Embedded atom method (EAM) has been successfully applied to investigate surface properties and simulate diffusion phenomena, which do a great help to study micro-joining process. However, because its theory and parameterization are based on bulk system it fails in solving some problems of non-bulk system. In order to increase the applicability of EAM to non-bulk system, a new scheme of EAM, DR-EAM is proposed. In this scheme, the dimer structure is selected as a common reference structure and the parameters are derived from dimer and some bulk properties. In this work, the DR-EAM parameters of 7 kinds of FCC metals are renewed by revising the modeling system and including some experimental data of dimer. The features of their energy-distance curves are discussed and it shows the need of including angular dependency of electronic density. The vacancy formation energies, which play an important role in the diffusion process, are calculated and compared with experimental data.

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In the tandem pulsed GMA welding, the occurrence of arc interruption by the electromagnetic interference between the two adjacent arcs becomes a problem. In order to clarify this problem, effects of inter-wire distance and Ar+CO₂ gas mixture ratio on an abnormal arc voltage and arc interruption by the electromagnetic interference, are investigated. The abnormal arc voltage and the arc interruption frequently occur with pulse peak currents are supplied alternately to two wires. In addition, both phenomena occur in trailing arc which is located on molten pool at base current duration remarkably. There are most number of abnormal arc voltage and arc interruption times in trailing when the inter-distance is 10 mm because a displacement of trailing arc by the electromagnetic interference becomes the longest. Moreover, the CO₂ mixture ratio affects the occurrence of abnormal arc voltage and arc interruption. The abnormal arc voltage and arc interruption do not occur when CO₂ gas mixture ratio is equal to or less than 5%. However, number of abnormal arc voltage and arc interruption times increase rapidly with increasing CO₂ gas mixture ratio when CO₂ gas mixture ratio is over 10%.

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In tandem pulsed GMA welding, it is most important issue to prevent arc interruption caused by electromagnetic interference between the two arcs. Pulse timing control can reduce arc interference in tandem pulsed GMA welding. One effective way is to delay the pulse end timing of trailing arc by 0.4-0.5 ms from that of leading arc. In addition, arc length control is assured by pulse frequency modulation (PFM) for the leading wire and pulse peak modulation (PPM) for the trailing wire with the pulse timing synchronized with the leading pulse. Consequently, leading and trailing arcs are maintained stable without arc interruption and a stable arc length control is established, which is hardly affected by fluctuations of wire feed rate and extension length.

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The minor alloying element in steel, such as sulfur has been of great concern in welding, because sulfur is an element that significantly changes the weld penetration. To analyze the "minor element effect" experimentally, accurate information on the molten pool, such as temperature or fluid flow, is essential. The radiation thermometer is commonly used to obtain the temperature field of the object. However, the emissivity problem occurs in measuring temperature of molten metals. The emissivity fluctuation causes a noticeable error in temperature indicated by radiation thermometers. As described in the previous paper, the UV (ultra-violet) radiation thermometer is useful to solve the emissivity problem.
In the present paper, the influence of sulfur content on temperature field of molten pool is discussed. In order to avoid interference from arc plasma, the measurement was carried out from the bottom side. It was found that temperature distribution was asymmetrical, and minor changes in sulfur content can change molten pool formation. As a result, it was made clear that changes in sulfur content also affected temperature field of molten pool.

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In order to obtain guidelines for the choice of appropriate welding processes for the tailored blanks using advanced high strength steel sheets, the formability of welded steel sheets were investigated. The purposes of this work are 1) to determine an upper steel strength limit that can be welded and 2) to investigate the deformation behavior of welded high strength steel sheets. Laser, mash seam, and plasma arc welding were employed up to 1180MPa high strength steel sheets. The results suggested that laser welding was the best welding process because of its small heat input and narrow weld width. It could be applied to 980MPa or 1180MPa steel sheets. On the other hands, 590MPa was the maximum strength grade to which mash seam and plasma arc welding could be applied. The reason is that the mash seam and the plasma arc welding thermal cycles softened the HAZ of high strength steel.

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The formability and fatigue performance of welded advanced high strength steel sheets were studied at the viewpoints of industrial tailored blank application. In order to obtain material requirements for the formability of tailored blanks, five different 590MPa steels were evaluated with laser welding and plasma arc welding. In addition, appropriate welding parameters were investigated. The material property of base metal for improving the formability of laser welded steel sheets was high elongation. The material requirement for plasma arc welding was low weld hardness. Low carbon content was a favorable composition for plasma arc welding. Welding speed was important for the formability of laser welded ultra high strength steel sheets. Low welding speed should be avoided to prevent a softening in HAZ. The clamping position of jig for plasma arc welding could affect the formability, because of the changing a cooling effect by the clamping jig. The fatigue strength of laser and arc welded joints using AHSS were higher than that of mash seam welded joints.

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Hardness distribution in welds of Ultra-Narrow Gap GMA welding has been evaluated with numerical simulation. Estimation of hardness distribution has been performed considering weld thermal cycle and microstructure in welds. In order to investigate temperature profile of welds precisely, detailed measurement has been performed. Welding wire is oscillated vertically in the Ultra-Narrow Gap GMA welding process, however, temperature profile in heat affected zone during cooling has not been affected by the oscillation. Therefore the finite element heat conduction analysis has been possible to perform without considering the effect of oscillation of welding wire. Changes in microstructure have also been calculated based on the continuous cooling transformation diagram of the material. Hardness distribution in welds has been estimated based on the calculated weld thermal cycle and fraction of microstructure with the rule of mixture. Estimated distribution of hardness in welds was in good agreement with measured result. The method is applicable to evaluation of performance of welded joint.

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Hardness distribution of multipass weld metal has been evaluated with numerical simulation. Fraction of microstructure and cooling rate during weld thermal cycle has been taken into consideration in evaluation of hardness. Continuous cooling transformation diagram (CCT diagram) has been used in calculation of microstructure. Simulated weld thermal cycle tests has been conducted to create CCT diagram for multipass weld metal. Microstructure of the weld metal after reheating has been more dependent on the peak temperature than the previous microstructure before reheating. Therefore calculation of microstructure has been conducted based on two different CCT diagram for the different peak temperature. Hardness distribution has been estimated based on the rule of mixture. Fraction of each microstructure has been calculated with CCT diagram, and hardness of each microstructure has been estimated from cooling rate. Hardness distribution of multipass weld metal has nicely reproduced with numerical simulation.

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In the production of small electronic and medical devices, it is often necessary to produce crossed wire joints in fine nickel wires. Therefore, commercial purity nickel fine wire was welded by the resistance microwelding. A study has been performed to improve the welding process considering the welding mechanism. A welding mechanism consists of cold wire collapse, surface melting, liquid phase squeezed out and solid state bonding. Set down which indicates the depth of penetration for wires was not adequate to evaluate the joint quality. Sufficient local heat generation as well as set down is the key to high quality welds, first to generate enough transient surface melting, second to facilitate plastic deformation in order to squeezed out liquid and expand the bonded area. This requires a proper balance of high initial contact resistance and sufficient high welding force, which could be obtained by setting a low firing force.

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This report describes the effect of friction time and friction pressure on tensile strength of carbon steel welded joint by a low heat input friction welding method (LHI method) that was developed by authors. Medium (0.35%) and high (0.55%) carbon steel joints were made by friction speed of 27.5s-1 through a continuous drive friction welding machine with an electromagnetic clutch in order to prevent braking deformation during rotation stop.
The experiments produced the following summarized results.
(1) Medium carbon steel joint did not obtain the same tensile strength as that of the base metal, and the fracture occurred at the welded interface when it was made at friction time up to the initial torque by friction pressure of 30MPa. However, the joint obtained the same tensile strength as that of the base metal, and the fracture occurred at the base metal when it was made at friction time up to the initial torque by 90MPa.
(2) Medium carbon steel joint by 30MPa fractured at the welded interface due to the unjoined region, which was produced at the peripheral region of the welded interface during friction process. The unjoined region had not produced with increasing friction pressure.
(3) High carbon steel joint did not obtain the same tensile strength as that of the base metal, and the fracture occurred at the welded interface when it was made at friction time up to the initial torque by 30 and 90MPa.
(4) High carbon steel joint fractured at the welded interface due to the quench crack, which was produced during cooling stage after welding. The quench crack reduced with increasing friction pressure. However, the joint by high friction pressure (300MPa) did not obtain the same tensile strength as that of the base metal, and it fractured at the base metal, because the center portion at the welded interface hardly joined after the initial torque.

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The laser brazing of Inconel600 alloy with precious filler metals has been conducted using a diode laser generator with single beam and tandem beams for preheating and brazing. Inconel 600 alloy of 1mm thickness was butt-brazed using Au-18%Ni, Ag-10%Pd and Ag-21%Cu-25%Pd filler metals with diameter of 0.5mm by using brazing flux. We make butt-joint with single beam brazing for various brazing clearance and use tandem beam brazing to improve the wettability of melted filler metal. Experimental results have shown that the sound butt joints which are free from defect such as porosity and lack of penetration could be obtained with brazing clearance of 0.1-1.5mm. The joint strengths of brazed joint which were produced in the optimal condition by using Au-Ni and Ag-Cu-Pd filler metals were comparable to the base metal strength in the joints with the brazing clearance between 0.1-1.5mm, whereas those of the joints which were brazed by using Ag-Pd filler metal increased with decreasing the brazing clearance and attained about 70% of the base metal strength at brazing clearance of 0.1mm.

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Erosion phenomenon during the diode laser brazing of Inconel600 alloy with Au-18%Ni filler metal has been investigated. The erosion kinetics of isothermal and thermal cycle process was examined and analyzed with the Nernst-Brunner equation. The calculated amount of erosion in base metal by using single beam and tandem beam for preheating and brazing was compared with the experimental ; microstructural observation for the joint has shown that the amount of erosion increased with increasing the laser power. The calculated thickness of erosion was in good agreement with experimental value. It followed that the Nernst-Brunner equation seemed to be applicable to predict the erosion during laser brazing process. And the amount of erosion with tandem beam in the optimum condition was less than that with single beam brazing in spite of good wettability and spreadability against the base metal.

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The authors tried to butt-joint weld an aluminum alloy plate to a mild steel plate, which had thick oxide film on the faying surfaces, using friction stir welding in order to investigate the behavior of the oxide film and the process for the formation of Fe fragments scattering in Al matrix of a joint and to examine the joining process. The following results were obtained.
A rotating pin rubs the Fe faying surface and removes the oxide film from the surface, resulting in making the Fe faying surface activated. The rotating pin draws a part of Fe nearby an interface into the Al matrix by a whirling motion of the pin, and the Fe part drawn into the Al scatters in the Al matrix as fragments. At the moment that a part of Fe is drawn into the Al matrix, the oxide films on the Fe and Al are broken, and the broken oxide films remain around the Fe fragments scattered in the Al matrix. A rotating pin transfers the softened Al in a plastic flow state by frictional heat onto the activated Fe faying surface, resulting in adhering between the Fe and the Al.

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Aluminum nitride (AlN) is one of the attractive ceramics in respect of its excellent properties. It was possible to fabricate Al/AlN composite coatings onto carbon steel substrate by reactive RF plasma spray process. However, most of the coatings peeled off due to the difference of thermal expansion coefficient between AlN and carbon steel. It might be possible to prevent peeling of AlN coatings with formation of Al layer between substrate and AlN coating. To fabricate these coatings, it was necessary to control the nitriding reaction. Thus, nitriding process of aluminum powder in the reactive RF plasma spray process was investigated. Then the coatings were fabricated with changing nitrogen flow rate in the plasma gas. Although it was difficult to fabricate functionally graded AlN coating, it was possible to prevent the peeling off of the coatings with formation of Al layer between carbon steel substrate and AlN layer on the surface of the coatings.

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Microbially influenced corrosion (MIC) of stainless steel welds has been confirmed not only in natural sea water but also in fresh water recently. In this study, four kinds of SUS304 test coupons, like as base metal, as welded by GTAW without filler metal, as welded by GTAW with filler metal and polished after welding by GTAW, have been prepared. These test coupons have been exposed to the dam water, which caused the MIC failure on the SUS304 piping in the last few years, for 550 days in order to investigate the relation between the adhesion of biofilms and MIC. Visual inspection, measuring of adherent bacteria, proteins and corrosion potential have been executed on the immersed test coupons for each exposure times of 170, 420 and 550 days. The adhesion quantity of biofilms on the test coupons exposed for 170 and 550 days have increased much more than that on the test coupons exposed for 420 days. The effect on the adhesion of biofilms based on the difference of the surface treatments between polishing and as welded has not been confirmed. The ennoblement of the corrosion potential has a connection with not only the adhesion of biofilms but also algae and sedimentation. Furthermore, all test coupons have been observed by SEM. No corrosion pits has been observed on any test coupons of the base metal. On the other hand, some corrosion pits has been confirmed on the test coupons of welds regardless of reinforcement or oxide scale in this study.

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The microstructure of friction-welded interfaces of commercially pure aluminum A1070 to mild steel S10C has been investigated mainly by TEM observations to reveal metallographic factors controlling the bond strength. The bond strength was estimated from the tensile strength of a specimen with a circumferential notch at the interface. A maximum strength of 124 MPa was obtained at a friction time of 0.5 s (rotation speed = 20 s^-1, friction pressure = 20 MPa, and forge pressure = 100 MPa). At this friction time, no IMC(Intermetallic compound) layer could detected at the interface, and fracture occurred in the aluminum base metal. With an increase in friction time, the bond strength was decreased, and brittle fracture at the interface came to occupy the almost whole area of the fractured surface at friction times more than 2 s. At a friction time of 2 s, an interfacial layer about 30-100 nm wide that consisted of Fe₂Al₅ was formed at the interface. TEM observations of the cross-sectional microstructure of fractured surfaces revealed that the brittle fracture occurred in the IMC layer. The bond strength of these joints decreased with an increase in the width of the IMC layer. The width of the IMC layer was increased almost linearly with an increase in friction time. The kinetics of the growth of the IMC layer suggests that it is controlled by mechanical mixing of Al and Fe at the interface as well as their diffusion.

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