Long-term creep strength of Cr-Mo steel with fully annealed ferrite and pearlite microstructure is higher than those of the steels with martensite, tempered martensite and bainite microstructures. In this study, influence of post weld heat treatment at the temperature of austenite single phase region(full annealing PWHT) on long-term creep strength at 550°C has been investigated on 2.25Cr-1Mo steel, in comparison with that of normal post weld heat treatment at the temperature below A _C1 (standard PWHT). Creep rupture strength of the welded joint subjected to standard PWHT is lower than those of base metal and weld metal in the short-term. However, such differences in creep rupture strength disappear after long-term creep exposure for about 50,000h at 550°C. On the other hand, creep strength of the welded joint subjected to full annealing PWHT is obviously higher than those of the welded joint subjected to standard PWHT, base metal and weld metal in the long-term condition. It may be also expected to reduce a probability of mechanical damage such as Type IV cracking by the full annealing PWHT, since inhomogeneous microstructures in weld metal and heat affected zone have been essentially extinguished. Full annealing at the temperature of austenite single phase region has been proposed as a new post weld heat treatment condition available to obtain high creep strength and to improve a reliability of welded joint for low alloy Cr-Mo steels.

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Welding of Al-Mg and Al-Mg-Si alloys was performed using twin spot beams formed with two Nd: YAG lasers for the purpose of reducing porosities and humping caused due to unstable phenomena of weld bead during welding of aluminum alloys. The relationship between the configuration of the twin spot beam and the quality of the weld beads was investigated using X-ray and high-speed camera observations of the keyhole shapes and weld beads. X-ray observation of the weld beads showed that beam distance had a strong influence on the amount of porosities. At a shorter beam distance, porosities were apt to occurred in the weld. The amount of porosities decreased with an increase of beam distance. Beam distance affects keyhole shapes. The amount of porosities was clearly related to the ratio of keyhole depth to keyhole opening. Larger keyhole opening and/or shallower keyhole depth, smaller amount of porosities caused by instability of the weld pool.

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Welding of aluminum alloys (Al-Mg and Al-Mg-Si series) was performed using 2-kW and 3-kW continuous wave Nd: YAG lasers. Two beams were delivered by optical cables 0.6 mm in diameter and focused on the surface of the specimens as twin spots. Overlap joints of 2-mm-thick sheets were made at various welding parameters, including beam distance, beam arrangement and welding speed. The quality of the bead, including its appearance and macrostructure, and the tensile strength of the joints were investigated. At a shorter beam distance of 0.36 mm, the weld bead surface was humped, making it unacceptable in terms of quality. Sound weld beads were obtained at beam distances of 0.6 mm and 1.0 mm. As the beam distance increased, the penetration depth became shallower. At a beam distance of 1.0 mm, the area of fusion zone was too small to provide sufficient strength. The influence of the beam arrangement on penetration depth became larger with a longer beam distance. With inline beam welding at a longer beam distance, the penetration depth became much shallower than cross beam arrangement. In both beam arrangement, twin spot beam with beam distance of 0.6 mm provides higher and stable strength for lap joint of 2 mm-thick aluminum sheets.

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The development of welding technologies with high productivity is very important in the field of welding. Flash welding is essentially very fast welding and can meet this requirement. However, their weld properties, especially fracture toughness, have not been acceptable and thus the application has been limited to the field that low temperature toughness is not required.
In this study, factors governing flash weld toughness have been studied using API X80 grade high strength steel. Results obtained have been summarized as follows ; (1)Weld defect at the weld line has been minimized with the increase of upsetting length. The use of pre-heating process is also effective. (2)Despite of such defect delimitation, the weld toughness has still shown lower values. (3)Welded HAZ showed coarse-grained upper bainite in the as-welded condition, whose vTrs showed worse values when the notch position become closer to the weld line. Good correlation between vTrs and the prior γ grain size has been observed in the synthetic HAZ. In case of actual FBW-HAZ, while the vTrs at 3 mm or 4 mm apart from the weld line showed similar correlation with that of the synthetic HAZ, the vTrs at the weld line was much higher than this correlation line, which is caused by the upsetting deformation. (5)According to the upsetting simulation based on the hot forging, its toughness sharply declined as the increase of upsetting length especially in the steel containing large number of inclusions. It has been considered that the weld toughness degrade due to the existence of the inclusions at the weld interface and this degradation is multiplied by the micro-structural factors, such as HAZ microstructure, grain size and localized softening region.

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This paper deals with the problem concerning the modeling and sensing of the weld pool. In order to obtain the high quality of the welding result, it is important to measure and control the penetration depth in the robotic welding. Since it is difficult to directly measure the penetration depth and the relationship among the penetration depth, the welding current and other weld pool parameters such as the surface shape of the weld pool and the gap width, is non-liner, the weld pool model is constructed by neural network to estimate the penetration depth. The input and output variables of neural network model are determined from the information of the deferential equation for the penetration depth and the weld pool widths. Some steady state data and a transient response data of the fundamental experiment result are used as the training data of neural network model. The neural network model with less error can be learned by using these training data.
When the experimental data of the weld pool widths and the gap width are input to the neural network model, this model is performed as the sensor to estimate the penetration depth. The validity of the neural network model was verified by the welding experiment.

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The authors have carried out the human task simulation by a computer about the shipbuilding works in order to increase the productivity and safety so far, corresponding to the recent situation of the Japanese shipbuilding industry; decreasing of skilled workes and increasing of old workers.
This report picked up the welding works for hull construction as a main job in shipbuilding. For the typical welding postures such as flat, horizontal, vertical, and overhead position, the human body analysis has been carried out, using general-purpose human simulation software “Jack”; static strength prediction of human body, lumbar force analysis, and energy consumption analysis. Moreover, the necessary rest time to recover the fatigue has been studied, and the optimal work cycle is examined.
In addition, “Kansei Engineering” technique has been introduced for the evaluation of the results by the biomechanical analysis. SD (Semantic Differential) evaluation has been carried out concerning the body feeling of welding workers, and then principle factor analysis, one of multivariate analysis, has been executed, according to the process of “Kansei Engineering”.
As the result, it can be concluded that the flat position to weld, widely applied posture, is most severe for static load on lower body as well as energy expenditure, and that the rule of “short breaks and often” is preferable from the viewpoint of recovery of the fatigue, especially for the works such that the consumption energy is large. Furthermore, it can be concluded that overhead position to weld is severe in the evaluation of “Kansei Engineering”, because of the fatigue on upper body, and that “the weariness of the hand” causes the working fatigue by the welding works.

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Resistance spot welding was carried out for ultra-fine grained high strength steel sheets to investigate the characteristics of the weld. The steel sheets have a low carbon equivalent in order to retain good weldability and realize high strength by means of their ultra-fine grained microstructure. Ultra-fine grained high strength steel sheets require higher welding current in comparison with normal high strength steel sheets in order to create the same size nugget. It depends upon the difference of resistivity and strength at high temperature. On condition that the same size nugget is created, there is no difference of tensile shear strength and cross tention strength between ultra-fine grained high strength steel sheets and normal high strength steel sheets.

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This study was carried out to clarify whether it is possible or not to improve the mechanical properties of welds by means of applying ultrasonic vibration to the weld metal. Autogenous weld by TIG process was made on an austenitic stainless steel plate vibrated ultrasonically. The effects of the ultrasonic vibration on the solidification structure and the mechanical properties of the weld were examined. The main results obtained in this study are as follows.
The ultrasonic vibration applied to the weld metal makes the tensile strength of the weld higher than that of the conventional weld. By the ultrasonic vibration, the width of an austenite columnar crystal in the weld metal is decreased and this is likely to be one of the factors to increase the tensile strength of the weld.
The ultrasonic vibration changes the microstructure in the weld metal from cellular dendrite morphology in non-vibrated weld metal to subgrain morphology aligned in the growth direction. The existence of the subgrain boundary is also likely to be one of the factors to increase the tensile strength of the weld.
The ultrasonic vibration decreased the primary dendrite arm spacing in the weld metal, causing the weld strength to increase. The larger the ultrasonic vibration amplitude is, the more conspicuous the above phenomena are.

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Monte Carlo simulation of grain growth and recrystallization behaviors in HAZ was carried out for GTA and LB weldments of carbon steel and nickel on the basis of the Potts model. The temperature distribution in HAZ was analytically computed by quasi-stationary heat conduction equations. The coarsening of grain in HAZ increased markedly in the vicinity of fusion boundary in the GTA weldments compared with LB weldments of carbon steel. The recrystallized zone spread over the HAZ along the fusion boundary in GTA and LB weldments of nickel. The recrystallized grains near the fusion boundary were also coarsened with increasing the heat input while the grains around the interface between recrystallized and un-recrystallized zones were refined. It was elucidated that the grain growth and recrystallization behaviors in HAZ could be well visualized by the Monte Carlo simulation.

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HAZ characteristics of laser welded ultra-fine grained steels were investigated using a 20 kW CO₂ laser facility. A narrow softening region was clearly observed in HAZ welded at low welding speed of less than 0.017 m/s. The softening was caused by coarsening of ferrite grains which were heated above Ac1. On the other hand, HAZ softening was suppressed at higher welding speeds of more than 0.033 m/s due to the formation of tiny dispersed MA constituents and reducing grain coarsening. The prior austenite grain size was also examined at various welding speeds. The result indicate that the prior austenite grain size decreased with increasing the welding speed and with decreasing the initial ferrite grain size.

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There is an increasing demand for evaluation of fracture toughness of long time used pressure vessels. This paper proposes a small-size specimen, which uses parallel laser welds of small-sample extracted from running vessels. The constraint effect by hard region of laser welds is strong enough to cause brittle fracture in the small-size specimen. Sometimes the constraint of laser welds in the small-size specimen is higher than that of the full-thick base metal specimen. To estimate the fracture toughness of the thick base metal specimen, this paper applies the local approach, which employs the Weibull stress as a fracture driving force of the material. It has been shown that the distribution of the critical Weibull stress at brittle fracture initiation for the small-size specimen with hard zones and homogeneous specimens are almost the same. This means that the critical Weibull stress is a material property independent of the specimen type. Based on the Weibull stress criterion, the fracture toughness of every thick base metal specimen is expected to be predicted from the fracture toughness results of the small-size specimen. The procedure proposed in this paper is useful to evaluate the fracture toughness of pressure vessels in service from the test results by small sample extracted from the vessels.

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Laser welding process is widely used in industry. However, its use in joining dissimilar metals like steel and aluminium is recently suggested. Dissimilar joint between aluminium and steel is required to achieve reduced weight in structure with better strength, but formation of brittle intermetallic compounds(IMC)at the joint interface reduces the strength considerably. By controlling laser power and interaction time with the material, there is a possibility of achieving joints with reduced thickness of IMC.
In this work, laser roll bonding of aluminium and steel sheets was fundamentally investigated. As basis study, Time-Temperature-Phase(TTP)diagram was constructed for diffusion-bonded joints under different compressive pressures. Steel sheet was heated with CO₂ laser beam, immediately followed by pressing it against the underlying aluminium sheet by means of a roll. Joints were made by inserting aluminium-brazing flux and shielding with argon gas to avoid surface oxidation. Temperature profiles across the section were simulated to identify suitable laser power and travel speed. The interfaces of joints were analyzed by using EPMA. Tensile shear strength of the joints was measured and the sheared surface was analyzed by XRD. There is an optimum range of laser power, travel speed and roll pressure in which appreciable joint strength results. It is found that reduction in the thickness of interface layer and hence improvement in the joint strength is possible by laser roll bonding process.

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In order to improve the ultrasonic weld strength of Aluminum(Al)-Magnesium(Mg)alloy, the effects of pre-heating the specimen and the putting a drop of alcohol on the welding surface on the weldability were examined. Al-4.1 mass% Mg alloy (A5086)with 1.2 mm thickness was lap-welded. The following results were obtained in this study.
Pre-heating the specimen increases the strength of the weld. The adhesion of the alcohol on the welding surface causes the specimen temperature during welding to increase by 130 K higher than that without the alcohol adhesion, because the large relative motion between the specimens was induced at the incipient stage of the welding process due to the alcohol adhesion. The amplified relative motion and softening of the specimen because of the temperature rise broken down the Mg-segregated region preventing the faying surface from welding and resulted in increasing the weld strength about twice as high as that without alcohol. Furthermore, the adhesion of the alcohol causes the welding time necessary to achieve the sound welding to shorten below 1 second.

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In this study, diffusion bondings of tantalum to titanium were performed. Effect of the bonding temperature on the joint strength has been investigated. Main results obtained are as follows. β-(Ti-Ta)solid was formed in the bonded zone of the tantalum and the titanium, and(α+β)Ti solid was formed in the titanium base metal away from the bond interface. In the bonded zone of the tantalum to the titanium, Kirkendall voids were not observed though the diffusion of Ta into the titanium occurred over a wide range. The reason why the joint strength increased with bonding temperature is that the non-bonded areas were decreased with an increase in the bonding temperature. The joints of the tantalum and the titanium fractured at the base metal of the tantalum on the tensile test when the bonding was performed at temperatures ranging from 1173 K to 1473 K for a bonding time of 1.8 ks and a bonding pressure of 5.6 MPa. The strength of the joints of the tantalum and the titanium also increased with bonding pressure up to 20 MPa owing to a decrease of non-bonded area when bonding temperature was 1073K.

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In welding of a thin plate, welding deformation is a severe problem. It is difficult to correct the welding deformation after welding. Therefore, the welding deformation should be avoided before or during welding. As the welding deformation is generated by thermal cycle during welding, it is thought low heat input joining process is efficient to decrease the welding deformation. In this study, friction welding was proposed as lower heat input welding process than arc welding. MAG welding was also carried out to compare welding deformation and heat input. However, it is difficult to compare heat input between friction welding and arc welding. Because, there in no general heat input calculation in friction welding and does not define the weld line in friction welding. To compare the heat input directly, pipe circumference was assumed to weld line and calculated heat input by simplified heat input rate, which was proposed by authors.
The appearance of friction welded joint was sound and tensile tests revealed the fracture occurred at the base metal of plate. In friction welding, the deformation of plate was smaller than that in MAG welded joints. Heat input in friction welding was 10-30% in MAG welding by direct comparison for same dimension of joints. The time for welding was also smaller in friction welding. Friction welding is small deformation and efficiency process for joining of pipe and thin plate.

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