Laser peening can introduce compressive residual stress to the surface and, therefore, is effective in enhancing the fatigue strength. In this study, we conducted laser peening in air with a water film formed by a nozzle and examined whether the distribution of residual stress along the thickness was different from that achieved laser peening in water in our previous studies. We also assessed the resultant residual stress and fatigue life when the pulse energy was reduced for developing a simple method to conduct laser peening on large structures. Compressive residual stress equivalent to that observed after laser peening in water was obtained in nozzle-type laser peening in terms of magnitude and depth. With the reduction of pulse energy, it was observed that the depth of the compressive residual stress tended to decrease significantly and the fatigue life also tended to reduce. The results indicate that the depth of the compressive residual stress has a considerable effect on the fatigue life of welded structures as well as the magnitude of the surface residual stress.

In previous studies, many fatigue tests have been performed to investigate the fatigue behavior of out-of-plane gusset welded joints. However, fatigue behavior of out-of-plane gusset joints with one side fillet weld has not yet been clarified sufficiently. In this research, in order to quantitatively clarify the behavior, welding residual stress at the weld toes of the joint was measured by a cutting method. In addition to this, fatigue tests were carried out on the joints with one side weld and turn around weld. The plate thickness is 9 mm and the weld size is 10mm. Moreover, FE stress analyses were conducted on the test specimen models and parametric models with different main plate and gusset plate thickness. Their results show that fatigue cracks for out-of-plane gusset joints with one side fillet weld initiate from both weld toes at start and end points of weld beads and non-penetration weld of the gusset side. And, the crack initiation points are different depending on the weld arrangement, however, their fatigue strengths are almost same.

Effect of chemical compositions and microstructures on hydrogen embrittlement of austenitic stainless steel weld metals in high pressure hydrogen gas was surveyed by using the Slow Strain Rate Test.
As a result, hydrogen emblittlement of weld metal was hardly influenced by delta ferrite in weld metal, but by stability of austenite phase, which was estimated by Md₃₀ value or Ni equivalent. In the weld metal with poor stability of austenite, α' martensite was formed near crack induced by SSRT. Additionally, though the crystal structure of α' martensite is as same as delta ferrite, susceptibility of hydrogen emblittlement became higher with the increase of α' martensite.
The mechanism to explain the difference between delta ferrite and α' martensite was considered as following. The hardness, which increases the hydrogen embrittlement susceptibility in bcc structure, is higher in α' martensite than in delta ferrite. In addition, α' martensite might be formed continuously with propagation of crack. Therefore, the effect of α' martensite on hydrogen embrittlement could be larger compared with delta ferrite.

A series of tensile tests were carried out on fillet welded lap joints assisted with bonding for investigating the static tensile strength characteristics of the joints from the viewpoints of stress reduction effect around the welded part due to bonding. It was confirmed that the mechanical properties of epoxy resin bonding used in this study were not deteriorated by heating less than 150 degrees Celsius. When the fillet welded lap joints with bonding were assembled, the bond layer in 20 mm from the weld toe was subjected to heating over 150 degrees Celsius. In other words, the mechanical properties in that region were deteriorated. The strengths of elastic limit of specimens with welding and bonding were higher than those of specimens with only welding by from 60 to 100 MPa. The ultimate tensile strengths of them were almost the same because they were broken at the base plate. The strains around the weld toe and the root of specimens with welding and bonding were smaller than those of specimens with welding by around 13 % in elastic region. The strengths of specimens with only bonding were 170 MPa, which could be explained by a theory of elastic stress distribution. Even if the bond layer in 20 mm from weld toe of the specimens with welding and bonding was thermally damaged, the possibility was confirmed that the residual bond layer had around 100 MPa in strength. It could be concluded that the strength of the residual bonding assisted to decrease the stress around the welded part of the specimens with welding and bonding.

The effect of sodium on repair-weldability of SUS316FR steel under the remaining sodium environment was investigated by transverse-Varestraint and laser cladding tests. Solidification brittle temperature range (BTR) of SUS316FR steel with AF solidification mode was 37 K. However, BTR was expanded from 37 to 67 K, as amount of surface-adhered sodium increased from 0 to 7.99 mg/cm². From microstructural observation of the weld metal, there would be a possibility that metallic sodium existed at cell boundaries in the weld metal during welding solidification. According to the thermodynamic calculation, the sodium would expand the solid-liquid coexistence temperature range. It could be concluded that the enhanced solidification cracking susceptibility under sodium environment would be attributed to the enlargement of the solid-liquid coexistence temperature range. Finally, it was confirmed that any solidification cracks and blowholes did not occur in the overlaid weld metal through multipass laser cladding tests. Namely, it could be confirmed that SUS316FR steel possessed superior repair-weldability under the sodium environment.

The cause of hot cracking in Co-Cr-Mo (CCM) alloy induced by surface melting by electron beam (EB) irradiation was investigated for different EB currents. A regular pattern of linear grooves and ridges was formed by horizontal EB scanning with a square raster pattern. However, some irregular-shaped grooves also occurred. As the EB current was increased, these irregular grooves became larger and cracks appeared within them. Cross-sectional observations showed that the cracks occurred mainly in the heat-affected zone (HAZ) along grain boundaries and extended into the fused zone (FZ). The HAZ cracks terminated at grain boundary precipitates. The fractured surface of the cracks exhibited well-developed cellular-dendritic solidification structures in the FZ, indicating that these were solidification cracks. In contrast, the cracks in the HAZ had an immature dendritic structure with a relatively flat surface, typical of liquation cracks. It can be deduced that the cracks were caused by a liquid film remaining at the grain boundaries, and the driving force for crack propagation was shrinkage distortion caused by the fusion-solidification process.

Welding processes often induce inaccuracies for ship-building fabrication such as an excessive root gap due to the level of cutting precision and welding deformation, etc. In the Japanese ship-building industry, the decision to allow a root gap, for example, depends on the accuracy states in the Japan Ship-building Quality Standard (JSQS). The JSQS is easy to use because it gives tolerance limits and countermeasures for various issues that occur in the ship construction, such as root gap, misalignment, and angular distortion. However, the tolerance limits and countermeasures of the JSQS might be insufficient, because they still focus on the shielded metal arc welding (SMAW), which was the primary welding process used when the first version of JSQS was established around 50 years ago. The validity of widely applying the JSQS to joint welds by currently used welding processes, such as gas metal arc welding (GMAW) and laser beam welding, has not been officially confirmed and not been included in the latest JSQS. It is necessary to learn the effects of many factors, such as the amount of root gap, countermeasures, and heat input, on the static strength, fatigue strength and corrosion of ship structures when recent weld processes such as GMAW are used in order to apply the current welding processes to a new JSQS or something like it. In this study, experiments on the static strength for fillet joints are carried out by changing the amount of root gap, plate thickness, and fillet weld leg length, and the relation between the static strength and the amount of root gap is established.

Twin wire submerged arc welding (SAW) is widely used in the joining of oil or gas pipelines because of its high productivity. However, when it is applied to join the high strength steel pipe, the heat affected zone (HAZ), especially the coarse grain zone, becomes very brittle. To investigate the strength and toughness of HAZ, thermal cycles in HAZ due to twin wire SAW must be studied. In present studies, theoretical equations of twin wire welding thermal cycles for both a thick plate and a thin plate were developed. The critical thicknesses for both a thick plate and a thin plate were determined. The welding thermal cycles for a medium thickness plate were predicted by linear interpolation using results of both a thick plate and a thin plate. Through the theoretical equations, thermal cycles of twin wire SAW with actual welding conditions used in pipe fabrication were predicted and verified by both FEM and experiment. A good agreement among theoretical results, FEM and experimental results was achieved. Furthermore, the microstructures and Vicker's hardness of HAZ was predicted based on CCT diagram and thermal cycles calculated by the proposed theoretical equations. Predicted microstructures and hardness were compared with the experimental ones. A good agreement was also obtained.

In the present study, a semi-destructive method for estimating the mechanical properties, including plastic strain, of materials was developed using the instrumental indentation technique (IIT). The formula for estimating plastic strain was derived using true stress-true strain curves that were measured by IIT. The developed method requires only one parameter, namely, the work hardening coefficient, for estimating the plastic strain of pre-strained materials. The usefulness of the developed method was evaluated using pre-strained specimens. The dimension of the plastic strain estimated using the developed method generally agrees with the dimension of the plastic strain of the pre-strained materials. The developed method was used to evaluate the distribution of plastic strain in the notched region of pre-strained materials. The distributions of plastic strain estimated by the developed method were validated by comparison with FE simulation results. The results indicate that the distribution of plastic strain can be estimated conveniently using the developed method.

Surface finishing methods, such as Water Jet Peening (WJP), have been applied to welds in some major components of nuclear power plants as a countermeasure against Primary Water Stress Corrosion Cracking (PWSCC). In addition, buffering, one of the methods of surface finishing, is being standardized, and thus the buffing has been also recognized as the well-established method of improving stress. On the other hand, the long-term reliability of peening techniques has been confirmed by accelerated test. However, the effectiveness of stress improvement by surface finishing is limited to thin layers and the effect of complicated residual stress distribution in the weld metal beneath the surface is not strictly taken into account for long-term reliability. This paper, therefore, describes the accelerated tests, which confirmed that the long-term reliability of the layer subjected to buffing was equal to that subjected to WJP. The long-term reliability of very thin stress improved layer was also confirmed through a trial evaluation by thermal elastic-plastic creep analysis, even if the effect of complicated residual stress distribution in the weld metal was excessively taken into account. Considering the above findings, an approach is proposed for constructing the prediction method of the long-term reliability of stress improvement by surface finishing.

The deep hole drilling (DHD) technique has received much attention in recent years as a method for measuring through-thickness residual stresses. However, some accuracy problems occur with the DHD technique. One reason is that the traditional evaluation formula assumes that the stress condition around the reference hole is two-dimensional plane stress. In this study, a new evaluation formula and procedure are proposed using three-dimensional stress functions to evaluate the residual stress more accurately. Then, a known stress field is evaluated by the traditional formula and the new formula using the finite element method to compare the accuracy of the both results. These results indicate that the proposed formula can evaluate the residual stress better than the traditional formula can.

Weld residual stress analysis is an effective method for integrity evaluation of welding structures. However, faster computation techniques have been demanded because of the huge calculation time needed for the analysis. Recently, accurate weld residual stress evaluation based on faster three-dimensional analysis has become available through development of the iterative substructure method. In this study, computing performance of the three-dimensional weld residual stress analysis using the iterative substructure method was examined for a large-diameter thick-walled stainless steel pipe joint and then the analysis accuracy was validated by the weld residual stress distribution measured using the inherent strain method. These results demonstrated that the analysis technique using the iterative substructure method could accurately calculate large-scale welding problems within practical times.

In order to predict deformation during line heating accurately, thermal elastic-plastic FEM with remeshing technique is developed. The model is represented by a fine mesh near the heat source and much coarser mesh around the fine area. Mesh of the model is updated with the movement of heat source, and solution variables are mapping from the old mesh to the new mesh. The analysis procedure after each remeshing step is the same as conventional method. Results of a typical line heating example show that the remeshing technique ensures the accuracy and accelerates computing speed greatly.

In recent years, it has been possible to predict the welding deformation by only performing elastic analysis using inherent strain. In elastic analysis with inherent strain, Static implicit FEM is generally used. In Static implicit FEM, huge computing resources are necessary to predict the welding deformation of the large scale structure and it is very difficult to analyze. Therefore, in order to achieve shorter computing time and lower memory consumption, the authors developed a new analysis method using inherent strain based on Idealized explicit FEM.
The developed method is applied to the welding deformation analysis of the fundamental welding structure to verify the validity of proposed method. As a result, it is found that the developed method has almost the same accuracy as Static implicit FEM. In addition, the developed method can analyze the problem of 571,176 degrees of freedom 4 times smaller in computing time and 10 times less in memory consumption than Static implicit FEM. Thus, it can be concluded that the developed method is very effective method in the welding deformation analysis of large scale structure.

In this study, a numerical model coupling weld bead formation and thermal conduction was developed for more accurate simulation of the temperature distribution during temper bead welding. Most of the welding parameters used in the analysis were estimated from experimental welding conditions based on experiments and numerical simulations of previous research works. The simulation results were compared with the experimental results under the same welding conditions to confirm the usefulness of the developed numerical model. Except for the weld penetration shape, the analytical results of the bead surface shape and the temperature distribution, such as the A_c1 lines, were in good agreement with the experimental results. The developed model has the potential to be effective and more precise in predicting and evaluating properties such as the metallographic structure and the hardness of the HAZ and the resulting weld residual stress by temper bead welding with the appropriate computational cost.

Cold cracks in welds are the result of the formation of brittle microstructure as martensite in the presence of diffusible hydrogen as well as of tensile stresses. Cold cracks occur when the combination of cold crack influence parameters (CCIP) overcomes a critical limit. In this study, critical combinations of CCIP were identified with an enhanced test procedure under welding conditions. The test is based on the physical simulation technique of heat affected zone (HAZ) and it is carried out using the test and simulation center Gleeble 3500. Thereby, laboratory special specimens are charged with hydrogen from pure hydrogen atmosphere in the initial stage of the test. Rigidly restraint specimen section is subjected to different weld temperature cycles. Through the thermal exposure, the desired microstructure of HAZ is set in the test zone of the restraint specimen section. A plastic deformation takes place owing to the prevented expansion during heating and contraction of the specimen during cooling. Consequently, compressive or tensile reactions stresses arise in the deformed zone. Cracked specimen represents a critical combination of the CCIP. The quantitative cold crack criterion separates the cold crack susceptible combinations from those non susceptible.

It is problem that nitrogen absorption in gas tungsten arc (GTA) weld metal. Recently, various arc welding simulation techniques have been developed. However, there are few models dealing with the absorption and a mixture of a shielding gas, an atmosphere and a metal vapor.. In this study, intruding behaviors of the atmospheric gas (N₂) into the molten poor and nitrogen transportation phenomenon in the molten poor during GTA welding was investigated using a unified numerical model. This model includes the tungsten cathode, arc plasma and base metal. We take atmosphere (N₂), shielding gas from the gas nozzle and metal vapor from the molten poor into consideration. As a result, we show nitrogen concentration distribution and nitrogen transportation in the molten poor. Additionally, we clarify that these behaviors are affected by the arc plasma characteristics.

Welding is widely used for assembling steel structures such as ships and bridges. Quantitative prediction and effective control are required to minimize residual stress and welding distortion. Heat conduction Finite Element (FE) analysis and thermal elastic plastic FE analysis are generally used in welding simulations. However, these analyses have very large computing time and memory consumption that are proportional to the second or third power of the number of degrees of freedom of the analysis model. To accomplish smaller computing time and memory consumption for thermal elastic plastic analysis, we have developed Idealized explicit Finite Element Method (FEM). In this study, Idealized explicit FEM for heat conduction analysis is developed and applied to series of computations for bead-on-plate welding and tandem fillet welding of a large-scale stiffened plate. As a result, it is found that Idealized explicit FEM can reduce the computing time and memory consumption of heat conduction analysis especially for large-scale problems.

This study examines the control of the residual stress in resistance spot welds by controlling the electrode force profile during the holding time. To examine the residual stress, numerical simulations of spot welding were performed. First, the effect of the electrode force value during the holding time on the residual stress distribution was investigated. The results show that an increase in the value of the electrode force tends to reduce the tensile residual stress. Second, the effect of the time duration of the applied electrode force on the residual stress distribution was investigated. The results show that it is important to apply the electrode force during the transformation to reduce the tensile stress. Finally, as an example, a controlling electrode force profile was proposed that reduces the tensile residual stresses in spot welds. The results indicate that the residual stresses in the spot welds can be reduced by increasing the electrode force during cooling and by applying the electrode force during the transformation.

Even when a macroscopically uniform deformation is applied to a polycrystalline material, the generated microscopic strain distribution will not be uniform due to factors such as the non-uniform grain orientation distribution and grain boundaries. Despite the strong need to accurately measure strain distributions, suitable method is limited. This study proposes an experimental method for estimating strain distributions in polycrystalline materials. The in-plane displacement distribution was determined by analyzing microscopy images obtained before and after deformation and calculating the displacement differences. The active slip plane was then determined by comparing the angle of {111} planes in each grain and the slip line angle obtained from scanning electron microscope (SEM) images. Finally, six components of strain tensor were estimated by assuming a single slip system.

Through the isothermal ageing treatments at 873, 923, 973, and 1023K, precipitation of sigma (σ) phase in type 316FR stainless steel weld metal was examined based on the kinetics approach. Microstructural examination was performed by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The dominantly precipitated phases were sigma (σ) and chi (χ), nucleated at δ-ferrite/austenite (γ) interface or interior of the δ-ferrite grains, consuming the δ-ferrite during isothermal holding at each of ageing temperature. The total amount of intermetallic phases precipitated during isothermal ageing sigmoidally increased as a function of the ageing time. Precipitation of these intermetallic phases could be approximately followed by Johnson-Mehl equation. Based on the determined kinetic equation, precipitation behavior of intermetallic phases at service-exposure temperature could be successfully predicted.

The aged nuclear power plant needs to be repaired or maintained, and temper bead welding is one effective repair welding methods instead of post weld heat treatment. For temper bead welding, hardness and toughness are the key criteria to evaluate the tempering effect. A neural network-based method for hardness and toughness prediction in heat affected zone of low-alloy steel has been investigated to evaluate the tempering effect in temper bead welding. On the basis of experimentally obtained database, the new hardness and toughness prediction system was constructed by using RBF-neural network. With it, the hardness and toughness distribution in heat affected zone of temper bead welding was calculated based on the thermal cycles numerically obtained by Finite Element Method. The predicted hardness and toughness were in good accordance with the experimental results. It follows that our new prediction system is effective for estimating the tempering effect during temper bead welding and hence enables us to assess the effectiveness of temper bead welding before the actual repair welding.

The effects of ausforming on variant selection of martensite in Cr-Mo steel were investigated. The non-ausformed martensite and martensite ausformed at 850°C and 750°C were analyzed. Blocks were elongated in a single direction in the non-ausformed martensite specimens, however, blocks were elongated in multiple directions in ausformed martensite specimens. EBSD analyses indicated that non-ausformed martensite specimens contained only a single packet in a single γ, but that ausformed martensite specimens contained four packets in a single γ. The sizes of the blocks and packets decreased with ausforming. In situ observations revealed that blocks were formed only from γ grain boundaries in non-ausformed martensite specimens. In contrast, in the ausformed γ grains, sub-grain boundaries were observed, and martensite blocks were formed from γ grain boundaries as well as sub-grain boundaries. The blocks were not developed parallel to the slip band. The elongation of the blocks was stopped at sub-grain boundaries; as a result, packet size was considered to be smaller.

Tungsten coating with a thickness of 0.6 mm on reduced-activation ferritic/martensitic steel (RAF/M) F82H (Fe-8Cr-2W) have been produced by Vacuum Plasma Spraying (VPS). Heat flux experiments using an electron beam and quantitative analyses about temperature profiles and thermal stress using FEM have been carried out on the VPS-W coated F82H. In addition, behavior of hydrogen penetration/permeation on the VPS-W coated F82H has been investigated by the tritium (T) tracer technique.

The present paper reports the bondabiliy and the interfacial microstructure between copper ribbons and nickel-coated copper plates by ultrasonic bonding. The ultrasonic bondability of copper to nickel is significantly improved by heating up to 423 K. On the other hand, the bondability of copper to itself is deteriorated by surface oxidation. It is also shown that the nickel coating suppresses the deterioration. The nickel layer at the central part of the bond area shows S-curved morphology, whereas it is torn and/or folded in the peripheral part of the bond interface, indicating that a complicated deformation is induced during bonding. Although ultrasonic bonding at 423 K allows lowering of the ultrasonic power to one-third of that required for bonding at room temperature, it is difficult to avoid tearing of the Ni layer.

On TIG-MIG hybrid welding process, the MIG arc can keep stability even in pure inert shielding gas by the effect of hybridization with TIG arc and it becomes possible to achieve the both merits of high quality as well as TIG and high efficiency as well as MIG. In this report, we considered about optimum torch configuration on this process for practical application. As a result, following findings were obtained: (1) More than +45° MIG torch angle is needed to keep forehead angle of MIG arc under condition that quite large repulsion occurs. (2) Although there is not seen deciding difference for bead shape and penetration by the change of torch angle, TIG 0°/MIG+45° is the best configuration relatively for the penetration shape and flatness of bead shape. (3) Based on the consideration, original torch for TIG-MIG hybrid welding process was produced and it was confirmed the good weldability on practical joint.

Ultrasonic bonding can be of use in developing electronic packaging technology at lower temperatures to afford more reliable bonded joints. We bonded Cu and Ni sheets together and used high-temperature testing to evaluate the thermal reliability of the joints at 673 K. Furthermore, we observed the microstructure of the joint interface and evaluated its effect on the fracture load of the Cu-Ni interface. The fracture load of the Cu-Ni interface of the joints heated at 673 K temporarily increased at 300 h and then decreased after 1000 h, and the heated joints showed good thermal reliability. Obvious changes were detected in the microstructure of the interface in the heated joint. The bonded region had obviously expanded vertical to the direction of the ultrasonic bonding, and the Cu side of the joint was composed almost entirely of coarse grains because almost all the Cu grains grew during the high-temperature test. However, a finely recrystallized microstructure and a diffusion layer had formed, especially at the Cu-Ni bonding interface. We discussed the effects of the changes in the microstructure of the Cu-Ni interface and the bonding material on the fracture load of the Cu-Ni joint. The diffusion and recrystallization region was not always stronger than the Cu base metal, suggesting that expanding the bonded region is important to improve the fracture load of the Cu-Ni interface. The ratio of the unbonded region to the bonded one at the Cu-Ni interface decreased from 72 to 54 % after the high-temperature test, indicating that diffusion bonding had occurred during the high-temperature test performed at 673 K. Therefore, the fracture load of the interface between the bonded Cu and Ni was improved at 673 K.

Bondability under lower applied pressures (0.5 and 1 MPa) was investigated on the basis of the interaction force generated between the particles and the solvent used during sintering. In the case of single (unmixed) solvents, the shear strength decreased monotonously with an increase in the molecular weight of the solvent used, regardless of the size of the particles used. This was because the decomposition of solvents with high molecular weight was not complete till bonding at 300°C. In the case of mixed solvents, the shear strength achieved using a mix of triethylene glycolol (TEG) and polyethylene glycol (PEG) 200 increased and was greater than that achieved using conventional solders. The results obtained could be explained well by the dynamics of the solvent and the particles. Thus, it was surmised that the capillary force can be effective in joining larger particles together under lower applied pressures.

This study shows the time variation of temperature distribution and concentration distribution of metal vapor during gas tungsten arc welding. In this study, stationary gas tungsten arc welding of pure iron was conducted in helium as shielding gas. We investigated the influence of iron vapor on arc plasma and mechanism of iron vapor transportation from the weld pool into the arc plasma. We observed the arc plasma two-dimensionally with spectrometric method. We captured three different monochromatic images simultaneously by three monochromators with high speed video cameras. Thus, two inherence spectra of iron and also one inherence spectrum of helium in arc plasma were obtained. Using these spectra, plasma temperature distribution was obtained. In addition, concentration distribution of iron vapor was obtained by measuring electron density. It was concluded that plasma temperature decreased with increase of iron vapor concentration. Especially, the metal vapor concentration indicated the highest value near the base metal and the temperature around the weld pool decreased rapidly.

Hardfacing weld techniques apply mainly to improve the service life of engineering components either by rebuilding or fabricating a composite layer, which will protect the components from wear, erosion, and corrosion. However, hardfaced deposits are susceptible to cracks compared to the base metal due to thermal cycles and different chemical composition of hardfaced consumables. In addition, this method yields only marginal improvements in wear resistance performance. This investigation was attempted to eliminate the cracks by depositing a soft buttering layer by using austenitic stainless steel consumable in the area between base metal and hardfaced layer. Preheating was also applied at the joint. The result reveals that buttering and preheating reduce crack susceptibility and increase wear performance of hardfaced HSLA steel deposit.

TIG-MIG hybrid welding process has a possibility as a new type of welding process with high quality and high efficiency. However, the mechanism of this process is not clear due to complex interaction of TIG arc and MIG arc. In this study, the calculation of arc phenomena in TIG-MIG hybrid welding process was performed by using three-dimensional numerical model, in which torch angle was changed. Consequently, it was found that the expansion of high temperature plasma between the electrodes and the current between the electrodes increased with torch angle, and the balance determined by the stiffness and repulsion of both arc was important for the convergence of heat flux. It was also found that TIG-MIG hybrid welding process had the possibility to optimize the plasma property and the heat source property by adjusting TIG and MIG torch angles.

An accurate control over the rate of heat input and material deposition is essential in gas metal arc welding for its greater use in joining of sheet metals. Although pulsed current gas metal arc welding facilitates excellent control over the rate of material deposition, greater rate of heat input due to high peak pulses remains critical. Gas metal arc welding with conventional short-circuiting mode of metal transfer provides a significant reduction in the rate of heat input while an uninterrupted and spatter-free material deposition is too difficult to achieve. A novel low energy input short-circuiting gas metal arc welding is proposed here that facilitates short-circuiting mode of metal transfer with a very low power detachment phase. Here we present an investigation on the current, voltage and consequent metal transfer sequences in pulsed current, and the conventional and the novel low energy input short-circuiting gas metal arc welding processes of high strength automotive steel sheets. It is realized that the low energy input short-circuiting process could provide uninterrupted and nearly spatter-free metal transfer at significantly reduced electrical power in comparison to both pulsed current and conventional short-circuiting gas metal arc welding. The low energy input short-circuiting process could also facilitate fairly small angular distortion of weld joints.

This study aims to clarify fume formation mechanism theoretically through visualization of fume shape. In this paper, influence of quenching rate on fume formation process was investigated through numerical analysis with the fume formation model. The fume formation model consisting of heterogeneous condensation model, homogeneous nucleation model and coagulation model has been developed. The fume formation process under the condition assuming fume formation from iron vapor in MIG welding was visualized through visualization with numerical simulation. As a result, it was found that around 1000K, diameters of secondary particles approach an order of hundred nanometer at maximum. Those typically become chain shape consisting of primary particles. The diameters of primary particles become larger for lower quenching rate because of occurrence of coagulation among particles in liquid phase.

In this paper, individual effects of quenching rate and pressure of metal vapor on fume formation process were investigated through experimental observation with the equipment which enables to produce a simple one-dimensional arc plasma stream and to discuss both effects individually and numerical simulation. Consequently, it was clarified that high pressure of metal vapor enlarged size of fume particles. In addition, it was also found that high quenching rate generated large amount of particles because much more metal vapor was consumed by homogeneous nucleation. As a result, typical particle shape becomes like chain.

During welding, the weld pool has a fluid flow with thermal conduction and a free surface. The dropped metal also supplies a free surface. These complex phenomena are difficult to simulate using the grid method. On the other hand, particle methods, such as the smoothed-particle hydrodynamics (SPH) method or the moving particle semi-implicit (MPS) method, can evaluate the fluid flow and the thermal conduction with a free surface more easily than grid methods. In the present study, for high-speed and effective simulation of the welding process, including the weld pool, a hybrid particle and grid method with explicit MPS is performed. In this hybrid method, particles can be used in the weld pool and the area located near the weld pool, whereas grid elements are used in the other areas. Furthermore, in order to consider interface effects, a light source model for the welding process simulation is developed. As a result, numerical simulation of the heat conduction in the weld joint, including the fluid flow in the weld pool with a free surface, is performed.

The formation of stable penetration beads in joining of the thick materials is important in order to achieve high quality welded metal joints. Since plasma welding employs the high welding current density, it is suitable in joining of the thick materials. The relationship between the voltage and the keyhole is unclear. It is important to keep the standoff in a high quality of the plasma welding. But it is difficult to estimate the standoff by using the voltage and the welding current. The authors tried to observe the weld pool on top side of the base metal, directly. For this purpose, the authors tried to apply a CCD camera with an external trigger. Moreover, the keyhole depends on the standoff of the plasma torch. When the base metal was inclined due to the setting in the robotic welding, a good quality of welding cannot be obtained. In the teaching of the robot, it took much time to adjust the standoff, since the electrode of the plasma torch was hidden into the plasma tip. In order to save the teaching time, the authors tried to control standoff of torch by processing the weld pool images taken with the CCD camera during the welding.

A new longitudinal electromagnetic hybrid TIG welding-brazing method (LE-TIG-B welding) is applied to light alloy / steel dissimilar metals joining experiments. The microstructure and composition analyses of the different brazed joints are examined using OM, SEM, EDS and Vickers microhardness measurement techniques. The elements distribution, microhardness, welding defect and wetting angle are investigated by the comparison of TIG welding-brazing (TIG-B welding) and LE-TIG-B welding. The result shows that 3003 aluminum alloy / Q235 carbon steel, AZ91 magnesium alloy / 304 stainless steel, AZ31 magnesium alloy / Q235 carbon steel can be formed the tight lap joint by LE-TIG-B welding. The longitudinal electromagnetic field can affect the flowability and surface wettability for the filler metal on the interface of light alloy / steel dissimilar metal joints. The tensile strength of the joints attains a maximum value of 198.6 MPa with the magnetic frequency and magnetic intensity set at 20Hz and 0.01T respectively. The additional longitudinal electromagnetic field can be an inverse affect on the process of light alloy / steel dissimilar metals joining, so the welding parameters need be optimized in LE-TIG-B welding.

In order to investigate effects of applied longitude magnetic field on the plasma in welding process, velocity and temperature distribution in argon Gas Tungsten Arc (GTA) plasma during welding is predicted by numerical simulation with and without applied magnetic field. Our results showed that arc plasma expand in the bottom and shrink in the upper part as a result of the Lorentz force induced by the longitude magnetic field. The arc plasma velocity also expands because the Lorentz force.

In this research, the hot-wire gas tungsten arc welding (GTAW) process was developed to create a sound bead having a high WC content ratio and uniform dispersion of WC particles in order to improve the abrasion resistance of a hard-faced layer. The effect of the welding parameter on welding phenomenon and weld bead properties, i.e. the content ratio and distribution of WC, were investigated in detail to achieve the optimum welding conditions. In addition, an abrasion test was carried out using the specimen produced using the obtained conditions to evaluate the abrasion resistance property. The weld bead using the hot-wire GTAW process by employing the optimum conditions showed the high content ratio and uniform distribution of WC particles, and it could achieve the high abrasion resistance.

In this study, plasma diagnostics in MIG welding of aluminum is performed. In MIG welding, the metal vapor has the large influences on the welding process since the content of the metal vapor in the arc plasma is large. The distribution of metal vapor varies dynamically by a metal transfer. Therefore, the dynamical distributions of temperature and concentration of metal vapor are obtained by the spectral images pictured by high-speed video cameras. Consequently, the metal vapor is distributed near the center axis and the temperature of the arc plasma decreases. Finally, we discuss plasma physics in the welding arc through numerical simulations, using the basic conservation equations of mass, energy, momentum, current and electron density of plasma physics. There is close interaction between the electrode, the arc plasma, the weld pool, and also the metal vapor, which constitute the welding process, and must be considered as a unified system. The simulation results also show the temperature of the arc plasma decreases near the center axis. This is caused by the fact that the energy loss by the radiation increases with increases with the metal content.

Resistance heat aided friction stir welding (RFSW) is a new hybrid welding technology which is based on Friction stir welding. This new technology has been invented in order to solve some challenges or questions of friction stir welding. In this paper, a 3D viscoplasticity thermo mechanical model of RFSW with threaded / unthreaded pin is built up, and the temperature, strain-stress, materials flow and welding defects have been achieved successfully by FEM simulation software. The effect of resistance heat is investigated through FSW and RFSW with different magnitudes of additional electronic current. The research results show that additional electronic current reduces the asymmetry of temperature distribution of FSW. RFSW appears as a bimodality strain curve, and the effective strain of the advancing side is larger than that of the retreating side. The threaded pin helps to achieve the material flow in the thickness direction of FSW joints by the tracking method. In addition, the welding defects of kiss, furrow and wormhole are can be predicted successfully. RFSW can make great improvements on the welding defects, which is in agreement with the experimental results.

Resistance heat aided friction stir welding (RFSW) was utilized in this experiment with the combined effects of friction heating and resistance heating by adding the direct electric current (0-160 A) to the AZ31B magnesium alloy butt joint. The welded joints were described by optical microscope observations. Analysis of the grain boundary character distribution by optical microscope showed that the weld zone defects decreased, and smoother and more compact joint appearance was accessed by combining the direct electric current with the conventional FSW. In addition, raising the value of electric current promoted the joint quality and refined the grain size in the weld nugget zone (WNZ) and thermo mechanically affected zone (TMAZ) dramatically. However, since there are more grain growth and secondary recrystallization in HAZ of RFSW joints, the average grain size is larger than that of conventional FSW joints. The travel speed of 30 mm/min and the current value of 160A were optimum options within the parameters investigated in this work.

Friction stir spot welding (FSSW) has already been used in many companies for joining light materials such as aluminum and magnesium alloys, and in recent years, this process is coming to use for dissimilar joints. It is important to understand the material flow during dissimilar FSSW to obtain a sound joint. In this study, the material flow and temperature history during dissimilar FSSW on magnesium alloys is investigated by experimental approaches. Results of the investigation indicate that the combination of the upper and lower sheet materials affects largely on the material flow and joint soundness. Furthermore, the selection of a tracer material to observe the material flow was discussed. It becomes also clear that the appropriate tracer material must be selected for the adequate investigation of the material flow during FSSW.

To use brass as a tribological material instead of Pb bronze, the feasibility of forming a lap joint of brass sheet on steel plate was investigated by using Friction Stir Welding, and it was proved that a lap joint with smooth surface and good joint strength was successfully made. Furthermore, the mechanism taking place at the lap joint of brass to steel was discussed by observing the interface.

The aim of this study is to develop the novel temperature measurement method during welding by using two high-speed cameras and the two-color thermometry method. This new method has some features, i.e. in-situ observation, high response speed, 2D measurement, wide temperature range and noncontact measurement. By using this method, the accurate in-situ temperature 2D distribution and history could be obtained during laser welding based on the selection of suitable band-pass filters for laser welding.

Three-dimensional finite element analysis with a special subroutine was carried out to obtain the temperature distribution during hot-wire laser welding with a narrow-gap joint. A moving heat source was modeled with the element rebirth technique, and laser reflection was modeled by heat flux distributed around the front of the weld pool. The model was used to reproduce the phenomenon of hot-wire laser welding with a narrow gap investigated by in-situ observation. The thermal strain for different weld shapes was then calculated to evaluate the susceptibility to solidification cracking. The simulation results were validated with experimental measurements in terms of the thermal cycle history of the molten pool, peak temperature distribution close to the fusion zone, and cross section of the weld bead. The thermal simulation was found to agree reasonably well with the experimental results and it was revealed that hot-wire laser welding is an interesting alternative process that reduces solidification cracking in a narrow-gap joint.

Hydroforming is a manufacturing process that uses a fluid medium to form a part by using high internal pressure. The bulging process is one of the important methods for manufacturing the serpentine flowing channel evaporation board for the ice machine. The serpentine flowing channel evaporation board is made by the laser welding, which is of a high and complicated nonlinear deformation characteristic in bulging. In this paper, one simple practical model is applied to simulate the deformation and check the reliability of welding joint on bulging process. The bulging key parameters--bulging pressure, size of flowing channels and loading path are determined and optimized in order to avoid the uneven bulging. The height change and thickness distribution of welding structure's bulging deformation are discussed respectively.

Control of beam focus position is important in the regime choice of electron beam welding (EBW). It is known that the methods of electron beam focus control in electron beam welding are done according to the parameters of secondary current signal collected in plasma. The purpose of the study is to develop a method for operational control of the electron beam focusing during welding in the deep penetration mode. The method uses an additional informational parameter of collected current signal in beam/work-piece plasma. This parameter can be used for creating the operational control methods for exact electron beam focusing during the process of welding. In addition, uses of this parameter allow a better study of the keyhole processes.

Friction stir welding (FSW) is a suitable solid-state joining process for dissimilar metal joining. We have performed metallographic characterizations to investigate the effects that an alloying element has on the interfacial microstructure between a dissimilar joint of AZ31 magnesium alloy and commercial pure titanium, produced by FSW. At the joint interface, an interfacial layer was observed with a thickness of less than 100 nm in a STEM-HAADF image produced with a transmission electron microscope (TEM) and an energy dispersive X-ray spectroscope (EDS). In this layer, TiAl intermetallic compound was observed with nano-beam diffraction (NBD).

In order to clarify the mechanism of metal vapor generation in helium (He) gas tungsten arc plasma, the phenomenon of heat transfer between the plasma and the base metal was examined. In addition to the argon (Ar) arc and He arc, the numerical analyses of imaginary arcs changing the thermal conductivities of respective arcs were conducted, and the plasma temperatures, molten pool temperatures, and the concentrations of metal vapor were compared. As a result, with the increase of the thermal conductivity of the plasma, the maximum temperature of the molten pool surface and the maximum iron (Fe) vapor concentration went up while the plasma temperature in the vicinity of the molten pool dropped. Therefore it is assumed that the phenomenon of Fe vapor generation in the He arc and the phenomenon of the plasma temperature drop near the molten pool are influenced greatly by the high thermal conductivity of the He plasma.