In this study, the effects of resistance spot welding conditions on the nugget diameter, which was one of the major influencing factors of resistance spot weld joint strength, was modeled by a machine learning method which had been proposed by the authors in recent years. Then, the applicability of constructed model and the effect of resistance spot welding conditions on the nugget diameters were discussed. The feature of the machine learning method used in this study was that the relationship between input and output could be derived as an easy-to-understand mathematical expression. A resistance spot welding condition-nugget diameter database was created through experiments using 590MPa class steel plates, and a nugget diameter prediction model was constructed to reproduce the database appropriately. As a result, it was indicated that the nugget diameter prediction model can predict the nugget diameter under welding conditions used for model construction and those not used precisely. Furthermore, it was found that the nugget diameter prediction model was composed of two terms that were presumed to reflect the spread of material melting due to heat input and the phenomenon at the beginning of energization.

Laser-arc hybrid welding is divided into laser-based and arc-based welding. In the arc-based welding, the laser beam irradiation provides the guiding effect of the arc plasma. Some studies have reported that an unstable arc plasma becomes stable due to this guiding effect. This is considered as the influence of metal vapor generation by laser beam irradiation, however, there are few reports on the measurement of metal vapor and arc plasma state during the guiding process. Therefore, the mechanism has been still not fully understood. In this study, the fundamental study of the guiding arc phenomenon was conducted in TIG arc welding (60A) with laser beam irradiation (170W) under relatively low power condition. When the base metal was stainless steel, the guiding arc by the laser beam irradiation was observed unlike the mild steel. Spectroscopic measurement for the guiding arc phenomenon in stainless steel showed that the metal vapor was composed of manganese and chromium, and was widely distributed near the irradiation point, but not concentrated. We also observed the decrement of plasma temperature, which is considered to be caused by distributed metal vapor and the resultant decrement of the current density.

Solidification cracking can be explained by the intersection between the high temperature ductility curve and thermal strain curves; thus, to predict the location and length of solidification cracking during arc welding, these two curves were calculated. The critical strain rate (ε_CSR) of the high temperature ductility curve was measured by an in-situ observation technique. The solidification initiation and completion temperature were calculated by the Kurz-Giovanola-Trivedi (KGT) and solidification segregation models respectively. The thermal strain curve was calculated using a finite element simulation model. The solidification cracking occurrence in the actual U-type hot cracking test during the arc welding corresponded with the prediction results estimated from the metallurgical and thermal elastic-plastic analysis models.

Solidification cracking susceptibility in arc & laser welding of DSSs (duplex stainless steels) was quantitatively evaluated and the numerical simulation of solidification cracking susceptibility has clarified the affecting factors. The BTRs (Brittle temperature range) of standard, lean and super DSSs in GTAW were 58K, 60K and 76K, respectively. The solidification cracking susceptibilities of DSSs were lower than those of austenitic SSs with A mode solidification modes. The BTRs of standard, lean and super DSSs in LBW were 40K, 45K and 56K in LBW, respectively. The BTRs in LBW were reduced by 15-20K compared to those in GTAW. These results suggested that DSSs had a significantly low risk of solidification cracking in LBW as well as GTAW. In order to clarify the affecting factors of solidification cracking, numerical simulation of solidification cracking susceptibility was carried out. The segregated concentrations of P, S and C in LBW were slightly lower than those in GTAW, suggesting that the solidification cracking susceptibility in LBW was reduced to GTAW attributed to the inhibition of solidification segregation because of the rapid solidification in LBW. In addition, we discussed that there is a difference in the hot cracking susceptibility compared with the austenitic stainless steel having the A mode. To explain this phenomenon, the segregation amounts of S and P by arc, laser and solidification modes were investigated. The segregation of Standard DSS was lower than that of Type310S when using same amount of P and S. This clearly indicates that F mode solidification was less segregated than A mode solidification. This is because the equilibrium partition coefficient and the diffusion coefficient of the standard DSS are larger than that of Type 310S.

The aim of this study is to propose the prediction approach of ductile crack growth resistance of cracked component with steels, based on ductile crack growth simulation using only mechanical properties of material obtained from tensile test for a round-bar specimen. Ductile crack growth was simulated in accordance with nonlinear damage accumulation model with stress triaxiality dependency of critical strain derived from void growth analyses of unit cell. In terms of simplicity, the prediction formula of the stress triaxiality dependency of critical strain using mechanical properties of material was proposed in this paper. In order to confirm the applicability of proposed prediction approach, ductile crack growth simulations were performed with elasto-plastic finite element analyses implementing nonlinear damage accumulation model using tensile test result for a round-bar specimen. As a result, it was found that ductile crack growth resistance could be predicted with high accuracy by the proposed approach. In addition, it was confirmed that the proposed approach was also applicable to steels with different mechanical properties and could reproduce the effect of plastic constraint related to specimen dimension to ductile crack growth resistance.

The optimum weld line shape for the remote laser welding were investigated to improve shear strength and peel strength of the lap joints of 980 and 1180MPa grade automotive high tensile strength steel sheets. Lap joints were prepared with two kinds of weld line shapes that were linear line and circle. These lap joints were exposed to three different tensile tests of tensile shear test, cross tension test and L-form tension test. In tensile shear test, weld line shapes didn’t affect on tensile shear strength (TSS), and TSS was proportional to weld line length. In cross tension test, weld line shapes much affected on cross tension strength (CTS). CTS of the circle weld line shape joints was much higher than that of the linear weld line shape joints. In L-form tension test, the weld line shapes also much affected on L-form tension strength (LTS). LTS of continuous linear line weld joints was much higher than that of circle shape weld joints. The factors which controlled TSS, CTS and LTS were discussed by conducting FEA of the tensile tests from a viewpoint of local stress concentration considering the loading types of shear stress and tensile stress.

Arc welding is important technology for manufacturing that is directly linked to quality and productivity. Especially GMAW with Ar or CO2 shielding gas is widely used in various industries because of its high productivity. To achieve high quality and productivity, in-process welding quality control technology will be required. In this study, the authors studies the welding phenomena observing technology for online detection of welding defects by using image sensing method. By selecting the transmission wavelength of band-path filter 1320nm, which is in the wavelength range where the brightness of arc light is weak and the brightness of moltenpool light is high, welding phenomena just near arc and moltenpool can be observed clearly.

Automated welding process is expected to realize a high-quality and high-productive welding process without skill of welders. It is sometimes difficult to obtain demanded welding quality because of any disturbance such as variation of the assembly accuracy in welding object. One of the solutions to overcome the problem is in-process monitoring of welding phenomena using various sensors. In this research, we focus on the visual sensor. The weld pool shape during the welding process is obtained by a compact CMOS camera and the relationship between the weld pool shape and weld quality is discussed. The monitoring technique is applied to CO2 welding on thin plate lap joint. In the experiment, deviation of the target position and the gap between the upper and lower plates are changed as disturbance. The correlation between them and the welding quality are determined from the cross-section shape. The weld quality depends on the target position, the gap conditions. From the obtained images by a CMOS camera, it’s confirmed that the molten pool tended to tilt toward the lower plate as the target position changes from the lower plate side to the upper plate side. Therefore, we developed an image processing program to extract the molten pool right and left area ratio (RR, RL) that indicates the inclination of the molten pool. Relationship between the average value of them during welding time and target position deviation, and the gap size are correlated respectively. As a result, the target position deviation and gap size can detect from the image of the weld pool. Moreover, the relationship between welding quality and RL/RR is discussed. There’s the correlation between RL and welding quality. This result shows that in-process monitoring of weld quality is possible by calculating RL.

Charpy impact tests are widely used for evaluating the notch toughness of materials and welded joints. Generally, the test results show a large statistical scatter in the ductile-to-brittle transition temperature range. Recent research found that the statistical distribution of the Charpy absorbed energy was characterized by a two-parameter Weibull distribution with a shape parameter of 2 under pure brittle fracture. In this study, a probabilistic model for evaluating notch toughness was applied. The Charpy impact test was conducted in the ductile-brittle transition temperature range, and the results showed a large statistical scatter. The Charpy absorbed energies at 1%, 50%, and 99% fracture probability estimated by the maximum likelihood method with the two-parameter Weibull distribution and a shape parameter of 2 showed good agreement with theexperimental data for pure brittle fracture. The temperature dependence of the scale parameter for the absorbed energy can be expressed as an exponential function. The absorbed energy predicted in the ductile-brittle transition temperature range by the probabilistic model showed good agreement with the experimental data under pure brittle fracture.

The multi arc-based cooperative metal additive manufacturing, which consists of two or three wire and arc systems, is more suitable for larger components because of higher deposition rates compared with wire arc additive manufacturing (WAAM). These wire and arc systems can work cooperatively in a concurrent manner in the process of manufacturing the components. In this paper, the residual stress of multi arc-based cooperative metal additive manufacturing is investigated with finite element (FE) simulations. Firstly, on the basis of considering the temperature-dependent thermal-physical properties, the double ellipsoid heat source model is used to establish the calculation model of the molten pool heat transfer in the multi arc-based cooperative metal additive manufacturing. Temperature field distribution of this process is firstly calculated and analyzed. These results are fed into the thermal elastic-plastic FE models and then the stress can be calculated. The analysis of stress evolution in different positions of the deposition layer indicate that the residual stress in the inter-bead is higher than that inside the welding bead.

The resulting properties of parts fabricated by wire and arc additive manufacturing (WAAW) processes are determined by their microstructure, local composition, and porosity. The objective of this work is to compare the difference of the solidification behaviors by means of numerical simulation when it precisely deposits material upon the single- and multiple-layer. Here, a three-dimensional (3D) heat transfer and fluid flow model of arc-based additive manufacturing to calculate thermal-flow fields, deposit shape and size, cooling rates and solidification parameters was developed. The calculated fusion zone geometries for the single- and multiple-layer deposition processes considering convective flow of melts agreed well with the corresponding experimental data for AA2319 deposits. It was found that under the same process parameters, the cooling rate and the solidification rate of the molten pool for the single-layer deposition process are always greater than that for the multiplelayer deposition process. Specifically, the refined equiaxed grains are more easily obtained at a cooling rate greater than 50 K ps−1 when the polarity is designated as direct current electrode positive (DCEP). Width of the deposited bead of multi-layer structures is significantly greater than that of single-layer structures due to the lack of material at the lateral sides and the ambient conditions for the heat loss.

Residual stresses developed during fusion welding operations may affect the fatigue life of the steel welded structures under dynamic loading conditions. This research aimed to investigate the residual stress distribution along the fusion welded joint. Neutron diffraction was used to measure internal stresses of flux-cored arc welded SM570-TMC steel plate, which fabricated using two different types of welding wire, E71 LT H4 (containing 0.4%Ni) and E81-Ni1 (containing 1%Ni). The longitudinal, normal, and transverse directions of residual stress were measured at the welded joint along lines 3 mm and 8 mm below the top surface. Neutron diffraction results show that residual stress in the longitudinal direction was higher than both normal and transverse directions. The residual stresses of weld using welding wire containing 1%Ni seem higher than 0.4%Ni. It may correspond to the formation of hard phases in the weld metal (WM) due to increasing nickel content.

To address the recent need for improved efficiency in thick plate welding, a high-current buried-arc welding system has been developed. In a high-current buried-arc welding process, it is necessary to adequately evaluate the thermal effect of high heat input on the welded joint. The mechanical properties have been verified1), and in the present study, we focused on observing microstructures. In some regions of the weld metal in the joint welded through high-current buried-arc welding, a characteristic metallographic structure was observed in which the macrostructure appeared extremely coarse. However, this coarse macrostructure was assumed to be a trace of the prior γ grain; the actual microstructure mainly consisted of fine intragranular idiomorphic ferrite and acicular ferrite. These two types of ferrite are known to precipitate at different temperature ranges; thus, their mixed structure is extremely characteristic and is formed because of high heat input.

Thermal elastic plastic analysis is used to predict welding deformation and residual stress. In the thermal elastic plastic analysis, the non-linear mechanical behavior of a welded is part consecutively analyzed. Therefore, to evaluate a complex structure, this analysis requires detailed modeling and a huge amount of computing time. However, the welded part is generally small compared to the whole structure, and it is very difficult to construct an analysis model for the whole structure that considers the welded part. In this research, the mesh superposition method is introduced to thermal elastic plastic analysis. The mesh superposition method makes it is possible to construct separate analysis models for the whole structure and the welded part. The applicability of the proposed method was investigated by comparison with a conventional analysis method. The result showed that the proposed analysis method has almost the same accuracy as the conventional method. The proposed method was also applied to the analysis of welding deformation in a structure consisting of multiple parts. The result indicated that the proposed method can effectively construct the complex analysis model by using a local mesh to join the multiple parts in a global mesh.

The objective of this study is to improve Gurson model by combining with cohesive traction-separation law to realize crack propagation associated with transition from ductile to brittle fracture. To embed the cohesive cracks into the Gurson model, five kinds of conditional equations are solved for the crack opening displacement and the plastic strain. One of the conditional equations correspond to the local balance equations between the cohesive tractions and the principal stresses and the others are the yield function, the isotropic hardening law, evolutional equation of void volume fraction and inequality constraint. The enhanced Gurson model allows us to represent the nucleation and propagation of the ductile crack along with the void nucleation and growth. Moreover, it is realized by the embedded cohesive traction-separation law that the stress rapidly drops down when the crack accelerates due to the transition from the ductile to brittle fracture. Throughout the numerical examples at several temperatures, it is confirmed that the proposed model enables us to realize load-displacement curves depending on temperature along with the ductile-brittle transition. Also, the proposed model has represented changes of crack propagation rate and void volume fraction by depending on temperature. Furthermore, the proposed model has capability of reproducing the crack propagation associated with the transition from the ductile to brittle fracture at -60 °C.

The welding with hybrid heat sources combined a high-power disk laser and a metal actives gas (MAG) arc was performed on the I-groove joint of 12 mm thick high-strength steel plates. To investigate the effect of laser focal position on joint formation, the combination effect of laser power and focal position on the cross section and the backing bead shape was investigated. The results showed that a good welded joint was obtained at the minimum laser power of 8 kW when focal position is 6 mm below the surface of the base material. At the minimum laser power, it was clarified that the welding penetration depth was deeper at focal position of the below the surface than at focal position of the above the surface.

The welding with hybrid heat sources combined a plasma arc and a metal inert gas (MIG) arc was performed on the I-groove joint of 9 mm thick high-strength steel plates. The plasma torch is set up in the leading position, while the MIG torch is set up in the trailing position. As a result of observing hybrid welding with a high-speed camera, the plasma arc and the MIG arc are repelled each other. The influence of the root gap on the cross section and the bead formation was investigated. As a result, defect-free beads were formed at the root gap of 1.5 mm.

Grinding ofweld toesis generally used toimprovethe fatigue strength of fillet weld joints. In this study,a technique employingadditional weldswas applied toout-of-plane gusset welded jointsand the relationship between the shape of the additional weld and fatigue strength was investigated. The shape of the additional weld was controlled by changing the aiming position. When there was more distance between the first weld toe and the aiming position of the additional weld, the flank angle and toe radius tended to be smaller and larger, respectively. In the nominal stress range of 150 MPa, fatigue life was extended to 2.7 times that of weld jointswithout additional welds. Although the toe radius of the additional weld was smaller than that created bygrinding, the resultant fatigue strength of the additional weld joint was almost equivalent to that of the grinding case. This can beexplained in terms of stress concentration, since the flank angle and toe radius in the additional weld toe canbe improved without reducing the throat thickness.

In the present study, a large-scale finite element analysis of welding thermal elastic-plastic behavior was conducted for the purpose of estimating the residual stress distribution at dissimilar welds between a cast-iron pipe and steel flange. Based on the calculation results, the effect of welding pass and heat input conditions on the distribution of residual stress was investigated. The results showed that tensile residual stress occurred along the interface between a cast-iron pipe and steel flange, regardless of the welding pass and heat input conditions. Note that the calculated tensile residual stress becomes smaller for a large-heat-input and small-pass-number condition, compared to a small-heat-input and large-pass-number condition. This is because transverse residual stress induced by welding is strongly affected by the balance of plate thickness and heat input, which is a cause of the bending moment at welds. Thus, we concluded that the transverse residual stress could be controlled along the interface between a cast-iron pipe and steel flange by optimizing the welding conditions according to the weld joint configuration. Through a welding test and an experiment using deep hole drilling, a relatively small tensile residual stress was obtained for a large-heat-input and small-pass-number condition.

Welding is often used to join automotive components during production. Deformation may occur during welding, which leads to increased production costs. Therefore, prediction of welding deformation is highly demanded in automobile production. To predict the welding deformation of real automotive components, it is necessary to conduct a numerical simulation within a reasonable computing time. In this study, an efficient simulation was achieved with a large-scale thermal elastic plastic analysis method, called the idealized explicit finite element method (IEFEM), to predict deformation of a side rail, which is a component of the rear suspension member of a car. In addition, the analysis method was also applied to the prediction of deformation in a front suspension member having more than 2 million degrees of freedom. The accuracy of the analyses was evaluated by comparison with experimental measurements. The results indicated that the IEFEM can accurately predict welding deformation and can be used to conduct simulations within a reasonable computing time.

A study was carried out to evaluate the relationship of microstructure and impact toughness for different nickel level of electrodes in multipass flux-cored arc welded SM570-TMC steel joint. The base metal used in this study was SM570-TMC plate with 16 mm thickness. The multipass welds were run by using flux-cored arc welding (FCAW) with a flat position (1G). Three SM570-TMC welded plates were fabricated with varying amounts of the nickel content of electrodes, 0.4, 1.0 and 1.5% Ni. The effects of nickel were studied on the weld metals. The investigations consist of observation on the microstructure and mechanical tests. The results indicated that at a temperature of 25 °C and 0 °C there was no obvious different impact energy value of weld metal by using electrodes 0.4 and 1.0% Ni. Besides, at a temperature of -20 °C the impact energy of weldmetalcontaining 1.0% Ni was superior to the other. It seems the acicular ferrite (AF) formation on the weld metal containing 1.0% Ni effectively improves low-temperature impact toughness. On the other hand, the impact energy of weld metal, 1.5% Ni was the lowest. It is found that the higher nickel content caused the microsegregation as observed by the electron probe micro analyzer (EPMA).

Blowhole (BH) contained in weld joints is known to be a cause of reduction in strength. Concerning the effect of BHs to fatigue performance of weld joints, their total number or volume are conventionally used for the mechanical design, since the available data are limited by the measuring precision and their costs. In other words, the mechanical design would be improved if the individual size or position of the BHs could be precisely determined since it would lead to a better knowledge of the material performance and therefore to a more precise maintenance of the welded components. The purpose of this study is to develop a fatigue performance evaluation method for aluminum alloy joints considering the effect of BHs. Several FE models of lap welded joints, with different BH distributions, were generated to simulate the BH effects under fatigue loading. The material model adopted for FE analyses is based on an unconventional plasticity model, calibrated to reproduce the cyclic plasticity behavior of aluminum A5083-O. The numerical local stress-strain relationship is analyzed in correspondence to the area with the higher plastic deformation. Therefore, the fatigue crack initiation life was calculated using a formula based on the experimental database. Predicted S-N curve on crack initiation life revealed that BHs in some cases would reduce the fatigue strength. Also, there was a significant correlation between the crack initiation life and the normalized distance between the toe and the BHs.

Fatigue crack is often initiated around weld toe, mainly caused by high stress concentration. Various techniques for improving the fatigue strength of joints have been proposed up to the present. Additional welding is known to be one of the effective techniques to improve the fatigue life of welded components. Recently, the authors have conducted fatigue tests on joints with additional weld. The tested welded joints were fabricated with different additional welding conditions. The tests revealed that smaller flank angles and bigger weld toe radii were obtained under certain additional weld conditions and the fatigue tests indicated that fatigue strength of out-of-plane gusset was remarkably improved by additional welding. In this study, 3D finite element (FE) analyses were conducted based on the in-house experimental campaign carried out to characterize the effect of additional welding on fatigue life. The crack initiation and crack propagation simulations were conducted using elastoplastic and linear fracture mechanics analyses, respectively. The results show that an additional weld treatment reduced remarkably the stress concentration at weld toe, leading to longer fatigue life.

The purpose of present study is to develop a numerical technique for the fatigue life assessment considering the effect of corrosion. For assessing the fatigue crack initiation life, an elastoplastic constitutive model was developed. The model incorporates a novel cyclic plasticity theory together with a crack initiation criteria, which is extended to consider the surface changes. On the other hand, the crack propagation was taken into account based on linear fracture mechanics considering SIF ranges and an extension of the Paris’ law to consider the corrosion effect. The numerical results showed a good agreement with the experimental data. Moreover, the model prediction was able to catch a reduction of the fatigue life due to the degradation of the mechanical properties in a corrosive environment.

The duplex current feeding metal inert gas (DCF-MIG) welding which has two current feeding points was developed. This DCF-MIG process is able to control the welding current and wire feeding speed independently by additional feeding current from the secondary tip below the conventional MIG tip. In the past DCF-MIG studies, only the studies using the steel or SUS welding wire material were reported. In this study, the influence of welding wire materials of DCF-MIG was investigated. As a result, the electrical conductivity of welding wire materials was found to have influence on the quantity of Joule heating between current feeding points. However, in the DCF-MIG with low electrical conductivity material, the average of primary current greatly decreases as the increase of the secondary current, consequently the total current of DCF-MIG was suppressed. It was found that there was the optimum value of electrical conductivity of welding material to obtain enough effects of DCF-MIG.

Welded joints are often characterized by fatigue failure due to a series of factors such as stress concentration caused by bead geometry, changes in material properties by weld heat, and the effect of residual stresses. Grinding, TIG treatment, peening, etc., are available to improve the fatigue strength of weld joints. Also, the application of additional welding has been reported as one of the methods for reducing the stress concentration at the weld toe, with the consequential extension of fatigue life. A previous work reported that an additional weld could improve fatigue life of non-load-carrying fillet joints. The benefits of additional weld are also confirmed for the pre-fatigue damaged joints. Besides, grinding treatment achieves further improvement of fatigue life. However, additional weld and grinding treatments introduce geometrical and material changes such as the modification of the bead shape, angular distortions and welding residual stresses. In this study, the fatigue performance, in terms of crack initiation and propagation life, was numerically evaluated and compared with experimental results. Fatigue crack initiation and propagation life are evaluated based on cyclic elastoplastic FE and X-FE analyses.

Fatigue damage has been recognized as one of the critical issues in the design of welded structures. Among different welded joints, one-sided welding of fillet joints is a common practice that has the advantages of an excellent workability and to reduce the manufacturing costs. However, it is characterized by the presence of weld roots. The same type of welding is often used for U-rib steel components, where, in many cases, fatigue cracks are initiated from the weld root. Subsequently, the cracks often propagate and penetrate the upper deck or the weld bead. These cracks, originated from weld root, are not easy to be detected even by using novel non-destructive inspection techniques. A novel method to improve the fatigue performance of the weld root of U-rib has been developed by the co-authors of this work. The new strategy consists in introducing a compressive residual stress field by applying a tensile pre-overload from inside of the U-rib structure. The effect of the pre-overload on fatigue performance was experimentally clarified in a previous work. However, the benefits of this expedient remain to be elucidated theoretically in order to optimize the pre-overloading process. The purpose of this study is to clarify the mechanism of the fatigue life extension due to the application of a pre-overload by means of FE analyses based on a novel cyclic elastoplastic model.

The present study investigates the dissimilar metal weld between SUS 316 and Naval Steel AH 36. The joints were fabricated with the gas metal arc welding with ER309L filler wire with two different heat inputs (0.8 kJ/mm and 1.5 kJ/mm). The weld joints were subjected to metallurgical and mechanical characterization as well as residual stress measurement by neutron diffraction several locations from the center of a weldment. The microstructural examination was carried out to reveal the defect in the weld joints and structural transformation in the weld zone. The results showed that the tensile residual stresses for the low heat input present in the axial direction and compressive residual stress in the direction of normal and transverse at stainless steels 316 welds. While the tensile residual stresses on the low heat input of marine steel AH 36 present in the direction of normal and transverse, but the compressive residual stress is in the axial direction. However, in contrast, the high heat input of dissimilar weld produces the compressive stress in the normal and transverse directions and tensile residual stress in the axial direction. The hardness value tends to proportional to the amount of residual stress. The dissimilar welds with low heat input produce a relatively higher residual stress compared to welds with high input that experienced balanced tensile and compressive residual stress.

Welding is an important joining technology for dissimilar materials, such as metals and polymers. However, the joint strength of the welded structure and the mechanical behavior of the polymer change significantly owing to the thermal history of the polymer during the welding process. In this study, polyamide 6 (PA6) plates were welded to stainless steel (SUS304) and aluminum (A5052) plates using arc welding at different welding conditions. The effects of the welding conditions and metal type on the joint strength were investigated using the tensile tests. The experimental results demonstrated that the joint strength of the welded structure strongly depended on the thermal history of PA6. When the temperature of the metal plate exceeded the melting temperature of PA6, the joint strength increased. However, the joint strength decreased considerably when the temperature exceeded the thermal decomposition temperature of PA6. The fracture of PA6 was classified into two categories depending on the amount of heat applied to PA6, namely, fracture of interface and fracture of polymer.

The transient transport process of the chromium vapor during Tungsten Inert Gas (TIG) welding was revealed by imaging spectroscopic analysis. During TIG welding, the metal vapor generated from the weld pool surface is transported to the arc plasma. The metal vapor affects the plasma properties such as electrical conductivity and radiation coefficient. However, the transient transport process of the metal vapors including ions inside TIG arc plasma has not been clarified. In this paper, the experiments were performed under the special conditions such as helium TIG welding on pure chromium. After the arc ignition, the chromium vapors were generated from a pure chromium base metal and transported through the inside of the arc plasma to the tungsten cathode. As time passed, the He I spectral intensity gradually decreased. In contrast, the Cr I and Cr II spectral intensities gradually increased near the weld pool, near the electrode and the intermediate region. At a certain time, the chromium vapor reached the center of the arc plasma when using an electrode with a flat tip. The metal vapor distribution inside the arc plasma transiently changed when the plasma flow slowed down due to the electrode deformation during TIG welding.

A gas metal arc welding phenomena simulator was developed, which simultaneously computed an arc plasma behavior and a weld pool formation process with the time evolution by alternately conducting the particle method and the grid method calculations. Also, the numerical simulation of this welding process was conducted. As a result, a weld pool was swelled up by the transportation of molten metal droplets with the time evolution and it was solidified after the heat source passed. The plasma temperature distribution in the welding direction temporally became asymmetry at the start of the welding. This was because the metal vapor evaporated from a wire surface was transported to forward, preferentially in the welding direction. Furthermore, the iron vapor concentration on the weld bead became lower than forward side. Therefore, the arc temperature at forward of the wire became lower than backward because the radiation loss increased with increase in the iron vapor concentration. A few seconds after the start of the welding, the change of the arc plasma temperature distribution became smaller because a sufficient amount of a molten metal was supplied by the molten metal droplet transfer and the dent on the weld pool surface reduced.

We have investigated factors affecting the short arc lamp characteristics using a unified numerical model bychanging filling gas species (Xe, Kr and Ar). The influence of the arc voltage, temperature and flow field, radiative efficiency and gas convective pattern and also the blackening location was examined. The arc voltage, temperature and flow field were found to be affected by the gasspecies. The thermodynamic and transport properties were in turn found to affect the radiative efficiency and gas convective pattern, and were thus finally reflected in the location of blackening position. This indicates that the light intensity and life time of the short arc lamp can be controlled by optimizing a mixture composition taking into account the arc characteristics such as arc volume, temperature, velocity and heat road to the electrodes. For example, mixing of Xe, which has a larger arc volume and a smaller heat load on the electrode, with Kr or Ar is suggested to lead to a higher arc temperature and gas velocity.

In the modified 9Cr-1Mo steel welds, Ni+Mn content of the filler metal and PWHT temperature range differ between U.S. and Japan. Hence, we investigated the influence of PWHT temperatures on the toughness of modified 9Cr-1Mo steel shielded metal arc weld metals withdifferent Ni+Mn content, it was clarified that the oxide particle density and the hardness of matrix affect the toughness. The weld metal with a high Ni+Mn content of 2.6% partially transformed into austenite during holding on 1073 K which was more than Ac1. As a result, the freshly formed martensite of same microstructure as-welded state was formed, the hardness increased and the toughness decreased. As evidence, part of fractured surface after Charpy test is the same as that of as-welded state, spherical oxide particles (almost without any cracks) were observed at the center of the dimple. Therefore, it was suggested that the toughness decreases due to the partial precipitation of freshly formed martensite by carrying out PWHT, and oxide particles affect the decreasing of the toughness.

A fully coupled plasma arc-keyhole-weld pool model was developed to investigate the keyhole behaviors and thermal fluid flow in high current plasma arc welding. The relationship between the forces distribution and fluid flow patterns was revealed. Owing to a constraint effect of the water cooling nozzle, the plasma arc flows downward with high energy and momentum, so a large keyhole forms. Inside the keyhole, the plasma arc mainly flows downward, and very few plasma arc flows upward along the keyhole wall. At the top surface of the keyhole, the plasma arc flows outward. The dominant downward plasma shear stress and negative arc pressure in the z direction cause a dominant anti-clockwise eddy in the weld pool. The outward plasma arc shear stress on the top weld pool surface causes a small clockwise eddy in the weld pool.

In the conventional MIG welding, it is difficult to control the droplet temperature because of the unique relationship between the welding current and the wire feed speed. This study aims to develop a novel MIG welding process with duplex current feeding (DCF-MIG), which enables to control the welding current and the wire feed speed independently by feeding the secondary current near the wire tip in addition to the conventional MIG current. In this paper, a feasibility to achieve lowering the droplet temperature using DCF-MIG welding was investigated through numerical analysis. Consequently, the decrease in the droplet temperature in the DCF-MIG welding was predicted to be feasible.

This paper presents the measurement of residual stress (RS) distribution at the weld root of a U-rib specimen using the contour method. A fully automated data processing code was developed to process the measured data over the cut surfaces. In addition, the effect of the applied boundary conditions (BCs) on the resulting energy and reproduced RS was examined using different BCs cases. Full 2D maps of RS distribution were obtained where a compressive RS was induced at the weld root. Moreover, it is found that the studied BCs caseshave a small influence on the resulting energy and negligible effect on the general distribution of reproduced RS.

Workpiece vibration has been applied during arc welding to make weld metal microstructure fine and reduce weld defects such as the blow holes. It was unexpectedly found that the continuous workpiece vibration utilizing a sine component with a specific frequency parallel to welding direction changed the penetration shape from finger-shape to pan-bottom shape in the pulsed metal active gas welding via 18% of CO2 shielding gas. A coupled Eulerian Lagrangian finite element model was employed especially for adding workspace vibration to the fluid flow of molten materials. The velocity ofmolten materials through the designated zone of the weld pool was investigated with and without the workpiece vibration. The simulation suggests that increase in the fluid flow velocity along the welding direction at a specific frequency of workpiece vibration led to bringing the high-temperature material to the position where the final penetration shape was determined. Consequently, the penetration bottom would be widened and the middle of fusion line would be deepened, leading to penetration shape change to relatively pan-bottom shape.

In an automatic welding system using a visual sensor, it is necessary to clearly observe the weld pool to control the welding phenomenon with a visual sensor. However, since the arc light is too strong, it is difficult to take clear weld pool images. It is important to reduce the influence of the arc light. Therefore, the spectral distribution of the weld pool and arc light in plasma arc welding and pulse MAG welding was measured changing the current. Based on the measurement results, the characteristics of the spectral distribution in each welding method were found. Using the results, a filter suitable for the weld pool observation was selected.

Since spot welding is quick in production and stable in quality, the joining of thin materials in automobiles and electrical products. The nugget in the joint part took places due to the joule effect of high current. Therefore, the current density at the contact between the base material and the electrode becomes high and weld expulsion is generated in welding for high tensile strength steels with high current during a short time. The authors introduced a numerical model of resistance spot welding and analyzed this phenomenon. In the numerical model, the temperature characteristics of the material were considered. The contact state between the electrode and the base material and that between the base materials depend on the yield stress of the material and the pressure at the contact part. Moreover, the contact resistance depends on the radius of the electrode. The contact resistance was confirmed by performing simulation when the radius of the electrode was changed. The relationship between the electrode radius and the temperature distribution was investigated. The welding condition of two pulsed welding current are induced analyzing its relationship.

Gas metal arc welding is typically performed by adding a minimal amount of oxygen to the shielding gas to protect the arc; this affects the toughness of the welded joints. This can be avoided by employing pure argon gas as the shielding environment. However, this leads to beadmeandering because the cathode spot on the base metal moves past the molten pool in search of O2 outside the shielding gas. In this study, we propose a method for controlling the cathode spot by using an external magnetic field for suppressing bead-meandering, the proposed method does not require changing the electrode wire or the shielding gas. Horizontal and vertical external magnetic fields are obtained when using DC and AC external magnetic fields, respectively. A bead-on-plate welding experiment is performed by applying an external magnetic field and using pure Ar as the shielding gas. The effect of the external magnetic field on the stability of the arc, shape of the bead, and penetration depth is evaluated, and the, effectiveness of pure Ar-MIG arc welding using external magnetic field is examined.

Characteristics of oxide inclusions in steel weld metals with varying acicular ferrite (AF) fractions, which were produced by gas metal arc welding using controlled CO2 (10%, 30%, and 50%) and titanium contents (by placing ultra-fine Ti wire), were statistically investigated. The correlation between identified phases in the oxide inclusions and AF formation was discussed from the viewpoint of AF formation mechanism. For high AF fraction samples, we confirmed that all oxide inclusions include Mn-Si-Al-Ti-O amorphous phases. By contrast, an amorphous phase was never observed for low AF fraction samples. Additionally, we confirmed that a few of the amorphous oxides created a Mn-depleted zone (MDZ), which suggested that the MDZ formed by Mn-Si-Al-Ti-O amorphous phase stimulates AF formation.

This study aims to clarify extinction phenomena of air arc plasma through magnetic blowout process in electrical contacts. For this purpose, numerical simulation taking into account ablation of polymer vapour was carried out. As a result, increase in the pressure due to polymer ablation was found to strongly affect the arc extinction phenomena. Especially, the pressure above 6 atm prevented arc re-ignition due to decrease in arc temperature which lowered electrical conductivity in the electrode gap.

Ag-Cu and Ag-Cu-Pd interlayers improved the joint strength between friction-welded Ti-6Al-4V and type-718 Ni-based alloys. Frictional heat melted the Ag-Cu interlayer and partially melted the Ag-Cu-Pd interlayer, and the interlayers were conjointly ejected. This suppressed the formation of intermetallic Ti-Ni compounds at the interface. The tensile strength of the joint increased from 698 MPa (Au-Ni) to 774 MPa (Ag-Cu) and 776 MPa (Ag-Cu-Pd). The underlying mechanism of this strength improvement stemmed from a combination of interlayer brazing and solute diffusion from the base metal to the interlayer. This effect was larger with the Ag-Cu and Ag-Cu-Pd interlayers compared to the Au-Ni interlayer owing to the near absence of Ni-Ti intermetallic compound formation.

A molten metal droplet transfer processes were simulated by a numerical model using a three-dimensional smoothed particle hydrodynamics method in order to clarify the flux column formation mechanism at the tip of a wire during a flux cored arc welding process. This study focuses on the flux cored arc welding with TiO2 based wire.As a result, although the average droplet size obtained by the computation was larger than that by the experiment, the average length of the flux column obtained by the computation showed agreement with that by the experiment, which supports supported validity of this computational model. Moreover, the melting rate of the metal-pipe around flux was higher than flux. The flux column was formed at the tip of the wire. The simulations with different values of a specific heat and a thermal conductivity were performed to investigate the effect of the heat conduction in the wire on the flux column formation. The unmelted flux column was formed at the tip of the wire when the specific heat of the flux component was smaller and the thermal conductivity was higher than those of TiO2. The result indicated that the heat conduction in flux played an important role in the flux column formation during flux cored arc welding.

The material flow in the mechanical clinching process with preforming of the lower sheet was improved to increase joint strength for joining an aluminium sheet and an ultra-high strength steel sheet. In this process, the material flow to the radius direction in the final compression increases due to the increased upper sheet material in the cavity of the preformed lower sheet, and then the interlock increases. Finally, the joint strength increases because of the increased interlock.

In the present study, feasibility of joining 1.2 mm thick AA5052 aluminum alloy to 1.2 mm thick galvannealed (GA) high strength steel DP590 by direct current pulsed gas metal arc welding (DC pulsed GMAW) is studied through experimental and numerical analysis. A comparative study in joining of dissimilar materials by DC pulsed GMAW is performed to realize the effect of different welding parameters on the thermal characteristics of the welded joints and growth of IMC layer thickness. A 3D heat transfer model is developed to estimate the temperature distribution, temperature histories, and the IMC layer thickness of the welded joints. The numerical result showed the fair agreement with experimentally measured average IMC layer thickness within the standard deviation values. An increase of welding current induced longer wet length with lower bead height.

Submerged arc welding (SAW) is widely used for butt welding of thick plates in large steel structures because of its high deposition rate and high weld quality. 1-pass per layer narrow gap welding with a narrow root width is an effective process that reduces welding time and deformation. However, compared with conventional grooves, it is at risk to occur lack of fusion due to narrow gap. And, the degradation of mechanical properties is a concern because the reheated region becomes thinner. In this study, a welding condition optimization program to control the weld shape for narrow gap SAW was developed. First, welds were conducted on plate under different welding conditions, and a weld shape model was established. Next, an optimization algorithm for deciding welding conditions that can achieve the target weld shape using the weld shape model was established. Then, welding conditions for achieving different layer thicknesses were calculated using the optimization method. The performance of the program was verified by multi-layer welding under the decided conditions.