Shuho Tsubota

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.

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.

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.

Development of laser welding technology for fully austenitic stainless steel

Abstract The radial stainless steel plates (RPs) used for Toroidal field (TF) coils in the International Thermonuclear Experimental Reactor (ITER) are 13 m long, 9 m wide and 10 cm thick, which are quite large. Even though they are very large structures, high manufacturing tolerances and high mechanical strength at 4 K are required. It is also required that each RP should be fabricated every three weeks. Therefore, the authors intend to develop efficient manufacturing methods for an ITER TF coil RP. Laser welding is then selected as a welding method for RP. Especially, the development of high technology laser welding is necessary to prevent hot cracking in the material used for the RP; namely, fully austenitic stainless steel with high nitrogen content. The authors carried out trial laser welding experiments aiming at its application to RP. As a result, it was effective to make the angle of back inclination of the weld head at a uniform welding speed. It also seemed that the sensitivity of hot cracking could be reduced by optimizing the chemical compositions of material used for RP. The base material and the welded joints satisfied mechanical properties in 4 K. The application of the laser welding technology to the fully austenitic stainless steel was therefore demonstrated.

Application of YAG laser welding to gas turbine components

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.

Development of 10kW class YAG laser welding technology

Recently laser power is increased rapidly and the laser welding ability is advanced, then we can use it for thick plate welding in heavy industries. In such a background, we aimed to apply YAG laser welding to manufacture our products, and introduced 10kW and 7kW class YAG laser processing system. Then we have developed the optical fiber transmitting technology for high power beam, and welding technology which could make over 20mmt 1pass weld joint. To achieve such welding ability, we optimized welding parameters and controlled keyhole state, penetration shape and welding efficiency. Then we could get the high quality weld efficiently. After we confirmed the mechanical property of weld joint based on the examination standard for power plants, we applied the YAG laser welding to manufacture stainless vessels for reprocessing plants.Recently laser power is increased rapidly and the laser welding ability is advanced, then we can use it for thick plate welding in heavy industries. In such a background, we aimed to apply YAG laser welding to manufacture our products, and introduced 10kW and 7kW class YAG laser processing system. Then we have developed the optical fiber transmitting technology for high power beam, and welding technology which could make over 20mmt 1pass weld joint. To achieve such welding ability, we optimized welding parameters and controlled keyhole state, penetration shape and welding efficiency. Then we could get the high quality weld efficiently. After we confirmed the mechanical property of weld joint based on the examination standard for power plants, we applied the YAG laser welding to manufacture stainless vessels for reprocessing plants.