Takashi Ishide

Modelling of keyhole dynamics and porosity formation considering the adaptive keyhole shape and three-phase coupling during deep-penetration laser welding

The joint quality of deep-penetration laser beam welding is related to the keyhole behaviour, e.g. keyhole-induced porosities. In this paper, a model which considered the existence of three phases, including plasma gas, liquid metal and solid metal, was proposed to describe the keyhole phenomena of laser welding. The forces of interaction of fluid dynamics in the keyhole and molten pool were modelled using the CFD software, and an adaptive heat source model was proposed for the absorption of laser energy. The molten pool and keyhole phenomena of laser beam welding were simulated using the developed model, as well as the formation of keyhole-induced porosities. It was found that the keyhole depth self-fluctuates in continuous laser welding, and the bubbles formed from keyhole collapse and shrinkage are the cause of keyhole-induced porosity.

Latest MIG, TIG arc-YAG laser hybrid welding systems for various welding products

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.

10-kW-class YAG laser application for heavy components

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

Optical fiber transmission of 2 kW cw YAG laser and its practical application to welding

The YAG laser has an excellent characteristic that makes beam transmission with optical fibers. In addition, the oscillator of the 2 kW class has recently been developed, and so its applications are changing largely from conventional fine material processing. The authors have adopted the YAG laser welding that uses optical fibers favorable to application to narrow, complicated areas. First, the authors have analyzed the beam intensity distribution near the focusing point using ray tracing method, and manufactured the compact condensing optical lens systems, and metal mirrors which have high reflectivity and durability that can be inserted in the tubes of about 16 mm I.D. to weld them. And also have developed the beam transmission system that can transmit beam for a long distance of about 200 m using small diameter optical fibers. In addition, with these systems the welding conditions to get about 2 mm penetration for stable welding has been obtained and applied YAG laser welding of steam generator tubes in practical nuclear power plants.

Development of TIG-YAG and MIG-YAG hybrid welding

Laser welding is seeing progressively widespread applications as a low-strain and high-efficiency welding technique. The YAG laser offers easier beam transmission and more stable beam quality than the CO 2 laser. Available applications include welding of nuclear power plant related components with a 7 kW class system, use of a 10 kW class laser, for cutting in decommissioning of nuclear reactors, and welding of gas turbine components at plate thicknesses of up to 25 mm with a 10 kW class laser. Such applications, however, require a high degree of weld processing accuracy, being limited to products with high groove processing accuracy and fit-up accuracy. For components with unsatisfactory processing accuracy, it is necessary to arrange the working environment so that the lasers can be applied at stages ahead of the welding process. Such obstacles have previously limited laser welding applications. The authors have sought to achieve substantial expansion of laser welding at currently available levels of high quality and high efficiency by facilitating laser application to conventionally arc-welded products through arc/laser hybridisation. In this context, they have investigated hybrid welding systems involving non-coaxial (tandem) configuration of the arc and laser and coaxial configuration of the arc and laser, the latter to achieve a more compact welding head and to allow application to three-dimensional components. This article profiles the characteristics of the newly developed systems and also adduces specific results demonstrating their superiority.

Coaxial TIG‐YAG & MIG‐YAG welding methods

The practical use of laser welding has been promoted as a low strain and highly efficient welding process; in comparison with the CO2 laser, the YAG laser facilitates beam transmission and has a stable beam quality, so its practical use in the following areas has been advanced: welding of nuclear power related apparatus using a 7 kW class system, application to decommissioning cutting techniques using a 10 kW class laser and welding of gas turbine components using a 10 kW class YAG laser. However, the application of these techniques is restricted to certain products with high work accuracy for welds, high groove work accuracy and high butting accuracy. In cases where components have no high product work accuracy it is necessary to prepare the environment at the preceding stage of the welding process so that laser can be applied; this is one of the factors which inhibits the practical use of laser welding. Accordingly, the authors have facilitated the application of lasers to products, to which welding was conventionally carried out by arc welding, by improving the hybridisation of the arc and laser to a practical level with the aim of the expansion of a further application of laser welding capable of achieving high quality and highly efficient welding. In particular, attempts were made, by means of hybridisation, of relaxing target groove tolerance, of achieving higher quality by controlling porosity, of stabilising the MIG arc under a pure Ar atmosphere and also of facilitating high speed welding. In addition, with the aim of applying this technique to three dimensional shaped sections and to develop a compact welding head, an investigation was carried out into coaxial TIG-YAG and MIG-YAG welding systems where the arc and laser beam are arranged to be coaxial. In this report, the characteristics of the systems developed are described and the advantages of such processes are demonstrated.

Visualization for Interaction between Laser Beam and YAG-Laser-Induced Plume

This study was performed to obtain a fundamental knowledge of the effect of a laser-induced plume on the absorption or attenuation, refraction and defocusing of the incident laser beam during welding. The interaction of the near-infrared laser beams such as a YAG laser and a fiber laser with the YAG laser-induced plume was investigated by measuring the passing-through power and by visualizing the beam behavior in terms of the attenuation and the refraction of the laser beam. The attenuation of the probe laser was confirmed to increase with a decrease in the wavelength, and the maximum attenuation of the fiber laser was about 5 %. Such attenuation was attributed to Rayleigh scattering. On the other hand, the laser beam was bent only about 0.6 mrad by the refraction. The refraction was further evaluated by observing the behavior of a fiber laser beam through the plume. The visualization of the spot behavior revealed that the interaction between the near-infrared laser beam and the YAG laser-induced plume was considerably small. It was consequently concluded that the fiber and YAG lasers were recognized to deliver a desirable wavelength with the slight interaction against the laser-induced plume.This study was performed to obtain a fundamental knowledge of the effect of a laser-induced plume on the absorption or attenuation, refraction and defocusing of the incident laser beam during welding. The interaction of the near-infrared laser beams such as a YAG laser and a fiber laser with the YAG laser-induced plume was investigated by measuring the passing-through power and by visualizing the beam behavior in terms of the attenuation and the refraction of the laser beam. The attenuation of the probe laser was confirmed to increase with a decrease in the wavelength, and the maximum attenuation of the fiber laser was about 5 %. Such attenuation was attributed to Rayleigh scattering. On the other hand, the laser beam was bent only about 0.6 mrad by the refraction. The refraction was further evaluated by observing the behavior of a fiber laser beam through the plume. The visualization of the spot behavior revealed that the interaction between the near-infrared laser beam and the YAG laser-induced plume wa...

Application of 7 kW class high power yttrium–aluminum–garnet laser welding to stainless steel tanks

Laser beam welding is used for precise parts such as core internal parts in nuclear power plants that require high quality. To weld large-scale and thick-wall products, the high power laser beam must be transferred and a deep penetration welding procedure must be developed. In this article, therefore, an optical fiber transmission system for 7 kW class high power yttrium–aluminum–garnet (YAG) laser and pulse modulated laser welding techniques were developed to obtain deep penetration. The detailed observation of the weld pool and keyhole dynamics using a high-speed camera and x-ray transmission system was carried out to understand high power YAG laser welding phenomena. It has been clarified that there are proper pulse duty and pulse duration for optimum welding condition to obtain sound and efficient weld. After confirmation of the high power YAG laser welding joint performance, this procedure has been applied to the welding of stainless steel tanks for the nuclear fuel reprocessing plant.

Application of pulse-modulated high-power YAG laser to welding of heavy plates

Lasers operating in air at high energy density are extensively used in a wide variety of industrial fields for thermal processing, such as welding, cutting, etc. The authors have long recognised the superior qualities (low distortion, low heat input) and high efficiency of laser welding and have previously sought to apply a 5 kW CO 2 laser in manufacturing processes in the nuclear power field governed by high precision and rigorous quality standards. For welding of heavy plates targeted by the authors, however, laser powers must be substantially uprated. High power uprating of the CO 2 laser has progressed apace. These advances have, for example, led to application of a 45 kW CO 2 laser in steelmaking. From the perspective of high available power, the conventional CO 2 laser has also seen applications in laser welding of heavy plates. In present circumstances, however, CO 2 laser welding faces some uncertainty in terms of weld quality assurance through the need to incorporate measures for laser-generated plasma control during welding of heavy plates and through measures needed to counteract the changes in beam quality due to the thermal lens effect of transmission-type optical components. Depending on mirror transmission, the CO 2 laser optical system is highly complex, and the beam intensity distribution system in the processing zone is also liable to vary because of the high beam path length. Another drawback of the CO 2 laser is somewhat limited application to heavy industry with its emphasis on numerous large-sized machine components manufactured as one-off items or in very short series. The high-power YAG laser, on the other hand, facilitates optical fibre transmission and is therefore used in the manufacture of large-sized products with complex shapes. This type of laser is also reported to be superior in process terms through being less sensitive to laser-generated plasma and the thermal lens effect of optical components. In recent years, the YAG laser has seen good high power uprating progress, with 3–5 kW class systems being used in the automotive field. While the continuous pulse oscillation type of laser has been focused on heavy plate applications, 7–10 kW class pulse-modulated laser oscillators have been developed for pulse wave oscillation applications with peak powers up to 3-fold of average powers. Through establishment of optical fibre transmission technology and a deep-penetration welding procedure for the high-power YAG laser, it is expected to be possible at a stroke to extend applications of this laser to welding of heavy plates, including areas so far dominated by the conventional CO 2 laser. This article describes application of a ‘world’s biggest’ 7 kW class pulse-modulated YAG laser with a maximum peak power of 21 kW on an actual production line. Through application of optical fibre transmission of optimisation of pulse welding conditions in the high-power range, a technique for deep-penetration welding of heavy plates is established. The keyhole behaviour during welding is examined with a high-speed video camera. An X-ray transmission system is also used to investigate the deeppenetration mechanism and the mechanisms of weld defect generation and prevention during welding of heavy plates. Full-sized application of the newly developed high-power pulse-modulated YAG laser to welding of tanks for a nuclear fuel reprocessing plant is finally reported.

In-process monitoring of weld qualities using multi photo-sensor system in pulsed Nd:YAG laser welding

We developed a robust and economical technique capable of estimating penetration depth and distinguishing between full- and partial- penetration welding based on the light emission from keyhole. Light intensities of the vapor inside and above the keyhole could be measured by using multiple photo-diodes arranged at different aiming angles. High-speed photographs were also taken in synchronization with photo-diodes to observe dynamic behaviors of the plume and the keyhole opening and to assist the interpretation of the photo-diode signals. The keyhole was divided into two parts, a cup part and a finger part. These two parts could be then detected separately by using the multi photo-sensor system. The results show that light emission from the cup part and the finger part can be separately detected; the finger part emission can be used to estimate the penetration depth, while the cup part emission can be used to distinguish between full and partial- penetration welding.We developed a robust and economical technique capable of estimating penetration depth and distinguishing between full- and partial- penetration welding based on the light emission from keyhole. Light intensities of the vapor inside and above the keyhole could be measured by using multiple photo-diodes arranged at different aiming angles. High-speed photographs were also taken in synchronization with photo-diodes to observe dynamic behaviors of the plume and the keyhole opening and to assist the interpretation of the photo-diode signals. The keyhole was divided into two parts, a cup part and a finger part. These two parts could be then detected separately by using the multi photo-sensor system. The results show that light emission from the cup part and the finger part can be separately detected; the finger part emission can be used to estimate the penetration depth, while the cup part emission can be used to distinguish between full and partial- penetration welding.