A novel butt-joining technology of Cu and Al thin plates was developed, in which the edge of a Cu plate over the edge of an Al plate were pressed by the rotary tool with a truncated cone-shape in the atmosphere. Joining experiments were performed using 1mm thick C1020 plates and 1mm thick A1050 plates. The joint quality was investigated by observation and analysis of the joining interface and by tensile testing of dumbbell specimens fabricated with the Cu-Al joined bodies. The results clarified that the eutectic melting between Cu and Al was caused by pressure and frictional heat, and that the overlapped portion deformed into a butt joining shape while extruding the eutectic melt from the joining interface. A slant joining interface was formed between the C1020 and A1050 plates, and the film-like layer presumed to be consisted of CuAl₂ and CuAl were generated at the entire joining interface. The thickness of the layer ranged from 0.6 to 1.3μm. The average ultimate tensile strength of the dumbbell specimens was 92MPa, and the section of A1050 approximately 2mm away from the joint fractured in the tensile testing. This indicates that the Cu-Al joint had a higher strength than 92MPa.

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The purpose of this study is to develop a simulation method for predicting the effect of shape and dimension on shear strength, deformability and fracture behavior of knock-off bolts with circumferential notch and axial hole. Shearing tests for four types of knock-off bolts with various notch depth and axial hole diameter for constant cross-sectional area are conducted. Shear strength of these bolts is almost same value due to same cross-sectional area. On the other hand, the knock-off bolt with a shallower notch has larger deformability. Ductile cracking from notch of knock-off bolts unloaded at maximum load is observed. It is necessary for predicting shear strength and deformability of knock-off bolts to consider ductile crack initiation and growth. Stress states near notch tip of knock-off bolts depend on the angle to loading direction and are influenced by notch depth. The tensile/shear combined stress state dependent ductile damage model is proposed for predicting ductile crack initiation and growth from notch of knock-off bolts. The shear strength and deformability predicted by simulation based on the proposed ductile damage model present good agreement with experimental results. The effect of shape and dimension of knock-off bolt on shear strength and deformability is estimated by means of the developed simulation method. It is demonstrated that cross-sectional area and ratio of notch depth to bolt radius are dominant factor for shear strength and deformability, respectively.

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In order to investigate the effect of gas cutting during removal of restraining jig on deformation after submerged arc welding, the authors have conducted the experiments on a series of processes using a constraint jig plates in a test specimen modeling the deck of a steel bridge. In this paper, an Idealized explicit FEM developed by the authors is applied to investigate in detail the factors that cause springback deformation during the gas cutting of the constraint jig plates. A series of welding and cutting processes using a constraint jig plates were modeled, and then a three-dimensional thermo-elastic-plastic analysis was performed using the Idealized explicit FEM. As a result, it is confirmed that the experimental and analytical results were in good agreement, and the validity of the analysis method was demonstrated. In addition, it is found that the inherent deformation during gas cutting had a significant influence on the springback deformation by the numerical analysis. Then, we proposed a simplified mechanical model to estimate the deformation during gas cutting. As a result, it was confirmed that the proposed mechanical model showed quantitatively good agreement with the springback deformation.

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Friction stir welding (FSW) has been widely used in welding of low melting point metal, such as Al alloy or Cu alloy. However, high melting point metal, such as Fe alloy, it is difficult to be welded by FSW because of short life or high price of FSW tools. Silicon nitride tool can have both long life and low cost. In order to obtain design guideline for silicon nitride FSW tool, the relationship between the mechanical properties and tool life was examined. Two silicon nitride materials which had different microstructure were prepared. One of them had lower high temperature mechanical properties and the other had higher properties. Tools made by them could join the SUS 304 plate under the following conditions; welding speed was lower than 1000mm/min, and revolutional pitch was lower than 1.8mm/r. Even the tool which had excellent mechanical properties at high temperature worn faster when the joining temperature was 950℃ or higher. The tool with excellent high temperature mechanical properties had a tool life 6 times longer than the other tool. It is considered the life of the tool was improved when the crystallization of the grain boundary phase promoted the rise in the melting point of the grain boundary phase and the precipitation of silicon oxide during FSW.

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Demand for heavy thickness plate is increasing in offshore structure application and fracture toughness represented by CTOD is one of the most important properties for these heavy thickness plate. CTOD tests shall be conducted as full thickness of the material but it takes a lot of time and cost especially with the thickness exceeds 100mm. It is important to estimate the CTOD of full thickness plate using reduced thickness CTOD tests to accelerate material development in the laboratory. As a conventional estimating method for full thickness CTOD using the reduced thickness CTOD test, only the difference of crack driving force between full thickness and reduced thickness specimens have been considered. However, it is necessary to consider crack driving force in plate thickness direction for full thickness and reduced thickness specimens. As the first step, the influence of parameters on effective thickness considering crack driving force in plate thickness direction for the base metal is investigated in this study. Finally, formulation of effective thickness is proposed on the base of the influential parameters.

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A method is proposed to evaluate surface tension for axisymmetric meniscus in gravity. The method requires positional coordinates and tangential angles of arbitrary two points on liquid surface. A theory of the liquid in gravity is used to derive a differential equation. All of the equations are normalized by a parameter determined from the gravity, the density, and the surface tension of the liquid. Numerical calculation is carried out to show that the present model expresses various kinds of axisymmetric meniscus. The present method is proposed based on the theory and investigated by the experiment using pure water with a borosilicate glass rod. The positional coordinates and the tangential angle of the surface points are obtained from a photograph of the meniscus. The method shows a successful evaluation of the surface tension. The present method is compared with conventional methods and discussed from a view point of minimum information required to evaluate the surface tension. It is expressed that the Hutzler’s method is a special case of the previous method and the previous method is a special case of the present method. The present method is suggested useful in various meniscus tests, such as sessile drop tests, pendant drop tests, Wilhelmy rod tests, wetting balance tests, meniscograph tests, etc., since it requires only information on meniscus shape, i.e., the position and the angle of arbitrary two point on it.

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Advanced high strength steel sheet (AHSS) is widely used in the automotive body for weight reduction and the improvement of crash performance. In general, weldability and formability of steel sheet tends to decline as steel strength increases. Hot stamping is a technique that achieves both formability and strength of steel sheet, and TS1500MPa class hot stamped steel sheet is applied to many vehicle models. On the other hand, since the joint strength characteristics of the spot weld are dominated by the melt-solidified zone and weld heat affected zone, there is a concern that performance of spot welded joints will decline as steel strength increases. In other words, in order to further expand the application of AHSS, it is very important to have knowledge about the characteristics of spot welded joints. In this report, we investigated the TSS of the TS1500MPa class hot stamped steel sheet, focusing on the characteristics of tensile shear joints. Then, we compared the joint strength and failure position of the same kind material joints and the dissimilar material joints with the TS1500MPa class hot stamped steel sheet in case of the plug failure. As a result, it was found that TSS of dissimilar material joints depend on base metal strength on lower strength steel side. However, fracture occurred on the TS1500MPa class hot stamped steel side except for joints with large difference of base metal strength. The mechanism is thought that the effect of strain concentration at the edge of nugget on the TS1500MPa class hot stamped steel side due to the increase of rotational deformation around the nugget.

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Advanced high strength steel sheet (AHSS) is widely used in the automotive body for weight reduction and the improvement of crash performance. Peel tensile strength of spot welds tends to decrease as the steel sheet strength increases. Therefore, in order to further expand the application of AHSS, technology to prevent fracture of spot welds is required. In order to achieve that, it is necessary to understand the characteristics of spot welded joints and clarify the controlling factors. However, most of the conventional joint strength evaluations have been conducted on the same kind material joints, and there are few systematic studies on the joint strength and fracture mode of dissimilar material joints that are often used in actual vehicles. Therefore, the purpose of this study is to obtain a guideline for grasping the characteristics of dissimilar joints in case of the plug failure. Using various joint shapes, we compared the peel tensile strength and failure position of the same kind material joints and the dissimilar material joints with the TS1500MPa class hot stamped steel sheet. As a result, it was found that the deformation behavior of the test piece affects the joint strength and fracture position of the dissimilar material joints. Specifically, in dissimilar material joints, cross tension joints and double cup-shape tension joints tended to break on the TS1500MPa class hot stamped steel side. On the other hand, L-shape tension joints broke on the lower strength material side. From these results, it is considered important to understand their deformation behavior in order to analyze and predict the fracture phenomenon of spot welds in actual vehicles and automotive parts.

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It is difficult to examine the relation between the root notch welding residual stress (WRS) and the root-failure fatigue strength because of the difficulty in the direct root WRS measurement. The root WRS can be calculated by thermal elastic plastic finite element analysis (TEPFEA), but its accuracy has not been fully verified yet. In this study, root-failure fatigue tests, in which direct root WRS measurement can be performed, are carried out. Plug welded (PW) specimens with backing plates are used in those tests, and the root WRS in as welded (AW) and stress relieved (SR=PWHT) specimens are measured using X-ray diffraction (XRD) method by cutting off the backing plate. The measured WRSs are compared with those calculated by TEPFEA. It is found that the root WRSs in the PW specimens estimated by TEPFEA become much larger than those measured when creep strain is neglected. The SR specimen’s fatigue strength improvement ratio is estimated by using the WRS effect formulas developed for toe-failure cases (IIW Fatigue Recommendations and MIL-HDBK-5D). The estimated improvement ratio shows fair agreement with that measured.

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It has been reported that hydrogen dissolves in the weld bead in laser welding. If the hydrogen concentration in the weld bead is high, there is concern that the weld bead may fracture during press forming of tailored blanks. Therefore, assuming that 1ppm of hydrogen is dissolved in the weld bead at welding, the change in hydrogen concentration over time is estimated by a two-dimensional difference calculation. In the calculations, hydrogen is assumed to follow the Fick’s diffusion equation, and the diffusion coefficient at room temperatures is the value determined by the authors for a 980MPa steel laser welds in the past. The diffusion coefficients at high temperatures were determined from several references. As a result, it was estimated that about 30% of the hydrogen had effused from the surface by the time the weld bead cooled to room temperature. The mechanical properties of the weld bead are considered to be governed by the maximum hydrogen concentration at the center of the weld bead cross-section, but this value cannot be measured directly. Therefore, assuming that all the hydrogen contained in the entire model is in the weld bead and considering the average hydrogen concentration in the weld bead, the ratio of the maximum hydrogen concentration to the average hydrogen concentration is around 1 up to 6h after welding, indicating that the average hydrogen concentration is a good indicator of the maximum hydrogen concentration. It was also confirmed that the logarithm of the average hydrogen concentration is proportional to time as time passes after welding and that the hydrogen concentration distribution in the thickness direction behaves as a solution of a one-dimensional diffusion equation. These results suggest that the hydrogen concentration in the laser weld bead can be considered as an analytical solution of the one-dimensional diffusion equation.

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Stainless steel is applied in a wide range of fields as a material with high heat and corrosion resistance. Welding, brazing, and solid-phase bonding are the most common joining techniques for stainless steel. Conventional joining methods have issues such as local thermal distortion of the joining area and the need for high-temperature heat treatment at 800-1000°C. In this paper, we investigated a new joining technique for stainless steel. With the aim of establishing a new joining technology for stainless steel, we joined stainless steel using the Ni Micro-Plating Bonding method (NMPB) and evaluated the shear stress and the tensile stress of the joints. The joining process is performed at about 55°C, followed by heat treatment at a relatively low temperature of 300°C or higher. Shear and tensile tests were conducted on the NMPB-joined SUS304, and the results showed that the shear stress was 114MPa and the tensile stress was 77MPa without annealing. After heat treatment at 300-400°C for 1.5h, the shear stress of 200MPa and the tensile stress of 240MPa were obtained. In addition, the heat treatment at 600-800°C for 1.5h resulted in the shear stress of 230MPa and the tensile stress of 400MPa. The plating crystallographic structure before heat treatment showed preferential orientation to <001>, but after heat treatment, recrystallization progressed beyond the boundary interface at 400°C or higher, and a non-oriented crystalline was observed.

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Effect of Al or Ti addition on the ferrite formation in ultra-low oxygen (about a few tens ppm) weld metal of low carbon steels were investigated. A bead-on-plate welding experiments were performed with a Tungsten Inert Gas (TIG) welding system. The double nozzle was attached to the welding torch used, Ar was passed along the outer nozzle and He is added along the inner nozzle. Optical micrographs showed that primary ferrite appeared along prior austenite grain boundaries and that as the temperature is decreased ferrite side plates were grew into grain interior. Their formation positions were approached to the welding end as the decreasing Al and increasing Ti content. This change in ferrite formation position attributed on the ferrite formation temperature, which was assumed to increase as the decreasing Al and increasing Ti content. Energy dispersive X-ray spectroscopy analysis of inclusions showed that Al and Ti were mainly associated with O, it was obvious that the inclusions in the steels were Al- and Ti- oxide inclusions. Al-oxide inclusions were not favourable acicular ferrite nucleation sites and that the inclusions contributing to acicular ferrite formation were Ti-oxide ones. The number density of acicular ferrite was small. This indicated that acicular ferrite formation was suppressed because the number density of inclusions was low under the ultra-low oxygen welding condition.

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The Widmanstätten austenite is well known precipitates at grain boundaries in duplex stainless steel welds, making the performance of welded joints degraded. That is affected by its morphology. However, the three dimensional microstructure has not been made abundantly clear. Almost all the studies on the morphology of Widmanstätten right now were based two dimensional only. In order to identify the morphology of Widmanstätten austenite, the three dimensional structure was observed by the serial sectioning three dimensional method. The Widmanstätten austenite in the same area observed on different cross-sections shows different morphologies and arrangements. And through the EBSD analysis, the relationship between part of the granular austenite in the ferrite grain and the Widmanstätten austenite precipitated at the grain boundary was determined.

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Extending the service life of structures and decreasing the damage caused by fractures or deterioration are important. Previous studies have confirmed that adhesive-bonding-assisted-fillet-welding in cover plate joints has a positive impact on stress mitigation near weld roots and also presents a possibility for improving fatigue life. To investigate the effects of fillet welding combined with adhesive bonding on the fatigue life of weld roots in steel bridge members, several experiments were performed on the specimens, modelling the welds between the flange and the sole plate. The possibility of defect occurrence was reduced by inserting a heat-resistant rubber into the edge of the adhesive bond part of the specimen. A cyclic tensile stress was applied to the weld root of the specimen to generate fatigue cracks from the weld root. Compared with the only-weld specimens, the specimens with welding and bonding demonstrated evident stress mitigation and extension of fatigue life. In addition, the incomplete filling of the bond could possibly reduce the effect of fatigue life improvement.

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Arc efficiency of gas metal arc welding was measured in changing welding current. Increasing welding current, arc efficiency showed convex curve. Estimating amount of heat transferred by metal droplets, it was concluded that increase of arc efficiency in the welding current less than 186 A was caused by heat of metal transfer, while decrease in the welding current more than 186 A was caused by overwhelming arc radiation.

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