Our previous work has demonstrated that, when welding a large diameter steel stud in the horizontal position, it is necessary to prevent molten metal from dripping from the weld pool. The formation of blowholes in the weld metal must also be suppressed to maintain the soundness of the joint. In addition, it was found that a joint can be considered suitable if its defect ratio does not exceed a threshold level of 10%, and that this can be achieved by optimizing various welding parameters, such as the arc holding time and current.
In the present study, two processes were used to further improve the soundness of joints stud-welded in the horizontal position. First, the ferrule was inverted in order to reduce the interstices between it and the stud, and to eliminate gas through-grooves. Both factors are deleterious because they can allow molten metal to hang down from the weld pool. Inversion of the ferrule was found to be effective at supporting the molten metal and ensuring a suitable degree of fusion. Second, the welding atmosphere was controlled by employing aluminum as a deoxidant in conjunction with argon gas to shield the welded region from ambient air, thus reducing the formation of blowholes in the weld metal. The formation of blowholes was significantly reduced by the combined effects of these two methods.

View full abstract

Resistance spot welding of three steel sheets is used in automotive manufacturing. The determination of welding conditions for three steel sheets welding is more difficult than that for two steel sheets welding because in the former case, two interfaces are simultaneously welded. The welding condition to improve the weld strength of thin and thick sheets' interface is required for thin, thick, and thick sheets welding. In addition, avoiding weld spatter is an important technical concern for the three sheets welding.
This paper discusses the triply coupled effects of elasto-plastic large deformation contact, electric current, and thermal conduction for the three sheets welding via triply coupled finite element analyses. The triply coupled effects of thin and thick sheets' interface and those of thick and thick sheets' interface are compared to discuss the special characteristics of the triply coupled effects of three sheets welding.
The nugget diameter of the thick and thick sheets' interface is larger than that of the thin and thick sheets' interface because the electrical contact resistance and the base electrical resistance of high-strength steels are higher than those of general-strength steels. Furthermore, the chain of the coupled effects of electrical resistances, Joule heat generation, temperatures, and electrical resistances is strong. As the effect of welding current on temperature by the coupling effect for the thick and thick sheets' interface is stronger than that for the thin and thick sheets' interface, the possibility of spatter increases. Increasing the nugget diameter solely for the thin and thick sheets' interface using electrode force is difficult because the effect of the electrode force on the nugget diameter is approximately the same for both interfaces. On the contrary, material characteristics can affect the nugget diameter solely for either of the interfaces. The possibility of spatter increases for the welding condition with a sheet gap because the temperature of the contact edge for the sheets' interface increases.

View full abstract

Consumption of tungsten electrode during TIG welding process is one of unavoidable problems and many studies have been progressed to improve the consumption resistance. In general, some kinds of oxides (ThO₂, La₂O₃, Ce₂O₃) are added to the tungsten electrode in order to make the thermionic emission easy and control the electrode temperature below the melting point of tungsten. However, the lifetime of electrode is still limited within a few hours because the additives evaporate and a lack of additives is caused on the electrode surface. In this study, numerical simulations which focus on the evaporation and diffusion phenomena of additives in the electrode were performed in order to clarify a consumption mechanism and identify effective factors for the lifetime of electrode.
As time passed, the mass concentration of additive decreased due to an evaporation phenomenon whereas the additive was supplied from inside to outside of electrode by a diffusion phenomenon. When the degree of coverage of additive decreased, the electrode temperature quickly increased and it reached the melting point of tungsten. The lifetime of electrode was strongly depended on the physical properties of additives such as diffusion constant and melting point of their oxides.

View full abstract

Magnesium, which is the lightest among the metals used as structural materials, has several advantages such as high strength-to-weight ratio and high recyclability. Recently, the demands of dissimilar metal joint of magnesium alloy to steel have arisen for weight reduction in transportation vehicle industry. However, it is well known that joining of magnesium alloy to steel is difficult because of differences in melting point and thermal conductivity between both metals. Therefore, new welding processes with high reliability and productivity for these dissimilar materials are demanded. Laser roll welding is one of the candidates, which is effective for joining of dissimilar metals. In the present work, laser roll welding of magnesium alloy to steel was tried to investigate the effects of the process parameters on the microstructure of the joint and the mechanical properties. As a result, existence of the interface layer consisting mainly of Fe and Al was confirmed, and increase in welding speed led to decrease in the layer thickness. In addition, increase of bonding area at the joint interface led to increase of the joint strength.

View full abstract

Computational model of submerged arc welding (SAW) was developed to clarify the arc phenomena and the heat source characteristics. Furthermore, the weld part during SAW was observed using an X-ray transmission system. To investigate the effect of the closed space on arc phenomena, the heat flux and current density distributions on a base metal surface during closed gas tungsten arc welding were measured by split anode method. Computational results showed that the metal vapor concentration in arc space during SAW became higher because the arc was generated in the closed space. However, its heat input to the base metal was almost the same as that of gas metal arc welding. Meanwhile, the welding current and arc voltage which largely affect characteristics of arc plasma agreed with the experiment results. The slag thickness obtained from the computation was also the same as that of the experiment, which supports the validity of this computational model. Moreover, comparative experiments with gas tungsten arc and closed gas tungsten arc showed that the closed space environment around the weld part in SAW did not affect heat characteristics of arc plasma. It was clarified that heat transfer and radiation energy of arc play different roles in SAW. Penetration of a base metal was formed by the heating from arc. Flux was melted by the radiative heating from arc, and it formed slag. It was suggested that the high heat efficiency from 90 to 99% during SAW was obtained since the radiation energy was given to a base metal indirectly through the slag.

View full abstract

It is well-known that diffusible hydrogen strongly influences cold cracking. However, it isn't understood well what kinds of factors in welding consumables affect diffusible hydrogen in weld metal. For example, it is thought that seamed flux cored wire has more amount of diffusible hydrogen than seamless flux cored wire. This study pointed out a misunderstanding of diffusible hydrogen content generated by FCAW. As a result of investigation, there was no correlation between wire structure and diffusible hydrogen content. Furthermore, welding-wire-related factors on GMAW and FCAW were classified clearly into the follows. (a) Surface lubricant, (b) Initial moisture of flux and (c) Moisture absorbed after production. Various welding wires were prepared and measured the diffusible hydrogen content in order to compare the effect of the each factor. As a result, the most influential factor is (b). The second is (c). The effect of (a) is the smallest.

View full abstract

Thermal nanoparticle spraying has been developed to create fine ceramic layers with a high deposition rate. Micro composite fragments containing ceramic nanoparticles could be successfully introduced into a plasma flame using a conventional powder feeding equipment. Yttria-stabilized zirconia (YSZ) nanoparticles with an average diameter of 200 nm were dispersed in a thermosetting acrylic liquid resin at a volume fraction of 40-60 %. The paste material was placed into a sealed container having an inner capacity of 150 cm³. Dispersing and degassing were performed using a planetary mixer. The mixed paste was solidified by heating at 150°C for 30 min. The composite bulk was crushed using a high-speed vibrating milling machine. The obtained micro composite fragments having particle sizes in the range of 45-106μm were separated by sieving. The moving behaviors of the micro composite fragments in the plasma flame were visualized using a high-speed camera. In the plasma flame, the resin matrix was burned down, and the heated nanoparticles were deposited on a metal substrate. The microstructure of the coated layers was examined using scanning electron microscopy (SEM). The hardness distributions of the coated layers were measured using a micro Vickers hardness tester.

View full abstract

Al-Si-Mn weathering steel is a promising material for long-life social infrastructure. This steel shows excellent weathering ability without rare elements. When this steel is applied to welded structure, toughness at HAZ (Heat Affected Zone) bond in weld joint drops. It is becoming clear that doping of B at ppm-level to the steel improves the toughness, but the effective range of B content is limited. So precision quantitative microanalysis of B for control of the effect is necessary. In this study, we applied laser ablation ICP mass spectrometry (LA-ICP-MS) to precision microanalysis of B in the steel. This method is rapid process in comparison with various wet chemical analyses which are prescribed for quantitative microanalysis. The values of B ion intensity by LA-ICP-MS are correlated closely with B concentration by wet chemical microanalysis. This excellent correlation suggests LA-ICP-MS is a promising method for quantitative microanalysis of B in Al-Si-Mn weathering steel.

View full abstract

In the current standard, diameters of upset weldable steel bar are restricted 16mm or less. In this study, we visualized the changes of welded part and the changes of temperature in the upset welding by using a high-speed camera. The specimens used in the experiment are SD390 steels with diameters of 19mm or more. Thus, in the upset welding of shear reinforcement with diameter of 32mm, it is possible to achieve a 100% joint efficiency by ensuring the optimum Joule heat by adjusting the welding pressure and the electric current. The two-color thermometry revealed that ensuring the joining temperature range (1300°C-1400°C) is important to secure the safe welding.

View full abstract

Stress corrosion cracking (SCC) has recently been observed in Ni-based alloy welds in nuclear power plants, despite countermeasures to prevent it. In the repair welding, temper bead welding (TBW) is applicable when post weld heat treatment (PWHT) is difficult to carry out. In TBW, the welding process variables are appropriately controlled so that the thermal cycles of the subsequent passes provide the area hardened by previous passes with the temper effect, so the hardened area size and thermal cycle tempering parameter (TCTP) are very important. However, very few studies have discussed the quantitative relationships among the hardened area size, TCTP and the welding process variables. In this study, we conducted a combined weld bead surface profile and heat conduction analysis for simulating temper bead welding with Ni-based filler metal to SQV2A plate under various welding conditions. We quantified the hardened area size and TCTP as linear functions of the welding process variable parameter, which was derived from welding heat conduction theory with a moving heat source. Consequently, we concluded that the appropriate welding conditions in temper bead repair welding can be determined based on the welding process variable parameter.

View full abstract

In nuclear power plants, the high tensile residual stress introduced by welding after surface machining may be an important factor in the occurrence of stress corrosion cracking in welds of low-carbon austenitic stainless steel type SUS316L. This study investigated the effect of machining conditions on variations in the distributions of residual stress introduced through surface machining and sequential welding. First, in experiments, test specimens were prepared by surface machining under different machining conditions. Then, bead-on-plate welding under the same welding conditions was conducted on the test specimens with different surface machined layers. The residual stress variations were evaluated by X-ray diffraction method. As a result, the residual stress introduced by surface machining showed nearly uniform distributions and varied drastically depending on the machining conditions. After welding, the welding longitudinal residual stress had a maximum tensile stress in the heat affected zone and the values of residual stress were independent of the residual stress due to surface machining. Based on the Vickers hardness testing results, we concluded that the magnitude of the maximum residual stress introduced by welding after surface machining is strongly dependent on the degree of work hardening due to surface machining.

View full abstract

Hydrogen induced cracking is usually evaluated by totally charged hydrogen concentration and macroscopically applied stress. However, microscopic distributions of stress and hydrogen concentration should be formed at the scale of microstructure. The stress concentration and hydrogen accumulation would be remarkable in duplex stainless steels and its welds, because they usually consist of ferritic and austenitic phases with different strength and diffusion properties. In this study, the effect of microstructure in duplex stainless steel on hydrogen diffusion behavior was investigated as a first step toward the evaluation of hydrogen induced fracture at microscopic scale. A series of finite element simulations have been performed under microstructure models considering duplex stainless steels and its weld metals. The hydrogen diffusion behavior was strongly influenced by the continuity of austenite phases which have lower diffusion constant compared to ferrite phases. The diffusible hydrogen accumulated to the location where stress concentration occurred due to microscopically different mechanical properties, however, the effect of stress concentration at microscopic scale was not significant for the overall diffusion behavior when the stress gradient at macroscopic scale was relatively small.

View full abstract

Recently, weight saving of car bodies have been required due to the enhancement of regulations for emissions of CO2, and to improve collision safety, high tensile steel is widely used on car bodies. However, joint strength may decrease on high tensile steel. In addition, joint strength depends on fracture mode. Therefore, it is necessary to predict crack propagation direction and investigate the factors which influence the crack propagation direction. In this research, the author has proposed ductile crack analysis method based on continuum damage mechanics. In ductile crack propagation analysis, various potential models have been proposed as damage term. However, in such models, it is difficult to obtain convergence on the calculation of nonlinear constitutive relation. Therefore, in this study, a simplified ductile fracture evaluation method, which is based on the elastic-plastic analysis considering large deformation and strain, was proposed. The proposed method was applied to the analysis of cross tension test on spot weld joints. As a result, it was found that crack propagates along the outer periphery of the weld nugget on larger nugget diameter. These fracture modes are called plug fracture and higher cross tensile strength can be obtained in plug fracture. However, in smaller nugget diameter, crack propagates across the internal nugget. It is called interface fracture and joint strength decreases in this fracture mode. These tendencies agree with the results obtained by experiments. Therefore, in this study, it can be said that crack propagation direction can be predicted on cross tension test on spot weld joints by using the proposed method.

View full abstract

Metal transfer behavior in Plasma MIG welding process was observed by means of shadow graph method. Laser system was used in order to eliminate the arc effect, and the metal transfer was clearly observed by using high speed video camera. In this study, metal transfer in conventional MIG welding was also observed for comparison between both processes. As a result, under the same MIG current setting, the transfer type for both Plasma MIG welding and conventional MIG welding were observed to be in different mode, in which Plasma MIG welding behaved as a globular mode, whereas conventional MIG welding has already became spray mode. It is suggested that, the presence of plasma current in Plasma MIG welding process affected the MIG current distribution for not concentrated at wire tip but dispersed to some ranges on wire between plasma electrodes and wire tip. These phenomena which led to the dispersion of force concentration resulted in a lower transfer frequency and bigger droplet size. These factors then were identified as dominant factors that influenced the droplet temperature.

View full abstract

Simulation of molten flux flow during submerged arc welding was carried out by a hybrid method using Discrete Element Method and Incompressible Smoothed Particle Hydrodynamics method. As a result, flux melting process was calculated with time evolution. Moreover, it was also simulated that molten flux was re-solidified at a backward of a heat source. In this simulation, the average thickness of this slag at z = 5.0±0.25 mm (the center of the wire in the z direction) was estimated to be 4.8 mm. These results showed that the potential and the usefulness of this hybrid method to simulate the flux melting during SAW.

View full abstract

Pure copper (Cu) were joined to polyamide 6 (PA6) directly by friction lap joining (FLJ) at the joining speed of 200-1600 mm/min with a constant rotation rate of 1000 rpm. As the joining speed increased, the tensile shear force increased first, and decreased thereafter, and the maximum tensile shear force at 800 mm/min could approach to 1 kN. The effect of the joining area, the strength of the melting plastic matrix, and the bubbles on the tensile shear force was discussed.

View full abstract

The cleaning action with AC Tungsten Inert Gas (TIG) welding is attributed to the removal process of oxide film caused by cathode spots. In this study, a double shielding gas torch, which used the internal shielding gas of helium and the external shielding gas of helium or helium with 5.0 % oxygen, was employed to investigate influence of admixture of oxygen into the shielding gas on the cathode spot behavior in AC TIG welding. The cathode spots were photographed by a high-speed video camera and their distribution and velocity are discussed. Consequently, it was found that when the oxygen was mixed into the shielding gas, the number of cathode spots seemed to increase. Furthermore, the average velocity of cathode spots was considerably decreased to approximately 50 m/s near the center of the weld pool compared with 100m/s in case of pure helium shielding gas.

View full abstract

MIG welding under pure argon shielding gas atmosphere (pure argon-MIG welding) is suitable to obtain a high-strength and high toughness welded joint. However, it is difficult to apply pure argon-MIG welding practically to welding structure because of arc instability. In order to perform stable pure argon-MIG welding, duplex current feeding MIG welding has been developed. The duplex current feeding MIG welding consists of primary MIG welding current by constant-voltage power source and secondary current by constant-current power resource. In previous experimental study, it was found that the temperature of a droplet by duplex current feeding MIG welding was higher than the conventional MIG welding. In this paper, influence of electrical conductivity of the wire on basic characteristics of duplex current feeding MIG welding are discussed by numerical analysis. Consequently, it was clarified that the increase in the droplet temperature in the duplex current feeding MIG welding was significant especially in case of a wire material with lower electrical conductivity.

View full abstract

Gas Metal Arc Welding (GMAW) under pure Argon shielding gas atmosphere (pure Argon-GMAW) is suitable to obtain a high-strength and high toughness welded joint. However, it is difficult that pure Argon-GMA welding is applied practically welding structure because of arc instability. In order to perform stable pure Argon-GMA welding, duplex current feeding GMAW (DCF-GMAW) has been developed. The DCF-GMAW consists of primary GMA welding current and secondary welding current by constant-current power resource. DFC-GMAW can feed larger current near wire tip. This effect makes that weld penetration depth is deeper, weld bead shape is improved using DCF-GMAW.

View full abstract

In order to examine the difference between the sensitivity of Charpy absorbed energy, _VE, and critical CTOD, δ_cr, to strength mismatch in welds, the numerical analysis was conducted. This study adopted the evaluation method based on the Weibull stress criterion for calculation of _VE and δ_cr assuming that the critical Weibull stress at brittle fracture is independent of the strength mismatch as well as the loading rate. This analysis showed that the _VE and δ_cr decreased as the location of the V-notch or crack approached welded interface. The change in _VE was larger than that in δ_cr, because the strength mismatch caused acceleration of strain rate and decrease in temperature rise near the V-notch as well as plastic constraint. This result indicates that the sensitivity of the _VE and δ_cr to strength mismatch is different depending on the loading rate.

View full abstract

This study investigates the effects of maximum temperature and temperature history during post-heating treatment on residual stress of resistance spot welded high-strength steel sheets. To examine the effects of post-heating treatment, numerical simulations were performed. First, the effect of the phase proportion of tempered martensite and maximum temperature was investigated. This result shows that both of the phase proportion of tempered martensite and maximum temperature affect the residual stress. Second, only the effect of the maximum temperature was examined when the almost all martensite changed to tempered martensite. This result shows that the low maximum temperature decreases the residual stress. Finally, as an example, controlling the temperature history was proposed to satisfy both of the controlling the residual stress and shortening the tempering time. This result shows that controlling temperature history can decrease the maximum temperature in short tempering time and control the tensile residual stress.

View full abstract

The core structure of a plate-fin type heat exchanger is composite of brazed plates and fins. Header tanks are welded on the core as inlet and outlet paths. Heat exchangers are essential in the energy system and their importance will continue to grow in the future. In recent years, improvement performance of this heat exchanger is expected; therefore, the number of streams introduced into a core and the operating pressure are expected to increase. This leads to the increase of the number and the thickness of header tanks used. As the result, the effect of welding heat input will become significant. The weld induced deformation and residual stress will affect the structural integrity of the heat exchanger, however, the effect of the welding of header tanks has not been well investigated. The information on the relationship between member dimension and weld induced stress will support the design and fabrication process. In this study, a quasi-three-dimensional simulation model of welding of header tanks on core structure was developed. The effect of member dimensions on weld-induced stress was investigated systematically by the proposed simulation model.

View full abstract

To prevent stress corrosion cracking or to extend the fatigue life of structures, various peening techniques are employed. In this research, to reproduce the residual stress distribution after shot peening, the residual stress due to multi-pass welding was predicted and the analysis of shot peening considering the welding residual stress distribution was conducted using the analysis system proposed by the authors, which was based on high-speed non-linear FE analysis method named Idealized Explicit FEM. The analyzed residual stress distribution due to the shot peening was compared with the experimental measurement using X-ray diffraction. As a result, it was found that the predicted residual stress distribution well reproduces the residual stress distribution after shot peening. To evaluate the effect of shot peening under the various loading conditions, tensile and compressive cyclic load was applied to the pipe joint on the numerical analysis considering the residual stress after shot peening. As a result, it was found that the influence of tensile cyclic load on the residual stress distribution is small while the compressive residual stress due to peening is decreased by the compressive cyclic load in the targeted pipe joint.

View full abstract

This study investigated the influence of impact velocity for the V-notched Charpy specimen on the transition time, t_T, which is defined as the time when the kinetic energy is equal to the deformation energy of the specimen. The t_T indicates the point in the response after which inertial effects diminish rapidly. In this study, dynamic stress/strain fields in the V-notched Charpy specimen were numerically analyzed by means of three-dimensional dynamic explicit finite element (FE) analysis. This analysis considered the effect of high-speed strain rate on the flow stress and the increase in temperature during impact loading. The contact problem between the specimen and the striker of Charpy testing machine was solved using the Hertzian contact theory. This FE-analysis shows that the t_T decreases slightly with increasing impact velocity over the range from 1 to 10 m/s. The deformation energy increases more rapidly than the kinetic energy with increasing impact velocity. The increase in the deformation energy leads to shorter t_T. The t_T depends on the strength class of steels. The t_T decreased with decreasing in strength of steels, because of the reduction of kinetic energy.

View full abstract

Friction stir welding (FSW) is a solid-state joining process which employs a lower joining temperature than that used in fusion welding. In this study, the weldability and the effects on the interfacial microstructure of alloying elements in dissimilar metal lap joints between commercial pure titanium and nickel-based alloy (Inconel 625) were examined by friction stir welding. The thickness of the stir zone on the titanium side decreased with increasing tool rotation speed. At high rotation speeds, the titanium adhered to the surface of the FSW tool's shoulder and probe. Fracture occurred in the region in the stir zone of titanium where its thickness decreased by tensile share test. A thin reaction layer was formed at the joint interface under suitable joining conditions. Transmission electron microscopy revealed two types of layers in this reaction layer. The layer on the titanium side was less than 100 nm thick and contained nickel, titanium and a small amount of iron; the other layer, on the Inconel 625 side, was less than 50 nm thick and contained all the alloying elements in Inconel 625, plus titanium. A quantitative analysis of the titanium-side layer using TEM-EDX showed its composition to be 68.9% Ti and 29.2% Ni (by atomic percentage), suggesting that the layer is comprised of the intermetallic compound Ti₂Ni.

View full abstract

Relationship between oxidation of Ag-Cu-Ti braze alloy and oxygen contents was investigated in the laser brazing of a hexagonal boron nitride block to cemented carbide plate in Ar atmosphere. The laser brazing was conducted in Ar-flow atmosphere in a vacuum chamber, and changing Ar-flow rate with and without pre-evacuation. No oxidation of the braze alloy was found after 3-min Ar-flow atmosphere over 5 L/min without pre-evacuation and at 5 L/min with pre-evacuation. When the Ar flow rate increased to 10 L/min without pre-evacuation, the oxygen content decreased to 3.8 ppm. In this case, the average shear strength of the brazed joint was about 8 MPa and a fracture occurred at the hexagonal, boron nitride side of the specimen, near the interface.

View full abstract

This study aims to develop a new oxygen plasma cutting process for a thick steel plate. In this process, the oxidation reaction is enhanced by preheating the cut surface near the bottom by an additional heat source to realize cutting of the thick plate. In this paper, influence of the preheating process on cutting performance was discussed. Consequently, it was found that the maximum cutting velocity increased with the assistance of the preheating, because the oxidation reaction increased also near the bottom surface. However, in order to decrease the surface roughness and the taper angle, the preheating temperature and the preheating method should be improved in the future.

View full abstract

This investigation is purposed to clarify the influence of pilot gas composition to convective pattern on the weld pool surface in Plasma keyhole arc welding process. In order to clarify this, the convective pattern on the top surface of weld pool was investigated in both cases of pilot gas: pure Ar and Ar mixture with 10% hydrogen. For estimating the convective pattern, the zirconia particles with diameter of 0.03 mm were utilized as tracers. After welding, using the software (Dipp-motion, Detect Co., Ltd) for analyzing the movement of zirconia particles, the convective pattern was estimated. In case of pure Ar, zirconia particles were in circulated motions just behind keyhole on the top surface of weld pool, and transported to rear part of weld pool on the bottom surface. In case of Ar mixture with 10% hydrogen, zirconia particles on both surfaces (top surface and bottom surface) of weld pool were in circulated motions behind keyhole. Furthermore, the weld bead shape was narrow on both top surface and bottom surface in case of pure Ar. Meanwhile, the weld bead shape was wide on both top surface and bottom surface in case of Ar mixture with 10% hydrogen.

View full abstract

In this study, the heat source characteristics of arc plasma in GMAW using 100% CO₂ shielding gas. In particular, arc temperature, Fe vapor concentration and electrical conductivity and metal droplet temperature of a YGW11 wire electrode were measured by optical emission spectroscopic methods. The plasma temperature reached over 8000 K at the center of the arc plasma. The droplet temperature near the arc root was higher than other areas.

View full abstract

Hydrogen related delayed fracture is a critical issue to be solved to enhance the application of high-strength steels. This hold true with the high strength steels and its welds used in automotive applications. The trend of the increase of the strength of steels used will continue, therefore, it is important to investigate the hydrogen related fracture of high-strength automotive steels. In this study, hydrogen diffusion and accumulation behavior in the high-strength steel welds has been investigated by numerical simulation. The residual stress distribution of the resistance spot welds was considered as the driving force of hydrogen diffusion. The hydrogen diffusion was simulated under residual stress field with different initial hydrogen distributions and boundary conditions. When the diffusible hydrogen distributed uniformly in the weld metal just after welding, hydrogen diffused rapidly after welding and retained slightly at the center of the weld metal. When the diffusible hydrogen was introduced through the surface of the welded joint, diffusible hydrogen accumulated in the area of high tensile stress. In addition, the rate of increase of the hydrogen concentration was dependent on the distance from the surface of the welded joint.

View full abstract

Residual stress is one of important issues for optimizing the manufacturing process and ensuring the structural integrity of welded components. Many results on the issue have been reported, however, the residual stress relaxation mechanism of dissimilar weld joint by the thermal cycle is not yet enough clarified. In this study, the residual stress evolution process of a dissimilar weld joint during thermal cycle was investigated using in-situ neutron diffraction technique and the Idealized Explicit Finite Element Method (IEFEM). The base materials of the dissimilar weld specimen were a SUS316L stainless steel and a NCF600 nickel alloy. The obtained results clearly show that the residual stress especially in the NCF600 side decreased in the first thermal cycle due to yielding of the NCF600. This yielding was caused by an estimation that the total stress, which is the sum of the initial welding residual stress and the thermal stress, exceeded the yield strength of NCF600 during the heating process.

View full abstract

Multi-layered pulse MAG welding by robots is used for large structures. The sensing technology for the detection of defects is crucial towards improving the productivity and quality of robotic welding. The authors have developed a system that detects welding defects (blowholes) by utilizing welding current and voltage waveform data. By processing the measurement data, it is possible to detect welding defects with a probability of 95% or better. The developed defect detection system has been introduced in the company's robotic welding systems.

View full abstract

The influence of specimen size of Charpy impact test on the absorbed energy characteristic was investigated for evaluating the toughness of steel obtained from the actual bridge member with fire damage. V-notch Charpy specimens with different thickness were extracted from the steel applied a thermal cycle simulating fire. Charpy impact test was carried out on the specimens with considering the concept of the fire damage by using transition temperature shift concept. On the other hand, the absorbed energy per unit area for 5 mm thickness sub-size specimen with the fire damage was higher than that for full-size specimen, even though the test temperature was shifted. The results indicated that the degree of shift temperature was different between the steels with and without the fire damage.

View full abstract

In this study, we have investigated the effect of thermal aging on impact strength of Sn-Ag-Cu solder bumps in fluxless soldering process using formic acid atmosphere. We conducted impact tests on as-reflowed and thermally-aged solder bumps, which were fabricated on either formic acid atmosphere or a liquid flux. The results indicated that the morphology of the intermetallic compound layers in samples soldered using formic acid was similar to those soldered with the liquid flux, after soldering and after thermal aging. The fluxless solder bumps, using formic acid atmosphere, have impact strengths similar to the solder bumps obtained using the liquid flux in nitrogen atmosphere

View full abstract

This investigation aims to develop a new Plasma-MIG hybrid welding process for butting welding joints of large thickness. In this welding process, Plasma torch and MIG torch are connected in electrode negative (EN) and in electrode positive (EP), respectively. Plasma torch is set up in the leading position, meanwhile the MIG torch is set up in the trailing position. The plasma welding is utilized in keyhole model. The results show that the successful single-sided welding in one pass is fully penetrated. The wettability of welding joints is improved and the penetration is increased in comparison with only conventional MIG welding process and only conventional Plasma welding process. In addition, in order to discuss the research results, the temperature field on the surface of weld pool is also measured. As a result, the temperature on the weld pool surface in Plasma-MIG hybrid welding case is higher in comparison with conventional MIG welding case, especially near the leading edge of weld pool.

View full abstract

It has been reported that welding distortion can be well reproduced using inherent deformation by performing the simple elastic finite element analysis. To efficiently predict the welding distortion of large structures, the inherent deformation database in the design space of the shrinkage and bending control parameters for each basic welded joint was established through experimental measurements and nonlinear transient simulations by thermal elastoplastic FEM. Then, when welding conditions and constraint conditions used in the real production lines are different from those employed in established inherent deformation database, the estimating method of inherent deformation for practical applications is recommended. With the aid of the established inherent deformation database, the welding distortion of large structure models with/without jig constraints for the construction machinery is predicted using the enhanced elastic FEM solver and compared with experimental measurements. The efficiency and validity of the enhanced solver and the inherent deformation database are clearly demonstrated.

View full abstract

In case of welding of thick plates, it is necessary to apply appropriate groove shape for preventing welding distortion and defects. It is difficult to define an appropriate welding condition of inexperienced structure. In this study, the optimization method for welding condition on T-joint weld was investigated. The groove shape (groove angles and position of root edge) and a welding sequence were defined as design variables. The distortion was defined as the objective function and it was minimized. The response surface methodology was used for this optimization. For the purpose of confirming the validity of this optimization method, numerical analysis and actual weld experiment with optimized welding condition were performed. As a result of numerical analysis, the residual distortion was sufficiently small. Moreover, it was confirmed that the residual distortion of actual weld experiment was smaller than acceptable value.

View full abstract

The large deformation including bucking may occur due to welding on thin-plate structures. The prediction of welding distortion on thin-plate structures is difficult since change of deformation mode may occur. In this research, in order to evaluate the welding distortion of thin-plate structures, an analysis method that considers large deformation theory in thermal elastic plastic analysis using Idealized Explicit FEM with solid elements was developed. The developed method was applied to the analysis of stiffened thin-plate structures. The influence of heat input Q and welding speed v on large deformation were investigated in the analysis. As a result, it was clear that a buckling-type deformation occurs with larger heat input Q. In addition, it was also found that the bucking-type deformation mode varies with the above factors. It can be assumed that these factors affect the transient deformation behavior on welding and this causes the various generating process of buckling-type deformation. In addition, to discuss the analysis accuracy, the welding distortion of stiffened structure was analyzed and the analyzed results were compared with the experimental measurements. As a result, it was found that both deformations agree with each other. From these results, it was clearly seen that the developed method can stably analyze the welding deformation of stiffened thin-plate structures with large deformation including bucking without introducing initial imperfections.

View full abstract

Laser shock peening attracts increasing attention as one of techniques to improve fatigue life of materials and structures. The application of this technique generally induces a compressive residual stress field, modifies the geometrical shape and alters the material hardening. Nanosecond laser peening, which uses nanosecond laser pulse is necessary to be carried out in immersed water for the confinement of the plasma. On the other hand, femtosecond laser peening, which uses femtosecond laser pulse, can be carried out in the air because of its high laser intensity. However, both techniques proceed under a very high strain rate, making difficult to completely understand the whole peening process, therefore the best conditions for laser peening and the mechanism of creation of residual stress field have not been clarified well yet. In this study, finite element analyses are carried out to examine the residual stress field, displacement and distribution of equivalent plastic strain in order to examine the effect of laser peening on the material properties. Each technique was simulated by means of a single laser shot irradiating to the center of the specimen surface. We discuss the numerical results of the residual stress, displacement and equivalent plastic strain fields.

View full abstract

Hot-wire laser brazing was proposed for attaining a lap fillet dissimilar joint of galvanized (GI) steel sheet / A5052 sheet. Our results revealed that relatively sound brazing beads with/without a few defects were obtained at high brazing speeds of up to 4.0 m/min. Furthermore, the average thickness of the brittle intermetallic compounds formed at the joint interface was carefully controlled and ranged from < 1 μm to several μm. Moreover, a high tensile strength of the joint was realized; this strength is 80∼90% of the base galvanized (GI) steel strength.

View full abstract

Droplet temperature for Metal Inert Gas welding process was measured by using optical pyrometry; two colors temperature measurementmethod. High speed color video camera was used to capture an image of metal droplet during transfer to base metal. As a result, the movement of an arc to cover the droplet from bottom half to the entire droplet by increase of welding current had allowed the change in droplet heating process from concentrated to uniform heating. The concentrated heating in globular transfer mode had caused a higher current density at the bottom of the droplet. The change of heating process from concentrated heating in globular mode to a rather uniform heating during transition resulted in a smaller current density and reduced the heat flux into droplet, therefore decreased the droplet temperature. The further increase in welding current led the increase of droplet temperature and produced the minimum temperature in the middle of transition mode.

View full abstract

The objective of this study is to propose new anisotropic damage constitutive law that represents the separation process on cleavage plane in a polycrystalline aggregate. The proposed law is formulated by embedding of an exponential type of the cohesive zone model (CZM) in a standard crystal plasticity constitutive law. The separation of the cleavage plane can be realized by exponential type of the CZM based on an atomic potential. On the other hands, the crystal plasticity constitutive law is used to simulate the deformation due to the crystalline slip on each crystallographic system. Thus, the proposed damage constitutive law is capable of representing the microscopic mechanism characterized by both the fracture behavior of the cleavage plane and the plastic deformation of the crystallographic slip. Several numerical simulations are conducted to demonstrate the capability of the proposed damage constitutive law. In particular, it is confirmed that the proposed model enables us to simulate the crack propagation in arbitrary directions, and the resultant anisotropic strength in a single crystal grain.

View full abstract

Fatigue tests and numerical analyses were carried out in order to investigate the effect of hammer peening on fatigue life of U-rib structure. The fatigue test results showed that the fatigue life of peened specimens are longer than the non-peened ones. In addition, the numerical results showed that compressive residual stresses were induced at weld toe and root by hammer peening, moreover that the accumulated plastic strains during cyclic loading can explain the experimental results in terms of fatigue life extension.

View full abstract

Since arc narrows in the plasma welding, heat concentration ratio is higher than other welding. This heat source generates the keyhole in the high current region, which penetrates base metal. The base metals are joined by closing the keyhole. The quality of welding depends on state of the keyhole. However, the keyhole may become unstable due to behavior of molten metal. In this research, observations of the keyhole and molten metal under various welding conditions were performed by using high speed video camera to investigate the behavior of the keyhole in the plasma welding. Length from torch cap to bottom edge of the keyhole in high speed video camera picture was proportional to plasma gas flow rate. On the other hand, the length was mostly kept constant regardless of change of welding current. When plasma gas flow rate was changed during the welding in real time, appearance of the keyhole and shape of bead were changed. Control of stabilization of the keyhole is tried based on relationship between the length found by image processing and plasma gas flow rate. The authors tried to design the digital controller controlling the length by plasma gas flow rate to obtain the stable keyhole.

View full abstract

Resistance spot welding, hereinafter referred to as spot welding, is one of the most important methods for metal sheet welding used in industries such as the automobile, electronics, and so on. This welding is faster than other welding process such as arc welding process, laser welding process. Since high efficiency is required in this welding, welding in a short period is, required. Conventional resistance spot welding takes several hundred mili seconds to weld. Welding method in a short period less than 20 ms is developed. In order to find the optimum welding condition, 3D simulation model of spot welding in a short period was made in this research. The model based on electrical, thermal, and structural phenomena is required in this simulations. Numerical simulations were carried out by using Marc/Mentat software which had advantage of nonlinear analysis. The materials parameters were determined based on a contact domain between base metal and electrode. The validity of the 3D numerical model was verified by comparing the numerical simulations and the experimental results. The nugget size under various welding current was investigated. Since the experimental results and analytical results of this study showed almost the similar results, the validity of the proposed model was verified.

View full abstract

The present paper reports the results of experimental tensile tests, geometrical measurements and Vickers hardness tests in order to characterize the deformation process and the material failure of a HT780 butt welded joint specimens. In addition, the distribution of the strain fields on the surface of the samples is obtained by means of the digital image correlation technique. Furthermore, the relationship between the local stress-strain behavior and hardness is discussed from the results.

View full abstract

Damage in a structure is caused by material degradation due to initiation, growth and coalescence of micro-cracks/voids. In the recent years this topic acquired great importance in order to obtain a better design and to prevent the failure of components and structures. The present work aims to consider the influence of the stress triaxiality and the Lode angle effect on the ductile damage evolution, since it has been experimentally proved that the loading conditions highly affect the effective strain (i.e. cumulative plastic strain at failure). An ad hoc Lode angle function is adopted, together with the ductile damage evolution law proposed by Lemaitre, associating the decrease of the Lode angle influence with the generation of large plastic strain. On the other hands the choice of coupling the continuum damage mechanics framework together with an unconventional plasticity model is functional to investigate cyclic loading problems, since conventional plastic algorithms tend to overestimate the material ratcheting, leading to a wrong accumulation of the damage through cycles.

View full abstract