Infrared CO₂ and YAG lasers have great difficulty in melting metals such as pure aluminum and copper possessing high reflectivity and high thermal conductivity. On the other hand, the laser absorption of such metals generally increases with a decrease in the wavelength, and thus the development of a laser of higher power and shorter wavelength is promising. Recently, Q-switched SHG (Second Harmonic Generation) YAG laser of the maximum power of 50 W, the wavelength of 532 nm and the maximum repetition rate of 50 kHz has been developed; however, this laser has never been employed as a welding heat source. In this study, therefore, coaxially focusing optical system, in which a pulsed YAG laser beam was combined with a Q-switched SHG laser beam, was designed and constructed to evaluate melting characteristics of various metals with respective laser beams and especially the effect of the combined SHG laser. A deep penetration type of a needle-like weld bead could be produced with the SHG laser alone. It was also found that the penetration of the weld fusion zones produced with the combined laser beams of different wavelengths was 2 to 4 times deeper than that with pulsed YAG laser only. The absorption efficiency of pulsed YAG laser in a metal was confirmed to greatly increase by the combination of the Q-switched SHG YAG laser. The increases in penetration and beam absorption were attributed to the effect of keyholes produced by the high peak power density of Q-switched SHG laser.

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This paper deals with the behavior of a keyhole and a laser-induced plasma observed during laser welding of several metals with a single YAG laser or superimposed two beams through the ultra high-speed video camera, with the objective of understanding enhanced beam absorption due to two different wavelength laser beams. Moreover, reflected YAG laser beams were measured by photodiodes, and the relationship between the keyhole behavior and the reflected-laser photodiode signal was investigated to clarify rapid welding phenomena. It was observed that the behavior of a keyhole and a laser-induced plasma during laser welding was considerably different between a single beam and combined two different beams. And the reflected light intensity during welding with the superimposed two different laser beams lowered in the earlier stage than that with a single pulsed YAG laser beam.

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It is well known that arc welding of thin metallic sheets in difficult due to their easier distortion and deformation. On the other hand, in laser welding, such distortion and deformation is greatly reduced due to a smaller spot size and smaller heat input than in arc welding. Even in laser welding, thinner metallic sheets become more difficult to weld due to the loss of materials and the difficulty in supporting a shallow molten pool against evaporation and recoil pressure, leading to the formation of cut or drilled welds. In this study, therefore, lap welding characteristics of thin metallic sheets were investigated by utilizing combined beams or respective single beam of normal pulsed YAG laser and Q-switched SHG YAG laser. It was consequently found that the formation conditions for good lap welds in thin stainless steel sheets were broadened by using combined laser beams. Especially, lap welding of pure copper sheets was very difficult with single pulsed YAG laser; however, it was confirmed to be possible to weld them with combined laser beams of two different wavelengths in the air environment.

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Laser beam welding is used for small and thin parts that are required the high quality and precision. To weld large-scale and thick-wall products, high power laser beam must be transferred freely and deep penetration welding procedure must be developed. This paper will describe an optical fiber transmission system for 7 kW-class high power YAG laser and pulse modulated laser welding techniques to obtain deep penetration. The detailed observation of the weld pool and keyhole dynamics using high-speed camera and X-ray transmission system is 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 weld. Moreover it was revealed that bubbles, which were generated by the evaporation of metals near the bottom of keyhole, merged and disappeared into molten pool under N₂ shielding gas. After confirmation of the high power YAG laser welding joint performance of stainless steel Type 304, this procedure has been applied to the welding of tanks for the nuclear fuel reprocessing plant.

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The butt welding of very thin aluminium sheets by the ordinary inverter controlled TIG welder are performed at ultra high welding speed 24, 000 mm/min. In the experiment, the polarity is DCEN, fine 2% ThO₂-W electrodes and the magnetic control of arc are used. The base metals are clamped between two heavy copper plate to reduce the heat flow to the preceding area which causes deflection of the thin base metal. The results obtained are as follows:
1) As the arc is dragged backward at the welding speed 6, 000 mm/min, the welding can be done at the narrow range of current. But the arc can be corrected by the magnetic control, consequently the welding is possible at the wide range of current.
2) To correct arc deflection the rectangular electromagnetic tip is more efficient than the circular one.
3) In the butt welding results of the electrode tip angles from 30° to 120° with the magnetically controlled arc, the angle 60° of electrode tip is found to be suitable for 6, 000 mm/min welding speed.
4) Without magnetic control the butt welding of 0.3 mm thin aluminium sheets is impossible at more than 12, 000 mm/min speed, but becomes possible at 12, 000∼24, 000 mm/min speed with magnetic control.

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We have investigated the V-I characteristic of plasma in the glow-to-arc transition at atmospheric pressure in argon. Plasma is generated between the electrodes with the gap spacing of 1 mm. Both of the electrodes are the ThO₂-W rods, which are 1 mm in a diameter and needle-shape. The rods are located in a quartz tube coaxially. A DC power source is supplying constant voltage of 600 V during the discharge. The voltage between the electrode and the current into plasma are measured at the sampling rate of 2.5 kHz. As a result of the measurements, the V-I characteristic of plasma in the current range from the arc to the glow discharge is obtained. Plasma in a transition changes some metastable states cyclically. The cathode spot has been observed on the tip of the cathode in a metastable state and it has not been done in another metastable state. Our observation suggests that the metastable state of the plasma in the glow-to-arc transition will be affected on the boundary condition at the cathode.

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The stable back bead in a first layer of the one-side multilayer welding is important to obtain a good quality of welding. For this purpose, the switch back welding method was applied for the root gap from 1 mm to 5 mm in V groove welding without the backing plate. This method requires the cooperative control of power source, wire feed motor and robot manipulator by the personal computers. The validity of welding conditions for each root gap were confirmed by observing the arc and the weld pool, and the back bead appearance. Next, the feed-forward control of the back bead for the root gap variation was discussed. The root gap on start and end points of welding were detected by using the spot laser sensor. The welding conditions, welding current, wire feed rate, weaving width, forward/backward stroke of the switch back motion and stroke speed, were continuously controlled corresponding to the root gap variation, and the stable back bead was obtained by the fundamental experiment regardless of changing the root gap from 4.4 mm to 1.9 mm.

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Quality of a multi-layer welding depends on a welding in first layer. In order to get a high quality of the welding, it is necessary to make a stable back bead and to melt the base metals. In narrow gap welding, it is difficult to control the bead height (deposited metal) and the back bead (the melted area of the backside) simultaneously by using conventional welding. To improve this problem, authors propose a cooperative control of the welding equipments including the welding power source, the electrode wire feeder and the welding robot, i.e., the welding torch is not only oscillating on the groove, but also moving back and forth. To control the heat input, the power source is controlled according to the torch motion (robot motion), too. In the forward process, the arc (the heat) is given to the root edges to get a wide back bead. In the backward process, the suitable bead height is obtained. The idea of its robot motion comes from the knowledge of the skilled welding workers. Since the arc length must be kept short during the welding, the power source with a nonlinear power source characteristic is used. Welding conditions are investigated for 4 mm root gap, 5 mm root gap and 6 mm root gap by observing arc position with a high speed motion video camera, and observing the weld pool with a CCD camera. The welding conditions are stored in a database. Before the welding, the root gap at the beginning and at the end on the base metal is detected by a slit laser light and the CCD camera, and is stored into a memory in the personal computer. During the welding, the personal computer detects the torch position from the encoder pulses of the welding robot, finds the root gap at the torch position and calculates the welding conditions by using the data base, i.e., the adaptive control for the gap variation is applied. The welding experiment under the root gap variation from 6 mm to 4 mm is carried out to verify the validity of the cooperative control method.

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Microstructures and joint strengths of SUS316 steel brazed with Cu-7P filler were investigated. When Cu plating treatment was performed on the surface of SUS316 steel over 15 μm, the reaction of SUS316 steel and Cu-7P filler didn’t occur so that the formation of the harmful Fe-P compound was prevented. These brazed samples showed excellent joint strengths as same as a conventional SUS316 joint brazed with pure Cu. On the other hand, when the molten Cu-7P filler reacted on SUS316 steel directly, the Fe-P-Cr-Ni compound formed at the brazed interface. This compound contains approximately 40 at% Fe and 40 at%P. The Vickers hardness of the Fe-P-Cr-Ni compound was evaluated as 522, which is approximately three times that of SUS316 steel. When the brittle Fe-P compound forms at the brazed interface, the joint strength decreases because the brittle fracture becomes to occur easily at the brazed interface.

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The authors have studied on the tensile characteristics of the joints of the ferritic spheroidal graphite cast iron bonded with Ni-base brazing sheets and found that the tensile characteristics of the joints were comparable to those of the base metal. For application of the joints to the mechanical parts, it is necessary to make clear the dynamic characteristics of the joints. In this study, the ferritic spheroidal graphite cast irons were bonded with Ni-Si-B system brazing sheet at 1313 K in vacuum, argon gas and air. Effects of bonding time and bonding atmosphere on absorbed energy of the joints were investigated by Charpy impact test. Main results obtained are as follows: (1) When the cast irons were bonded for 2 minutes in vacuum, the absorbed energy of the joints was 12.6 J. This value was nearly equal to the Japanese Industrial Standard (JIS) value and nearly equal to that of the base metal. (2) When the cast irons were bonded for 10 and 30 minutes in vacuum, the scattering range of the absorbed energy was 5-6 J, the value was larger than that of the joints bonded for 2 minutes, and the average values of the absorbed energy were about 1/2 of that of the base metal. Because Fe₃(C, B)s were precipitated in the base metal near the bond interface of the joints bonded for 10 and 30 minutes, the crack on the Charpy impact test was propagated in the precipitation zone of Fe₃(C, B)s and the brittle fracture was caused. (3) When the cast irons were bonded for 2 minutes in argon and air, the absorbed energies of the joints were a small value of about 4.5 J. Because the liquid of the brazing metal emitted to the outside surface of the specimen was oxidized by air during the initial stage of the bonding, and the amount of the emission of the brazing metal was less than that of the joints bonded in vacuum. So, brazed metals contained B were remained in the bond interface and the fraction of Fe₃(C, B) was increased. The cracks were propagated in the precipitation zone of Fe₃(C, B) by Charpy impact test. So, the absorbed energy was decreased compared to that of the joints bonded in vacuum.

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The authors have been trying to apply the laser speckle method in the strain measurement at a high temperature such as in the case of welding process. We already reported that in-situ strain measurement over 1000°C is possible and some phase-transformation during cooling of a weld metal has been successfully measured, for example. However, strong light emission from the welding arc restricted us to measurements from the bottom surface. In the present report, we first extended the period of measurement from the bottom surface to include the hottest thermal cycle when the metal at the measuring position is molten. By using optical filters to improve the S/N ratio of laser speckle pattern against arc light and using signal processing to eliminate fluctuation due to thermal convection, we could measure strain behavior on the top surface with reasonable accuracy. With these improvements, the instrument has become more adapted to actual welding process.

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Tensile properties of a fusion welded joint depend highly on the characteristics of the heat-affected-zone (HAZ). The HAZ characteristics show much variation according to the welding procedures. Numerical analysis is a powerful and convenient method for designing the high strength joints, and the three-dimensional finite element analysis (3-D FEA) has been the only reliable method for the wide plate weld joints. However, the 3-D FEA is too time-consuming; hence, a more efficient method is pursued in this study. Two 2-D FEA models, the plane strain model and the generalized plane strain model, are compared with the 3-D model. Variables for the over-match or the even-match weld metal, U-type or V-type profile of the softened HAZ strength, and the HAZ thickness and width are investigated. The results show that the joint strength and the distribution of the equivalent plastic strain are almost similar for the 2-D generalized plane strain model and the 3-D FEA.

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The authors have clarified the joining mechanism of the first phase of friction stage during friction welding process for similar materials. The present paper describes the simulated friction torque in the first phase during friction stage for joining of mild steels. The joining model of the wear stage and the seizure stage were constructed from the joining phenomenon that was obtained by experiment. In actual simulation, the real contact area and the thermal stress at welded interface in the wear stage, and the seizure temperature and the relative speed of welded interface in the seizure stage were taken into consideration. The calculations of the friction torque and the temperature of welded interface were carried out under the conditions that were coupled with the friction pressure, the relative speed and the yield strengths of base metals. The following are concluded.
(1) The calculated values of friction torque, initial torque and elapsed time for initial torque were in approximate agreement with the experimental values. The calculated values of temperature at welded interface of center portion were higher than that of periphery portion at the time that the friction torque reached to the initial torque.
(2) It was clarified that the initial torque value depends on the seizure temperature at welded interface, and the yield strength that correspond to the seizure temperature.
(3) Simple calculation for required level to capacities of the friction welding machine can be estimated the initial torque by the seizure temperature.

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The behavior of magnesium (Mg) in the ultrasonic welding process of aluminum (Al) alloy containing Mg was investigated to understand the reason for the poor weldability. Commercially pure Al (A1050), Al-2.6 mass% Mg alloy (A5052), and Al-4.1 mass% Mg alloy (A5086), 1.2 mm thick each, were lap-welded and the fracture surfaces were examined by SEM observation and AES analysis. For all of these materials, the tensile load of the weld increased with an increase in the clamping load or in the welding time. In the case of Al-Mg alloys, higher Mg concentration required higher clamping load or longer welding time to obtain the tensile load equivalent to A1050. SEM observation of the fracture surface of the weld indicated that dimple pattern occupied the almost whole fracture surface of A1050. On the other hand, upon Al-Mg alloys, scraped fracture pattern, dimple pattern and brittle fracture pattern were observed and the scraped pattern occupied the largest area in the fracture surface. AES analysis of each fracture surface showed that Mg was segregated at the surface presenting the scraped and the brittle pattern. Especially at the scraped pattern surface, welding for short time of 0.5 s induced the segregation of Mg about twice as high as the concentration before welding. When the specimen having the surface segregation of Mg formed by the pre-heat treatment was welded, the scraped pattern was observed on the almost whole fracture surface and the tensile load of the weld decreased. Conclusively, it was found out that the surface segregation of Mg induced during the process of the welding caused the ultrasonic weldability of Al-Mg alloy to deteriorate.

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In the previous reports 1 and 2, the authors have clarified the joining mechanism in the first phase of friction stage during friction welding process. The present paper describes the relationship between the friction time, friction torque, and joint properties of friction welding for a low heat input friction welding method. The materials joined were mild steels (same materials), and a brake type friction welding machine was used for joining. In actual experiment, i.e., the low heat input friction welding method (LHI method), the electromagnetic clutch was used in order to exclude braking deformation at rotation stopping. The following are concluded.
(1) The joints obtained only in the first stage (up to initial peak torque) had 100% joint efficiency and 90 degrees bend ductility with no crack. It was determined that friction welded joints with 100% joint efficiency and good bend ductility could be obtained by using only the friction stage up to initial peak torque and without the need for the forging (upsetting) stage.
(2) The fracture occurred at the substrates (not at the welded interface) in welded joints when friction torque was close to initial peak torque. It was clarified that those joints has less width of heat affected zone and less width of hardening.
(3) It was clarified that the friction welded joints without using the forging stage (the friction welded joints by using the LHI method) have same mechanical properties as those welded by the conventional friction welding process including that stage. The LHI method has more advantages, i.e., less burn-off and less burr.

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