This paper deals with the welding properties of semi-automatic arc welding for hot dip galvanizing rebar to discuss the applicability of hot dip galvanizing steel to concrete rebar. The concrete rebar has many projection points to assure its adhesive property with concrete. The influence of these projections at the flare groove joint arrangement on the weldability of semi-automatic arc welding has been mainly investigated. As a result, the suitable combination of shielding gas and electrode wire on flare groove joint which shows less cavity formation and high joint shear strength has been made clear.

Laser peening which introduces compressive residual stress on surface is effective in extending fatigue lives of welded components, because tensile residual stress after welding is one of the most important factors reducing the fatigue lives. Steels for structures are widely used for bridges, buildings, etc., however the effects of laser peening on steels for structures are not well established. In this study, the residual stress, Vickers hardness and fatigue strength of the toe of butt welded joints pretreated by laser peening were examined and compared to those without laser peening. These results were further compared to those of annealed specimens in order to clarify the main factor of improving the fatigue strength. Main results are summarized as follows. 1) Large and deep compressive residual stress was generated around the toe of the butt welded joints by laser peening. 2) Vickers hardness also increased by laser peening but the incremental level was small. 3) Fatigue strength at 10⁷ cycles of the butt welded joints became much higher by laser peening. 4) It can be considered that the main factor of improving fatigue strength is the generation of compressive residual stress.

In temper bead welding, hardness is one of the key criteria to evaluate the tempering effect. A neural network-based method for hardness prediction in the heat affected zone (HAZ) of low-alloy steel has been investigated in the present study to evaluate the tempering effect in temper bead welding. The new hardness prediction system was constructed by using a neural network based on the experimentally obtained hardness database. On the basis of the thermal cycles numerically obtained by FEM, hardness distribution in HAZ of low alloy steel welded with temper bead welding method was calculated. The predicted hardness was in good accordance with the experimental results. It follows that our new prediction system is effective for estimating the tempering effect in HAZ during temper bead welding and hence enables us to assess the effectiveness of temper bead welding.

Low cycle fatigue strengths of load-carrying cruciform joints with incomplete penetration and strength under-match between base metal and deposit metal were studied. Fatigue tests were performed on specimens with five matching conditions and two sizes of incomplete penetration. Test results revealed that failure life was governed by crack propagation, and that crack propagation paths differed by matching and loading conditions. Also, fatigue strength of the joints was compared regarding the degree of strength matching and size of incomplete penetration. It was found that the effect of the strength matching on the fatigue strength becomes large in low cycle fatigue region by significantly reducing fatigue life of the specimen in the case of under-matched joints.

This paper deals with the characteristics of bead formation in horizontal fillet T-joint of 12 mm thick SS400 plate using high power fiber laser and pulsed MAG arc hybrid welding. One pass fully-penetrated sound weld bead without void or pit was successfully obtained in laser leading condition at the optimized welding conditions including laser aiming offset on the vertical plate from joint line and laser irradiation angle. In the case of arc leading void or pit was likely to form in the weld bead, and sound weld bead was difficult to obtain. It was also revealed that metal supplied by welding wire was concentrated on the upper side of weld bead, which was made mainly by arc welding, and difficult to mix into the bottom part of weld bead, which was made mainly by laser welding, in both arc and laser leading conditions. Lack of fusion and hot cracking occurred, but can be suppressed by setting the optimum welding speed and arc current to modify the shape of weld penetration.

Hybrid welding consisted of high power fiber laser and pulsed MAG arc was conducted on square-groove butt joint for SS400 of 12 mm thickness to investigate the optimum welding parameters for fully-penetrated weld bead formations by single pass. As a result, a sound fullypenetrated weld bead can not be obtained in laser leading-arc trailing process because irregular melt-through was observed in the bottom side weld beads under the all evaluated welding parameters. However, the formation of the irregular melt-through was suppressed effectively in arc leading-laser trailing process and sound fully-penetrated weld beads without any apparent defects was obtained successfully at the root gap up to 0.5 mm. Moreover, by using a high speed camera, it was revealed that the irregular melt-through was formed when amount of spatter was fluctuated on bottom side weld bead in laser leading-arc trailing process. However, in arc leading-laser trailing process, spattering constantly occurred and a stable molten metal pool was observed on bottom side weld bead. It was also found that the optimum laser and arc distance was below 4 mm, in which laser beam was irradiated onto the weld pool under the arc. From study results, the effect of hybrid welding parameters on the full penetration of 12 mm thick SS400 plate was systemically evaluated.

The hot cracking susceptibility of type 25Cr-35Ni extra high-purity stainless steels was evaluated by the transverse-Varestraint test with varying the contents of Cr as well as P and S. Two types of hot cracks occurred in these steels by Varestraint test; solidification and ductility-dip cracks. The solidification cracking susceptibility was reduced as the amount of Cr in steels increased, and 25Cr-35Ni stainless steel was negligibly susceptible to solidification cracking. On the other hand, the ductility-dip cracking susceptibility adversely increased with an increase in Cr content as well as P and S contents in steels. There was a good linear relationship between the compositional parameter of (P+1.22S) and the DTR, as well as between (P+1.19S) and the BTR in 25Cr-35Ni stainless steel welds. Accordingly, the quantitative influence of S to an increase in the ductility-dip as well as solidification cracking susceptibility was approx. 1.2 times as large as that of P. The amount of P+1.22S in steels should be limited to approx. 90ppm in order to obtain the sufficiently low ductility-dip cracking susceptibility in 25Cr-35Ni stainless steel welds. Numerical simulation of segregation behaviours of P and S revealed that they were segregated at the grain boundary in the ductility-dip temperature range during welding. Molecular orbital analysis suggested that ductility-dip cracking was attributed to the grain boundary embrittlement due to the grain boundary segregation of P and S. In order to further inhibit the ductility-dip cracking in 25Cr-35Ni extra high-purity stainless steel welds, the effect of La addition on hot cracking susceptibility was investigated. The solidification and ductility-dip cracking susceptibilities could be improved by adding 20-70ppmLa to the extra high-purity steel containing P and S of 6-8ppm. The ductility-dip cracking susceptibility was decreased as a result of the desegregation of P and S to grain boundaries due to the scavenging effect of La.

The numerical simulation by multi-phase field method (MPFM) has been performed in association with calculation of phase diagram. Microstructure formations during solidification, cooling and heat treatment in SUS304 are simulated in conditions of multi-component and multi-phase alloy. It is confirmed that dendrite growth and δ/γ transformation can be predicted and reasonable agreements with dendrite growth theory and previous works are obtained. Moreover, dissolution behavior of δ-ferrite during heat treatment can be also evaluated by MPFM. It will be a useful tool in analyzing the microstructure formation in welding process.

The present study was aimed at producing new Ag-based filler metals that have a melting point lower than that of a conventional Ag-based filler metal (BAg-24) for brazing cemented carbide and possess the ability to provide a high strength joint of cemented carbide. Using an Ag-based filler metal with a lower melting point (BAg0) that was a quaternary alloy of Ag-Cu-Zn-Sn system previously developed by authors, and Ag-based filler metals with Ni or Co element added into the BAg0, cemented carbide rods were brazed. Bending strength of the joint and the brazed layer microstructure were investigated. The following results were obtained in this study.
Bending strength of a joint brazed at 650°C using the BAg0 was about 85% that of a joint brazed at 750°C using the BAg-24. When Ni element was added into the BAg0, intermetallic compounds were formed in the brazed layer, and bending strength of a brazed joint was decreased. By adding a small amount of Co element into the BAg0, bending strength of a joint brazed at 650°C was improved and was equivalent to that of a joint brazed at 750°C using the BAg-24. However, excessive addition of the Co element made the bending strength of a joint decreased.

This report deals with effects of heating direction on laser forming which is used to form curved surface with thin plate. When we conduct laser forming to create suitable results, it is one of some important problems where and how much we should put in-plane strain into a thin plate. Some researchers have presented a new development method with curvature lines as a solution to that problem. Curvature line can be obtained as a result to combine primary curvature directions. In-plane strain is calculated from distance change of these lines. Heating direction is chosen parallel to curvature line because in-plane strain is put as transverse shrinkage caused by heating. In this case, heating direction is fixed on the plate, not changeable, even if side lines of a product is changed, because curvature line direction is unique for the curved surface.
Geodesic in-plane method we have proposed can show in-plane strain for every direction, so that we can select heating direction as you want to do.
In this report, we study experimentally the effects of heating direction on laser forming of saddle curved surface for a situation where we chose heating direction freely based on the geodesic in-plane method. As a result, we learn following three points, saddle surface can be created along another heating direction of curvature line, bend effect exists because of transverse shrinkage, and symmetrical heating line set can avoid curved surface shift.

Prevention of weld cracking is primely important for ensuring the integrity of steel construction. Especially, for high strength steel weld, which is susceptible to hydrogen cold cracking, it is important to clarify the critical condition for cracking in order for expanding use of high strength steel applications. In this study, critical conditions of hydrogen content and residual stress in the high strength steel weld with tensile strength level of over 980MPa were investigated. Cold cracking test was conducted using Y-grooved constraint weld joint by intentionally introducing hydrogen gas, and critical hydrogen content in the weld metal to cold cracking was determined. Cold cracking occurred in the weld metal from the "root", bottom portion in the weld metal, when hydrogen content exceeds about 2ppm, and fracture surface was typical grain boundary fracture caused by hydrogen embrittlement. Then, residual stress distribution in the weld meal was measured by neutron diffraction technique. Same Y-grooved weld joint with no cold cracking was used for stress measurement. Neutron beam was filtered by 2mm×2mm slit to measure the residual stress in the small region. Reference sample without residual stress was prepared by introducing multiple saw cuts with small intervals in the same weld joint, and lattice spacing of stress free condition was first measured. Then, lattice spacing in the three different directions along the centerline of the weld metal were measured to calculate residual stress in the three directions. All portions in the weld metal center showed tensile residual stress. The root portion showed highest tensile stress of over 1110MPa in the transverse direction, which is the same as the crack opening direction. Therefore, it is considered that high level of tensile residual stress is the key factor for cold cracking. Hydrogen concentration to the root region by pressure stress was discussed in terms of local hydrogen content condition for cracking.

The microcracking susceptibility in dissimilar multipass welds of alloy 690 to type 316L stainless steel was investigated by the Varestraint test and the multipass welding test using the four different stainless steels varying the amounts of impurity elements such as P and S, and using the nine different filler metals of alloy 690 varying the REM(La) content. In order to simulate the dissimilar weld metals, stainless steels were gas tungsten arc (GTA) welded using alloy 690 filler metals with varying the dilution ratio. Microcracks occurring in the reheated weld metals were classified into ductility dip, liquation and solidification cracks. The ductility dip cracking susceptibility decreased with increasing the La content in the weld metal, while the liquation and solidification cracking susceptibilities increased contrarily when the La content in the weld metal exceeded ∼0.02 mass%. The relation among the microcracking susceptibility, the (P+S) and La contents in every weld pass of the multipass welding was investigated. Ductility dip cracks occurred in the compositional range (atomic ratio) of La/(P+S)<0.13 and solidification/liquation cracks occurred in that of La/(P+S)>0.55, while any cracks did not occur at La/(P+S) being between 0.13-0.55. The ductility dip cracking susceptibility could be improved by adding La due to the scavenging of the impurity elements. The excessive La addition to the weld metal resulted in the solidification/liquation cracking alternatively attributed to the formation of Ni-La intermetallic compounds with low eutectic point. The optimal filler metal for the dissimilar multipass welding of alloy 690 to stainless steel was selected as La/(P+S) would be 0.13-0.55 for every weld pass.

In order to elucidate welding distortion and residual stress generated by laser-arc hybrid welding for high strength steel (HT780), a series of experiments and analyses were carried out. A bead-on-plate welding was carried out by using a fiber laser and CO2 MAG arc welding. For simulating the laser-arc hybrid welding by thermal elastic-plastic analysis, a method for heat input was proposed by considering the penetration shape by laser and arc separately. With using this model and the idealized mechanical properties by considering the phase transformation in cooling stage, welding distortion and residual stress obtained by the experiment were simulated. Synthetically evaluating the results obtained from the proposed heat input model for laser-arc hybrid welding, it was verified that the experimental result could be accurately simulated by the analysis. Even though the dual heat source was used in welding, it was known that welding distortion and residual stress generated by laser-arc hybrid welding for HT780 and their production mechanisms were basically the same as those by laser beam welding.

In order to evaluate the occurrence of cracks such as SCC, it is important to estimate microscopic stress distribution. The microscopic stress distribution is generated due to microscopic inhomogeneity. In this study a numerical simulation method to evaluate the effect of crystal orientation was proposed. The method consists of measurement of crystal orientation by EBSP method and microscopic stress analysis based on the theory of crystal plasticity considering EBSP measurement result. The method was applied to the microscopic stress calculation of the weld metal band electrode submerged arc welding of Ni-base alloy. It was demonstrated that microscopic stress concentration occurs around the grain boundary of the weld metal.

The cooling channels have to be installed in the first and side walls in current design of Japanese test blanket module for the international thermonuclear experimental reactor (ITER) and these walls are planned to be joined by electron beam (EB). Since the EB weld joint is intended near the cooling channels in the first wall, thermal elastic-plastic finite element analyses were conducted to examine the influence of cooling channels on the welding residual stress. From these results, it was found that there would be a risk of stress corrosion cracking (SCC) on the surface of cooling channels due to large tensile residual stress. As a result of serial numerical studies by using simplified model for examining the position of weld joint, it was revealed that the residual stress on the surface of channels would decrease by changing the distance between weld line and cooling channels from 10 to 16 mm. Also, the movement of weld line would make the plastic strain distribution on the surface of channels uniform and let the distribution to weld line symmetric. Then, it can be concluded that the uniformity and symmetrical distribution might make a selection of appropriate condition for post weld heat treatment simpler.

In order to demonstrate not only the deformation of grain but also the opening and/or sliding at grain boundary, the interface element was introduced into the ordinary finite element method, and this numerical method was applied for examining the microstructural fracture behavior in two-dimensional ideal microstructure obtained through Voronoi tessellations. As for the grain, the anisotropy in elastic modulus due to the grain orientation was taken into account, while the fracture strength at grain boundary was assumed to be related to the boundary energy which could be determined by the atomic disorder at the boundary. From the serial computational results for examining the influences of elastic properties in grain (isotropy and anisotropy), mechanical property at grain boundary (interaction between opening and sliding deformation), and grain configurations, it was revealed that all the factors varied in this research might affect the microstructural fracture behavior. Also, it can be concluded that this numerical method with the interface element can be useful for demonstrating the microstructural fracture behavior including the deformation at grain boundary.

Friction stir welding (FSW) is an energy efficient and environmentally benign welding process. The FSW is welded as a solid-state joining process under a combination of extruding and forging. The weld joint property strongly depends on the heat input and material flow during welding. The region of weld defects of butt weld was investigated by material flow around tool. The material flow direction during welding was investigated using a high speed video camera between transparent Poly-vinyl chloride (PVC) materials as simulation experiments of aluminum alloys. Additionally, the material flow velocity was measured by particle image velocimetry (PIV) method. Also, the material flow was numerically simulated which is finite element method (FEM) by plastic forming software DEFORM-3D. From PIV and FEM analysis, material flow directions were observed different type of flow direction at advancing side (AS) and retreating side (RS). One is that the material flow direction was corresponded to the tool rotation at RS. Other is that the flow direction was in the reverse direction of the tool rotation at AS. On the other hand, the material flow velocities of PIV analysis ranges from about 2 to 20 mm/s at AS. The material flow velocities of PIV analysis ranges from 1 to 5 mm/s at RS.

In this study, the theoretical tool design was proposed based on the material flow during friction stir spot welding (FSSW). At first, the fracture mode in a cross tensile test and a tensile shear test was investigated in detail. Then, the ideal profile of the welded joint was proposed to accomplish the high joint strength. The high joint strength can be acquired when the tip of the partially bonded interface is located at distance from the periphery of the keyhole and the surface of the top sheet. Based on the material flow model, it became clear that the strong inwards flow produced by the shoulder or the increase of the heat input to soften material around the stir zone leaded to producing the ideal joint. Thus four types of tools with different shoulder profiles were investigated. The investigation of the material flow and tensile tests were carried out. 15mm-spiral tool, which was designed to facilitate the material flow beneath the shoulder efficiently, attained the high joint strength in both a cross tensile and a tensile shear test.

In order to investigate the defects in the intersection of variable polarity plasma arc (VPPA) welding seam with the friction stir welding seam. Different kinds of specimens with crossing and overlapping welding seams were prepared. Defects in the overlapping and crossing welding seam were detected. The mechanism of defects was further explored. At last, the schematic of welding porosity forming were given.

Resistance spot welding is a very complicated process involving electro-magnetic, thermal, fluid flow, mechanical and metallurgical variables. Since weld nugget area is close and unobservable with experimental means, numerical methods are mainly used to reveal the nugget formation mechanism. So far, heat transfer behaviors in the weld nugget have been systematically studied with traditional electro-thermal model, however, the model cannot model the fluid flow caused by induced magnetic field in the molten nugget. In this paper, a multi-physics model, which comprehensively considers the coupling of electric, magnetic, thermal and flow fields during RSW, is used to investigate the fluid flow and heat transfer laws in the weld nugget. Results show that molten metal in the nugget makes high speed rotation motion in four cores, moreover, the rotation motion significantly changes the profile of the nugget and the thermal field evolution, compared with the traditional electro-thermal model. Because the fluid flow and heat transfer jointly determines the crystallization process and final microstructure, as a result, when predicting crystal growth process, the effects of the fluid flow during resistance spot welding should be considered in order to obtain a more accurate prediction results.

In order to investigate the weld nugget shifting phenomenon in resistance spot welding, a combined thermal-mechanical-electrical finite element model was developed to analyze the weld nugget formation process during welding. It was found that when the thickness of the bottom sheet increases, the degree of weld nugget shifting would become serious which will lead to the unqualified weld nugget. For the welding joint with material combination of SAE1004 + SAE1004 + DP600, the critical thickness combination is 0.6 + 1.8 + 2.0 mm. To improve the weld nugget shifting of the multi-stackup sheets joint, different electrode tips were used in the welding process. The effectiveness of this method was validated by the metallographic pictures.

Ultrasonic bonding process for wiring an Al ribbon to electric pads on substrate is affected by ribbon deformation and thermal behavior, i.e., (temperature rise of the materials during bonding). In the present study, the deformation and thermal behavior are analyzed. Numerical simulations of temperature rise and the deformation of the ribbon (wiring materials) were carried out by finite difference and element methods, respectively. As a result, the temperature rise and deformation processes were visualized by graphic images. It was suggested that the temperature of ribbon rose up to greater than 373 K during bonding. The ribbon deformation and the frictional slip behaviors influenced each other. Ultrasonic vibration enhances the equivalent stress in the Al ribbon very largely.

A solder joint is required to have a impact reliability for use in portable electronic products. In general, there is a correlation between the impact reliability and the morphology and thickness of the reaction layer formed at the solder/under bump metallurgy (UBM) interface. The characteristics of the reaction layer depend on the UBM. This study aims to clarify the effect of UBMs on the reaction layer formed at the solder/UBM interface. For this purpose, Sn-3.0 mass%Ag-0.5 mass%Cu was soldered on UBMs such as electroless Co-P plating (Co-P) and Ni-Co-P plating (Ni-Co-P). An electroless Ni-P plating (Ni-P) substrate was used as a reference substrate. The peak temperature of the preset reflow profile was 523 K for 60 s. Then, some of the specimens were subjected to aging at 423 K for 168-1008 h. After soldering, the spreading area of the solder on the UBMs was measured using an optical microscope (OM). The spreading area of the solder on Co-P was larger than that of the solder on Ni-Co-P and Ni-P. Therefore, the solder on Co-P had better wetting characteristics than that on Ni-Co-P and Ni-P. Further, the reaction layer at the solder/UBM interface was observed using a scanning electron microscope (SEM). A fine and a large needle-like intermetallic compound (IMC) were formed at the solder/Co-P and the solder/Ni-Co-P interfaces, respectively, whereas a layer-like IMC was formed at the solder/Ni-P interface. After aging for 1008 h, the thicknesses of the continuous IMC layers for Co-P and Ni-Co-P were approximately 10 μm and 5 μm, respectively, whereas the thickness of the continuous IMC layer for Ni-P was only approximately 3 μm.

To extend the engineering applications of bulk metallic glasses (BMGs), it is necessary to create process to form appropriate BMG/BMG and BMG/crystalline metal joint. In this study, hydrofluoric acid (HF) solution treatment and a laser soldering process were employed to enhance the solderability of Cu₆₀Zr₃₀Ti₁₀ BMG. The surface treatment in HF solution successfully dissolved surface Zr and Ti, resulting in concentration of Cu at the surface. A reflow soldering process using Sn-3.0 mass% Ag-0.5 mass% Cu (SAC) solder on this surface showed obvious dewetting because the thin Cu-rich layer dissolved into the molten solder. Laser soldering with a 0.01 s irradiation time, which produces rapid heating and cooling rates, offered good wettability of SAC solder and Sn-57 mass% Bi (SB) solder. At the interface between the SB solder and the HF-treated BMG foil, an intermetallic compound (IMC) layer was observed by scanning electron microscopy (SEM).

Generally, ThO₂ or La₂O₃ are added to a tungsten cathode for prompting electron emission from the cathode and preventing the cathode from consumption. In this study, influence of adding 2%ThO to the tungsten cathode as emitter material on arc properties and weld pool formation mechanism in TIG welding was investigated by employing numerical simulation model considering heat and mass transfers among the cathode, the arc and the weld pool. As a result, it was found that although the maximum cathode jet velocity in case of 2%ThO₂-W cathode was 497 m/s, that in case of pure W cathode was only 156 m/s because of low current density caused by flatten surface of the melting cathode tip. In case of 2%ThO₂-W cathode, the depth and the width of the weld pool became shallow wide than those in case of pure W cathode. For a reason, it is considered that because shearing force from the arc to the weld pool was larger than that of pure W cathode due to higher cathode jet velocity, the heat transport in the weld pool near the surface in radial outward direction increased

Cu powders were thermally sprayed onto AISI304 substrate surface under various ambient pressures, the splat shape on the substrate had a transitional changing tendency from a splash splat to a disk one with decreasing the ambient pressure. Millimeter-sized molten Cu droplets were deposited on AISI304 substrate surface by free-falling experiment, the heat transfer from splat to substrate was enhanced with the decrease of ambient pressure, which can be attributed to the good contact at splat-substrate interface. The shear adhesion strength of coating fabricated on blasted AISI304 substrate corresponded quite well to that of the splat shape. Therefore, control of thermal spray process through observation on individual splat behavior is meaningful.

Pulsed tungsten inert gas (TIG) welding is used to improve the stability and speed of arc welding, and to provide greater control over the heat input to the weld. The temperature and the radiation power density of the pulsed arc vary as a function of time, as does the distribution of metal vapor. They also affect the arc. A self-consistent two-dimensional model of the arc and electrodes is used to calculate the arc properties as a function of time. Self-absorption of radiation is treated in three directions considering absorption throughout the plasma. The relation between the metal vapor and the radiation power density is analyzed by calculating the gas flow velocity affected by the radiation absorption with the iron vapor distribution. The temperature, iron vapor, and radiation power density distributions depend on the self-absorption model that is used. The temperature distribution becomes broader when self-absorption of radiation from all directions is considered. Results show that the temperature distribution is affected strongly by the fast gas flow velocity during the peak current period. During the base current period, it expands to the radial direction.

Arc properties in Gas Tungsten Arc (GTA) strongly depend on welding conditions such as an arc current, an arc length and so on. In GTA, the arc current and an arc voltage are determined by an external characteristic of a power source and an electrical characteristic of the arc. There are two kinds of external characteristics of power sources, namely, Constant Current (CC) and Constant Voltage (CV) characteristics. The electrical characteristic of the arc depends on the arc length. In this study, dependences of the arc properties and relationships between the current and the voltage on the arc length in GTA employing power sources with the CC and the CV characteristics were numerically analyzed. As a result, it was found that the voltage and power of the arc decreased with decrease of the arc length in case of the CC characteristic and the current and the power of the arc increase dramatically with decrease of the arc length in case of the CV characteristic. Furthermore, with variation of the arc length, the arc power hardly changes for the CC characteristic, although the arc power largely changes for the CV characteristic.

TIG pulsed-arc welding is suitable for back-bead welding, thin plate welding and so on, because the heat source properties can be controlled by current waveform. The heat flux onto the base metal is affected mainly by thermal conduction and electron condensation from the arc. Both factors strongly depend on the temperature distribution and current path in the arc. In order to clarify the heat source properties of TIG pulsed-arc, dynamic variation in two-dimensional temperature distribution of TIG pulsed-arc was measured through Fowler-Milne method with a high speed video camera as a first step of the study. As a result, it was found that the arc column was expanded in radial direction and the maximum arc temperature was 20,000K during the peak current of 200A. On the other hand, the width of the arc column decreased especially in the downstream region of the arc and the maximum arc temperature fell to 17,500K during the base current of 50A.

Fe nanoparticles were generated by DC arc plasma method under atmospheric He-H₂ mixture gas with different H₂ concentration. The arc plasma was well constricted due to thermal pinching when H₂ was mixed. The generation rate of the Fe nanoparticles significantly enhanced as H₂ concentration increased to 40 %, and it was about 0.16 g/min for He-40 % H₂ arc plasma. The vaporization rate of Fe did not significantly increase any more as H₂ concentration exceeds 40 %. Numerical simulation based on magnetohydrodynamics (MHD) was performed and indicated that the Fe anode can be efficiently heated due to the H₂ induced thermal pinching. The maximum anode temperature was calculated close to boiling point of Fe when heated by He-40 % H₂ arc plasma. The specific surface area of Fe nanoparticles produced from He-40 % H₂ arc plasma was about 11.7 m²/g and its corresponding diameter was 65 nm.

The accuracy of dimension in the steel structure after welding becomes an important problem in the product cost. The control of the welding distortion is demanded in the steel structure welding to improve the productivity. For this purpose, the estimation of the amount of the deformation is needed and its behavior is investigated. In this paper, the numerical model of the heat source was proposed in the GMA welding. The fundamental welding experiments in the bead on welding were carried out to improve the accuracy of the numerical model. The cross section in the simulation result was agreed with the experimental results. During the welding, the behavior of the deformation and the temperature distribution was investigated by using the laser sensor and a thermoelectric couple. Dynamic behavior of the deformation was investigated during the welding. The visualizations of the dynamic behavior of deformation were tried by using the numerical analysis. The behaviors were good agreement with the experimental results.

The wire melting behavior in pure Ar shielding gas can be controlled with a coaxial hybrid solid (CHS) wire which has a coaxial double structure with a different composition in its inner and outer parts. This wire can prevent the generation of a column of liquid metal (CLM) at the wire tip due to the difference in the materials properties of the inner and outer parts and stabilize MIG welding in pure Ar shielding gas (Ar-MIG welding). We examine the effects of the material properties (melting temperature, specific heat and thermal conductivity) on the wire melting behavior, then propose and show the effectiveness of a design guide for the CHS wire by carrying out a simulation and a welding examination on the new CHS wire which was developed based on this design guide.

In plasma MIG welding process, because shielding gas is ionized in advance by a plasma electrode before supply, the shape of arc is easily controlled through electromagnetic force. Therefore the controllability of the arc is remarkably enhanced compared with that in a conventional MIG welding process. Because of this advantage, it can be employed for MIG welding in pure inert gas atmosphere. Although experimental results on plasma MIG welding in pure argon atmosphere were reported, further improvement of the welding process is required because it is difficult to form bead stably due to lack of the wettability. We have developed a new torch for plasma MIG welding which has shorter distance between a contact tip and the top of a nozzle than that of the conventional torch. In this paper, experimental results on welding of steel plates with V groove in pure argon atmosphere employing the new torch are discussed compared with those for the conventional torch. As a result, it was found that the bead formation was improved due to high wettability in case of the new torch, because melting volume of the base metal increased due to increase in averaged MIG current and heat flux.

As for standardization of a repair technology for nuclear reactors with a laser welding technique, a numerical study was performed to investigate precisely phase change phenomena between the solid and liquid state of micron-sized metallic particles by the laser irradiation. The metallic particles are embedded in the crack on the outer surface of nuclear components and then the laser is irradiated from the upper to them. The metallic particles melt and the crack is restored by solidifying of the melted metallic particles. This paper describes the numerical results on the phase changel phenomenon during melting and solidification of the metallic particles.

Toward a standardization of laser welding repair processes and controlling a residual stress which is induced by laser welding, we constructed the fully parallelized laser welding simulation model using one-fluid model (in the simulation, solid, liquid and gas phase are simultaneously calculated by one set of governing equations) and some advanced numerical models. In the simulation, the base material is a pure aluminum which was included to the code as a physical parameter and we considered the surface tension force and its effect of a temperature gradient named Marangoni stress. As a result, reasonable results were obtained that is welding bead which is one of the representative behavior of a low power density laser welding and the appropriate shape of inside the molten pool. Therefore, the model can be applied to be practical laser welding problems.

The effect of direction of unidirectional pores on the fusion zone shape, which produced by laser welding, of lotus-type porous iron was investigated through the numerical simulation of temperature distribution. Three-dimensional heat-transfer analyses, which take into account the thermal properties of the lotus-type porous iron depending on the direction and volume fraction of unidirectional pores, were performed by the ABAQUS FE code with user-defined subroutines. These results indicated that the lotus-type porous iron has little anisotropy of melting property. The calculated shape of weld fusion zone is in good agreement with the cross-sectional view obtained by experiments. In order to clarify the reason of these results, anisotropy of thermal diffusivity in the lotus-type porous copper, magnesium, and iron used and anisotropy of laser energy absorption coefficient for these metals were estimated. As a result, the lotus-type porous iron used has a little difference of thermal diffusivity and laser energy absorption coefficient due to its low thermal conductivity, low porosity, large average pore diameters, and high laser absorption coefficient of base metal.

In this study, hot-wire laser welding was used to develop the welding method for the lap joint of 980MPa class high-strength steel sheets (thickness: 1 mm) with a wide gap. Detailed phenomena of wire melting and bead formation during welding were understood using the high-speed camera, and then the optimum welding condition and the mechanical properties of weld joints were investigated. The fiber laser with hot wire was used for welding the lap joint with the gap of 1 mm. The welding parameters such as the laser output of 3kW, spot diameter of 1.5mmφ, welding speed of 1.5m/min and the wire feeding speed of 3.8 m/min were fixed. The welding parameters of hot wire such as wire insertion angle, wire insertion position, and current of wire were varied. Moreover, shear test of the weld joints were done. As a result, upper and lower sheets with gap of 1 mm were well joined without weld defects in the welding parameters such as the wire insertion positions (from the laser spot center) of 3mm and the wire insertion angle of 70° and the wire currents of 102∼116A. Moreover, the tensile shear strengths of weld joints were 500∼600MPa regardless of the wire current.

Since it is indispensable to remove oxide layer on aluminum for realizing high quality weld joint in arc welding of aluminum plates, AC TIG welding is generally employed. In periods of EP (Electrode Positive) of AC TIG welding, the oxide layer is removed by cleaning action of cathode spots. In this paper, results on experimental observations of the cleaning action of the oxide layer by cathode spots in AC TIG welding of the aluminum plate with a high speed video camera will be reported. As a result, it was found that cathode spots moved slowly on the oxide layer and their averaged velocity was 26.1m/s. On the other hand, cathode spots near the center of the weld pool, where the oxide layer had been mostly removed, moved at high speed and their averaged velocity reached 121.9 m/s. Furthermore, the velocity greatly changed depending on the position of the cathode spot and exceeded 1 km/s at the maximum.

The aim of this paper is to develop the high efficiency and high quality fillet welding technique using the combination of hot wire system and Laser welding. The melting of filler wire depends on wire current and there is an adequate vale of the current in each wire feeding speed. The leg length increases by the addition of filler wire compared with the non filler welding. The length is also larger than the laser irradiated area. This must be caused by the laser beam reflection on the molten pool surface. The results reveal that the parameter optimization leads to the stationary welding phenomenon. Besides, obtained welds have attractive properties, namely low heat input and low dilution.

Drilling is widely applied in materials processing. Novel drilling methods, including electrical discharge machining, laser drilling, and plasma arc drilling, have emerged in recent years to overcome such problems as tool wear and low efficiency in the drilling of thick plates or difficult-to-machine materials. Plasma arc drilling is an effective and high-speed drilling method for thick plates. However, the mechanisms of plasma arc drilling, such as the hole formation and dross formation processes, have not yet been fully elucidated. In this study, a number of experiments under different drilling conditions were conducted to investigate the hole formation process in mild steel plate of 12 mm in thickness, using a high-speed video camera. Hole formation in plasma arc drilling is a highly complex process since both melting and vaporization are involved. The mechanisms of molten metal ejection as a material removal process were verified. The formation and removal of dross at the entrance and exit sides were elucidated by our observations of molten metal ejection. Further experiments were carried out to investigate the effect of the anode motion peculiar to plasma arc drilling. The results clarified the hole formation process and some specific related phenomena. The insights gained into the hole formation process will be useful in improving hole quality and reducing dross around the hole.

The Tungsten Inert Gas Arc Welding (TIG) hybrid an electromagnetic field (abbr. FM-TIG) is an effective method to improve the welding quality of steel, and it is also an accepted joining technique in a wide range of non-ferrous metal manufacturing application. In order to study the forming inception of welding pool in TIG hybrid longitudinal electromagnetic field (abbr. LMF-TIG), the process of breeding, growing and developing of LMF-TIG welding pool is simulated by finite element method (abbr. FEM). Our study shows that LMF-TIG is significantly different from TIG welding in the welding pool formation process. The molten liquid metal flowing actions have these characteristics in LMF-TIG welding pool: the incubation stage has the "step" type unsteady flowing property, the growth stage shows the "double loop" flowing pattern, and a unique "stirring" movement mode occurs in the whole developing process. The experimental results indicate that the FEM simulation is effective and reliable.

In order to meet requirements of hot forge mould in the plastic manufacturing fields, the gas metal arc welding (GMAW) with a longitudinal electromagnetic field (LMF-GMAW) is applied to manufacture the bimetal thermal forming mould and repair the old die. The microstructure and mechanical properties are analyzed by SEM, EDS, micro-hardness, wear-resistance and thermal physical simulation testing methods. Our study shows that the LMF-GMAW method can increase the wear resistance property of the surfacing layer, enhance the interface bonding ability and improve the thermal mechanical strength of bimetal overlay work pieces.

Energy dispersive synchrotron diffraction (EDXRD) analysis and 3 dimensional digital image correlations were conducted to investigate the stress and strain effected transformation behavior during tensile loading of low alloyed TRansformation Induced Plasticity (TRIP) steel. This technique allowed for phase specific stress measurement during certain tensile load steps in the elastic and also plastic regime. Additionally the simultaneous determination of the load dependent phase content was realized. The results show that the martensite transformation starts only after exceeding the overall yield point and is finished before reaching the uniform elongation, whereas a large portion of the austenite remains unchanged in the structure. Furthermore, the martensite transformation related to the stress in the γ-phase and α-phase was analyzed and quantified.

Inclusions contributing to acicular ferrite nucleation were investigated from a crystallographic point of view to understand the formation mechanism for acicular ferrite microstructure in low alloy steel laser weld metals. The sample was low carbon Ti-B weld metals with an oxygen content of 480 ppm. In this sample, intragranular acicular ferrite formation was observed from some inclusions and acicular ferrite had Kurdjumov-Sachs orientation relationship with austenite matrix. The multi-phase inclusions contributed to nucleation of acicular ferrite. They were surrounded by a Ti-enriched layer. It was confirmed by selected area diffraction and EDS analysis that the Ti-enriched layer was TiO. The acicular ferrite had Baker-Nutting orientation relationship with TiO layer on the inclusion surface. The lattice misfit was 3.0 % at the interface between the acicular ferrite and TiO.
Therefore, it is considered that the TiO on the inclusion surface contributes to the heterogeneous nucleation of acicular ferrite by small lattice misfit.

In this study, the effect of grain size on solidification cracking susceptibility of Type 347 stainless steel was investigated by using U-type hot cracking test with developed in-situ observation method and the effect of grain size of the weld metal on critical strain for solidification cracking was evaluated quantitatively. The grain size of the weld metal varied from about 70 to 210μm by changing in the grain size of the base metal. Consequently, it was concluded that according to the measurement of CST (Critical Strain Temperature), solidification cracking susceptibility increased with an increase in grain size of weld metal. Moreover, the area of the grain boundary for each unit area decreased when the grain size increased, and then strain which was applied for grain boundary increased and then critical strain for solidification cracking of grain boundary decreased with grain size. Therefore, it was guessed that the solidification cracking susceptibility increased with an increase in grain size.

Finite element simulation of welding process has been wildly employed in engineering where welding deformation and residual stress are considered. One big problem during simulation is the severe demands of capacity of hard disk and computation time, especially for large structures with long weld length or multi-pass welding. To minimize the number of unknowns in finite element model, a rezoning technique is developed to simulate the multi-pass welding process. The local nonlinear zone around the current pass is modeled with a dense finite element mesh, while the region of other passes and nearby is modeled with a coarser mesh. Then the model is redefined layer by layer to represent the filling of the welding pass and the motion of the weld arc. The rezoning procedure is implemented using a reverse mapping algorithm with Newton method to solve the high order simultaneous equations. Three-dimensional finite element simulations with and without rezoning technique are performed to obtain transient temperature, welding deformation and residual stress. By comparing the results of the model, computational efficiency and accuracy of the proposed method is confirmed.

The heat source properties of TIG arc strongly depend on composition of shielding gas. For example, since the arc column is constricted due to low electrical conductivity of the helium arc, heat flux onto a base metal in case of helium TIG arc is higher than that of argon TIG arc. The heat source properties can be controlled also by current waveform. Pulsed TIG welding is suitable for back-bead welding and thin plate welding, because the heat flux onto the base metal can be controlled by adjusting peak / base current ratio and frequency. A number of results on experimental and theoretical investigations of the heat source properties of DC TIG arc have been reported. However, those of pulsed TIG arc are still not fully understood because of the complexity of the phenomenon. In this paper, numerical simulation result of the heat source properties of pulsed TIG arc for various shielding gas composition will be reported. As a result, it was found that the heat flux increased immediately after transition from base current to peak current because of the thermal pinch effect. Furthermore, temperature distribution of helium arc changes following change in current immediately due to high thermal conductivity