Since Tungsten Inert Gas (TIG) arc enables to produce arc plasma with high energy density, it is suitable as a heat source especially for processes to require concentrating the heat input at a point. On the other hand, Tube Cathode Arc (TCA) to be a kind of TIG produces the arc plasma by introducing shielding gas through the central hole of the tube cathode. In the present paper, basic heat source property of argon TCA at the atmospheric pressure was numerically analyzed and compared with that for the conventional TIG arc. As a result, it was found that TCA is suitable for processes such as brazing or buildup, since it enables to heat a target material uniformly by controlling inner shielding gas flow rate. Furthermore, TCA is expected to realize high-speed welding because the arc pressure to a base metal reaches only 30% of TIG due to lower current density.

According to the specification for Highway Bridges, the preheat temperature is determined by thickness of plate, PCM and diffusible hydrogen. The standard of the minimum preheat temperature has been determined based on the results of y-groove weld cracking test. Therefore, it is considered to require the superfluous preheat temperature, when this standard is applied to the fillet welding. However, the discussion of preheat temperature with the results of fillet weld cracking test has been still insufficient. In this paper, to discuss the standard for fillet welding, we examine the cold cracking susceptibility and the HAZ hardness distributions of the fillet welds using 50mm thick plates whose PCM ranges from 0.24% to 0.27% with various preheat temperatures. Using the present results, we propose a modified standard of the minimum preheat temperature for fillet welding.

A high-power fiber laser can produce an ultra-high peak power density of MW/mm² level corresponding to a focused electron beam, and is promising as one of the desirable heat sources for high-speed and deep-penetration welding. In this study, therefore, fiber laser welding and simultaneous observation with high-speed video cameras and X-ray transmission real-time imaging system were performed with the objective of obtaining a fundamental knowledge of welding phenomena and defect formation mechanisms in producing deep and narrow weld beads in Type 304 stainless steel. A 6 kW fiber laser beam through φ 100 μm feeding fiber or φ 300 μm process fiber could be focused at 1.1 MW/mm² or 120 kW/mm² peak power density, respectively. Consequently, deeply penetrated weld beads with narrow width were formed, and fully penetrated welds in 8 mm thick plate could be produced at less than 2 and 4.5 m/min for 120 kW/mm² and 1.1 MW/mm², respectively. In the case of the ultra-high power density, humping weld beads were formed at more than 6 m/min. This reason was attributed to the intermittent melt ejection from a small keyhole inlet and the subsequent melt flows on the narrow molten pool surface. On the other hand, in the case of the high power density, weld fusion zones with porosity, sound partial penetration welds and underfilled weld beads with large spatters were obtained at the welding speeds below 3 m/min, between 4.5 to 8 m/min and above 10 m/min, respectively. These formation mechanisms were interpreted by considering bubbles formation from a keyhole tip leading to pores captured by the solidifying front, the formation of a long molten pool suppressing and accommodating spattering, and the production of a narrow, short molten pool causing easy generation of spattering, respectively.

The possibility and the mechanical properties of Al/Cu dissimilar bonding with liquefaction by the reaction diffusion in air were investigated at the temperature range between the melting point of aluminum and the eutectic temperature of Al-Cu system equilibrium diagram. The surface of each specimen was prepared by a simple preprocessing with the polishing and the cleaning by acetone. The Al/Cu dissimilar joint could be obtained by this bonding process in air. The bonding pressure is an important factor for this bonding process. The increment of bonding temperature and the decrement of oxygen concentration of air also promote solid state diffusion at Al/Cu interface. All bonded specimens in this study were fractured by the brittle fracture mode like cleavage fracture. The phases observed mainly in fracture surface is θ phase. The tensile strength of specimen bonded in air is similar with that of specimen by the diffusion bonding in vacuum.

Ni-base single crystal superalloy CMSX-4 has been widely used for turbine blades and vanes because of its superior heat-resistance. However, since they are usually used under severe environments, cracks sometime occur. Since this material is so expensive, there exists strong demand for repairing technique which enables to form singlecrystal in the repaired region to prevent deterioration of fatigue resistance. Therefore, in this study, the applicability of repairing technique utilizing iso-thermal solidification treatment with using Ni-Cr-B and Ni-Cr-Si alloys as filler metals to cracks occurred in single crystal superalloy CMSX-4 was investigated. The microstructure and crystal orientaion in the repaired regions under various heat treatment conditions were examined by SEM and EBSP. SEM and EBSP analyses have revealed that isothermal solidification and single-crystallization in the repaired region was achieved by repairing with using Ni-4Cr-2.5B filler metal under the condition of 1448K×176.4ks. The same result could be obtained also by using Ni-4Cr-10Si filler metal under the condition of 1473K×90ks. Creep rupture test for the repaired regions with Ni-4Cr-2.5B and Ni-4Cr-10Si filler metals has shown that the creep rupture strentgh of repaired region with Ni-4Cr-10Si filler metal reached about 2/3 of that of the base metal.

Ni-base single crystal superalloy CMSX-4 has been widely used for turbine blades and vanes. However, since they are used under severe environments, the blades sometime suffer from erosion. Therfore, it is of great interest to develop repairing techniques that allow the life of single crystal turbine blades and vanes to be extended. So, in this study, the applicability of repairing techniques of the eroded blades by directional solidification treatment was investigated. Ni-Cr-B and Ni-Cr-Si filler metals were used as built-up repairing for the eroded region of the blade. The built-up repairing treatment was developed by utilizing unidirectionally solidification of a repaierd region under temperature gradient using a vertical type furnace with a movable heater. Microstructure and crystal orientaion in the repaired regions under various heat treatment conditions were examined by SEM and EBSP. The microstructure of the repaired region could be classified into several zones; eutectic zone, normally solidified zone, isothermally solidified zone and substrate. The solidification morphology of the normally solidified zone changes from dendoritic to planar with decreasing the moving speed of the heater. The single-crystallization could be achieved in the isothermally solidified zone and the normally solidified zone when its solidification morphology was planar. The thickness of the built-up region of about 2590μm can be obtained when the moving speed of heater is 0.28μm/s using Ni-Cr-Si filler metal.

To improve the quality of a joint composed of metallic micro-structural parts, a novel resistance brazing method called the "two-step resistance brazing method" has been developed. The target application of the two-step resistance brazing is the fabrications of medical devices and micro-machines. In this method, resistance brazing is completed in two steps. The first step is performed with relatively low electric energy input so that the uniformity of the surface contact condition between two parts can be improved due to local melting and subsequent solidification, and so that the optimal energizing condition for the second step of resistance brazing can be determined through the electric current data acquired during the first step. The second step is performed with the determined electric energy input so that brazing filler metal between two parts can be melted adequately.
To prove the efficacy of the two-step resistance brazing method for joining metallic micro-structural parts, type 304 stainless steel wire with a diameter of 100μm was butt-joint brazed with gold and copper brazing filler metal plated individually on each surface of the base metals. The plated filler metals were alloyed due to the Joule's heat, resulting in the formation of an interlayer between the base metals. All the brazed joints which fractured in the base metals but not in the joined region during tension testing had an interlayer of gold-rich alloy including dispersed dendrites, and consequently the tensile strengths ranged from 624 to 707 MPa. The high strength of the brazed joints is caused by the less deterioration of the base metals during the two-step resistance brazing.

Al/Cu bonding in the air can be accomplished at the temperature range between eutectic temperature of Al-Cu system and melting point of Al. The change of microstructure in Al/Cu bond was investigated by SEM observation and EPMA analysis. The observed specimens were obtained by the difference of displacement of bonded specimen. Mushroom shape of L is observed in Al side at the first step of the initial stage of bonding process. The phase in Cu side begins to be observed significantly at the second step of the initial stage. The time difference is brougt by the difference of each diffusion coefficient. The ratio of bonding area of this bonding method depends on the formation ratio of the phase in Cu side. L phase begins to be removed by the bonding load after the initial stage. L, α, θ(Al₂Cu) and η₂ (AlCu) are observed in the bond as the major microstructure of the bond at room temperature. Formation mode of the pre-existed θ layer affects the shape of next θ grain formed from liquid. Al oxide film can glow by the absorption of oxygen from the resolved Cu oxide film. The diffusion route is made by the separation of Al and Cu oxide film at the first step of the initial stage. Reaction diffusion occurs through the area where the separation of oxide film occurs and forms liquid in bond during this bonding process.

The effect of martensitic transformation start temperature and transformation expansion strain on welding angular distortion of welded joints was investigated. T-joints were fabricated with welding wires of which Ni and Cr content were varied and the residual angular distortion of the T-joints was measured. The residual angular distortion decreased when the martensitic transformation start temperature increased in the range from 100°C to 405°C. A numerical simulation with consideration of the effect of phase transformation was performed to investigate the relationship between the transformation expansion of the weld metal and the angular distortion of a T-joint. In order to clarify the effect of the transformation temperature and the transformation expansion strain separately, numerical simulations were carried out with hypothetical properties of transformation expansion. Evaluated were the relationship between the reduction of the angular distortion by the transformation expansion of the weld metal and properties of transformation expansion: transformation start temperature and transformation expansion strain. The reduction increased when the transformation temperature increased under hypothetical conditions that the constant transformation expansion occurred regardless of transformation temperatures. It also increased when the transformation expansion strain increased. In actual materials, as the transformation expansion strain is dependent on the transformation temperature, an optimum temperature range for reducing angular distortion exists.

The effect of heat input parameter on residual angular distortion and reduction by transformation expansion of weld metals was investigated by means of numerical simulation. The combinations of heat input Q and plate thickness h were varied to change the heat input parameter Q/h². Consequently, the residual angular distortion could be parameterized by the heat input parameter. On the other hand, the reduction of the angular distortion by the transformation expansion was not parameterized by heat input parameter. A parameter b/h, ratio of penetration depth to plate thickness was proposed to consider the effect of welding condition on the reduction of angular distortion. Reduction of angular distortion was linearly proportional to the ratio of the penetration depth to the plate thickness (b/h).

Fillet welded T-joints with the low-transformation-temperature welding wire was fabricated under conditions that the generation of the angular distortion was prevented. In one condition of restraint, the T-joint was welded with its flange plate fixed by clamping tools during welding and released after the joint was cooled to the room temperature. In the other condition, rib-plates were arranged orthogonal to the welding direction to prevent the generation of the angular distortion. Numerical simulations of these two joints were also carried out. Results of numerical simulation showed that the transformation expansion of the weld metal was effective for reducing the angular distortion even under the restraint conditions.

Unidirectional solidification process of weld metal for low-carbon steel was characterized by using time-resolved X-ray diffraction system. Ultra bright undulator beam was used as the probe. Since the incident undulator beam was both monochromatic and highly collimated, the reflection plane that satisfied the Bragg condition could reflect the crystal configuration in fusion zone very well. It made it possible crystal orientation analysis under rapid cooling rate. Then the growth morphology and phase transformation for dendrites in fusion zone were directly analyzed. Furthermore it was suggested that nucleation site in solid-state transformation could be elucidated using laser scanning confocal microscopy. It made it possible for complicated microstructure classification to be clear. Developed characterization techniques have potential as a tool for comprehensive analysis for weld solidification and phase transformation in order to progress weld mechanical property.

The formation of stable back beads in the first layer weld during one side multilayer welding is important to achieve high quality welded metal joints. The authors employed the switch back welding method for V groove joints without backing plates. In this welding method, the torch is moved backward and forward like switch back. First, the torch is moved forward and backward with high speed to discharge the arc to the root edges of the base metal. Next, the torch is moved forward with slow speed to form the weld bead. The mentioned torch motion is repeated. The authors tried to apply the switch back welding method to the butt welding to joint the disc with the flat metal plate without the backing plates. The tip of the electrode wire is changing due to a curve of its wire extension in accordance with a rotation of the torch axis. Even if the teaching is achieved before the welding, the weaving center is shifted from the gap center. The controller of the seam tracking was designed to prevent steady state error by using the B spline function to estimate the variation of the welding line. The burn through takes places during the welding. In order to investigate temperature distribution, the numerical simulation of the switch back welding was carried out. From its result, the gap was changed due to the heat distortion.
During the welding, the authors propose a method to detect the gap by processing the weld pool image taken with a CCD camera. In the robotic welding system, the switch back welding is carried out by using giving the gap and the weaving center at the beginning of the welding. The performance of this welding method was verified by carrying out the welding experiments.

Back heating method is proposed for angular distortion decreasing technique. Back heating is expected for angular distortion decrease to the following two effects; that is, bending in the opposite direction and homogenization of temperature distribution through thickness direction. Best condition of temperature distribution control for angular distortion decrease has been evaluated with numerical simulation. Back heating source is appropriately arranged to welding heat source, then, angular distortion is decreased more effective than the above-mentioned effects, and perfectly decreased by back heating of ten percent to welding heat input. In back heating, angular distortion can be decreased by the change in generation behavior of inherent strain according to nonconventional temperature history. Angular distortion decreasing technique is effective in any condition concerning heat input parameter and the best condition concerning multiple-heat-source arrangement is decided to be dependent on the size of mechanical molten pool.

Recently, the utility of VLFS (Very Large Floating Structures) attracts great deal of public attention. In order to construct VLFS, however, some technical problems remain. Because it is too large to build at any building dock or other site on the ground, the construction by joining floating units together on the sea is inevitable. Some advanced engineering for constructing operations on the sea are necessary, such as maintaining precise measurement of each unit, joining units in waves, underwater welding techniques.
Demonstration with a large floating model was carried out by Technological Research Association of Mega-Float (TRAM) from 1995 to 2001. During construction of floating model, we found out it is important to estimate the welding distortion precisely, because it causes distortion of VLFS itself and the misalignment among units. Since complete restriction of the distortion at sea is impossible, precise estimation of the distortion become essential for super-sizing VLFS. In order to solve this problem, we had trial estimation by FEM analysis.
In this paper, concerning on welding distortion, comparisons between experimental results and estimation by FEM analysis are shown. As a result, we show it is possible to estimate the welding distortion of VLFS under construction precisely. Moreover, we show FEM analysis can suggest the procedure to make geometrical inaccuracy and misalignment among units minimum by controlling the shapes of units and their placing position.

This paper deals with the thermal distortion and thermal stress of very large floating structures caused by sunshine during joining floating structures each other. In case of constructing very large floating structures, we have to join many floating units each other. The floating structure on the sea is deformed easily by thermal distribution in the structure. At daytime, the deck plate of the floating structure is heated by sunshine and the temperature of the deck plate rises about 30 degrees higher than that of the bottom plate. This thermal distribution causes thermal distortion and induces thermal stress in the floating structure. This thermal distortion causes not only geometrical inaccuracy of the floating structure but also results in wide opening between the bottom plates of the floating units. Therefore, precise estimation of this thermal distortion is very important for joining operation. In case of construction of the floating airport model, we estimated this thermal distortion and thermal stress by 3D FEM analysis and we decide the joining procedure by using analyzed results.

Understanding and controlling solidification and phase transformation process of weld metal is essential for forming the microstructure with superior mechanical property. Recent evolution of analysis technique makes for solidification and phase transformation process to be in-situ analyzed, in direct and reciprocal lattice space.
In the present work, unidirectional-solidification and phase transformation in the weld metal of commercial pure-titanium in Gas Tungsten Arc welding was in-situ observed by using Time-Resolved X-Ray Diffraction system with two-dimensional pixel detector. An undulator beam was used as a probe. Larger diffraction area could be detected in the time-resolution of 0.05 seconds, in unidirectional solidification and subsequent phase transformation process of pure-titanium weld metal. Furthermore, the microstructure formation during β-α phase transformation was in situ observed with High temperature Laser Scanning Confocal Microscopy.
The crystal configurations in unidirectional solidification of weld metal and rapid change of phase ratio in reconstructive phase transformation were clearly analyzed.

This paper deals with the welding properties of shielded metal 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 shielded metal arc welding has been mainly investigated. As a result, the optimum arrangement of projection points on flare groove joint which shows less cavity formation and high joint shear strength has been made clear.

The Friction Stir Welding (FSW) with heating of A5052-H34 aluminum alloy, which is a narrow welding condition material in FSW, was conducted to clarify the improvement of welding rate by heating effect. At first, the FSW with heating on temperatures of 150 and 300°C was examined at welding rates of 600 to 900mm/min. And the joint integrity was evaluated by observation of appearances of welded part and its cross section. Good joints in FSW with heating were obtained at welding rate up to 700mm/min for heating of 150°C, and up to 900mm/min for heating of 300°C. In addition, the hardness profile and tensile strength of joints by FSW with heating showed a good level as much as non-heating one with low welding rate. As the result, welding rate in FSW with heating could expand a conventional welding rate. Therefore, the improvement of welding rate in FSW with heating clarified would be feasible.

A numerical model to predict the solidification microstructure and solidification cracking susceptibility of austenitic stainless steel weld metal is proposed by taking into account the non-equilibrium theory. It was predicted using the developed numerical model that Fe-20Cr-14.4Ni, Fe-20Cr-12.7Ni, Fe-20Cr-11.5Ni and Fe-20Cr-9.8Ni weld metals solidified as A mode (L→L +γ→γ), AF mode (L→L +γ→L +γ+δ→γ+δ), FA mode (L→L +δ→L +δ+γ→δ+γ) and FA mode, respectively. Also, it was found that the effect of Ni content on the solidification cracking susceptibility of Fe-20Cr-(9.8-14.4)Ni weld metal, determined via Trans Varestraint testing, agreed with the results calculated using the model. Furthermore, it was found that the content of δ ferrite at just solidifying calculated using the model agreed with the results measured using Fe-20Cr-(11.5-14.4)Ni weld metals quenched in liquid Sn. These agreements support the validity of the developed numerical model.

The significance of fracture toughness test results of laser welds of structural steels has been discussed. In the laser welds of structural steels, the weld metal corresponds to a locally hardened zone and generally has a reduced fracture toughness. Nevertheless, the weld metal toughness measured in the toughness test, where a notch is located in the center of laser welds, is essentially being overestimated. This is due to a development of plastic zone in the base metal region adjacent to the hardened welds. However, such "reduced fracture toughness" of the laser weld metal does not necessarily mean a poor fracture performance of the welded joints. The welded joints subjected to tension exhibit a significant amount of constraint loss near the near crack-tip, whereas the fracture toughness specimen holds a constrained stress state due to bending loading. Besides, the plastic deformation of the base metal region adjacent to the welds provides a shielding effect on the CTOD of the crack in the weld metal. The equivalent CTOD concept, that links the fracture toughness specimen and the structural component at the same level of the Weibull stress, is applied for the evaluation of the constraint loss in the laser welded joints.

The formation of stable back beads in the first layer weld during one side multilayer welding is important to achieve high quality welded metal joints. The authors thus employed the switch back welding method for welding of V groove joints, from 2mm to 4.5mm root gap, without using backing plates. In this method, a power source, a wire feed unit and robot manipulators are the computer based cooperative control. In this robotic welding system, there are 4 personal computers, which control the welding robot, the digital welding power source and the wire feeder unit. Each unit is connected with Ethernet and UDP, User Datagram Protocol. By using this system, the authors investigate the relationship between the torch weaving and the pulsed welding current. For this purpose, the arc behaviors are taken with a high speed video camera. By analyzing these images, the waveform of the pulsed welding current is determined, i.e., in order to get a good back bead, it is important to discharge the arc to the root edge and the groove surface. The weaving frequency is up to 3Hz so that the switch back welding is applied to V groove backing less welding by using conventional welding robots.In the weaving frequency of 10Hz, 5Hz and 2.5Hz, the relationship between the pulsed welding current and the back bead shape are investigated. The suitability of the welding conditions for each root gap was verified by observation of the arc, molten pool and external appearance of back beads. A good quality of the welding was obtained under 2.5Hz weaving frequency. The feed-forward controller was designed to get a good back bead regardless of the gap variation. In order to verify the validity of the controller, the welding experiments were carried out. A wide and stable back bead was obtained regardless of the gap variation from 4.5 to 2mm.

Fracture path deviation (FPD) often occurs in standard V-notch Charpy (STD-Cv) test for laser welds because plastic deformation takes place preferably in the soft heat affected zone adjacent to the very narrow and hard weld metal. It is very important to measure the toughness of the laser welds by the test method without FPD. Among several methods, which have been developed to avoid the FPD, the side grooved Charpy (SG-Cv) test is considered to be the most simple and effective way. A decrease in the absorbed energy and an upward shift of the energy transition curve are observed in the SG-Cv test, compared with the STD-Cv test. In the previous paper, the difference between these two Charpy tests was discussed by analyzing fracture appearance. In this study, the lateral contraction of the Charpy specimen is used as a more simple measure to clarify such difference in Charpy test results. The method of evaluating the STD-Cv toughness from the SG-Cv test results is proposed based on the correlation between the lateral contraction and the Charpy energy. It is demonstrated that the proposed method gives a good estimation of STD-Cv energy.

Recently, the demands of dissimilar metal joint have been increasing in industry, especially in automobile industry. Therefore, research and development of dissimilar metal joining is accelerated. The Laser Roll Welding has been developed by M. Kutsuna and M. Rathod. Joining of dissimilar metals, such as Fe-Al and Fe-Cu has been studied. And titanium has superior corrosion resistance; it is used for the clad plate for splash zone of bridge pier, the sea-water plant, the condenser of a nuclear power plant and so on. It is well known that fusion welding of steel and titanium is difficult due to the brittle intermetallic compound formation. In the present work, the effect of process parameters on the formation of intermetallic compound at the interlayer was investigated to get a sound joint. The mechanical properties and microstructures of welded joint were also studied. The results of tensile shear test showed that the strength of joint increased with the decrease in interlayer thickness. When the interlayer thickness was less than 25 μm, tensile shear test specimens were failed in base metal of low carbon steel.

Ultrasonic Test (UT) method is the most appropriate method in many kinds of non-destructive test methods because of capabilities for detection and depth sizing of flaw. But it is generally said that the flaw detection in nickel base alloy welds is more difficult than that in ferritic steel welds because of dispersion and attenuation of the ultrasonic wave. Therefore, influences of welding conditions on flaw detectability and signal to noise ratio (SN ratio) of UT in nickel base alloy welds were investigated. Tested weld joints were prepared with multi-pass gas tungsten arc method. Partial weld joints were applied to weaving or magnetic stirring when that welded. Specimens having an electrical discharge machining slit in weld metal or in heat affected zone were tested by a longitudinal wave of 5 MHz in frequency and 45 degree in incident angle. Obtained results are as follows. Welded joints having columnar structures with the same growth direction showed high SN ratio that means good flaw detectability. Effective welding conditions, which make the columnar structure to be the same growth direction, were found to be the application of weaving or magnetic stirring. Same growth direction in columnar structures means same angle between the columnar structure growth direction and the ultrasonic wave propagating direction. Then, ultrasonic wave has almost same velocity in the weld metal, and the acoustic impedance will be almost same. High SN ratio may be caused by above phenomena. The SN ratio of weld joints by different base metal can be well explained by the difference of acoustic impedance at the boundary columnar structure and base metal.

GMA welding under pure Ar shielding gas atmosphere (pure Ar-MIG arc welding) is effective to obtain a high-quality welded joint. However, the pure Ar-MIG arc welding cannot be applied practically because of arc instability. In order to perform stable pure Ar-MIG welding, a new welding process has been required.
We analyzed the cause of arc instability by high-speed video camera system. From video images, it was found that long column of liquid metal (CLM) formed at melting wire tip moves irregularly. Therefore, it was expected that unstable MIG arc is due to the behavior of the CLM, although the cathode spots moves irregularly in front of weld pool.
Next, new welding process was developed, in which the small amount of oxygen can be added near the melting wire tip through the wire contact tip. In the developed process, the relationship between arc instability and oxygen content mixed in shielding gas was discussed. In the case of the introduction of small amount of oxygen, the length of CLM becomes effectively short and arc becomes stable, but the cathode spot moving area is not decreasing enough. Therefore, it was concluded that pure Ar-MIG arc instability depended on the CLM behavior.
Finally, the oxygen content in weld metal was able to reduce to less than 100 ppm in the process, and then high ductility weld metal was successfully obtained with low transformation-temperature welding wire.

The effect of the shielding gas concentration on the weld shape is studied for the moving bead-on-plate TIG welding of SUS304 stainless under He-O₂ mixed shielding. The small addition of oxygen to the helium base shielding gas can effectively adjust the oxygen content in the liquid pool and the weld shape. Oxygen is a surface active element for stainless steel. When the oxygen content in the liquid pool is over the critical value, around 70ppm, the weld shape suddenly changes from a wide shallow shape to a deep narrow shape due to the change in the Marangoni convection from the outward to inward direction on the liquid pool surface. Weld shape variations at different welding parameters including welding speed, welding current and electrode gap under pure He and He-0.4%O₂ mixed shielding are systematically investigated. The results showed that the final TIG weld shape depends to a large extent on the pattern and strength of the Marangoni convection on the pool surface, which is determined by the combinations of the oxygen content in the liquid pool, temperature coefficient of the surface tension (dσ/dT) and the temperature gradient on the pool surface (dT/dr). Different welding parameters will change the temperature distribution and gradient on the pool surface, and thus, affect the strength of the Marangoni convection and final weld shape.

In recent years, some fatigue tests have been performed to investigate the fatigue behavior of friction stir butt-welded joints in aluminum alloy. However, the fatigue behavior of the joints with thick plate of around 10 mm rarely has been investigated. In addition, the influence of the inclined angle of friction stir welding (FSW) line to applied stress has not yet been clarified in the previous studies. In this research, fatigue tests were carried out on four types of model specimens. The specimens have FSW of inclined angles of 0°, 30°, 60° and 90° to perpendicular direction of the applied stress. The observations of fatigue failure condition and fracture surfaces indicate that fatigue crack mainly initiates and propagates at the heat affected zone (HAZ) when the inclined angle is 0°. For the angle of 30°, 60° and 90°, the cracks tend to initiate at the stirred zone (SZ) and then the cracks propagate through the thickness of the specimen in the direction of perpendicular to the applied stress. The tests results shows that the change in the fatigue strength is small, despite of the different FSW line angles.

Friction stir spot welding of thixomolded and as-cast Mg-alloys AZ91 is investigated. The temperature cycles within the stir zone and in the TMAZ region are examined using thermocouples. The measured temperatures in the stir zone range from 437°C (710K) to 460°C (733K) (0.98Ts, where Ts is the solidus temperature in degrees Kelvin) in AZ91 spot welds. The stir zone microstructures in AZ91 spot welds typically comprise fine equiaxed α-Mg grains. However, spot welds made at room temperature with a thermal insulating layer introduced between the bottom of the AZ91 test specimen and the upper surface of the steel anvil showed evidence of eutectic segregation, liquid penetration and Liquid Penetration Induced (LPI) cracking in the stir zone.

In order to verify the validity of the developed numerical model to predict the solidification microstructure of austenitic stainless steel weld metal proposed in the previous report, spatially resolved X-ray diffraction measurements using synchrotron radiation have been carried out for Fe-20Cr-11.5Ni and Fe-20Cr-12.7Ni weld metals, quenched in liquid tin. X-ray diffraction analysis of Fe-20Cr-11.5Ni quenched weld metal, solidifying in FA mode (L→L +δ→L +δ+γ→δ+γ), showed that the leading γ phase crystallized in a eutectic growth mode down to a temperature drop of 6 K from the initiation of solidification. Also, from X-ray diffraction analysis of Fe-20Cr-12.7Ni quenched weld metal, which solidified in AF mode (L→L +γ→L +γ+δ→γ+δ), it was found that the leading δ phase crystallized in a eutectic growth mode within the temperature drop range between 18 and 24 K from the initiation of solidification. The crystallization temperatures predicted by the developed numerical model for leading γ and δ phases in Fe-20Cr-11.5Ni and Fe-20Cr-12.7Ni weld metals agreed with experimental data. These agreements support the validity of the developed numerical model. Furthermore, time-resolved in-situ observation of welding solidification process using synchrotron radiation have been carried out for Fe-20Cr-12.5Ni and Fe-20Cr-11.4Ni weld metals, it was found that γ and δ phase crystallized down to a temperature drop within about 47 K and 55 K from the initiation of solidification respectively. These results also coincided with the calculated results for Fe-20Cr-12.5Ni and Fe-20Cr-11.4Ni weld metal using the developed numerical model.

In this study, the fatigue behaviour of friction stir welded (FSW) joints of AZ61 magnesium alloy was investigated. AZ61 plates were joined with a welding speed of 100 mm/min and a tool rotational speed of 1200 rpm. The microstructure of the weld zone was classified into three regions: stir zone (SZ), thermo-mechanically affected zone (TMAZ) and heat affected zone (HAZ). In the SZ, fine equiaxed grains resulting from dynamic recrystallization were observed, where Vickers hardness was higher than in the parent metal. Fatigue tests were conducted under fully reversed axial loading (R = -1). The fatigue strength of FSW joints was higher than that of the parent metal, which could be attributed to the fine grain structure in the weld zone. Fatigue crack initiated at the boundary between SZ and TMAZ on the advancing side of the bottom plane. Such predominant crack initiation at the boundary was due to the texture developed during the welding process.

In this study, new evaluation method of local strain for the fracture assessment of the crack in the strain concentration was proposed. This method based on the relationship between the definition of local strain applied to the assessment method for brittle fracture of steel-frame structure WES2808-03 and liner fracture mechanics.
2-dimensional FE-analyses of the cross weld joint models with weld toe crack were conducted. Local strains at the stress concentration were calculated by the proposed method and CTODs were measured. And the relationships between local strains and CTODs were discussed. These results were compared with the CTOD design curve employed in WES2805-97 and the Level 2 FAD employed in BS7910-99, and the accuracy of assessment curves was investigated. The results demonstrate that the CTOD design curve gives slightly conservative estimation but FAD gives unsafe estimation in the case of Lr > 1.
The relationship among stress concentration factor and crack size and K-value was also investigated, and the suitability of estimation equation of strain concentration factor employed in WES2805 was discussed. The equation gives smaller strain concentration factor, but the influence of the results on the CTOD estimation by CTOD design curve is not serious.