New peening technology is proposed to improve the fatigue strength of welded joints. By using this technology, compressive residual stress is introduced at weld toe by the developed peening procedure which plastic deformation is only applied to the base material near the weld toe. In this study, improvement mechanism of fatigue strength of weld joints by hammer peening on base metal was clarified by FEA. It was clarified that increasing of stress at weld toe is controlled by depression formed near the weld toe. Not only compressive residual stress at weld toe but also decreasing stress concentration at weld toe by plastic deformation on base metal was indicated as the factor of improving fatigue strength of weld joints.
For gas metal arc welding, the effect of CO2 mixture in a shielding gas on a metal transfer process was investigated through the observation of the plasma characteristics and dynamic behavior at the droplet's growth-separation-transfer by the temperature measurement methods which were suitable respectively to the argon plasma region and the metal plasma region. At the present experimental conditions, the metal transfer process was a spray transfer type with 100%Ar shielding gas. On the other hand, with 85%Ar+15%CO2 shielding gas, the metal transfer process was a globular transfer type in which the arc length was shorter, the width was narrower, and the time interval of the droplet separation was longer. For both shielding gases, the metal plasma region near the arc central axis exhibited 6500-7500 K which was lower than the argon plasma region. With 85%Ar+15%CO2 shielding gas, when the metal droplet grew below the electrode wire, the region below the droplet has a high plasma temperature and a high concentration of iron vapor which surrounded the droplet. The region also exhibited a remarkably high electron number density. At the spray transfer process, the argon plasma region had the electron number density twice higher than the metal plasma region. Meanwhile, at the globular transfer process, the metal plasma region had a higher electron number density than the argon plasma region, which corresponded to a higher electrical conductivity near the arc axis. This means that the electric current goes through the arc axis easier than the spray transfer process. This condition increases the temperature below the droplet. The thermal expansion increases the force preventing the droplet from falling down. In consequence, the metal transfer takes the globular transfer type.
The radial stainless steel plates (RPs) used for Toroidal field (TF) coil in ITER are 13 m long, 9 m wide and 10 cm thick, which are quite large. Even though they are very large structures, high manufacturing tolerances and high mechanical strength at 4 K are required. It is also required that each RP should be fabricated every three weeks. Therefore, the authors intend to develop efficient manufacturing methods for ITER TF coil RP. The laser welding is then selected as a welding method for RP. Especially, the development of high technology laser welding is necessary to prevent hot cracking in the material used for the RP, namely, fully austenitic stainless steel with high nitrogen content. The authors carried out trial laser welding experiments aiming at its application to RP. As a result, it was effective to make the angle of back inclination of the weld head at the uniform welding speed. It also seemed that the sensitivity of hot cracking could be reduced by optimizing the chemical compositions of material used for RP. The base material and the welded joints satisfied mechanical properties in 4K. The application of the laser welding technology to the fully austenitic stainless steel was therefore demonstrated.
In order to achieve lighter and stronger car bodies by applying high strength steel sheets, one of key technologies is enhancement of joints strength. In this study, we investigated dependence of strength and fracture behavior on chemical compositions of the steels in spot-welded L-type joints in detail. Consequently, the following experimental results were obtained: 1) Maximum load of the joint decreased with increase of carbon (C) and phosphorous (P). The maximum load was decreased by 0.4 to 0.7kN with increase of 0.1% in C, with C content ranging from 0.03% to 0.5%, and 0.5kN with increase of 0.01% in P, with P content ranging from 0% to 0.03%, 2) Fracture portion changed from the outside to the inside of weld metal with increase of C and P. 3) Fracture path was estimated to accord with solidification segregation site in the weld metal, in case of a steel of 0.2%C-0.03%P. 4) By implementation of an appropriate post heat during spot welding process for the steel of 0.2%C-0.03%P, the degree of solidification segregation was clearly reduced and the maximum load of the joins was improved by 70%.
Hot stamping spot welding tailored blank technology is a process to produce spot welded automotive body parts by the following process: Spot welding steel sheets in lap configuration → Hot stamping (Heating to about 900°C → Quenching and forming in water-cooled die→ Shot blasting to remove scale). This process has the advantage of producing high strength lap welded automotive body parts without increasing the number of forming dies. In this study, the mechanical properties of the hot stamped spot weld (spot welding →hot stamping) and conventional spot weld (hot stamping →spot welding) of the 1500MPa class uncoated boron steel sheets were compared. The obtained results are as follows. The tensile shear strength (TSS) of the hot stamped spot weld and conventional spot weld were comparable and the fracture modes were the same. On the other hand, the cross tension strength (CTS) of hot stamped spot weld was significantly higher than that of the conventional spot weld. The fracture position of the hot stamped spot weld was outside the nugget and conventional spot weld was inside the nugget. The high CTS of the hot stamped spot welds might be caused by the improvement of the fracture toughness of the nugget, which was caused by reduction of the solidification segregation of the phosphorus. It is assumed that the heating process after spot welding leads to the reduction of the solidification segregation. For the tension test, because there was no HAZ softening in the hot stamped spot weld, no fracture was observed in HAZ and a higher elongation was obtained.
Shielded metal arc weld metal for type 600 nickel base alloy (alloy 182) is used for weld components in nuclear power plants. To evaluate the intergranular corrosion resistance of alloy 182 after application of shot peening and subsequent thermal aging treatment at 593-793 K, we conducted the corrosion test (immersed in boiled 16% sulfuric acid + 5.7% copper sulfate aqueous solution at 57.6 ks) using specimens of alloy 182. The results show that the intergranular corrosion resistance of alloy 182 subjected to heat treatment at 893 K for 72 ks was improved by shot peening. Also, the intergranular corrosion resistance was not changed by thermal aging treatment at 593-793 K subsequent to shot peening. Because remaining chromium depletion layers along grain boundaries were still observed by transmission electron microscope (TEM) after shot peening, disappearance of chromium depletion layers cannot be a factor in the improvement of the intergranular corrosion resistance. The results of measurement of surface residual stress by the X-ray diffraction method show that the compressive residual stress introduced by shot peening still remained on the surface of the specimens. Based on these observations, we assumed that chromium depletion layers along grain boundaries near the surface were dissolved by the environment of the corrosion test, the dissolved regions were closed by the compressive residual stress on the surface, and then the remaining chromium depletion layers were protected from the corrosive environment. This assumption explains why the intergranular corrosion resistance was improved although chromium depletion layers remained.
Application of advanced high strength steel sheet (AHSS) to car bodies is expanding to achieve lighter and stronger bodies. However, it sometimes becomes a problem that spot welded joints of AHSS exhibit low CTS (= Cross tension strength). Therefore some studies have been made on enhancement of CTS for AHSS. Welding with post heat of a short time is one of effective methods for improving CTS. In this study, we experimentally clarified appropriate welding conditions using hot-stamped steel sheets of 1470MPa grade first. And then we tried temperature estimation and phenomenon analysis by analytic solution of heat conduction equation, in order to derive decision guidelines for welding condition. Consequently, the following results were obtained: 1) Improvement of CTS by over 30% in comparison with that without post heat was obtained when appropriate 1st cooling time; tc1 and post heat time; th2 were chosen depending on the sheet thickness, and also current ratio (= current in post heat; Ih2 / current in 1st welding; Ih1) was around 0.9. 2) By means of the analytic solution, equations of temperature estimation for four steps (1st welding, 1st cooling, post heat and final cooling) were gained. 3) The phenomenon analysis with the temperature estimation led to each recommended condition for parameters such as tc1, th2 and Ih2/Ih1 on each sheet thickness. 4) These conditions analytically recommended showed a good accordance with them experimentally clarified.
Al-rich Zn/Al/Zn clad solder were developed as Pb-free solder for a die-attachment. The Zn/Al/Zn clad solder was produced by clad rolling of Zn and Al strips in order to prevent Al from oxidation and improve wettability. The Zn/Al/Zn clad solder melted at 382°C after solid-state interdiffusion of the Zn and Al layers. Bonding was successfully achieved with bonding pressure of a few kPa. Thermal cycle life of Invar-to-Cu substrate joint using the Zn/Al/Zn clad solder was longer than that of Pb-Sn-Ag solder. No kirken-dall voids were observed in the vicinity of the bonded interface after aging at 250°C for 1000 h.
Scatter of fracture toughness of welds is significantly large due to intrinsic nature of cleavage fracture and stochastic nature of fatigue-crack sampling of a local brittle zone. Under such circumstance, it is often desired to predict a distribution of fracture toughness of the local brittle zone, only. The present paper proposes a method of predicting critical CTOD distribution of a specific microstructure, e.g. coarse-grained HAZ, from weld HAZ CTOD tests. The method is based on the weakest-link assumption and uses a likelihood function which is defined by considering critical CTOD values, as well as crack initiation sites. The proposed method was examined by Monte-Carlo simulations.
Lap joints of an upper Al alloy (1.0-mm-thick A5052) and lower hot dip galvanized steel (1.2-mm-thick GI steel) were welded by a novel spot welding process for dissimilar metal lap joints using a new tool with the tip made of spherical ceramics, i.e., "Friction Anchor Welding." As a result, the rotating tool was plunged only to 1.3-1.4 mm from the Al alloy surface, and accordingly, a steel projection was not formed in the Al alloy sheet. Further, the Al alloy near the rotating tool was removed. However, near this removed area, the Zn layer on the GI steel melted and was removed by friction heat, and consequently, the GI steel and the Al alloy were welded. Thus, the tensile shear strength reached about 2.6 kN/point.
The reducing effect of the angular distortion in Tee type full penetrated welded joint fabricated by the laser-arc hybrid welding was investigated. The Angular distortion caused by the laser-arc hybrid welding were measured and compared with one caused by the CO2 gas shielded arc welding. As a result, the amount of angular distortion by the laser-arc hybrid welding was the same level as one in case of forming the leg size of F1 grade regulated by Common Structural Rules for Bulk Carriers (CSR-B) by applying arc welding. In addition, numerical analysis of the welding deformation was carried out by the thermal elastic-plastic finite element analysis and these results were generally consistent with the measured results. Moreover the effect on FE result of the penetrated shape was discussed.
The residual stress behaviors in fillet welded lap joints of sheet metal have been researched in the systematic testing procedure with varied steel types, steel thicknesses, and welding wires having different transformation points. Consequently, under the simulated fabrication welding conditions (with a constant amount of deposited metal), the transverse residual stress at the weld toe, which is deemed critical in fatigue strength, has been found almost invariable to a change in steel type and thickness, but it has been clarified to become compressive to a greater extent as the wire's transformation point is lower. Moreover, as for the residual stress inside the weld metal, the compressive residual stress area has been found to expand as the welding wire's transformation point reduces, from the results of the thermo-elastic-plastic analysis.
To make clear the effect of strain occurred in weld metal of Type 316L stainless steel during laser beam welding (LBW) on center-line cracking in LBWs, side-bead test was adopted to evaluate the susceptibility by measurement of the crack length. The stainless steels varied with impurity elements such as P and S were used as the materials for the test. From the side-bead test, the crack length decrease with increase in the laser welding velocity. Compared with high and low amounts of impurity elements, the crack length occurred in welds metals with higher contents of impurity elements were remarkably longer than those in lower ones. To discuss the phenomena, the thermal-elastic-plastic FEM analysis considering with temperature dependent interface element has been conducted to simulate the side-bead test. From the calculation, the cracking behavior recognized in the actual side-bead test was relatively in accordance with calculation. On the basis of these results, the side-bead test was a useful test method to evaluate the crack susceptibility. In addition, it seems to be suggested that the susceptibility may be possible to predict by the FEM analysis due to use correct physical constants of the materials.
To evaluate the effect of the dual-phase microstructure on hydrogen diffusion behavior, a numerical simulation using a two-dimensional dual-phase model was performed. The effect of the dual-phase microstructure was evaluated by changing the morphological parameters such as the shape and geometrical distribution of the austenite phase using two different phase fractions. The results indicated that the hydrogen does not diffuse through the higher coefficient phase on a preferential basis; instead, the diffusion occurs with the same time dependency in both phases. In addition, the layered pattern of the austenitic phase and the higher phase fraction of the austenite phase reduced the degree of hydrogen diffusion. The hydrogen diffusion in dual-phase steel is significantly affected by the difference in the hydrogen diffusion constants and morphology of the microstructure of the dual-phase steel.
During ultra high frequency pulsed gas tungsten arc welding (UHFP-GTAW), the root radius of arc decreased with arc constriction that caused the narrower weld bead. The larger arc force created the more depression of surface, which had been important to increasing of weld penetration. Simultaneously, molten liquid metal is the key factor during welding process, and the mobility of molten pool was enhanced by weld behavior compared with conventional gas tungsten arc welding (C-GTAW). In the paper, the characteristics of arc behavior and fluid of molten pool was discussed with UHFP-GTAW. The results indicated the significant effect of the arc plasma on heat and force of molten pool with UHFP-GTAW. Further, the temperature diffusion of molten pool would reduce, which can limit the impact of arc heat on base metal effectively.
In order to assess the applicability of the 3DP-DHD technique, which could consider the effects of three-dimensional nature and plastic deformation around reference hole, as the measuring method of welding residual stress, the technique was applied to measurement of residual stress in multi-pass weld joint. In addition, evaluation results were compared with those obtained by the stress relief technique and finite element analysis. As a result, evaluation results obtained by the 3DP-DHD technique were in good agreement with those of other techniques generally.
For quantitative evaluation of solidification cracking susceptibility in two kinds of type 316FR stainless steel (316FR-A and 316FR-B) laser welds, laser beam welding (LBW) transverse-Varestraint test was performed. As the welding speed increased from 1.67 to 40.0 mm/s, enlargement range of solidification brittle temperature range (BTR) for 316FR-B (from 14 to 40 K) was larger than that for 316FR-A (from 37 to 46 K), respectively. Based on theoretical calculations for solid/liquid coexistence temperature range by using Kurz-Giovanola-Trivedi and solidification segregation models, the reason for larger increment of the BTR for 316FR-B could be regarded as larger decrement of δ-ferrite amount during the welding solidification than that for 316FR-A, affecting severe increment of the impurities' segregation (S and P), thereby more enlarging the solid/liquid coexistence temperature range as compared with that of 316FR-A.
Evaluating the residual stress in welds is great important, and X-ray stress measurements are superior to other stress measurement methods because they are nondestructive and provide high-resolution results. However, it is well known that texture in welds is found in some materials such as austenitic stainless steel, and that such texture can greatly reduce the measurement accuracy. Although much knowledge regarding the application of X-ray stress measurement to materials with texture has been accumulated, most studies have considered texture under uniaxial stress. However, weld residual stress is typically multi-axial stress. In this study, an attempt was made to improve the accuracy of X-ray residual-stress measurements for welds with texture. To evaluate the stress in welds with texture under multi-axial stress, a new method was developed for evaluating plane stress from the elastic compliance, the value fraction of the crystal, and measurements in two orthogonal directions. A new method was also developed for evaluating the value fraction of the crystal in welds with texture. Moreover, in a SUS316L weld specimen, the residual stress evaluated by the proposed method agreed very well with that evaluated by the stress relief method. Thus, the proposed method has the potential to be effective for measuring residual stress in welds with texture.
Alumina (Al2O3) coatings by two types of plasma spraying method (APS: atmospheric plasma spraying and LPS: low pressure plasma spraying) were observed at the view point of coating structure in order to confirm superiority for electric isolation application. A breakdown voltage of dielectric layer is due to interlamellar gap continuous across Al2O3 coatings. The micron size evaluation of this gap was confirmed by macroscopic structure observation. The cross-sectional macroscopic structure was examined by copper electrolytic plating for visualization and by a scanning electron microscopy (SEM). Furthermore, dielectric volume resistivity was dominantly derived from microscopic structure and crystallinity of the coatings. The cross-sectional microscopic structure of the coatings was observed by a field emission SEM (FE-SEM). The crystallographic analyses were performed by electron back scatter diffraction (EBSD). As an evaluation result, there are no differences in macroscopic structures with gap of coatings between two spraying methods. The Al2O3 particles partially melted in spraying and made cracks in solidifying. The gap was derived from the cracking of molten particles and also exhibited the same structures between two spraying methods. Coating electric properties had no significant differences between two spraying methods. In this study, the factors which so significant differences were not observed in electric properties each spraying methods are investigated macroscopically and microscopically.
An SPH simulation of a welding process is performed for TIG welding, taking account the melting and solidification of the anode metal, the free surface movement of the liquid, and the four dominant flow-driving forces; gradient of surface tension (Marangoni effect), gas drag on the liquid surface, buoyancy, and electromagnetic force (Lorentz force). The process of welding two metal blocks is computed, including the dynamic formation of weld pool which joins the gap between the blocks. The difference of material's surface tension dependency to temperature causes a significant difference in the weld pool penetration. These results show the usefulness of this particle method in arc welding simulations.
Solidification cracking occurs as a result of thermal strain and solidification behavior that is determined by welding conditions, especially welding speed. Brittle temperature range (BTR), an important index for evaluating solidification cracking, is used to quantity the effect of welding speed on susceptibility to solidification cracking. However, measurement of BTR using Trans-Varestraint test during laser welding has rarely been studied. The purpose of this work is to investigate the effect of welding speed on solidification cracking susceptibility in type 310S stainless steel using Trans-Varestraint test during laser welding. Solidification cracking and ductility-dip cracking were distinguished by observing fracture surfaces. Compared with traditional Trans-Varestraint test carried out for gas tungsten arc welding, the number of solidification cracks and total crack lengths during laser welding were lower; however, the number density of solidification cracks and total crack length per bead width during laser welding were higher. Both of these values had a tendency to first increase and then decrease slightly with increasing welding speed: the maximum values occurred at approximately 1.5 m/min. Temperature profiles at 0.2 and 1.0 m/min during laser welding were measured by an optical-fiber radiation thermometer combined with in-situ observation using a high-speed camera. BTR was measured using the center crack length along the heat flow in the welding direction. BTR at 1.0 m/min was less than that at 0.2 m/min during laser welding because the maximum crack length appeared at the side of molten pool at welding speeds greater than 1.0 m/min.
A multi-phase AC arc has various advantages such as high energy efficiency, large plasma volume and low gas velocity. Therefore, the multi-phase AC arc is suitable to massive powder processing like nanomaterial fabrication processes and innovative in-flight glass melting technology. However, the understanding of discharge behavior of multi-phase AC arc still remains to be improved for the practical use. In particular, electrode erosion is one of the most important issues to be solved. The purpose of this study is to investigate the droplet ejection mechanism of the multi-phase AC arc based on high-speed visualization by the high-speed camera and appropriate band-pass filters system. Obtained results indicated that droplet ejection was involved by variation in current and voltage during an AC cycle. The larger droplets were ejected at only cathodic period and the transition time from the cathodic to anodic AC period. Estimation of forces acting on the molten droplet revealed the electromagnetic force was most important force which leads to the detachment of the droplet from the electrode surface. Smaller droplet on the electrode tip leads to the droplet ejection due to the relatively stronger electromagnetic force than surface tension.
In a newly developed rod feeding gas flame process, solid resin rod including nanoparticles could be fed into a Rokide® flame gun successfully to create fine ceramics layers without micro cracks and pores applying for electric, magnetic and dielectric components. In this investigation, alumina particles of 170 nm in average diameter were dispersed into acrylic liquid resin at 40 % in volume fraction. The paste materials were injected into a brass mold of φ40×200 mm in inner dimension and thermal cured through heating at 120°C for 60 min. Formed solid rods were fed into oxyacetylene gas flame coaxially by using the Rokide® spraying gun system. Sprayed particles were cached by water bath for microstructure observations by a scanning electron microscope and crystal phase analyses by an X-ray diffraction spectroscopy. Fine ceramics layer formations will be discussed systematically by the feeding speed of solid rods and gas flame condition of air pressure and oxygen pressure.
Aluminum alloys are useful structural materials to make automobiles lighter. But, in the case of welding for aluminum alloys, large distortion occurs due to its large thermal conductivity and thermal expansion ratio. From the view point of heat input suppression, laser beam welding (LBW) is one of effective approach to put the alloys into practical use. However, one of other big problems to adopt the alloys as structural materials with welding is weld defects such as porosity, hot cracking, and so on. Thus, development of the method to draw proper welding method and its conditions more efficiently is demanded, in order to improve productivity due to breaking away from current search methods by trial and error. Therefore, in order to clarify the mechanism of porosity in weld metal on LBW process and suggest proper welding conditions without any defects, numerical analysis based on fluid dynamic theories and in-situ observation using X-ray transmission real-time imaging system were conducted in this study.
Both strength and ductility in bainitic steels have been analyzed as functions of size of bainite lath nucleated within austenite grain, hereafter referred to as BWING, the number of BWINGs contained in an aggregate of bainite laths with almost parallel slip systems between neighboring bainite laths, hereafter referred to as ALPS, and related factors. 0.2% proof stress increased with decreasing size of BWING. Dislocation network could be introduced into austenite formed with isothermal holding at 1023 K before austenitization above 1673 K and would act as nucleation site for BWING, resulting in size of BWING being smaller. It could be suggested that almost parallel slip systems in neighboring BWINGs induced large homogeneous deformation, and dense BWINGs in an ALPS increased total elongation. As the number of BWINGs in an ALPS increases, many BWINGs having the larger Schmid factors would exist at many spots, resulting in many initiation sites for deformation and in inducing sufficient deformation of an ALPS. Conclusion is that main factors controlling 0.2% proof stress and total elongation are size of BWING, parallelism of slip systems in neighboring BWINGs, size of ALPS under a fixed size of BWING and the number of BWINGs within an ALPS.
A fiber laser was used to achieve stable consumable-electrode welding with pure Ar shielding gas (Ar-GMA). Ar-GMA welding produces a long column of liquid molten metal (CLM) at the wire tip, which causes irregular short-circuit and an unstable arc. The length of the CLM must be kept short to ensure stable welds. We have developed a method for cutting the CLM by means of a fiber laser. We investigated the cutting conditions for cold wire without an arc and the cutting conditions using a fiber laser with a pulse peak power of 6.0 kW and peak pulse time of 10 ms. Our experiments revealed that the laser irradiation position at the wire was crucial for achieving stable cutting, mainly because the recoil pressure force bends the CLM. After determining the optimum irradiation positions for cutting the CLM, laser-enhanced Ar-GMA welding was carried out. Stable welding was possible and excellent joints were obtained.
The solid state bonding of fine-grained high carbon steel was carried out and the interfacial contacting mechanisms were experimentally investigated. It was found that superplastic deformation played a dominant role in the interfacial contacting process from a bonding ratio of 30% to 40%, under the bonding conditions of bonding temperature T = 1023∼1053K and bonding pressure P = 35∼45MPa. The stress exponent, n, was about 2.5 while the activation energy, Q, was about 151 kJmol-1. Predominant bonding mechanism controlled by superplastic flow was reproduced in the early bonding stage, under the conditions of T = 1023∼1053K and P = 35∼45MPa.
Whether dislocation network could be introduced into austenite (γ) even during austenitization or not was analyzed by optical microscopy. Austenitization at 1450°C for 300s dissolves MnS completely into γ. It is clarified that isothermal holding at 900°C for 1200s after austenitization forms MnS along small-angle boundary within γ grain. It suggests that dislocation network forming the small-angle boundary is introduced and stable even in the high temperature region of γ. Serial sectioning observations of bainite laths with optical microscope reveals that BWING (bainite lath within γ grain) could nucleate from the dislocation network. Dislocation network forms by reaction of dislocations on each slip system. Each dislocation in a network would have different Burgers vector. Therefore, climbing rate of each dislocation in a network due to absorbing or generating vacancies might be different, resulting in bowing-out of dislocation belonging to a network and in increase in self-energy of a dislocation network. It could be concluded that dislocation network is stable even in γ and the network acts as nucleation site for BWING. Based on the conclusion, nucleation of BWING is enhanced through introduction of dense dislocation networks into γ.
The high-temperature joining process is a key technology for electronic components and assemblies of automotive and other high-temperature applications. Recently, focusing on a sintering behavior of metal particles, the joining process using metal particles has been proposed as a solder alternative to establish a new joining technology for high-temperature applications. In this study, microscale Ag particle pastes were experimentally applied and the effect of the addition of nanoscale particles into microscale particle pastes on Cu-to-Cu joint has been studied to improve the shear strength of Cu-to-Cu joint using a microscale particle paste. Then, the effect of bonding condition on the shear strength of Cu-to-Cu joint has been investigated. It was found that the shear strength of the joint using a 50/50 mixed paste was more than 20 MPa at 300 °C for 600 s under joining pressure of 5 MPa in air.
Steel chimney stack shells with thinning caused by local corrosion are repaired using spliced joints. However, if workers do conventional repair welding, back coating film is damaged by heat input and needs repainting which causes increased costs. Therefore it was developed a low heat input welding method protect back coating film from heat damage using high speed robot welding. Because it is not possible to measure the backside temperature directly during welding, the authors developed a system to guarantee backside temperature when using the low heat input welding method. Backside temperature is guaranteed by using surface temperature data obtained from the thermal sensor attached to the torch tip of the welding robot. This method is applied to steel chimney repair welding at thermal power plants.
In order to improve electrical conductivity of Mg-doped p-GaN, the enhancement of H release from Mg-doped p-GaN were attempted by applying current flow and electrical field through the p-GaN substrates during low temperature annealing. The microstructure and electrical properties after annealing were then investigated by transmission electron microscopy observation, direct current conduction test and Hall effect measurement. The results reveal that no reaction occurs between deposited Au film and GaN substrate during annealing at 573 K for 3600 s. The electrical conductivity does not show any improvement only by applying electrical field or by annealing at 573 K for 3600 s without current flow. It is likely due to the limited mobility of H within p-GaN. However, by applying current flow under the constant voltage of 30 V during annealing at 573 K for 3600 s, a significant improvement of the electrical conductivity has been observed. By applying electrical field and current flow during annealing at 573 K for 3600 s, the H release from p-GaN was enhanced and Mg was activated, resulting in increasing carrier concentration of acceptor and improving the electrical conductivity.
In this study, the authors developed a three-dimensional coupling analysis method for spot welding by using Idealized Explicit FEM, which can analyze large-scale welding residual stress and deformation problems in practical computing time. This proposed method was applied to a spot welding problem to show its analysis accuracy. It was found that the nugget sizes obtained by experimental measurements and the method are in good agreement. In addition, to investigate the effect of shunt current on the nugget size in multiple-spot welding, the method was applied to three-dimensional spot welding with four points of welding. As a result, it was shown that the influence of shunt current in multiple-spot welding can be evaluated by using the proposed method in realistic computing time.
The aim of this study was to develop a temperature measurement technique during welding using a developed multi-sensor camera based on a two-color thermometry method. This technique has specific features, such as in-situ temperature measurement, high response speed, two-dimensional (2D) noncontact measurement, and flexibility. Accurate in-situ temperature 2D distribution and its history can be acquired based on the selection of a suitable combination of band-pass filters for laser welding and the welded material. In addition, a clear visible image can be captured with the 2D temperature distribution using the developed multi-sensor camera even when a high welding speed is employed.
Shop primer is a predominant cause of pits and blowholes during fillet welding using a conventional CO2 arc welding process. The hot-wire laser welding process for fillet welds employs a reflected laser beam incident on a molten pool for melting the base metal, and produces a weld bead with an extremely low dilution of base metal. In this study, weld-defect prevention in hot-wire laser welding as applied to steel plates coated with shop primer was investigated by examining the vaporization of the shop primer at the molten pool front and beneath the stiffener at the root region. A melting technique employing a reflected laser beam incident on the molten pool surface can efficiently remove the shop primer at the molten pool front during welding. The quantity of several gases and metallic vapors emitted during decomposition of the shop primer at a root was reduced markedly during hot-wire laser welding under extremely low base metal dilution. The above features of the hot-wire laser welding process achieved sound fillet welds without pits and blowholes.
One of the most challenging topic nowadays for the steel manufacturers is the investigation of the role of the retained austenitic grains inside a martensitic body. The phase transformation of the former, in fact, has been proved to alter the material properties in terms of toughness, ductility, resistance and also it has be indicated as one of the probable causes for the crack initiation. The work presented in this paper aims to study this phenomena by means of a thermodynamically based single crystal constitutive model, coupling the crystallographic slip together with the phase transformation.
We investigated a hot-wire laser welding method using a laser diode with a rectangular laser beam to weld a large-diameter pipe with a narrow gap. First, welding trials using plate specimens were performed to obtain the optimum welding conditions using a defocused beam. We achieved stable feeding of wire, a stable molten pool, and a well-formed weld bead, with few defects. Second, we welded a large-diameter pipe using optimum welding conditions discovered from the trials. Although some small lack of fusion occurrences were observed in the side bending test, we achieved a narrow gap, low dilution, narrow heat-affected zone weld without solidification cracks.
An unconventional plasticity model has been adopted to investigate the role of the residual stress and plastic strain fields on a welded plate subjected to a series of cyclic loads. The purpose of this study is to examine the effect of residual stress fields on the buckling behavior triggered as a consequence of the increase of cyclic number and accumulation of plastic contributions. Moreover the effect of the tangential stress rate, included in the return mapping formulation of the plasticity algorithm, has been taken into account for a more realistic modelling to include yield-vertex effect for the non-proportional loading behavior.
TIG welding process consists of close energy balance between arc plasma and weld pool which plays an important role of anode electrode. Although arc is a heat source of welding and then energy produced by the arc is transferred into the anode, energy transfer inside the weld pool as the anode is more important for welding due to formation of weld geometry. In particular, the difference of convective direction of fluid flow in the weld pool dramatically leads to different weld geometry. In this paper, mechanisms of weld pool formation in TIG welding process is discussed on the basis of numerical simulation which takes account of close interaction between the arc plasma and the anode electrode. We also discuss the influence of the data of surface tensions from literatures. It is shown that surface tension gradient with temperature strongly dominates the direction of convective flow in the weld pool due to Marangoni effect and then greatly affects the formation of weld pool due to change in energy transfer inside the weld pool.
Residual stress, considered to be an influential factor in fatigue strength, was investigated through X-ray stress measurements on electron-beam welding joints, made of sheet metal with a thickness of approximately 10 mm. The Finite-Element Method (FEM) was also used to analyze the experimental results so as to verify any residual stress characteristics. The residual stress near the weld toe in the transverse direction was confirmed to be greatly compressed when the yield strength of the testing material became much greater. As a result of the three-point bending fatigue tests, except for S50C, it was demonstrated that the greater the residual stress of the test specimens was, the more they exhibited a longer fatigue life. It is thought that the reason for this was because the local area of the welded part became a stress concentration point due to its being greatly hardened.
Effect of cold rolling before austenitization on mechanical properties in bainitic steels has been investigated. Cold rolling of ferrite (α) and isothermal holding above A1 before austenitization were done to harden ferrite and to form thin layer of austenite (γ) at/around both α grain boundaries and annealing twin-boundaries. Both coherency stress around (γ/α)-interface and transformation stress might introduce dislocations into γ. The dislocations within γ might react with each other, resulting in formation of dislocation network, whose constituent dislocations have different Burgers vectors with each other. In the high temperature region of γ, the climbing of constituent dislocations in dislocation network might occur through absorption of vacancies with an individual rate, resulting in bowing of each constituent dislocation out (increasing in dislocation energy). Therefore, the absorption of vacancies with the constituent dislocations in dislocation network would be suppressed, resulting in the dislocation network being stable. Nucleation of bainite laths might be enhanced to relax the strain field around dislocation network, resulting in refining bainite lath. The refining of bainite lath would improve both strength and ductility in bainitic steel.
This paper describes the parallel seam welding of aluminum sheets by a magnetic pulse welding method with a flat one-turn coil. The coil has a narrow central part and this part has two parallel upper parts. A discharge current from a capacitor bank flows along these parts. A magnetic flux having two high-density peaks is suddenly generated around these parts. The flux intersects the sheets which are overlapped on the coil. The resulting eddy currents are induced in the sheets, applying two parallel strong electromagnetic forces to them. The sheets with a gap collide with each other at high speed on two parallel places, and are welded. In this time, four metal jets occur just ahead of the collision front and four weld zones are made. Two metal jets occurring in the inside collide if the experimental conditions are suitable. The interfaces of such parallel seam welded-aluminum sheets are observed by an optical microscope. Two inside weld zones are connected to each other with a small cavity.
The cleaning action in AC TIG welding is attributed to the oxide film removal process caused by cathode spots. Clarifying the behavior mechanism of these cathode spots will enable the control of the cleaning action. However, the behavior mechanism of cathode spots has not yet been experimentally clarified. This study reports that observations by high-speed video camera revealed the absence of cathode spots at the center of the molten pool, when shot in the vicinity of a molten pool after AC TIG welding using helium as shield gas on aluminum plates.
In recent years, steel I-girder bridges with fewer main girders have been widely adopted in Japan. In this design, the weight of the bridge can be reduced, and the thickness of the main girder flange plate is increased. The main girder blocks are welded and assembled onsite instead of being friction joined using high-strength bolts. Since achieving dimensional accuracy is important, accurate prediction is required for predicting the deformations that occur during the welding process. Therefore, in the present study, the idealized explicit finite element method is adopted to accurately predict such deformations. Through the comparison between computational and experimental results, the mechanism of the deformation, which occurs in I-girder bridges, is examined.
The objective of this research was to develop a single-pass vertical welding process for thick steel plates using hot-wire laser welding to reduce the heat input. A laser diode with a large rectangular beam spot and twin hot-wire feeding system were used. Welding phenomena were investigated using stationary and weaving laser irradiation with small and large groove widths. Single-pass vertical welding for thick steel plates with a low heat input and low dilution could be achieved. A critical energy density existed for each welding speed at which fusion of the groove surface could commence. The energy density affected fusion of the groove surface compared with welding speed. Weaving laser irradiation made it possible to maintain a high energy density and accommodate the large gap variation.
Thermal nanoparticle spraying was developed to create fine ceramics layers on free surfaces. Resin slurry containing ceramics nanoparticles were atomized by a compressed air jet in a chamber, and the resulting micro mists of composited droplets were effectively injected from a tubular nozzle into the plasma flare. Alumina particles with average diameters of 200 nm were dispersed into the acrylic liquid resin at 40% volume ratio. The microsized mist droplets with average diameters of 20 μm were first formed using the compressed air jet under pressure of 2 atm, and then introduced into the arc plasma with an argon gas spray of 50 slpm in flow rate. The alumina layer of 300 μm thickness was formed successfully on the SUS-304 substrate of 50×50×5 mm in size, which was placed at a distance of 100 mm from the plasma gun, and a higher speed coating at the deposition rate of 50-100 μm/s was achieved. Crystal phases of the coated layer were analyzed by X-ray diffraction. The cross sectional microstructures were observed with optical and scanning electron microscopy. Porosities of the coated layers were estimated through dielectric constant measurements using terahertz wave spectroscopy.
The objective of this study is to develop a new cohesive zone model to assess the residual strength of the structure subject to local chemical action of hydrogen. A damage variable representing material degradations is introduced to a cohesive zone model by use of the thermodynamics approach. Our model, in which the decrease of the energy release rate caused by hydrogen is considered, enables us to evaluate the hydrogen embrittlement. Some representative numerical examples are presented to demonstrate the performance of the proposed model in simulating fracture processes accelerated by the hydrogen concentration.
In order to trace the welding line in the lap welding, weld pool in the pulsed MAG welding was taken by using the CCD camera. The weld pool was observed in the front and rear to the welding direction. The features of the weld pool were investigated to recognize the welding pool. The welding line was detected by using image processing and the digital controller was designed to trace the welding line.
The present work aims to investigate the response of thin wall pier subjected to non-proportional cyclic loading. An innovative procedure, inside a return mapping formulation for an unconventional plasticity model, has been taken into account to consider the effect of the tangential stress rate for the generation of irreversible contributions. The new numerical algorithm has been compared against experimental data to prove the reliability and validity of the procedure.
Hybrid materials of fiber composite and aluminum alloys have a great potential in reducing the weight of transportation vehicles. It has been become an alternative solution to steel panels because they can provide high strength and improving energy absorbing capability under impact and collision loading. Developing a new composite material based on experiment by changing mechanical properties is quite expensive and takes long time. Therefore, numerical simulation with the aide of FEM is often performed, since its results have been proved to be close to the experimental ones. There are many different ways in which the fiber can be combined into the resin. In this paper, the effect of lining arrangements of composite materials and the orientations of the layers on the strength was simulated using FEM in which the adhesive bond between fiber composite/metal is assumed as a tie constraint. Several plate panels with different lining arrangements and subjected to impact loads were investigated. The results of analyses are presented and their trends are discussed.
Stable formation of the back bead in joining of thick material is important so as to achieve high quality of welded joint. Since plasma welding employs high current density, it is suitable in the joining of thick material. The quality of plasma welding depends on keyhole which is generated from pilot gas and welding current. As a result, the relationship between welding voltage and keyhole is unclear. It is very difficult to estimate the keyhole by welding voltage and current. Authors have tried to observe the keyhole and weld pool from the top side of base metal directly. For this purpose, a CCD camera is applied with an external trigger. Moreover, the situation of keyhole is taken by high speed camera in order to determine the peak current and base current. Timing of the shutter in CCD camera is set up to take clear images of the weld pool. The formation of keyhole is investigated under two kinds of pilot gas. One is a mixture of Ar 93% and H2 7% and the other is Ar 100%. The welding condition and groove shape are found to observe the keyhole from the top side of base metal.