The evaluation method for the microstructural stress-strain relationship of materials using a triangular pyramidal indenter is proposed in order to investigate the mechanical properties of steels and weld zone. An existing evaluation method using a ball indenter is correspondingly applied to the evaluation method using a triangular pyramidal indenter because the strain distribution under the indenter or the indentation curve on the unloading process between the ball and pyramidal indentation has a similarity. A corresponding ball indenter whose projection area is equal to that of the triangular pyramidal indenter is used to replace the triangular pyramidal indentation to the ball indentation, and the representative stress and strain that express the complicated deformation under the indenter are determined. The stress-strain relationships of single phase steels in microstructural size are estimated by the proposed method, and averagely correspond with those measured by macro-tensile tests. The difference of the stress-strain relationships due to the difference of the crystal orientation of each grain is possible to be negligible in this method. It is expected to clearly estimate the difference of the stress-strain relationship of each phase in such as dual phase steels by the proposed method.

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In this study, the efficacy of pre-heating using solid friction heating was numerically investigated to improve the heating process of the friction stir welding with heating, which made the high speed welding by pre-heating the whole aluminum metal. At first, the solid friction heating by advancing rotating heating tool generated solid frictional heat on tool contact area was suggested. Then the temperature variation with time of aluminum metal under the solid friction heating was calculated based on the finite difference approach. As the result, the temperature calculated showed that the peak temperature increased as increasing contact area of heating tool and decreased with increasing advancing speed of heating tool. And the temperature by the heating tool rose sufficient to replace partially the heating process in the friction stir welding with heating. Furthermore, the possibility of the heating process using the solid friction approach for the friction stir welding with heating was discussed by comparing calculated temperature with weldable conditions.

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The effect of oxide inclusion compositions on microstructure and notch toughness of weld metal for HT780MPa class high strength steel was investigated. Three different types of weld metals were prepared by varying deoxidant elements. Intragranular bainite microstructure and dispersed oxide inclusions were observed and quantified by an optical microscope and electron backscattered diffraction (EBSD) analyses to reveal affectors of notch toughness.
It has been found that FATT of the weld metal depends on size of intragranular bainite (acicular ferrite) nucleated on oxide inclusions and that the upper shelf energy is affected by density of dispersive oxides inclusions. It has also been clarified that all oxide inclusions, which contribute to intragranular transformations, consisted of titanium-rich crystalline phases and manganese-rich amorphous phases. By detailed FE-TEM observation, the crystalline phases was identified as MnTi₂O₄ having B-N orientation relationship with the intragranular bainite. It is considered that the reason the bainite have the orientation relationships both MnTi₂O₄ and austenite is that the MnTi₂O₄ is generated at the interface between the amorphous phases and austenite previous to the intragranular transformation.

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To form curved surface, in-plane strain is introduced into a plate by line heating, press working or laser heating in shipbuilding or sheet metal working like nose shape forming for bullet trains. Laser forming could be a potential useful method for sheet metal forming as well as press working.
When forming a curved surface with thin plate in some steps, it is thought to be useful to measure in-plane strain distribution before the forming is finished, to know whereabouts and amount of shortage of in-plane strain. The process to measure in-plane strain in an actual curved surface seems still not to be reported.
In this report, in-plane strain distributions existing bowl shape thin plate made by laser heating is investigated with geodesic lines drawn numerically on the surface. The in-plane strain is calculated with distance change of two geodesic lines. This in-plane strain distribution agrees with a strain distribution measured by shrinkage of the plate when the plate length is long. When plate length is short, these strains do not agree with each other. In this case, existence of some elongations cancelling effective in-plane strains appears in width change of the plate. In a result, in-plane strain measured with geodesic lines is useful to indicate performance of curved surface forming process.

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When galvanized steel sheets are closely overlapped and welded by laser lap-welding, a large amount of molten metal spatters, resulting in a poor surface appearance of the weld and weakened strength of the welded joint, as compared with that of cold-rolled steel sheets. Whereas, in the case of Al-coated steel sheets, even when closely overlapped and welded by laser lap-welding, no spattering occurs. Thus, a good surface appearance of the weld is obtained, but the welded joint has lower strength. In both cases above, it is known that if a clearance of about 0.1mm is provided between steel sheets, laser lap-welding produces a good surface appearance of the weld and the welded-joint strength equal to that of cold-rolled steel sheets.
This report discusses specifically how, in laser lap-welding of overlapped Al-coated steel sheets, Al of the coated layer comes to enter the weld metal, also specifically how to reduce the joint strength, as well as what behaviors of Al when a clearance is provided between steel sheets. When steel sheets are closely overlapped and welded, Al becoming molten on the base-metal side of the bond of the overlapped face becomes swallowed up by the bath streams of the molten pool, flowing into the molten pool, then forming the Fe-Al intermetallic compound, while not being sufficiently stirred. It is considered that when subjected to the tensile shear test, the Fe-Al intermetallic compound starts to fracture, thereby causing a partial loss of the weld metal and a reduction in the joint strength. On the other hand, when a clearance is provided between steel sheets, it may be inferred that the fusion Al on the base-metal side of the bond stays in place without flowing into the molten pool, consequently not forming the Fe-Al intermetallic compounds within the weld metal.

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The technique which can measure the transient welding deformation directly is very important to investigate the mechanism of welding deformation. In this study, in-situ displacement measurement method using digital camera is self-developed. This system is non-contact type and it doesn't need any complicated optical systems but it can measure the in-situ displacement over the full-field in high accuracy by using digital image correlation technique. Therefore, it is considered to be useful and easy to apply to practical problems. The number of measuring points that can be obtained at a time is more than 10 million. It is the same as the available pixels of the digital camera. Furthermore, since the active light source is not necessary in this system, the influence of the fluctuation of the atmosphere caused by the high temperature area is small.
In this study, the detail of the proposed system is performed and it is applied to the transient in-plane deformation problem under very high brightness lightened by welding arc. Through the comparison between the experiment and Thermal-elastic-plastic FE analysis, the validity of the transient transverse shrinkage distribution which is measured by proposed system is verified. The residual deformation is also investigated to check the accuracy and usefulness of the proposed system.

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A plasma arc is a kind of the plasma torch. Since it has a water-cooled copper nozzle in front of the cathode to constrict the arc plasma, the arc plasma with high energy density and high arc pressure can be produced. Therefore it is suitable especially for welding and cutting. The heat source property of the plasma arc can be controlled by an input current waveform. DC arc is generally applicable to welding process for almost all kind of metal except for aluminum and magnesium. In case of aluminum and magnesium AC arc is utilized to remove an oxide layer on a base metal surface. However, the energy source property of AC plasma arc is still not fully understood because of the complexity. In this paper, numerical simulation result of heat source property of AC plasma arc welding is reported

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In the present paper, a TIG arc in helium or argon is modeled taking into account the contamination of the plasma by the metal vapor from a stainless-steel weld pool. Iron, chromium and manganese are considered as the metal vapor species in this model. A viscosity approximation is used to express the diffusion coefficient in terms of the viscosities of the shielding gas and the metal vapor. The time-dependent two-dimensional distributions of temperature, velocity and metal vapor concentrations of iron, chromium and manganese are predicted, together with the weld penetration as a function of time for a 150 A arc at atmospheric pressure, for both helium and argon shielding gases. The distribution of the metal vapors depends on the diffusion term and the convection term. Due to the cathode jet, the convection term has a strong effect. Consequently, it is found that the metal vapors expand in the radial direction and are concentrated around the weld pool surface. The concentration of manganese vapor is larger than those of iron and chromium vapors, despite the fact that the proportion of manganese in stainless steel is significantly smaller.

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Pulsed arc welding is used for improvement of welding quality and stability. However, welding defects readily occur when welding is used under inappropriate parameters. Properties of arc and energy balance are very important for TIG welding. However, the theoretical elucidation of a transient arc has not been described quantitatively. As described in this paper, the temperature distribution is calculated under consideration of Fe vapor, which changes the energy balance of the arc and anode surface condition as a function of current. Consequently, expansion and contraction of the high-temperature area depends on the Fe vapor. During the peak current period, the increment of the high-temperature area slows when Fe vapor is a contaminant. During the base current period, decrement of the high-temperature area occurs rapidly when Fe vapor is a contaminant. The increment of radiation power and electrical conductivity alters properties such as the temperature and the axial flow velocity. Effects of the Fe vapor depend on its distribution because the temperature distribution depends on the current waveform. The relation between the current waveform and Fe vapor distribution was assessed. Three calculation conditions of the peak and base current were calculated as parameters. During the peak current period, the Fe vapor distribution differs in the case of current waveform. However, it is almost identical during the base current period. For pulsed TIG welding, metal vapor distribution changes instantaneously; its effects for properties of transient arc change as well.

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Fe nanoparticles were prepared by DC arc plasma method under Ar, Ar-50%H₂ or H₂ flow. The arc plasma was well constricted due to thermal pinch effect when Ar-50%H₂ or H₂ was used, and the generation ration of the Fe nanoparticles was significantly enhanced with the pinched arc. Our numerical simulation indicated that the Fe anode can be efficiently heated (close to boiling point of Fe) due to the thermal pinch effect, and thereby the generation ratio can be increased. Since the thermal pinch effect enhanced the arc flow velocity as well, the size of the resulting Fe nanoparticles was also influenced.

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This paper presents a study on a new method of welding penetration control for aluminum pipe in Tungsten Inert Gas (TIG) welding using omnidirectional vision-based monitoring of molten pool. As circumferential butt-welded pipes and the application of aluminum alloy in pipe welding that has been use in various industrial sectors requires new technique of welding monitoring process. For pipe welding using constant arc current and welding speed, the bead width becomes wider as the circumferential welding of small diameter pipes progresses. In order to avoid the errors and to obtain the uniform weld bead over the entire circumference of the pipe, the welding conditions should be controlled. This research studies the intelligent welding process of aluminum alloy pipe 6063S-T5 in fixed position using the AC welding machine. The monitoring system used an omnidirectional camera to monitor backside image of molten pool. A new method of image processing algorithm was implemented to process the captured image and to recognize the edge of molten pool. Back bead width as the result of detection was delivered into fuzzy inference system to control welding speed. From the experimental results it shows the effectiveness of the control system that is confirmed by sound weld of experimental results.

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The purpose of this study is to develop the ultra-high-speed GTA welding process with hot-wire system using pulsed current to heat filler wire. For precise control of the ultra-high-speed (over 5 m/min) GTA welding, it is necessary to understand deeply the ultra-high-speed welding phenomenon. The melting phenomenon of filler wire was observed in detail by a high-speed camera and filler wire temperature was measured precisely by a radiation thermometer. As a result, precise temperature distribution of filler wire and clearly visualized wire melting phenomenon was obtained.

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Control of the distance between the tip of an electrode wire and a base metal is important to obtain a good welding quality in spite of an arc fluctuation. Therefore, it is necessary to estimate the welding voltage relating to its distance in not only the steady state but also the transient state. For this purpose, this paper proposes neural network models which output the present welding voltage from the data relating to wire melting, such as past current, past voltage and past wire feed rate. Since performance of the neural network model depends on threshold functions, authors investigated the performance of the neural network models based on both sigmoid function and radial base function. To confirm the validity of these systems, fundamental experiments were carried out. In this paper, performances of the neural network were investigated in pulsed current welding and switch back welding the output data from the neural network were compared with the measured data. It was found that the neural network model based on the radial base function is useful than the sigmoid function to estimate the welding voltage in the switch back welding because of better responses in the transient state and smaller steady state error.

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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 V groove joints without backing plates. The burn through may take places due to the disturbance, such as the change of the base metal. If the tip of the electrode gets near the root edges, the arc discharged to the root edges of the groove. As a heat of the arc is given to the root edges, the root edges were melted enough and it is difficult to support the weld pool. In order to avoid the burn through, the weaving width was adjusted by detecting the state of the surface of the weld pool, i.e., the authors proposes the control method of weld pool. The validity of the weaving control method was verified by observation of the weld pool and the external appearance of back beads.

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In order to get a high quality of the welding, it is necessary to make a stable back bead and to melt metal plates. In narrow gap welding, it is difficult to control the bead height and the back bead simultaneously by using conventional welding. In order to solve this problem, authors proposed a switch back welding method, in which the welding torch was moving back and forth along the welding line. In the forward torch movement of the torch, the arc heat was given to the root edges to get a wide back bead. In the backward torch movement, the suitable bead height was formed. The traveling distance of the torch becomes key-factor to get a high quality welding result in this switch back welding. By carrying out the fundamental experiments, the numerical model in this welding was prepared. The numerical simulations were performed in the several kinds of the torch movement. The optimum welding conditions was found from simulation results. Its validity was verified by carrying out the welding experiments

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This paper presents characteristics of ionized gas metal arc processing which consists of a torch, a welding wire, shielding gases and two power sources. One of power sources ionizes the shielding gases before feeding into the arc region and another stably produces the arc with melting wire electrode. The degree of ionization in the shielding gas is well regulated by the primary power source, and then the secondary power source is able to control the arc devotedly, including the production and detachment of molten droplet at tip of the wire electrode. It is concluded that the ionized gas metal arc can make a stable processing of welding without spatters and fumes in comparison with the conventional gas metal arc processing. Furthermore, it is suggested that the ionized gas metal arc is very useful for not only welding but also thermal spraying for producing a fine coating of metal surface.

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New coaxial plasma GMA welding system has been developed on the purposes of improvement of weld bead, reduction of spatter and fume generation. Welding power sources of GMA and plasma, wire feeding equipment and coaxial plasma GMA welding torch are described in detail. The metal transfer of plasma GMA welding is observed and compared with that of pulsed GMA welding. Although metal transfer in plasma GMA welding and pulsed GMA welding is globular transfer, plasma GMA welding shows smooth metal transfer than that in pulsed GMA welding. Low spatter and low fume welding is realized. The plasma welding current shows high effect on the melting characteristic of Al-Mg alloy wire (A5183-WY), however it shows low effect on that of the mild steel wire (YGW-15). The effect of plasma welding current on bead shape and penetration is examined by bead on plate welding using 4.0 mm thickness Al-Mg alloy sheet and lap joints using 3.2 mm thickness mild steel sheets. With the increase of plasma welding current, the weld bead becomes wide and flat.

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Surfacing is an indispensable measure both to conserve as well as to generate worth. Different welding methods have been created over the time, were established and have found their long-term utilization in numerous specific industrial applications. This paper is intended to basically focus on some of the most important overlay welding processes and an evaluation of most recent process developments and advancements respectively, standing for remarkable improvements in terms of weld quality and reproducibility. The emphasis should thereby being laid upon the fully mechanized Gas Shielded Tungsten Arc (Hot wire) Welding (GTAW) and the Gas Shielded Metal Arc Welding (GMAW). Especially the usage of the highly advanced Cold Metal Transfer Process (CMT) enables the user to achieve new levels in process stability and quality, also in the field of overlaying.

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Dissimilar metal joining of aluminum alloys to steel is generally difficult to be in practical use because of a formation of brittle intermetallic Fe-Al compound(IMC) at the interface of the joint. The authors have been researching in order to minimize the thickness of this brittle IMC and have found that the formation of this brittle IMC is prevented in the region that is effective in getting the good joint strength by using the advanced hot-dip-aluminized steel sheet and by adopting the suitable joining conditions.
In particular this paper focuses the joinability in case of MIG braze welding which is expected to be the main and important joining method for structures.

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The purpose of this study was to develop the remote laser welding technique for galvannealed steel sheets using the pre-deforming process by laser heating and to develop in-line inspection system for remote laser welding system using visible images. It was concluded that the weldabillity of lap welding joint of galvannealed steel sheets could be improved by making the sheet gap of more than 0.04mm using the laser pre-deforming process before laser welding and moreover the quality of lap joints could be evaluated by using the proposed in-situ monitoring system.

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Fiber laser has been received extensive attention in welding of thick section materials in terms of high power intensity and high beam quality. This paper investigated the influence of fiber laser welding conditions, including welding speed, beam spot size and defocused distance, on the weld bead geometry in welding of thick section stainless steels. The result showed that the beam spot size had significant influence on the penetration depth, and the penetration depth at 0.1 mm beam spot size was shallower than that at 0.2 mm beam spot size. The maximal weld penetration at 10 kW laser power and 0.3 m/min could be obtained at the defocused distance of about -7 mm. Wine-cup weld bead geometry with excessive wide bead surface was formed in range of low welding speeds (lower than 1 m/min). As a result, the decrease in welding speed induces considerable increase in bead width, but only slight increase in penetration depth. In order to obtain deeper penetration at low welding speed range, it is important to enlarge and deepen the keyhole opening and suppress the plume/plasma. Thus an ultra-deep penetration laser welding method, which is characterized by using a gas jet nozzle to modify the keyhole and melt flow behavior, was developed. With two high-speed CMOS cameras and X-ray transmission apparatus, the behaviors of the keyhole, the melt flow and the plume in gas jet assisted laser welding were observed. The effects of the gas jet parameters and welding conditions on the weld bead geometry were investigated. By adoption of the gas jet, the penetration depth was increased from 18.2 mm to 24.5mm in the conditions of the laser power of 10 kW and the welding speed of 0.3 m/min. Moreover, the instability of penetration depth induced by thermal lens effect was also discussed.

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Compared with conventional lasers, fiber laser welding is characterized by high melting efficiency, deferent keyhole modes and power density characteristics, which could affect the heat and melt flow of the molten pool during welding. The objective of the present work was to study the fiber laser weldability of 5 mm thick AISI 304 austenitic stainless steel plates; therefore, bead-on-plate welding was exploited on AISI 304 stainless steel plates with different laser powers, welding speeds, defocused distances with different types of shielding gas and their effects on the weld zone geometry and properties and final solidification microstructure at room temperature. Laser power, welding speed and defocused distance have a great effect on the bead appearance and weld zone shape while almost no significant effect on both the type of microstructure and mechanical properties of welds. The microstructure of all laser welds was always austenitic including about 3–5 % ferrite. However, the lower the laser power and/or the higher the welding speed, the finer solidification structure, primary ferrite or mixed-mode solidification resulted in crack-free welds.

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New brilliant high-power lasers such as disc laser and fibre laser open up new fields of applications for laser-hybrid welding, e.g. power generation, shipbuilding and pipeline construction. For the use of laser-hybrid welding in pipe laying, insensibility towards tolerances and the possibility of orbital welding are very important factors in addition to the weld seam quality.
Within the scope of a basic research project, first promising results for laser-hybrid welding of thick materials could be demonstrated. As base material, the typical pipeline steel API 5L X65 was selected. With the help of a 20 kW fibre laser in combination with an arc-welding process it was possible to produce high-quality welds in plates of up to 20 mm thickness in a single pass and of up to 32 mm in three to five passes, both welded in position PA. Various joint preparations were examined to weld 20 mm thick plates in one pass. Besides different welding positions, gap bridgeability and misalignment were studied. Results for microhardness and Charpy toughness are presented to proof the applicability of laser-hybrid welding for joining in pipe laying.

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Full penetration weld bead by one pass butt joint welding of 12 mm thick SM490 plate were produced using the hybrid welding system combined with 4 kW power class fiber laser and pulsed MAG arc. The fusion zone was believed to be formed by one weld pool. A sound one pass full penetration weld bead without inside defects was obtained at the groove angle of 20 degrees and the gap width of 0.7 mm. A gap width of 1.5 mm was bridged by the hybrid welding process, but underfilling occurred at the root gap width over 1.0 mm in all groove angles. In order to improve the underfilling, higher feeding rate of a filler wire was applied by increasing the arc current. However, it was not a successful method because more molten metal and higher arc pressure accelerated the melt-through on the bottom beads. According to microstructure observation and hardness profile on the cross section, there was no brittle phase caused by the laser welding.

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316L stainless steel plate was friction stir welded using PCBN tools. The effect of tool shoulder profile and tool probe profile on tool wear characterization and weld formation was investigated. Two different shoulder profiles (screw with different pitches) with four different tool probe profiles (two different probe end shapes and two different probe lengths) have been used to fabricate FSW zone. Experimental results show that the tools with narrow pitch screw shoulder profile produce deeper FSW zone compared to the tools with wide pitch. The tools with spiral probe profile produce deeper FSW zone compared to the tools with chamfer probe profile. The tools with wide pitch screw shoulder profile is apt to produce lower working loads. The wear resistant of tools with chamfer probe profile is significantly higher than that of tools with spiral probe profile. The relation between tool geometries and tool wear and weld formation is discussed.

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Spot welding by friction stirring between aluminum alloy (AA5052) and several kinds of steels was conducted. Several kinds of steels were mild steel (270MPa of yield strength), high tensile strength steel (HTS steel, 980MPa of yield strength). The variation of cross tensile and tensile shear strengths of weld joints has the similar tendency. After shown the maximum strength, both tensile strengths decrease with increasing plunge depth. Torque, vertical load and temperature measured during welding between AA5052 and HTS steel indicate a high value more comparatively in any measurements than another one measured during welding between AA5052 and mild steel. The plastic flow like a tornado in AA5052 is caused by combining some plastic flows that had different directions.

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Local melting and cracking are investigated during friction stir spot welding of binary Mg-Al alloys containing from 3% to 23% Al by using a combination of stir zone temperature, rotational torque and detailed metallographic. The stir zone temperatures during Mg-15%Al and Mg-23%Al alloys friction stir spot welding correspond with the (α-Mg + Mg₁₇Al₁₂) eutectic temperature in the binary Mg-Al equilibrium phase diagram. The stir zone microstructures of friction stir spot welds in Mg-15%Al and Mg-23%Al alloys comprise fine equiaxed α phases and Mg₁₇Al₁₂. Narrow stir zones and lower rotational torques are observed when Mg-15%Al and Mg-23%Al alloys are spot welded since energy generation is diminished by tool slippage due to existing eutectic liquid at contact interface. These results support that local melting of the (α-Mg + Mg₁₇Al₁₂) eutectic microstructure occurred at the contact interface between the tool surface and material when the stir zone temperature attained to the eutectic temperature in the binary Mg-Al equilibrium phase diagram.
Liquation cracking is observed in the location beneath the tool shoulder in friction stir spot welds of Mg-9%Al alloy. The crack surface morphology is formed of dendrite shape. It is confirmed that the cracking occurred during Mg-9%Al alloy friction stir spot welding is liquation cracking.

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Friction welding was carried out on austenitic and ferritic stainless steels (diameter: 3 mm). The practical process windows, that is, weld lobes, were shown for similar and dissimilar welding. Proper balance of heat and deformation was necessary to form a favorable weld interface that had no unbonded regions and oxide layers. The microstructural refinement of both stainless steels occurred near the weld interface because of dynamic recrystallization. In the case of dissimilar welding, a lamellar structure of alpha and gamma phases was formed in the microstructural refinement region of type 430 steel, and strain-induced martensite was formed in type 304 steel.

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In the early heating stage by friction of friction welding, friction stir welding, spot friction welding, surface modification, and so on, conductive heat transfer plays an important role in formation of temperature field in the system, which would have direct effects upon the successive plastic flow process. In the heat transfer process of such systems, the effective dynamic coefficients of friction μ between a rotating rod and a metal plate is regarded as one of essential key parameters. However, almost no coefficients for various important industrial materials seem to have been obtained so far. In the present study, from experiments on unsteady-state friction heating and corresponding numerical calculations, the effective dynamic coefficients of friction μ using a rotating rod with a spherical tip of high speed tool steel have been evaluated for the aluminum alloy A2024, A5052 and A6061.

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We welded commercially pure titanium sheet to aluminum alloy A5052 sheet using a method of resistance spot welding with a cover plate, and investigated the mechanical properties and interfacial microstructure of the joint. The interfacial microstructure was observed using transmission electron microscopy. An approximately 160 nm thick layer of Al solid solution supersaturated with Ti was observed at the welding interface, in which contain the precipitates TiAl₃. Mechanical property analysis suggests that the reaction layer has no effect on the tensile shear load of the joint.

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To clarify the mechanism of acicular ferrite formation in low carbon submerged arc weld metals of a Ti-B system with different aluminum contents, the lattice misfit between inclusion and acicular ferrite was investigated. The inclusions were directly sliced into thin foils by a focused ion beam device and crystallographic analyses were performed using a transmission electron microscope. These inclusions were surrounded by a narrow Ti-enriched layer which was identified as TiO. Our results showed that the Baker-Nutting orientation relationship was satisfied between the narrow TiO on the inclusion surface and acicular ferrite.

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The solidification-mode during GTA welding of martensitic stainless steel was directly judged by using TRXRD system for welding. The X-ray detector with wider area was applied and its usefulness for the monitor of solidification process of welding was presented in the point of accuracy of the mode-judgments. The solidification behavior observed was related to the result of the trans-varestraint test to relate the solicitation phases and the hot-cracking phenomena. Furthermore, the retaining behavior of delta-ferrite from the temperature at solidification process to the room temperature was chased in-situ.

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The inclusion which stimulated the ferrite nucleation was directly classified through the in-situ observation experiments in the HAZ of the Ti-killed steel weld. The chemical composition of the selected inclusion was analyzed and the nucleation potential was discussed. The potential for the ferrite nucleation is ranked as follows: grain boundary, Ti-X and MnS in order. Furthermore, the existence of effective and ineffective MnS for nucleation was made clear and the difference was estimated.

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In order to investigate the solidification cracking susceptibility of alloy tool steel under rapid solidification, the cracking behaviors were dynamically observed with the high speed camera. It was clarified from the experimental results that the reduction of Si, and P content can decrease the susceptibility in the indexes of the minimum ductility (ε_min), the critical strain rate for temperature drop (CST) and Brittle Temperature Range (BTR). This could be explained by the behavior of liquid phase estimated by the microsegregation model. It is understood that the film like region at the last stage of solidification was finished early due to the reduction of segregation. Therefore, the reduction of Si and P content can be effective in decreasing the risk of solidification crack.

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Reinforcement of titanium by using titanium mono-boride is being applied to weld metal of Commercial Pure-titanium. In this presentation, the effect of re-hating for the boron-added weld metal is accessed by taking into account the multi-pass welding. The boron-added weld metal is re-heated and its microstructure change is in-situ observed by using High temperature Laser Scanning Confocal Microscopy (HSLCM). The formed microstructure is also analyzed in detail and compared with CP-titaniumand boron-added weld.

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This study is purposed to develop an evaluation method of solidification cracking susceptibility for laser DWJ of different dilution ratios. By using U-type hot cracking test with in-situ observation, the critical strain for the initiation of solidification cracking was measured locally near the crack rather than the macro strain measured in most other hot cracking tests. Meanwhile, the temperature history during laser welding at the trailing region of the molten pool was measured experimentally. The high temperature ductility curve was achieved based on the above test results of the critical strain and the temperature history. The critical strain rate of temperature drop (CST) was used to evaluate the solidification cracking susceptibility. Moreover, the residual liquid metal at the solidification front during laser welding was observed directly by using in-situ observation with an optical microscope lens. Consequently, all the results showed that solidification cracking susceptibility is the highest when the ratio of Inconel600 takes up roughly 40% in the weld bead among the used Inconel600/SUS347 laser DWJ.

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This study is purposed to develop a systematic method for prediction of solidification cracking during laser welding by accurately obtaining the high temperature ductility and weld strain of weld metal. The U-type hot cracking test was carried out under various initial loads; solidification cracking behavior was in-situ observed by a high speed camera so that the critical strain, for the initiation of solidification crack, was measured precisely. With the critical strain and temperature history measured at the solidification crack, the high temperature ductility curve was achieved. Meanwhile, a three dimensional FEM-analysis was performed to obtain the detailed strain history at the crack initiation point during laser welding. In order to get the accurate strain history, it is necessary to obtain the accurate material properties of weld metal at the elevated temperature up to solidification temperature range. The FEM-analysis was carried out by using the obtained high temperature material properties, which were measured by a developed high temperature tensile test with in-situ observation. Therefore, it became possible to predict the occurrence of solidification cracking during laser welding accurately by using both of the detailed high temperature ductility curve measured experimentally and the strain history obtained by the 3D FEM-analysis.

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The effect of Ce addition on microcracking susceptibility in the multipass weld metal of alloy 690 was investigated in order to improve the microcracking susceptibility in it. The ductility-dip cracking susceptibility in the reheated weld metal could be greatly improved by adding 0.015-0.025mass%Ce to the weld metal. Conversely, the excessive Ce addition to the weld metal led to liquation and solidification cracking in the weld metal. Hot ductility of the weld metal at the cracking temperature was greatly improved by adding 0.01-0.03mass%Ce, implying that the ductility-dip cracking susceptibility was decreased as a result of the desegregation of impurity elements of P and S to grain boundaries due to the scavenging effect of Ce. The excessive Ce addition to the weld metal resulted in the liquation and solidification cracking attributed to the formation of liquefiable Ni-Ce intermetallic compound. The multipass welding test confirmed that microcracks in the multipass welds were completely prevented by using the filler metal added 0.032mass%Ce.

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Recently in Japan, boiler waterwall tube damage such as fireside corrosion and circumferential cracking in a low NOx environment has become a serious issue, despite the fact that relatively lower sulfur content coal is typically being used than in the US. Thermal spray coating is the most popular method for tube protection in Japan, and thermal spray coating tubes are used for this purpose.
However, extensive damage to thermal spray coating tubes from cracking and exfoliation has recently been experienced. It is reported that the thermal fluctuations that occur as a result of operating changes create alternating stress, leading to cracking and exfoliation of the thermal sprayed thin coating. Corrosion-resistant weld overlays, such as stainless steel type 309, alloy 625 and alloy 622, are now commonly used to protect corrosion of boiler tubes in low NOx coal fired boilers in the US. In order to develop a fundamental understanding of the high temperature corrosive behavior of alloy 622 weld overlay, gaseous corrosion testing and certain mechanical tests for consideration of long-term aging were undertaken. After one year of service in the low NOx combustion environment of a coal fired supercritical boiler, field tests on alloy 622 weld overlay panels are continuing.
This report describes the behavior of alloy 622 overlay panels in a field test installed in a Japanese supercritical boiler, the laboratory results of weight loss corrosion testing, and mechanical tests related to aging.

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In recent years, various steel structures tend to be larger and the recent trend of applying high strength steels makes it possible to reduce the structure weight. Notch toughness as well as strength is essential for materials used in the offshore structure and bridge construction industries and achieving both of them is a big challenge. While covered electrodes have been widely used in high strength steel structures in out-of-position welding, the usage of flux-cored wire (FCW) is desired because of its advantages over covered electrode in terms of operational efficiency and less skill requirement. TiO₂ type FCW is most popular for all positional FCWs for mild steel and HT590MPa class steel and its excellent welding usability can be obtained with FCW for high strength steels up to HT780MPa. However, the weld metal made by TiO₂ type FCW contains higher oxygen, making it more difficult to achieve superior notch toughness. By contrast, basic type FCW capable of achieving higher notch toughness is not good in usability in out-of-position welding. In this report, the effects of various oxides on microstructure, notch toughness, welding usability and oxygen content in the weld metal of high strength steel FCW are discussed.
It has been clarified that notch toughness can be improved by decreasing the TiO₂ activity and the density of dispersive oxide inclusions, leading to less oxygen content in the weld metal of TiO₂ type FCW. In contrast to this, the oxygen content of the weld metal made by Al₂O₃ type FCW is high, and thus notch toughness cannot be improved even if its microstructure is refined. Furthermore, thermo-mechanical calculation has proved that welding usability in the vertical position becomes better when the solid-liquid coexistence temperature range of slag is wider.

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A flux cored wire for to steel sheet welding in automobile industries has been developed. The present wire is designed to reduce the slag formation to the level of conventional solid wire by controlling oxides in the flux. The present wire also contains comparatively high C, Ni and Cr so as to improve fatigue strength of a lap joint. The experimental results of plane bending fatigue tests reveal that the fatigue strength at 2 million cyclic loading is about 50% higher than that of ordinary solid wire. The charpy tests using the 1/4-size V notch specimens show that the absorbed energy of the present wire's weld metal at —40°C is almost equivalent to that of the conventional solid wire. Hence, it is concluded that the flux cored wire developed in the present work can be applied to steel sheet welding in automobile industries with high fatigue strength and high charpy impact properties.

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As sulfur content increases in weld metals, hot cracking is likely to occur. Therefore, to prevent hot cracking, the sulfur content of the welding consumables is controlled to be as low as possible in general. On the other hand, sulfur can considerably reduce the surface tension of the molten iron. Utilizing the latter characteristics of sulfur, the authors have succeeded in the development of an innovative gas metal arc (GMA) welding wire that contains a higher amount of sulfur as compared with conventional wires and possesses the flowing characteristics of the molten pool more suitable for thin steel plate welding. It has also been found through this research that the hot crack resistance of the weld metal degraded by increasing sulfur can be canceled by decreasing the carbon content of the wire to a substantially low level. The newly developed wire offers the following advantages over conventional solid wires: (1) wider and more consistent bead shape, (2) less undercut at higher welding speeds, (3) easier setting of the welding conditions due to wider tolerability to voltage fluctuations, and (4) excellent paint adhesiveness due to less slag generation.

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A highly corrosion-resistant stainless steel for application to chemical cargo tankers and welding consumables for the steel were developed. The stainless steel and welding consumables were designed to exhibit good resistance to corrosion by sea water and sulfuric acid. The developed welding consumables, flux-cored wire for CO₂ welding and flux and solid wire for submerged arc welding, proved that it is possible to attain welded joints that satisfy required corrosion resistance and mechanical properties, and their actual use for construction of chemical cargo tankers began in June 2004.

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The effects of alloying elements on the interfacial microstructure of the dissimilar Al/steel joints were discussed using SEM, EPMA, TEM and tensile test. For Si (and Cu)-added 6000 system aluminum alloy, EPMA analysis showed that Si (and Cu) was enriched in the reaction layer, which were formed during diffusion bonding. SAED (Selected area electron diffraction) pattern identified the reaction layer at the interface as Al-Fe-Si system containing Cu. High bondability was achieved by adding both Si and Cu to 6000 system aluminum alloy. High carbon content in steels caused the poor bondability, however, Si and Mn addition to the steel with large amount of carbon was effective to suppress the negative effect of carbon. An incubation time until a reaction layer was formed was recognized at lower bonding temperature and the time depended on additional elements. The reaction rate equation taking into account of an incubation time was newly proposed. This estimated bonding range based on the proposed reaction rate equation shows a good agreement with the obtained experimental results.

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The concept of diffusion brazing is an interesting approach to overcome several principal problems when joining high-performance engineering materials. Typical obstacles are low wettability and low melting ranges of the substrate materials, intensive metallurgical interactions or the build-up of thermally induced stresses. In many cases commercially available filler alloys cannot be employed. However, it is crucial to have adequate joining technologies available in order to realize modular design concepts of hybride structures.
The objective of the research is to employ diffusion brazing techniques to join temperature sensitive Al-base alloys. Based on fundamental thermodynamic considerations regarding interfacial phase transformations and corresponding diffusion mechanisms appropriate diffusion couples have been selected. Those materials have been deposited by PVD-processes onto the base materials. Subsequently, the components to be joined are placed into a vacuum furnace and are exposed to a temperature regime significantly below the melting range of the substrates. An additional force guarantees that the joint is in intimate contact and diffusion mechanisms can form a temporary liquid phase which solidifies isothermally during the brazing process. Apart from the materials selected the dwell time is the key parameter that controls the joint quality as well as interfacial precipitations.
A broad set of experiments have been conducted using diffusion bonding agents in the system Al-Cu and Al-Si-Cu. Variations of dwell time and jacking force help to develop a deeper understanding of the underlying diffusion mechanisms which is the basis for further optimization of Al-based diffusion brazements.

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The effects of zinc insert and Al content in Mg alloy on the bondability in steel/Mg alloy joints were evaluated using SEM, EDX, XRD and tensile test. A cold rolled steel plate (SPCC), a zinc coated steel (GI), Mg-3mass% Al alloy (AZ31) and Mg-6mass% Al alloy (AZ61) were used in this study. The bondability of the joints using zinc insert were significantly improved, because Zn-Mg eutectic reaction efficiently removes the oxide films on the Mg alloy. While a thin Fe-Al type interfacial reaction layer was formed discontinuously in GI/AZ31 joint, a Fe-Al and Al-Mg combined interfacial reaction layer was continuously formed in GI/AZ61 joint. These microstructural changes leads to mechanical properties of the joints, that is, the joint strength was improved from 40MPa in GI/AZ31 joint to 70MPa in GI/AZ61 joint.

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The effects of Cu and Si in aluminum alloys on the joint properties of Aluminum alloys/steels were evaluated by correlating the interfacial microstructures with the joint strength using diffusion bonding. It was found that joint strength was improved by adding Cu to aluminum alloys, because Cu suppressed formation and growth of a reaction layer. Moreover, the effect was much larger by Cu and Si combined addition. The fracture for the joints with higher strength occurred within the base aluminum alloys. As a reaction layer is thicker, the fracture mode was changed from an interfacial fracture to an aluminum alloy matrix and a reaction layer fracture, regardless of containing copper. For all investigated alloys, the highest joint strength was obtained in a certain value of reaction layer thickness, in particular, the optimum thickness value with the highest joint strength for Cu and Si combined alloy was smallest. By Cu and Si combined addition, our proposed guideline of the interfacial structure to obtain the joints strength more than 70 MPa was satisfied more easily. It was concluded that alloying of both Cu and Si to aluminum alloys improves the bondability of the joints.

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Joinability of silicon carbide to glass by anodic bonding was evaluated. Polycrystalline silicon carbide was successfully bonded to borosilicate glass Corning 7740. No crack by the residual stress occurred in all bonding conditions adopted in the present study. Increase in the bonding temperature enhanced progress of bonding, and the joint strength increased with the bonding temperature and the voltage application time. But the shear fracture stress of the joints did not exceed 6 MPa, and all the joints broken at the joint interface. Formation of a carbon-rich amorphous layer at the bond interface was observed by transmission electron microscopy. It was suggested that this layer caused relatively low joint strength of silicon carbide/glass anodically-bonded joint.

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Adhesion between a flat silicon rubber and an aluminum body with sinusoidal surface roughness is investigated. Hysteresis loops are observed in the relation between the external pressure and the contact width. The hysteresis is considered as the effect of the surface sub-roughness, and is successfully interpreted by a theory of the elastic contact between an infinite elastic body and a rigid body with single sinusoidal surface roughness. The effect of the sub-roughness is approximately expressed using a treatment of the energy dissipation. The work of adhesion and the parameter corresponding to the energy dissipation ratio are obtained from the measurements. The pressure required to snap to perfect contact and to separate the contact can be well predicted using the work of adhesion and these parameters.

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The present study relates the vibration behavior at the tip of wedge-tool during ultrasonic wire bonding with the state of adhesion at the interface of aluminum wire and silicon substrate. Cyclic change in amplitude by applying time of ultrasonic vibration is successfully detected by monitoring the vibration at the tip of wedge-tool. The spectral analysis indicates that local adhesion is not formed during increasing and high-amplitude stage of the cycle, while that is formed during decreasing and low-amplitude stage. By repeating such adhesion and separation, wire is deformed to a sufficient level to form a macroscopic bond. The ultrasonic vibration enhanced deformation of wire possibly occurs when the local adhesion is achieved. The amplitude of vibration affects the deformation rate.

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