Narrow gap welding with oscillation laser beam is one of the effective processes for thick plate welding. To put this welding process into practical manufacturing, a groove sensing system is required for preventing the welding defects, especially lack of fusion. In this study, we have developed a real-time groove sensing system using the image processing for narrow gap welding with oscillation laser beam. This developed system uses still image of weld zone taken by coaxial CMOS camera. It can recognize the position of the groove wall by analyzing the brightness distribution in the still image. And then it can control the oscillation width and the laser irradiated area by calculating the groove width and the groove center position. Some narrow gap welding experiments were performed to evaluate the performance of the developed system. The results revealed that the developed system is effective for narrow gap welding with oscillation laser beam. By using this system, the narrow gap groove can be weld even if the groove width has changed during welding process.

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This paper deals with the strengthening of spot welded lap joints by increasing the opening angle on the nugget edge. Based on the equation proposed by past researchers to predict the strength, the factor necessary for the strengthening was extracted. And, based on this factor, the strengthening of spot welded lap joints was examined. First of all, in order to increase the opening angle on the nugget edge, the vicinity of the nugget edge was bended with a bending machine. Next, cross tension strength tests were conducted into the spot welded lap joints with various opening angles on the nugget edge. Finally, the relationship between cross tension strength and the opening angles on the nugget edge was clarified. As a result, by increasing the opening angles on the nugget edge, we found that the cross tension strength could be improved by 20%.

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In order to evaluate the temper effect during the multiple post weld heat treatment (PWHT) process, Thermal Cycle Tempering Parameter (TCTP) proposed by the authors, which is derived by extending Larson-Miller parameter to non-isothermal process, has been used to evaluate quantitatively the temper effect during heating and cooling processes in the process of PWHT. It has been clarified that the temper effect in the multiple PWHT process including long-time heating and cooling processes can be quantitatively evaluated by TCTP. On the basis of the present study, it might be possible to shorten the holding time of the heat treatment for multiple PWHT.

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Reduction or control of angular distortion without additional processes is demanded because it takes great time and effort to correct the angular distortion of fillet welded T-joint. In this study, the reduction or control of angular distortion of both sides fillet welded T-joint by welding with a trailing reverse-side gas heating was investigated through the welding experiment and its numerical simulation. First, the effect of gas heating position and intensity on the reduction of angular distortion was experimentally investigated using gas burner. As the results, angular distortion became the smallest when the reverse-side heating using gas burner was located at backward 50mm of the welding torch. Also, the concentrated gas flame with increased propane and oxygen gas flow was effective for reducing angular distortion. It was clarified that the angular distortion could be controlled completely with an appropriate reverse-side gas heating condition. Next, the numerical simulation model of welding and gas heating was constructed based on comparison with the measured temperature histories and angular distortion. Through the numerical simulation of welding with a trailing reverse-side gas, more detailed understanding on the effect of gas heating condition on reduction of angular distortion was developed. In addition, it was confirmed that the gas heating position for the smallest angular distortion is dependent on the temperature distribution along thickness of flange plate.

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The high frequency electric resistance welding (hereinafter referred to as the HFW) pipes and tubes are being used for high-grade line pipes. To cope with high need for weld seam reliability, the clarification of welding phenomena is important. To clarify the HFW phenomena, at first, we have developed HFW simulation system by using electro magnetic, heat conductive and elastic plastic finite element analysis (FEA) methods. A continuous electromagnetic and heat conduction analysis was conducted by subdividing the cross section containing the electrode and the welding point into a large number of two-dimensional models. The temperature distribution in the HFW pipe welding procedure can be successfully simulated by this system. The deformation behaviour in which a portion of the weld rises to the inner and outer surfaces as the result of pressurisation from welding rolls can be analysed with this system. Secondly, HFW phenomena have been visualized dynamically using high-speed video camera technique. The high-intensity part visualized by high-speed images has a good accordance with the temperature distribution of FEA result. High-speed images have visualized the dynamic phenomena of molten metal flow and sputtering of molten steel. Very rapid movement of molten metal in the forming direction along the welding line was observed at V-convergence point with a speed of 2m/s∼50m/s, which was far faster than welding speed. This rapidly-moved molten metal generated the peculiar sputtering which spread the molten metal particles as describing an arc perpendicular to the welding line. These phenomena were assumed to be caused by the electromagnetic force concerning the molten steel at V-convergence point.

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In order to achieve high joint strength in resistance spot welding of ultra high strength steel (UHSS), the effect of adding "pulsed current pattern" consisting the combination of short cool time and short-time high-current post-heating was investigated. Finite element analysis (FEA) for post-heating patterns and experimental evaluation for joint strength were conducted using 980N/mm² grade steel sheets. FEA shows that the short-time high-current post-heating leads to rapid heating in nugget and heat affected zone (HAZ) compared to conventional temper pattern consisting long-time low-current post-heating. In particular, the high-current post-heating reheats HAZ near the electrodes and that near sheet-sheet interface higher than the center of the nugget. Pulsed current pattern utilizes the effect of this high-current post-heating to properly reheat nugget and HAZ, which prevents brittle fracture through nugget without remelting the nugget even in a short cool time of 8 cycles. The experimental results exhibit the pulsed current pattern improves failure mode from partial plug failure to plug failure and increase cross tension strength (CTS). The pulsed current pattern does not decrease hardness of the nugget and results in retaining sufficient tensile shear strength (TSS), while softened nugget by the conventional temper pattern causes lowering TSS. Wider proper current range with high CTS over 10kN and plug failure can be obtained in pulsed current pattern than in conventional temper pattern.

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Fatigue crack retardation with the infiltrated SiC paste into a crack is examined in low carbon structural steel. Two different sizes of SiC powders, whose average diameters are 15μm and 53μm, are used. The SiC powder mixed with oil is infiltrated into a through thickness fatigue crack from the crack mouth. Fatigue crack growth retardation is examined by the ΔK increasing test of R=0.1, comparing with the base plate property, where ΔK is stress intensity factor range and R is stress ratio. Crack growth is retarded just after infiltrating SiC paste into the crack mouth, and the deceleration of crack growth rate to 1/50 of the base plate appears in the maximum. It is revealed that this crack retardation behavior results from the crack closure induced by the wedge effect of SiC particle into a crack. The crack retardation effect is investigated with several combinations of the SiC particle size and the cyclic stress conditions. The crack growth rate, da/dN and the stress intensity factor, K_cl for the crack closure depend on both of the maximum stress intensity factor, K_max and the stress ratio, R. While the better retardation effect can appear in the higher K_max and the higher R ratio, it disappears in the R ratio over 0.7. The SiC paste with 15μm powder stably brings the crack retardation effect in the wider cyclic stress condition than the SiC paste with 53μm powder.

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Currently, remote laser welding using a solid-state laser is widespread in the industry. Meanwhile, it is well-known that laser-induced plume blown up from the processing point affects the penetration in laser welding, through the attenuation and the refraction of the laser beam. These phenomena in carbon dioxide laser welding have been investigated well and it is widely recognized that using the shielding gas flow to blow away the laser-induced plume is very important. However, at remote laser welding, it is not easy to maintain the shielding gas flow to processing point. By the way these phenomena depend on the wavelength of the laser. So, the quantitative knowledge of the attenuation and the refraction of the solid state laser beam are necessary in achieving a stable penetration in remote laser welding with this laser.
This study was made to determine the attenuation coefficient and the amount of the effective focus shift caused by refraction of the laser beam in the plume, through the melt run experiments with YAG laser. The attenuation coefficient of the laser beam was estimated to be 0.00090 mm^-1, from the dependence of the cross-sectional area of weld metal on the laser power and the plume length. This value is about one twentieth of the attenuation coefficient of the carbon dioxide laser beam at welding, found in the literature. The amount of focus shift was estimated to be 0.67 mm per 100 mm length plume, from the dependency of penetration depth on the defocusing distance and the plume length. Comparing the 3 mm plume of length, this value is centesimal of the estimated value by Beck et al in CO₂ laser welding. Therefore, the solid-state laser like YAG laser is considered to be suitable laser source for remote laser welding.

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Indirect resistance spot welding process with single-side electrode access was developed for the automotive applications. The variable controls of electrode force and welding current during welding were studied in order to achieve the promotion of weld nugget formation and the suppression of expulsion without sacrificing the productivity and design flexibility of automobiles. The welding experiments were performed on lapped test coupons of 0.7mm thick cold-rolled sheet with tensile strength of 270N/mm² and 1.6mm thick cold-rolled sheet with tensile strength of 980N/mm² using a resistance spot welding system consisting of a servo-motor controlled welding gun and an inverter DC power supply. Welding experiments verified that the occurrence of expulsion and formation of molten nugget were significantly influenced by the heat generation and melting process at an initial period during welding and manageable by applying the variable patterns of electrode force and welding current. When the welding was performed under the large shunting condition simply with the constant force and current pattern of 400N in electrode force, the appropriate current range was less than 1kA. On the other hand, it extended to 2.6kA when performed with the variable force and current pattern of 800N in force and 4kA in current at the first stage and 400N in force at the second stage, confirming the fact that the variable pattern successfully suppressed the expulsion and promoted the nugget formation. Numerical simulations were conducted to compare the difference in welding phenomena between the constant force and current pattern and the variable force and current pattern and clarified that the effect of variable force and current pattern on the promotion of nugget formation and the suppression of expulsion.

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Through-silicon via (TSV) technology for 3D stacking is attracting much attention as a means of alleviating the miniaturization limits on advanced semiconductor devices. Despite a great deal of research, low load (<1 MPa), low temperature (<473 K) and short time (<300 s) solid phase bonding with high heat resistance (>623 K) to prevent the damage of weak low-k dielectric material etc. has not been realized. In this work, we examine a new Ag-Sn thin film bonding system to replace Cu-Cu direct bonding. It is found that Ag/Sn/nano Ag-nano Ag/Sn/Ag thin film bonding systems (especially when the film thickness of the surface Ag is controlled to around 10nm) is a promising approach because 1) it enables low load (<0.4 MPa), low temperature (<453 K) and short time (<300 s) bonding, and 2) the bonded interface has a high heat resistance (>673 K) and joint strength (>29 MPa). It is found that it may be possible to realize an optimal solid phase bonding system for wafer-level 3D-stacking for 3D-IC which can satisfy a hierarchical temperature based bonding method that include TSV formation.

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We investigate how welding conditions affect the arc in TIG welding with a constricted nozzle by numerical simulation using an axial-symmetric two-dimensional model. When helium is used as a shielding gas, the effect of the constricted nozzle is more remarkable than argon. The heat flux to the center of the anode surface increases more with a higher welding current. This feature is suitable for pulsed welding. Even if an electrode is consumed and its tip angle is changed, a stable arc plasma and heat intensity are obtained.

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