were mainly vehicle performance improvement and lightweight. And laser welding was applied to limited portion where shapes of panels are simple and easy to fit welded flanges. Toyota also has introduced laser welding onto 3 dimensional parts named trough panel since 1999. Our purpose of the introduction was common use of equipment. Trough panel has a complex shape and different shapes in each car type. In order to realize common use of welding equipment, we introduced parts locating equipment which had unique, small & simple jigs fo each car type and NC (Numerical Controlled) locators and air-cooled small laser head developed by ourselves to the trough welding process. Laser welding replaced spot welding and was applied linearly like stitches. Length of laser welding was determined according to comparison with statistic tensile strength and fatigue strength of spot welding.

YAG laser welding for body in white has been developed and applied to the joint of the roof and body side of the new Skyline. Several laser welding features, such as one-side welding and continuous welding can be used to, improve joint structure. To install laser facilities in body assembly shops, we have developed a reliable facility and gap control system.

In order to reduce the weight of automobiles, the laser transformation hardening response of ultra-low carbon steels (0.003%C) was investigated by comparison with the response of low carbon steels (0.08%C). The effects of the nitrogen and titanium content on hardness are discussed. In the low carbon steels, the hardness of the weld metal showed little change with increases in the titanium content. However, in the ultra-low carbon steels, hardness increased with increasing titanium content. In the low carbon steels, the microstructures of the weld metal, which were comprised mainly of martensite, changed little with increasing titanium content. On the other hand, in the utlra-low carbon steels, the microstructures were bainitic ferrite, and the grains were refined with increasing titanium content. With increasing N₂ content in the shielding gas, the hardness of the weld metal increased due to nitrogen absorption in the weld metal. Although the nitrogen contents of the ultra-low carbon steel welds were equivalent to those in the low carbon steel welds, the hardness increments of the former were larger than those of the latter. It was confirmed that increasing the N₂ content of the shielding gas is beneficial for strengthening ultra-low carbon steels by laser transformation hardening.

Laser technology in Bosch-production has an already over twenty-five year old tradition and with the advancement of laser beam sources and process engineering always new ranges of application have been found. The advantages of the laser as a tool for material processing lie in the fact that it fulfills the Bosch-specific requirements regarding the product spectrum (small, heat sensitive construction units) and serial production strategies (line manufacturing with high numbers of pieces) by favorable process advantages, like short process times, small energy input and good automizability. The main operational areas of the laser are at present in the product ranges automotive equipment and electronics. Especially in the fuel injection technology manufacturing is hard to imagine without laser material processing. Lasers are used with Bosch in various ways for welding, marking, surface treatment, soldering and drilling. In addition trimming and scribing of semiconductor elements is applied for controllers and sensors. Laser cutting -- with the exception of precision cutting of rotational symmetric parts -- is used only in the manufacturing of prototypes because it is uneconomic in mass production compared to punching and has no technological advantages. Future main areas of development will be microjoining and -ablation, joining of plastics and surface finishing.

Since the mid-1980s, Toyota Motor Corporation has applied CO₂ lasers and YAG lasers to machine (welding, piercing, cutting, surface modifying etc.) automobile parts. In recent years diode lasers, which are excellent in terms of cost performance, are now available on the market as a new type of oscillator and are expected to bring about a new age in laser technology. Two current problems with these lasers, however, are the lack of sufficient output and the difficulty in improving the focusing the beam, which is why it has not been easy to apply them to the machining of metal parts in the past. On the other hand, plastics can be joined with low energy because they have a lower melting point than metal and the rate of absorption of the laser is easy to control. Moreover, because the high degree of freedom in molding plastic parts results in many complex shapes that need to be welded, Toyota is looking into the use of diode lasers to weld plastic parts. This article will introduce the problems of plastics welding and the methods to solve them referring to actual examples.

The full-scale use of lasers in the steel industry began 25 years ago with their applications as controllable light sources. The laser systems contribute to increase efficiency and quality of the steel making processes, and also save energy of resources and labor. Laser applications in the steel making process generally require high input energy, however, it is essential to consider the interaction between the laser beam and materials. 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. We newly developed methods and systems of high power 45 kW CO₂ laser welding of hot steel specimens with their applications as welding characteristics of hot steel specimens that temperature is about 1000 degree C, have been examined. Using laser induced plasma as a secondary heat source, the penetration depth improves about 30% compared to that at room temperature. The bead width is also enlarged by 10%. The maximum depth is 38 mm at 1m/min welding velocity at 40 kW. A beam weaving method is adopted for further enlargement of bead width without degrading fusion efficiency. It is also effective for suppressing the bead depth deviation. Additionally, several new applications, for example, new type all-laser-welded honeycomb panels for high- speed civil transport, will be talked.

Influence of oxide film, such as laser cutting edge or mill-scale, on solidification crack formation in high power CO₂ laser of mild steel was investigated. Laser welding test of butt joints were performed. Examinations were made on the solidification crack formation. The results of these experiments lead to understanding that (1) excess oxygen supplied from oxide film causes solidification crack as well as porosity and (2) influence of oxygen content is bigger than that of sulfur when there is excess oxygen in the weld. It is considered that these are attributed to change of molten metal flow and drop of melting point such as sulfur. These results suggest that reduction of oxide film thickness or killing oxygen activity by supplementary deoxidizing elements may produce acceptable weld.

In the field of heavy industries, many products are made of thick metal parts. Nd:YAG laser has been recently developed up to 10 kW. Nd:YAG laser has the characteristics of the optical fiber transmittance and the good absorption by the metal surface, so that it is expected to apply Nd:YAG laser to thick plate welding. This study presents the thick plate welding with Nd:YAG laser and COIL (Chemical Oxygen-Iodine Laser). We have developed a coaxial beam combining system with beams of Nd:YAG laser and COIL. The maximum average power of the combined beam was 19 kW. Welding tests of 304 stainless steel plates were carried out. The effects of various welding parameters were investigated, such as the laser power and pulse modulation. As a result, it was clarified that the pulse wave has good efficiency of deeper penetration as compared to continuous wave at low welding speed. When the combined beam was used, 20 mm penetration depth on the stainless steel could be obtained in high aspect ratio at welding speed of 1m/min. When the combined beams and another Nd:YAG laser beam whose power was 4 kW were used, both side welding on 30 mm thickness plate could be achieved.

The use of a combination of 2 or 3 Nd:YAG lasers having each an output power of 3 or 4 kW is presented in the case of heavy section welding. We discuss the main difficulties that are occurring for these conditions. The strategy leading to the welding of sample thickness up to 60 mm is presented.

The strength of laser welding with lap joints depends on its welding length and width of bead at plates interface. Therefore, the narrow bead width of laser lap welding joints at plates interface forces to apply multi-times welding to get required strength. The goal of this paper is to increase interface bead width on thick plate laser lap welding. The authors turn their attention to the penetration shapes of high power CO₂ laser welding for achieve that. It is well known that penetration shapes are like a wine cup in the case of deep penetration welding using high power CO₂ laser. The penetration shape of the wine cup is suggested that a large amount of laser power is absorbed at the surface of the test pieces. Then, the authors have studied how to change the heat source distribution on laser lap welding, and have succeeded to increase the bead width at the interface by a new developed process.

An investigation to apply YAG laser welding to gas turbine components was carried out. The materials of gas turbine such as Ni base alloy are difficult to weld by conventional arc welding methods because of large heat affection. But laser welding can reduce heat input compared with conventional methods and keeps the good repeatability. The welding parameter survey was carried out to satisfy the designing requirements. The YAG laser welding under appropriate conditions enables to prevent welding defects such as HAZ cracks and improves the weld joints quality and performance. Tensile test and low cycle fatigue test were carried out. Tensile strength and fatigue life of laser weld joints are same or higher than that of conventional manual TIG weld joints. The Automatic YAG laser welding system was also developed and put into practical use.

Laser beam welding of primary aircraft structures manufactured from aluminum alloys is considered to have a great potential in cost saving. In order to evaluate this advantage, a technology program has been adopted at EADS, Military Aircraft. The goal was to manufacture air intake shells for the Eurofighter in a cost efficient way. Stretch formed skins and machined stiffeners are joined together with laser beam welding. The baseline for a comparison in terms of cost and weight was the conventional process based on stretch forming of thick plates and subsequent milling. The major tasks of the program have been the optimization of the twin focus laser beam welding process and the proof of the structural integrity including weld strength evaluation.

The purpose of this study is to evaluate the static strength of thick plate laser welded lap joint and the performance of thick sandwich panel for bridge deck structure. The strength of thin sheet laser welded lap joint whose thickness is less than 1.6 mm have been already reported, but the strength of thick plate laser welded lap joint whose thickness is more than 10 mm is not known. The authors believe this thick laser welded lap jont enable heavy industries to make a revolution on applying it for sandwich panel fabrication. Then, they have conducted mechanical tests of 10 mm + 6 mm thick laser welded lap joints to grasp their static strength. According to the obtained data, they fabricate a thick sandwich panel model of newly designed bridge deck structure for evaluation. The static lading test, cyclic loading test and collapsing test show that the thick sandwich panel has enough performance for the practical application.

Laser processing using the high-density energy beam concentrated on a very small area spot size has been advanced in the industrial fields of cutting and welding. Also of it, cutting with small heat-affect and narrow kerf width has become possible for high power CO₂ lasers. In recent years laser processing machines have replaced the conventional means of production in the sheet metal industry. With this has come a greater expectation for the laser processing machine to process the parts of large-item small-volume production with greater precision and better efficiency. Moreover, laser cutting, due to its greater precision and higher stability, is spreading to other fields such as the machine used for the construction of thick-plated equipment. This field has traditionally used gas cutting and plasma cutting in its manufacturing process. This paper will describe the current cutting technology of high power CO₂ laser. It will explain the relationship between the laser beam's characteristics and its quality performance of cutting. It will then go on to outline the current laser processing machines in which technology has been applied.

In nuclear power plants, irradiated materials like Control Rod (CR) should be stored underwater after service. Due to reducing the storage space, underwater cutting technology is expected. In this study, we developed underwater cutting technology of thick stainless steel with YAG laser in order to cut used CR. Preliminary tests were performed with flat plate test-pieces to optimize the cutting conditions. Due to creating a local dry area between nozzle and test-piece, high-pressure air was blown from the nozzle. Underwater laser cutting was carried out by laser irradiation power of 4 kW, changing the parameters of cutting speed, distance between the nozzle and test-piece, and thickness of the test-piece. We also investigated the wastes like dross and aerosols by laser cutting. Amount of dross was approximately 0.1 kg/m after cutting a 14 mm thick stainless steel plate, which is estimated to be less than other cutting method. Based on these results, we developed underwater cutting system of CR test-piece with YAG laser as a mock-up test. In the cutting torch, there was tracking system was introduced to keep the distance between the nozzle and the test-piece constant, and cutting monitor was also set-in to detect whether the test-piece was successfully cut or not. We have already tried to cut the CR test-piece with this facility and successfully cut in half.

Laser-based cutting and drilling of polymers and natural fiber composites has already covered a market niche, even though fundamentals of heat transfer and multiphase material balances are not well characterized up to now. Discussing the decomposition and vaporization balance, a recommendation on process parameters is given. Within this paper, criteria for laser-based production are given as well as requirements of the material. The investigation focus on strategies to control the pyrolysation and decomposition of the material as well as the prognosis of the process ability derived from laser Micropyrolysis-GC/MS correlated with industrial applications.

Laser shock processing (LSP) is an emerging industrial process in the field of surface treatment with particular application to the improvement of fatigue and corrosion properties. In the standard configuration, the metal sample is coated with a sacrificial layer in order to protect it from detrimental thermal effects, and a water overlay is used to improve the mechanical coupling by a confining like effect. Whereas the induced mechanical effects are now well understood, very few studies have been realized concerning the thermal effects. For this purpose, the knowledge of the confined plasma microscopic parameters has a great importance. A complete model describing the laser-liquid-metal interaction is presented. The model predicts the time evolution of the plasma parmmeters (temperature, density, ionization) and allows us to compute the induced pressure and temperature in the metal sample. By comparing the numerical results with various experimental measurements, predictions can be made concerning the best laser irradiation conditions for LSP.

Novel lightweight applications in the automotive and aircraft industries require advanced materials and techniques for surface protection as well as direct and rapid manufacturing of the related components and tools. The manufacturing processes presented in this paper are based on multiple additive and subtractive technologies such as laser cutting, laser welding, direct laser metal deposition, laser/plasma hybrid spraying technique or CNC milling. The process chain is similar to layer-based Rapid Prototyping Techniques. In the first step, the 3D CAD geometry is sliced into layers by a specially developed software. These slices are cut by high speed laser cutting and then joined together. In this way laminated tools or parts are built. To improve surface quality and to increase wear resistance a CNC machining center is used. The system consists of a CNC milling machine, in which a 3 kW Nd:YAG laser, a coaxial powder nozzle and a digitizing system are integrated. Using a new laser/plasma hybrid spraying technique, coatings can be deposited onto parts for surface protection. The layers show a low porosity and high adhesion strength, the thickness is up to 0.3 mm, and the lower effort for preliminary surface preparation reduces time and costs of the whole process.

Cladding of an austenitic stainless steel with the cobalt-based alloy Stellite 6, a trademark of Deloro Co, has been investigated by using both CO₂ and Nd:YAG laser beams. This material is used for hardfacing in a number of industries, notably power generation and heavy engineering. Alloy powder was fed into the laser beam by using argon as a carrier gas. Clads were produced with a range of processing parameters, and sectioned for metallographic examination. Hardness values measured in the clads increased to a maximum with an increase in powder feed rate. This correlated with a decrease in the dendrite arm spacing observed in the micrographs. Abrasive wear testing also indicated that a finer microstructure resulted in improved properties. The Nd:YAG laser beam was found to be more efficient for melting the powder because it is absorbed to a greater extent than the CO₂ laser beam. Cladding procedures were developed for both types of laser, and it is shown that in order to maximize in-service performance, the energy input of the process should be minimized.

Different weight ratio of nickel based alloy, titanium and graphite powders were mixed and then laser cladded onto carbon steel substrate to produce a surface metal matrix composite layer. The experimental results showed that the coating was uniform, continuous and free of cracks. An excellent bonding between the coating and the carbon steel substrate was ensured by the strong metallurgical interface. The microstructures of the coating were mainly composed of γ-Ni dendrite, M₂₃C₆, a small amount of CrB, and dispersed TiC particles, and the in-situ generated TiC_p/matrix interfaces were clean and free from deleterious surface reaction. The morphologies of TiC particles changed from the global, cluster to flower-like shape, the volume fraction of TiC_p and the microhardness gradually increased from the bottom to the top of the coating layer, and the maximum microhardness of the coating was about HV_0.2850, 3 times larger than that of steel substrate. The volume fraction of TiC particles increased with increasing of volume fraction of Ti and C too.

Laser direct deposition of metallic parts is a new manufacturing technology, which combines with computer-aided design, laser cladding and rapid prototyping. Fully dense metallic parts can be directly obtained through melting the coaxially fed powders with a high-power laser in a layer-by-layer manner. The process characteristics, system composition as well as some research and advancement on laser direct deposition are presented here. The microstructure and properties observation of laser direct formed 663 copper alloy, 316L stainless steel and Rene'95 nickel super alloy samples indicate that, the as-deposited microstructure is similar to rapidly solidified materials, with homogenous composition and free of defects. Under certain conditions, directionally solidified microstructure can be obtained. The as-formed mechanical properties are equal to or exceed those for casting and wrought annealed materials. At the same time, some sample parts with complicate shape are presented for technology demonstration. The formed parts show good surface quality and dimensional accuracy.

Surface modification of martensitic stainless steel 440C was achieved by laser surface melting using a 2-kW continuous wave Nd:YAG laser. The corrosion characteristics of laser-melted specimens fabricated under different processing parameters in 3.5% NaCl solution at 23°C were studied by potentiodynamic polarization technique. The pitting corrosion resistance of all laser-melted specimens was significantly improved, as evidenced by a shift of the pitting potential in the noble direction, a wider passive range, and a lower passive current density. The pitting potential of the laser-melted specimen fabricated under a power density of 4.2 kW/cm² and a scanning speed of 25 mm/s (specimen P12-440C-25) was increased to 200 mV (SCE), which was much higher than that of annealed and conventionally heat-treated 440C. The pitting corrosion characteristics of the laser-melted specimens were strongly dependent on the processing conditions which resulted in different microstructures. The enhanced pitting resistance resulted from the combined effect of dissolution or refinement of carbide particles and the presence of retained austenite. By using different processing parameters, it was found that the pitting resistance of the laser-melted specimen P12-440C-25 was the highest by virtue of a high volume fraction of retained austenite and a small amount of precipitated carbide.

TIG arc welding has been used to date as a method for clad welding of white metal as bearing material. We propose a new clad welding process that combines a CO₂ laser and a TIG arc, as a method for cladding at high speed. We hypothesized that this method would permit appropriate control of the melted quantity of base metal by varying the laser power. We carried out cladding while varying the laser power, and investigated the structure near the boundary between the clad layer and the base metal. Using the laser-TIG combined cladding, we found we were able to control appropriately the degree of dilution with the base metal. By applying this result to subsequent cladding, we were able to obtain a clad layer of high quality, which was slightly diluted with the base metal.

Quest for a material to suit the service performance is almost as old as human civilization. So far materials engineers have developed a series of alloys, polymers, ceramics, and composites to serve many of the performance requirements in a modern society. However, challenges appear when one needs to satisfy more than one boundary condition. For example, a component with negative Coefficient of Thermal Expansion (CTE) using a ductile metal was almost impossible until recently. Synthesis of various technologies such as Direct Metal Deposition (DMD) Homogenization Design Method (HDM) and mutli material Computer Aided Design (CAD) was necessary to achieve this goal. Rapid fabrication of three-dimensional shapes of engineering materials such as H13 tool steel and nickel super alloys are now possible using Direct Materials Deposition (DMD) technique as well as similar techniques such as Light Engineered New Shaping (LENS) or Directed Light Fabrication (DLF). However, DMD has closed loop capability that enables better dimension and thermal cycle control. This enables one to deposit different material at different pixels with a given height directly from a CAD drawing. The feedback loop also controls the thermal cycle. H13 tool steel is one of the difficult alloys for deposition due to residual stress accumulation from martensitic transformation. However, it is the material of choice for the die and tool industry. DMD has demonstrated successful fabrication of complicated shapes and dies and tools, even with H13 alloys. This process also offers copper chill blocks and water-cooling channels as the integral part of the tool. On the other hand ZrO₂ was co-deposited with nickel super alloys using DMD. Flexibility of the process is enormous and essentially it is an enabling technology to marterialize many a design. Using DMD in conjunction with HDM and multi-material CAD, one can produce components with predetermined performance such as negative co-efficient of expansion, by synthesis of designed microstructure. This paper briefly reviews the state of the art of DMD and describes the synthesis of three core technologies to produce designed materials with desired performance.

This paper describes the tailoring of thermoplastics and the laser assisted generation of active regions that catalyze a chemical metal deposition in a liquid plating solution on these thermoplastics. The plated regions may serve as an electronic circuit on the insulating thermoplastic substrate. Tailoring of the thermoplastic materials is achieved by addition of a special activator powder. This powder consists of micro-encapsulated catalytic core particles with a non-catalytic barrier layer on their surface. Different encapsulation methods are compared for a copper core material. Laser activation is examined using a cw CO₂ laser and an Nd:YAG laser. No activation is achieved with the CO₂ laser. With the Nd:YAG laser and suitable process parameters, activation is possible even at writing speeds of up to 650 mm/s. A qualitative model is presented that explains the basic mechanisms of the laser activation process.

The combination of dissimilar materials is a challenging goal for the development of parts with locally optimized properties. The main goal of these developments are weight reduction, optimization of properties or the tailoring of the properties for specific applications in combination with an efficient joining technology. However, using conventional high temperature joining technologies, the formation of intermetallic phases within the joining zone is a nearly unavoidable phenomenon when joining dissimilar material combinations. These phases cause a lack of the mechanical stability in the joining zone. By using an optimized laser joining technology for thin sheet materials this problem could be overcome. The localized energy input of the laser beam and a controlled heat distribution leads to minimized interaction of the joint materials. To overcome process instabilities a special working head technic was developed. At present, Fe-Al and Ti-Al material combinations are successfully joined by this process. Metallurgical and mechanical properties of a number of selected dissimilar material joints will be presented and applications discussed.

Since global purchasing in the automotive industry has been taken up all around the world there is one main key factor that makes a TB-supplier today successful: Producing highest quality at lowest cost. The fact that Tailored Blanks, which today may reach up to 1/3 of a car body weight, are purchased on the free market but from different steel suppliers, especially in Europe and NAFTA, the philosophy on OEM side has been changing gradually towards tough evaluation criteria. "No risk at the stamping side" calls for top quality Tailored- or Tubular Blank products. Outsourcing Tailored Blanks has been starting in Japan but up to now without any quality request from the OEM side like ISO 13919-1B (welding quality standard in Europe and USA). Increased competition will automatically push the quality level and the ongoing approach to combine high strength steel with Tailored- and Tubular Blanks will ask for even more reliable system concepts which enables to weld narrow seams at highest speed. Beside producing quality, which is the key to reduce one of the most important cost driver "material scrap," in-line quality systems with true and reliable evaluation is going to be a "must" on all weld systems. Traceability of all process related data submitted to interfaces according to customer request in combination with ghost-shift-operation of TB systems are tomorrow's state-of-the-art solutions of Tailored Blank-facilities.

In a EU Craft project including eight partners in five countries, the most important aspects regarding efficient and low cost manufacture by small and medium sized enterprises (SME) of tailored blanks has been systematically investigated. In this paper, small batch series laser welding of tailored blanks in SME will be described. This includes the design, development and systematic use of a flexible and low cost clamping device as well as the practical experience obtained on the job shop through systematic optimization of welding of tailored blanks with even and uneven thickness (0.75 and 1.25 mm). A clamping device that is able to hold finished parts up to 1 x 1 m is successfully manufactured and tested. A special arrangement with alignment needles along the weld line is used to precisely position the sheets. These needles are turned into the clamping device during welding, where root shielding is employed. Hydraulic presses hold down the sheets, so they move less than 0.01 mm during welding. High quality tailored blank welds are successfully manufactured in ten different combinations, including mild steel and medium strength steel with even and uneven thickness with and without zinc coating.

Laser direct-write and doping technique (LDWD) is used to introduce variations in electric properties of wide band gap materials such as SiC and diamond. Conductive, p-type doped, n-type doped and insulative tracks are created on different diamond and SiC substrates using this method. The effects of various processing parameters such as laser-matter interaction time, number of repeated exposures, and type of irradiation environment are investigated. SEM, SIMS, XPS and Raman spectroscopy are used to study the effect of laser irradiation on the microstructure, chemical binding and to obtain dopant depth profile in the substrates, respectively. LDWD technique proved to enhance the dopant (nitrogen) diffusivity into SiC resulted in a diffusion coefficient (available in paper)that is four orders of magnitudes faster than the reported value (5 x 10^-12 cm²s^-1). Process modeling is conducted to study the atomistic of laser-doping process and to utilize laser irradiation to increase both dopant penetration and concentration. Laser doping of nitrogen alters the Raman spectrum of the 4H-SiC suggesting that Raman spectroscopy can be used as a non-contact method to characterize the laser-doped SiC.