High-Mn austenitic steel is expected to be a next-generation structural material for cryogenic use by maintaining a stable fcc crystal structure even at cryogenic temperature by adding a large amount of C and Mn which are γ-stabilizing elements. In some cases, Cr is added together with C as a solid solution strengthening element to the high-Mn austenitic steel. Depending on the conditions of aging heat treatment, the formation of Cr carbide may affect the mechanical properties. In this study, the formation behavior of Cr carbide and the effect on cryogenic Charpy impact toughness in the aging heat treatment at from 773 to 1273K for 30s to 18ks of a high-Mn austenitic steel with 0.5%C-25%Mn-5%Cr.Regardless of the aging heat treatment conditions, no cleaved but 100% ductile fracture surface was observed in all samples after the Charpy impact test at 77K. On the other hand, M23C6 containing Cr was formed on the austenite grain boundaries in the sample aged for a long time at 1073K, and the Charpy absorbed energy showed lower than those of the other aging conditions. As the results of both detailed microstructure observation of M23C6 and adjacent austenite matrix and DICTRA simulation, it was clarified that M23C6 mainly act as a fracture initiation point but propagation to the matrix becomes ductile because no sharp alloy deficiency layer is formed at the M23C6 / austenite matrix interface. As a result of calculation of stacking fault energy using a thermochemical model, it was confirmed that γ stability at cryogenic temperature could be sufficiently secured even in the alloy-deficient region near M23C6 as well as in the stable matrix.

High Mn austenitic steel is expected to be a next-generation structural material for cryogenic use by maintaining a stable fcc crystal structure even at cryogenic temperatures by adding a large amount of Mn which are γ-stabilizing elements. In order to clarify the factors governing the cryogenic toughness of heat affected zone (HAZ) of welds, investigation of relationship between microstructure and Charpy impact toughness of multi-pass weld joint and a simulated thermal cycle test was conducted for high Mn steel containing 0.5%C-25%Mn-5%Cr.In the MAG welding, HAZ near fusion line showed high absorbed energy, whereas it was minimized at HAZ5mm reheated up to around 1073K. Furthermore, in a simulated thermal cycle test of continuous cooling, the absorbed energy was also minimized at 1073K. On the other hand, in the simulated thermal cycle test of quenching, the inverse dependence of the grain size on a conventional steel was shown, and the toughness increased as the grain size became coarse. Grain growth occurs according to the maximum temperature of HAZ, and M23C6 is formed on the grain boundary in the subsequent continuous cooling process. It was clarified that M23C6 became only initiation site of fracture under Charpy impact loading, and its existence density was the main factor of initiation energy. On the other hand, propagation process is a ductile fracture phenomenon in which γ stability is maintained regardless of the presence of M23C6.

[in Japanese]

Acicular ferrite(AF) formation mechanism in a GMA weld metal which is widely used for industry has investigated. Many precipitates (Mn-Al-Ti-O phases) were observed in the much AF-formed samples. The formation mechanism of AF on/around the precipitate was analyzed using TEM line analysis.The analysis results of the inclusion/ferrite interface indicated that both Mn and Ti were present at the edge of the precipitate, and Mn depleted zone (MDZ) was also observed near Mn and Ti rich area. Since MDZ was also found around other compounds such as amorphous and MnS, it is considered that MDZ contributes to the acicular ferrite formation rather than low lattice misfit.

A new resistance spot welding process with “pulsed current post-heating pattern” is developed for improving joint strength of ultra-high strength steel (UHSS) sheets. This post heating process consists of short-time high-current post-heating (pulsed current) and short-time cooling, which utilises the phenomenon that the pulsed current reheats especially near the outer peripheral regions of the contact area of electrode/steel interfaces and faying interface of steel sheets. The strength improvement can be obtained owing to two effects; the reduction of solidification segregation in the nugget and the appropriate softening of the heat affected zone (HAZ). Toughness of the nugget increases with alleviating segregation of embritting elements. Properly widened and softened HAZ would lead to large plastic deformation, which could reduce stress concentration at the edge of the nugget on the faying interface and prevent brittle fracture in the nugget during straining. This study aims to elucidate the microstructural change in the HAZ due to the pulsed post heating current pattern and its effect on hardness profiles, and to propose an estimation method of HAZ hardness. 1180MPa grade ultra-high strength steel sheet consisting of ferrite and martensite was used. Softened region was fairly widened with decreasing its minimum hardness when applying the pulsed current post heating. No significant difference was observed in the region where the maximum temperature is over Ac1 between with and without the pulsed current post heating pattern. This would be because the maximum temperature was the same in these regions even when applying the pulsed current. The HAZ was widened in the region where the maximum temperature was lower than Ac1. These regions have the same fractions of martensite and ferrite as the steel sheet before welding, but the martensite was much tempered when applying the pulsed current pattern. These tempered phenomena of the martensite can be well estimated using an accumulated tempering parameter for both tempering martensite and autotempering behaviour of martensite formed during cooling after welding.

Friction stir welding (FSW) was performed on thick plates of high-strength low-alloy (HSLA) steel with polycrystalline cubic boron nitride (PCBN) tools. To investigate the mechanism of microstructure formation in the stir zone (SZ), the prior austenite structure undergone through a thermo-mechanical history during the FSW process was investigated. The prior austenite structure was reconstructed by the local crystal orientations of prior austenite calculated from the bainite orientations by using the Kurdjumov–Sachs orientation relationship between the prior austenite and bainite. The reconstructed austenite revealed a heterogeneity in the SZ. The austenite grain size in the advancing side (AS) of SZ was larger than that of the retreating side (RS). Thus, a reheated temperature of AS was considered to be higher than that of RS. The textures of reconstructed austenite were also different in SZ. The rolling texture was observed in RS, but the shear texture was observed in the center and AS. The heterogeneous microstructure and texture in SZ was considered to be caused by the material flow behavior in FSW process.

Ultrahigh-purity aluminum sheets with excellent electrical and thermal conductivity at ultra-low temperature have uses of a heat dissipation component, e.g. for a superconducting equipment. Recently, development of welding methods for these sheets without degradation of the excellent conductivity has been required. This study focused on friction stir welding (FSW), which was conducted for 99.999wt.% (5N) Al thin sheets with thickness of 0.8mm. The FSW tool rotation speed was set to 3000rpm, and both the welding speed and tool insertion depth were changed. Decreasing welding speed suppressed the occurrence of a kissing bond on an FSW back side and increased the stir zone area having ultrafine grains, although residual strains in all the joints were more than tungsten inert gas (TIG)-welded joints which consisted of coarse grains similar to the base metal. Base metal fracture occurred in tensile tests of the FSW joints except that at the maximum welding speed, in contrast to rapture near the fusion line of the TIG-welded joint due to blowholes. On the other hand, residual resistivity ratio (RRR) of FSW and TIG-welded joints decreased to almost half of that of the base metal. The heat treatment at 500℃ for 3h increased RRR of the FSW joints to almost the same value as that of the base metal. This is attributed to significant reduction of residual strain as well as grain growth. Decreasing RRR of the TIG-welded joint seemed to be attributed to decrease in purity. In order to obtain high conductivity at ultra-low temperature in the 5N-Al thin sheet joints keeping higher joining strength, the FSW in conjunction with post weld heat treatment would be a better choice.

The formation mechanism of microstructure of duplex stainless steel weld metals with different N content and heat input and the effects of microstructural factors on pitting corrosion resistance of them were investigated. As N content of weld metal increased, ferrite content decreased and Cr nitride content increased. However, once N content exceeded a certain value, Cr nitride content decreased. As the ferrite content of the weld metal decreased, the critical pitting temperature (CPT) increased, and as N content increased, CPT increased. Thud, the pitting corrosion resistance of duplex stainless steel weld metals must be affected by both of N content and Cr nitride, and the precipitation of Cr nitride must be also affected by N content and the ferrite content due to N content and the weld heat input. The results indicated that these microstructural factors was important index to evaluate the pitting corrosion resistance of duplex stainless steel weld metals..

Diffusion controlled transformation simulations of carbon steel/nickel diffusion couple were performed and the results were compared with the experimental results previously reported, which show that the interdiffusion of Fe and Ni atoms occur and the hard layer forms around the bonding interface at two step heating. The simulation was divided into two heating steps. It was shown that the interdiffusion of Fe and Ni atoms occur at the first step heating (1250℃) and the carbon concentrated layer forms after the second step heating (600－800℃) at the carbon steel side near the interface, where Ni atoms diffuse from the nickel at the first step heating. The agreement between the simulation and the experimental results is good. The formation of the carbon concentrated layer is affected by the cementite precipitation in the carbon steel.

The aim of this study is to discuss and model the hot ductility curve during solidification in the welding from the metallurgical point of view. The contents in this study are as follows. The transverse Varestraint test was conducted to evaluate the solidification cracking, then the in-situ observation was applied on the Varestraint test in order to obtain the critical strain of solidification cracking occurrence. In addition, the U-type hot cracking test was also carried out for obtaining the critical strain, and the both critical strains were compared.Moreover, the numerical modeling was also performed in order to quantitatively explain the tendency of the relationships between the temperature and the strain. The solidification starting temperature at the non-equilibrium state was estimated based on the K-G-T model. The solidification segregation on the non-equilibrium solidification was calculated based on the mathematical analysis method proposed by T. Matsumiya. In addition, R-D-G model that is firstly suggested by M. Rappaz was used in order to explain the change in the strain depending on the progress in solidification. This paper finally mentioned the possibilities of the prediction of hot ductility curve by numerical simulation.

MPF (Multi-Phase Field) method was applied for the microstructure formation process of Fe-18%Cr-15%Ni-0.05%C alloy during weld solidification and critical strain rate due to solidification shrinkage in the mushy zone was calculated by RDG (Rappaz-Drezet-Gremaud) model. Estimated critical strain for the solidification cracking was corresponding to the experimental result of trans-varestraint test by adjusting the dendrite coalescence temperature and the vulnerable time. Initiation of solidification cracking was thought to be occurred near the solidification front, about one third of solidification temperature range. This result is corresponding to the previous work of in-situ observation. It is considered that the solidification shrinkage plays an important role for the appearance of solidification cracking. Further understanding of the metallurgical phenomenon in the initiation of welding solidification cracking could be obtained by the quantification of solid connection between dendrites at the vulnerable region.

In multi-pass welding of high manganese austenitic steel, weld metal is solidified in austenite single phase and hot cracking may occur depending on residual stress. In order to identify the cracking morphology, a fracture surface of the cracking of weld metal in multi-pass welding test was observed. The fracture surface indicated a dendritic morphology and a bunching pattern, therefore it is considered that the crack in the multi-pass weld metal is caused by residual liquid film or remelting liquid film. Varestraint test was carried out in order to clarify the characterization of the crack, and then the fracture surface of cracks obtained by the test was observed. Transverse-type and longitudinal Varestraint test were used for getting typical fracture surfaces of solidification cracking in weld metal, and liquation cracking in reheat weld metal and heat affected zone of base metal respectively. Comparing the fracture surfaces of cracks of Varestraint test and crack in practice, the fracture surface of the solidification cracking was most similar to the crack in multi-pass weld metal in this study. Thus, it is considered from comparing the fracture surfaces that solidification cracking might mainly occur in the multi-pass welding of the high manganese steel.

The aim of this study is to evaluate an effect of impurity elements on solidification cracking susceptibility of a high manganese austenitic steel for cryogenic use. An effect of phosphorus and sulfur was investigated by transverse-type Varestraint test. Increase in an amount of phosphorus enhanced the solidification cracking susceptibility and it was additionally induced the ductility-dip cracking on the Varestraint test. On the other hand, increase in an amount of sulfur didn’t enhance the solidification cracking susceptibility. Quenched microstructure around solid and liquid interface was analyzed by EBSD and EPMA. According to the results of analyses, the solidification mode of high manganese austenitic steel was fully austenite phase and it was assumed that MnS is forming during solidification.A numerical analysis considered a formation of MnS during welding solidification was carried out in order to quantitatively clarify the effect of phosphorus and sulfur on the susceptibility. According to the result, solidification completion temperature decreases with increasing the phosphorus content. On the other hands, increasing the sulfur content has an insignificant effect on the solidification completion temperature because of MnS formation. Thus, it can be considered that effect of sulfur for solidification cracking might be much smaller than the phosphorus in the high manganese steel.

Solidification cracking susceptibility of carbon steels was evaluated by the transverse-Varestraint test to reveal the fundamental effect of solute elements in the steel such as carbon, silicon, manganese, phosphorus and sulfur. Fracture surface of a pear-shaped bead cracking occurred in a weld metal of the carbon steel by GMAW exhibited the characteristic features of solidification cracking. The smallest solidification brittle temperature range was a chemical composition obtained by a flux cored wire, followed in order of SS400 and SM400. According to the EPMA results, a manganese sulfide was observed in weld metals of the pear-shaped bead cracking and Varestraint test weld metal.Numerical simulation considering FA mode solidification and the solidification segregation in carbon steels was carried out in order to clarify the effect of solute elements on solidification cracking. The solidification analysis suggested that carbon steels used in this study were solidified as fully delta ferrite until the solidification rate of 95%. The solid-liquid coexistence temperature range calculated by solidification analysis indicated similar tendency to the BTR evaluated by the transverse-Varestraint test. Then, the influence of solute element was virtually simulated by using the numerical simulation. It could be concluded that the solidification cracking susceptibility of carbon steels was attributed to the carbon amount at the terminal stage in solidification.

When conventional arc welding methods welded various kinds of duplex stainless steels, it is well known that the ferrite phase was enriched in welds. Notably, in the case of laser beam welding (LBW), due to high energy density and welding velocities, the phase balance might be remarkably changed from that in the base material. Also, it is concerned that such phase unbalances in welds because of the degradation of corrosion resistance, mechanical properties, and so on. Therefore, it is crucial to clarify the effect of alloy composition and welding heat cycle on microstructure change. Also, the morphology of phases formed in welds and mechanism of phase transformation during LBW thermal cycles should be clear, to predict the phases formed in LBW welds of duplex stainless steels and recovering method to maintain the phase balance. This experimental, detailed microstructure of austenite precipitated at ferrite grains in LB welds was investigated in this study. The morphology of widmanstätten austenite was observed by a scanning electron microscopy (SEM) after an immersion test using a sulfuric acid aqueous solution. The austenite was precipitated from grain boundaries growth with plate shape into the ferrite matrix. The elemental distributions in the welds were examined by an electron probe microanalyzer (EPMA). It was revealed that nitrogen was concentrated in austenite. Based on the result by electron backscattered diffraction pattern analysis (EBSD), it suggested that austenite precipitates at high-angle grain boundaries and widmanstätten austenite growth were maintaining some kinds of orientation relationship with ferrite matrix.

A simple method of repairing fatigue cracks using stop-holes reinforced with wedge members, that was previously proposed by the author, has been examined for the case of reinitiated cracks. By using this method, the stress intensity factor range around a reinitiated crack tip is expected to be reduced by the wedge load effect of the wedge members. The chief advantages of this method are that the repair work can be easily performed from only one side of a cracked structure, and that the wedge member can be set so adaptive as to maintain the wedge load automatically and effectively as the reinitiated crack grows. Specifically, slope-type wedge members have been adopted, and an adaptive mechanism of the wedge member has been devised using a pulley and a wire-type displacement meter. Fatigue tests were performed on a steel plate specimen with a drill hole and a notch, and validity of the above repair method was experimentally examined using both of simple and adaptive wedge members. In addition, an anti-fatigue smart paste, which consists of fine alumina particles and silicone grease, was applied to the periphery of the drill hole, and its effects on automatic restraint and visual detection of crack growth were investigated. FE analyses using contact elements were also carried out for a comparative study. As a result, it was found that the strain range on the specimen side is reduced to 44.0% and the fatigue life is prolonged by 12.4 times by application of the adaptive wedge member as compared with the case of the conventional stop-hole. As for the anti-fatigue smart paste, it was found that the fatigue life is further prolonged by about three times that in the case of the adaptive wedge member and the paste has an evident function to support visual detection of cracks.

Friction Stir Welding (FSW) has been attracting attention for its usefulness such as relatively easy joining compared to conventional arc welding, and its application to industries is being expanded, for example thick aluminum plates for infra-structures.FSW is known to yield defects called root flaws depending on welding conditions. In order to suppress the defects, it is necessary to optimally control the plastic flow and to stir the materials to be welded cooperatively. The present study tried to make a sound joint of 6061-T6 aluminum alloy sheets with 3mm in thickness using a newly designed tool called “Double Spiral Tool” which can increase the volume of the stir zone around the tool. Two types of tools were used: a normal type with M4 to M3 screws (called single spiral tool) and a “double spiral” type with the same pitch and twice amount of lead length. The welding tests revealed that the root flaw was completely disappeared with the double spiral tool under the welding condition of 900rpm-50mm/min, while it still remained in the joint produced by the single spiral tool. Thus, the present study successfully proved that the double spiral tool is effective to suppress the root flaw in FSW joints of 6061 aluminum alloy.

Submerged arc welding process which is applied to large constructions is a high heat input and high deposition rate process. To achieve a higher productive and higher quality process, the phenomena in the process have to be understood and controlled. However, the molten metal and the molten slag are covered by a flux during the process, and cannot be observed directly. In this study, a numerical simulation model of the weld pool formation in the submerged arc welding process is constructed to visualize the phenomena. In this model, both the molten metal and molten slag behavior are calculated and the vaporization of the molten slag is considered. The properties of the arc plasma which is a heat source of the process is given by a simplified heat source model in the simulation model. Radiative energy into the flux and pressure to keep an arc cavity are considered as heat source properties for SAW process simulation. The simulation model can reproduce the shape of the penetration and solidified slag covered the bead of the experimental result by adjusting the heat source parameters. Moreover, influence of the flux on the penetration formation is discussed by the model. When the flux exists, the bead width is wider and the bead height is lower than those without flux. This is because, the temperature of the side region of the bead becomes higher before the heat source passes, because the energy given into the flux is transported to the base metal when the flux exists.

It has been well known that, in GMA welding, various welding phenomena are largely affected by the type of shielding gas used. However, up to the present time, almost no attempt has been made to utilize ‘gas composition’ for controlling welding phenomena. This study focused on this point and, thus, an investigation was made to understand how dynamic gas compositional modification affected various welding phenomena. In this study, pulsed Ar gas was injected periodically into CO2 shielding gas so that the gas composition of arc atmosphere largely modified in a very short period of time. It was found that this quick gas compositional change from CO2 to Ar led to a large decrease in arc voltage and an arc shape change from a constricted shape to a flare shape and that a molten droplet formed at the electrode tip during the CO2 period was released due to a sudden change in force balance between the molten droplet and the arc. The increase in current stimulated by the voltage drop may have assisted the metal drop detachment due to an increase in electromagnetic pinch force. If the cycle of this periodical Ar gas injection was selected to be shorter than that of repelled metal transfer in CO2 atmosphere, the repelled transfer was avoided and stable drop transfer occurred instead with far less spatter. Appropriate selections of the duration time of pulsed Ar gas injections avoided spray transfer which might cause undesirable finger-like penetration profiles. This new conceptual welding method, namely ‘Pulsed Gas MAG Welding’, required only a small amount of Ar gas to obtain the effects mentioned above.

CO2 gas shielded arc welding, which is widely used in the industrial world, has been recognized as an efficient and economical GMA welding process. However, it’s characteristic repelled transfer produces a large amount of spatter which results in costly cleanup and, thus, practical methods to overcome this weakness have been highly demanded. In the series of this study, ‘Pulsed Gas MAG’ welding was proposed as a new approach to reducing spatter, in which Ar gas pulses were periodically injected into CO2 shielding gas so that gas composition of arc atmosphere was largely modified in a very short period of time and a metal droplet formed at the electrode tip during CO2 atmosphere was released before repelled transfer had occurred. In this report, the influence of parameters of Ar gas injections, such as frequencies, average gas flow rates and gas injection periods, on the metal transfer was investigated. It was found that synchronous metal transfer stimulated by pulsed Ar gas injections was achieved at any frequencies from 35 to 65Hz under the conditions of gas injection periods longer than 4ms and of Ar gas volumes larger than approximately 1mL/pulse. This is because Ar gas injections below these critical levels resulted in lack of time for arc shape changes and metal drop detachment. From these experimental results, a model to explain the metal transfer phenomena was developed. The amount of spatter was reduced to the levels as low as that of the conventional MAG welding with 80%Ar+20%CO2 shielding gas mix and penetration profiles were round shape like those obtained in CO2 arc welding.

Gas metal arc welding has wide industrial applications so that there is a high demand for increased efficiency and quality for it. A controlled short-circuit transfer process, which means a process realized by appropriate current and wire feed control in this paper, is a low heat input gas metal arc welding process. In this process, short-circuit transfer is stably and periodically repeated to enable the low heat input and the high deposition rate. Recent years, this type of process has been investigated to be applied to dissimilar materials welding, wire arc additive manufacturing and lap joint welding of thin plates of high tensile strength steel, these are current important topics. However, control factors of the process are not clear enough because of the complexity of its welding phenomena and the lack of investigations for them. In this study, a controlled short-circuit transfer process is modeled in two-dimensions to clarify the metal transfer and weld pool phenomena during the process. In this report, the effects of short-circuiting current on the process are investigated. We revealed that the electromagnetic force induced by short-circuiting current has a large effect on the weld formation process. Short-circuiting current induced the high pressure in the liquid column that is formed between electrode and base metal while short-circuiting, and the strong convection flow induced by the large pressure gradient between the liquid column and the weld pool promoted the weld formation. We also revealed that the electromagnetic force affects on the droplet transfer process. Short-circuit transfer cycles became more instable when the short-circuiting current is higher.

Most Japanese shipyards that build general merchant ships use arc welding, but they always have the problem that the accuracy of the hull construction is poor as a result of the thermal deformation caused by arc welding. As one method to overcome this problem, it is expected to adopt laser arc hybrid welding that can suppress the total heat input as a result of heat input concentration, but the initial introduction cost is high and this welding method has a poorer gap tolerance than conventional arc welding. In addition, few experiences to apply laser arc hybrid welding for constructing the joints at the plate thickness level used for hull members of general merchant ships. An improved procedure to fabricate a butt joint with a plate with a thickness of 15 to 20mm, which is used as a hull member of a general merchant ships is proposed in this study. Proposed procedure is that the Λ-shaped groove with a very small angle and one-pass welding by laser arc hybrid welding with CO2 gas shielded arc welding to satisfy the weld reinforcement on the front side weld bead. The performance of the joint manufactured by using our proposed procedure was confirmed according to the Guidelines for Laser Arc Hybrid Welding by a classification society Nippon Kaiji Kyokai (ClassNK). It is expected to be introduced as an alternative method of plate joint welding for hull members, especially submerged arc welding with large heat input.

Finite element model of gas pressure welding of steel bars is constructed in order to evaluate deformation behavior of the welded joint. The proposed model is based on a thermal-structural coupled analysis. Distribution of heat flux sourced from oxy-acetylene flame is assumed as a combination of two Gaussian functions, and heat losses due to convection and radiation are considered. Temperature dependences of thermal and mechanical properties of welding materials is modeled for the thermal-structural analysis. The analysis results show a good agreement with experimental results of variations of the temperature and the displacement on the weld center in the 20MPa to 40MPa range of the welding pressure. The proposed model revealed that tensile strain on the weld interface reaches larger value in the center than that in the outside of steel bar, depending on the temperature gradient. Additionally, the difference of the tensile strain between the center and the surface increases with increasing the upset force. It is considered that the temperature gradient on the weld interface is an important factor to control the deformation of gas pressure welded joint.

Finite-element analysis (FEA) of solder joints for ultrahigh-density packages must be conducted with an accurate understanding of the deformation characteristics of the Cu/Sn intermetallic compounds (IMCs): Cu3Sn and Cu6Sn5. Therefore, the authors proposed a method to estimate the tensile characteristics of these IMCs in the previous work. The method provided two new findings. One is that the breaking elongation of the Cu/Sn IMCs is approximately 1.4%. The other is that the stress-strain relation of the Cu/Sn IMCs shows nonlinearity. Since the Cu/Sn IMCs have been regarded as elastic materials for a long time, the latter finding should be verified in some way. Then, in this study, we conducted FEAs which simulate the shear tests using the copper-solder joint specimen whose solder joint has the Cu/Sn IMCs layers. The FEA for a loading condition was conducted in two different ways; i.e., two different constitutive models of an elastic model and an elastic-plastic model were employed to describe the deformation behavior of the IMCs for a loading condition. Actual shear tests were also conducted in the same loading conditions as the FEAs. Comparing the FEAs with the actual tests, we discussed the presence or absence of the material nonlinearity of the Cu/Sn IMCs. The result suggested that the Cu/Sn IMCs should have material nonlinearity.

In this study, to clarify effects of the rare earth metal (REM) on arc phenomena, observations of an arc appearance and a metal transfer during a gas metal arc welding using a REM added wire by a high-speed camera were conducted. Moreover, temperature measurements of the arc plasma were carried out using monochromators and high-speed cameras. As a result, the spray transfer mode was observed during the welding with the electrode negative polarity using the REM added wire when the shielding gas including CO2 gas was used. The globular transfer mode was obtained during the welding when 100%Ar gas was used as a shielding gas. These results showed the reverse tendency compared with a conventional gas metal arc welding using the electrode positive polarity. Moreover, the droplet frequency increased largely in the range of 220A to 260A during the gas metal arc welding with the electrode negative polarity using the REM added wire when the shielding gas including CO2 gas was used. From these results, it was suggested that the spray transfer mode during CO2 gas arc welding and metal active gas welding was obtained by two factors. One was the operation and stabilization of the hot cathode by the welding with the electrode negative polarity using the REM added wire, and the other was the increase of temperature of the molten metal droplet and constriction of arc and arc root by the thermal pinch effect.

This study aims to clarify the influence of shape around weld toe and hardness of weld metal on fatigue crack life in fillet lap joints for automotive chassis elements. Gas metal arc (GMA) welding was performed using 980MPa class hot rolled steel sheet as the test material. The shape around weld toe and hardness of weld metal were changed by the combination of the shield gas composition and the welding wire. The smoothing of the weld toe significantly improved the fatigue life compared to the hardened weld metal. The total fatigue life was separated into crack initiation life and crack propagation life by measuring the decreasing ratio in the stiffness of the specimen during the fatigue test. Then, the effect of smoothing the weld toe and hardening the weld metal on each life was examined. The changes in the propagating crack front shape and the propagation rate during fatigue test were examined by the beach mark technique. As a result, it was suggested that the smoothing of weld toe improves not only the crack initiation life but also the propagation life by delaying the crack propagation rate in the early stage of crack propagation. The mechanism of retardation of crack propagation rate due to smoothing of weld toe was discussed from the viewpoint of crack shape and aspect ratio.