We tried to bond A1050/A5052 and A1050/Cu by ultrasonic spot bonding and evaluated the bondability of joints by performing tensile shear test, the observation of microstructures and ultrasonic testing. In principle a good joint was obtained by ultrasonic bonding when we selected good bonding conditions and no intermetallic compound was observed at bond interface. It was observed that a crack initiated just before the maximum load near the boundary of the bond area of A1050 in both cases of A1050/A5052 and A1050/Cu by radiographic testing during shear test. Then bond area started to deform and changed its shape from circle to ellipse and the crack propagated further. The maximum load in shear test was obtained at the appropriate input energy depending on the cases of A1050/A5052 and A1050/Cu. The situation of bonds could be evaluated by ultrasonic testing. We could observe nano-crystals near the bond interface of A1050/Cu with TEM probably due to the rapid deformation during the bonding.
Energy coupling rate during welding of Al-Mg-Si alloy by single and twin spot Nd:YAG laser beams was measured calorimetrically at various input energies. Two beams were aligned parallel to welding direction and the distance was varied from 0.36 to 1.0 mm. The coupling rate in single spot beam welding was higher than that of twin spot beam at same power level. However increasing power, both were saturated in the same level at 80%. Energy coupling rate became larger as the beam distance became smaller. Measured coupling rates fit with modified Couffe-Dausinger model that determine coupling rate by penetration depth and focal spot size calculated from twin spot combined lineally. It is considered that modified focal spot size represents the keyhole opening size affects on reflection loss after multi-reflection in the keyhole.
For the purpose of improving the toughness of flash weld, the effect of upsetting conditions on the flash weld of 980MPa grade high strength steel bar has been studied. The main results obtained are as follows; (1) Characteristic features have been observed in the displacement profile when the upsetting condition is set to higher current and lower pressure during upsetting. In case of higher current conditions, the upsetting speed increases rapidly and then goes down in a short time, which phenomena is repeated several times until its upsetting process is finished. (2) According to high-speed camera observation, such rapid increase in the upsetting speed is attributed to the extrusion of molten metal layer formed at the weld interface due to excessive joule heating. The specimens showing the melting-extruding upsetting phenomena have revealed superior weld toughness rather than those produced by conventional upsetting conditions. (3) The reason of toughness improvement in the high current condition where the melting and extruding upsetting mode is observed is considered to be the extrusion of both oxides defects and base metal inclusions, and the achievement of small upset deformation due to the lower upsetting pressure.
In recent years, laser welding has begun to be used for assembly welding of automotive bodies and parts, although it has not yet to be used widely. One restraint is that in laser lap welding, the gap between the lapped sheets must be controlled very tightly. If the gap is wide, burn-through occurs, and, if the gap is excessive, the two sheets cannot be welded together. For this reason, the gap is generally restricted to 0.2 mm or less for laser welding. Laser lap welding of zinc-coated sheets poses additional problems. When lap welding of zinc-coated sheets is performed without any gap, the zinc evaporated between the sheets tends to blow off weld metal, or the zinc vapor tends to remain in the weld metal and form blowholes. Laser-arc hybrid welding was developed to solve these problems. This newly developed welding method maintains the features of laser welding, in that it is highly efficient and does not cause much thermal deformation due to its low heat input. Its proven characteristics are as follows: (1) The gap tolerance in hybrid lap welding is nearly equivalent to the thickness of the sheets to be welded, which is significantly larger than with laser welding. (2) The hybrid lap-welded joint can attain a strength that exceeds that of the base metal, because the bead width is wider than that in laser welding. (3) In hybrid lap welding of zinc-coated steel sheets, the formation of blowholes is restrained significantly compared to laser welding, even when the gap is 0 mm, so that a sound welded joint can be obtained.
There are few applications of laser welding to thick plates joining. One reason is attributed to narrow gap tolerance due to a small laser beam diameter leading to unacceptable seam defects such as underfill and burn through. Another is easy formation of weld imperfection such as porosity and cracking in a keyhole—type of deeply penetrated laser weld beads. Hybrid welding with a laser beam and an arc has been receiving considerable attention as one of the methods to suppress such defects and imperfection. This paper reports fundamental investigation results of hybrid welding with combined high power CO2 laser and Metal Inert Gas (MIG) arc, and especially hybrid welding results of thick plates with such a wide gap that the majority of a laser beam can pass through. The behavior of laser-induced and arc plasmas near the molten pool during the hybrid welding was observed with a black-and-white CCD camera. Penetration depth were confirmed to depend upon the distance between laser beam irradiation location and MIG wire target. The different depths were interpreted in terms of the interaction between laser-induced plasma and molten metal, melt flows, and so on. Moreover, under the formation conditions of the deepest weld beads, the wide gap groove was fully filled with the molten metal. The observation suggests that the recoil pressure due to the laser-induced evaporation should force the molten metal fed with a MIG wire to fill the leading gap and a fully penetrated keyhole was formed during hybrid welding. Consequently, a sound hybrid weld bead could be produced in joining the plates of 22 mm thickness with 4 mm gap, and moreover some weld beads indicated the best grade of the inspection test result according to JIS Z3104.
On maintenance of structures, there is a strong need to know stress condition of steel structures in use. Usually, strain gauges are used for stress measurement, but they can measure stress fluctuation only after pasting them to structures. So, stress already existed in structures beforehand cannot be measured. Stress measuring technique using the magneto-strictive effect senses magnetic field distortion by strain of steel itself. It has a possibility to measure stress already existed in structures beforehand, such as dead load stress or welding residual stress. We made experimental study on measuring stress in steel bridges using the magneto-strictive effect, and the following results are obtained. (1) Stress measuring technique using the magneto-strictive effect can measure stress in steel plates which have the same surface condition as that used in steel bridges. (2) The magneto-strictive effect has a possibility to measure welding residual stress in ‡T girder of steel bridges. (3) By using the magneto-strictive effect, it is possible to measure stress in steel with 200 μm thickness paint and to measure stress change of steel bridges during erection
Friction stir welding (FSW) shows significant advantages in the joining of aluminum alloys in recent years. In this paper, the feasibility of FSW for magnesium joints and magnesium to aluminum dissimilar joints was studied. Specimens were joined by FSW at the feed rate of 3.33-13.3 mm•s—1 and the tool rotational speed of 25 or 50 s—1. In the case of the butt welded magnesium alloy(AZ31), the average grain size in the stir zone is about 10μm that is smaller than that of in the parent metal. In the case of the dissimilar weld of magnesium alloy to aluminum alloy, mixing is limited and a more mechanical interlocking joint forms. However, the intermediate layer of about 2μm thickness is observed by TEM. Two kinds of phases are observed in the intermediate layer by the electron diffraction pattern. The electron diffraction suggests that one phase is Al12Mg17 intermetallic compound, while the other phase can not be defined. The mechanical properties of the dissimilar joint are inferior to those of the parent metals especially in the elongation because of the intermediate layer.
In this study, a ultrasonic evaluation was performed on the bonded areas in the semiconductor devices. The capillary or bonding tool fixed with epoxy resin holder was used for the transmission of ultrasound from the probe to wire bonded area and for the reception of reflected wave from bonded area. The ball shear test and EPMA analysis were also carried out for the evaluation of strength and wire boded area. The comparison between the ultrasonic test and shear test showed that the amplitude change from the wire bonded area is related with the very short time range and is small. The wave analyses of reflected wave proposed a available parameter for the nondestructive evaluation of wire bonded area, which showed good relation with the shear strength. It was also shown that this parameter depends upon the length of epoxy resin holder and was deeply related to the transmission of the ultrasound from capillary to holder in the measurement.
The lap welding of titanium sheet and mild steel sheet was performed with Cu-base filler foil as an insert metal using a seam welding method. The weldability was investigated in comparison with seam welds without the use of the insert metal. Without the insert metal, the seam welds were easily peeled off at the weld interface between the titanium and the mild steel. On the other hand, with the insert metal, the sound welds were produced at all welding conditions applied in this study. Therefore, it was concluded that the formation of intermetallic compounds, such as FeTi and Fe2Ti, were suppressed by using the insert metal. The result of the tensile shear test showed that the fractures occurred at the weld interface between the titanium and the mild steel, and the values of maximum tensile shear strength were approximately 100 to 120 MPa independent of the welding conditions. The XRD and EDX analyses on the fractured surfaces after performing the tensile shear test revealed that a very small amount of the insert metal remained at the weld interface between the titanium and the mild steel and also CuTi, Cu2Ti and β-Ti were formed at the interface. Consequently, the very small amount of the insert metal that remained at the weld interface between the titanium and the mild steel prevented the formation of brittle intermetallic compounds such as FeTi and Fe2Ti.
A tailor welded blank (TWB) consists of materials different in thickness, tensile strength, and other properties, and is now widely used for fabricating automotive bodies. TWBs are used not only to save materials and reduce cost in stamping and assembling, but also to improve fuel efficiency by reducing the weight of automobile bodies and to improve the stiffness. This report presents the conditions required at welds of TWBs composed of 590MPa class high strength steel sheets with high thickness ratios. Further, from the viewpoint of material properties, basic characteristics such as mechanical and fatigue properties and press formability of TWBs having a tensile strength of 590 MPa or more were clarified. The main conclusions obtained are as follows: (1) The weld thickness ratio TW/T1 must be kept at 0.8 or higher regardless of material properties and sheet thickness ratios, where TW is the thickness of the weld, and T1 is the thickness of the thinner base metal. (2)The elongation of weld joints is estimated systematically by the sum of weld metal hardness and base metal hardness. (3)The fatigue strength of welded joints is lower than that of base metal regardless of equal thickness or differential thickness welded joint. Also, as the thickness ratio increases for the differential thickness joints, the fatigue strength becomes low. (4)The formability of welded joints is inferior to that of the base metal because the weld is hard. When the weld heat input is high and thus the heat-affected zone is softened in case of 980MPa class high strength steel sheets, the formability decreases further.
To develop a new airtight seal parts for BWR in-core detector, brazing of silicon nitride and a metal(SUS304) was examined. In this paper, to clarify the relation between material properties of silicon nitride and bonding strength of the seal parts, tensile test of the seal parts specimen using various kind of silicon nitride was carried out. Cone shaped silicon nitride was applied at a center of the specimen and a copper interlayer which edge was tapered to 40° was inserted between silicon nitride and a metal(SUS304). Copper-titanium brazing alloy was used for brazing. The relation between mechanical properties of the silicon nitride and tensile strength of the joints indicated that the silicon nitride fracture toughness is more important than silicon nitride bending strength for determining over all joint strength. It is clarified that to obtain high tensile strength of the silicon nitride and stainless steel joints, that is to obtain highly reliable seal parts, it is necessary to use high fracture toughness silicon nitride in addition to an appropriate interlayer shape.
Hot dip galvanizing is considered to be effective for increasing corrosion resistance in steel bridges. The galvanizing makes possible to reduce the residual stress in the welded portion due to annealing effect, and stress concentration at the weld toe is reduced by the galvanizing if a part of stress is transmitted in the zinc layer. Therefore, it is considered that fatigue strength can be improved by hot dip galvanizing. These facts mean that the hot dip galvanizing is one of useful countermeasure against the fatigue and corrosion, which are regarded as weaknesses of a steel bridge. However, it has not been clear how the galvanizing influences the fatigue strength of a welded joint. This study aims at clarifying fatigue strength of welded joints galvanized. For this purpose, web-gusset welded joints that have especially low fatigue strength in the joint employed in steel bridges and are worried about occurrence of fatigue cracks are taken up, and model specimens were made of SM490YA and were galvanized under standard condition of hot dip galvanizing, then fatigue tests were carried out on these specimens.
Galvanizing is one of the effective corrosions protection methods, and many galvanizing steel bridges have been constructed until now. It is also considered that residual stress of welded portion is reduced by the annealing effect due to immersing welded member into hot dip galvanizing tank of about 450°C. The stress concentration at the weld toe as an origin of fatigue crack is reduced if a part of stress is transmitted in the galvanizing layer. Authors already confined facts that tensile residual stress of the welded portion was significantly relieved by immersing in galvanizing tank. It was also recognized by finite element analysis that stress concentration factor at steel was reduced by galvanizing layer. However, the fatigue strength of the galvanized joints became low compared with the as-welded joints. As the reason why the fatigue strength lowers, it was considered that the fatigue crack initiates in an early stage from the discontinuity at the alloy layer or rough boundary of galvanizing layer and steel. This study aims at clarifying fatigue strength of welded joints by galvanizing at Report 1. For this purpose, web-gusset welded joints and scallop details that have especially low fatigue strength in the joint used for steel bridges and are worried about occurrence of fatigue cracks are taken up, and the large-scale girder specimens were made of SM490YB and were galvanized under standard condition of hot dip galvanizing, then fatigue tests were carried out on these specimens. Furthermore, microstructures on the alloy layer between zinc and steel or welds have been observed and fatigue cracks origin has been closely examined.
The evaluation of ductile crack initiation in steel welded structures under seismic loading is crucial for structural design or safety assessment to prevent ductile cracking induced brittle fracture. The main purpose of this study is to propose a transferable criterion for evaluation of ductile cracking in welded structures under large scale cyclic loading from small scale tensile test results. On the basis of the observation of ductile crack initiation behavior of round-bar specimen with/without circumferential notches tested in single tension, the main controlling factor for ductile cracking in two-phase steel used was found to be a nucleation of micro-voids in soft phase (Ferrite phase) near Ferrite-Pearlite interface after a large extent of plastic straining. The material damage concept under reverse loading which correlates the material damage for micro-voids nucleation to macro-scale mechanical parameters was proposed by taking into account two aspects of Bauschinger effect : (a)mechanical aspect which influences deformation and stress/strain behaviors, (b) material damage aspect caused by dislocation behavior. A new criterion for ductile cracking of structural members under cyclic loading was proposed on the basis of that proposed effective damage concept and “two-parameter criterion” which can be applied to the steel structures under increasing load in single direction. The validity of the advanced two-parameter criterion was verified by conducting cyclic loading tests in axial direction for round-bar specimen and cyclic 3-point bending tests for cross-shaped specimens. Consequently, the advanced two-parameter ductile cracking criterion was found to be a transferable criterion for the evaluation of critical loading cycle of structural members from small scale tensile test results.
The fatigue strength of non-load carrying cruciform welded joints were investigated on small welded specimens by the testing method keeping the maximum stress to be the yielding strength of base metal. This stress pattern modifies the existence of welding tensile residual stress in real welded structures. Some fatigue tests were performed at R=0. Slit welded specimens of 250 mm width were prepared to realize the residual stress existed in real welded structures. The tensile residual stress at slit welded part reached to the yield strength. The tensile residual stress on small welded specimens was smaller than the yield strength. The fatigue strength of small welded specimens at R=0 was larger than that of slit welded specimen near fatigue limit region. The fatigue strength of small welded specimens by σ max=σ y tests coincided well with that of slit welded specimen. The knee point appeared around 2x107 cycles for slit welded specimens (R=0) and small welded specimens (σmax=σy). It is pointed out that the design S-N curves in JSSC have same concept with this paper.
Mechanical alloying (MA) method was introduced to produce a nano-grain WC powder as a source material and HVOF spraying of mechanical alloyed WC-Co powder was tried to fabricate a coating with nano-structure. Mechanical properties of the coatings obtained were evaluated and compared with the coating made from the commercially available conventional powder. The grain size of WC powder had a decreasing tendency up to nano meter order with increase of the alloying period. It was found that the grain size reached finally about 5nm after 100h MA treatment estimated by Scherrer’s equation. TEM observation result for the alloyed powder substantiated the estimation result of Scherrer’s equation. HVOF coating of the mechanically alloyed powder had a dense and lamellar structure, and the fine carbide particles distributed homogeneously in the coating. The grain size of the coating was estimated about 9nm and it was found that the grain size is not affected so much by the spraying. Hardness test was performed both on the surface and the cross section of the coating. The coatings made from the powders for 25h, 50h and 100h MA showed higher value in the cross section micro hardness by 20.2%, 9.2% and —18.2% compared to that made from the conventional powder. On the other hand, the hardness for the coating surface revealed lower value in the coating made from MA powder. The improvement of toughness value by 21.5% was performed in the coating made from 100h MA powder compared to that made from the conventional powder. From the observation of the crack pass in the coating, it was found that the higher toughness value of the coating made from MA powder was attributed to the more complex crack pass in this coating.
In the previous reports1-4), the authors had clarified the joining mechanism during the first phase of the friction stage during friction welding process. The present paper describes the effect of friction pressure and friction speed on initial seizure portion at the welded interface of mild carbon steels joint. The following are concluded. (1) When the substrates were welded under high friction pressure and high friction speed, i.e., 90MPa and 27.5s-1, almost no wearing existed during the first phase of the friction stage. The friction torque rapidly increased and reached to the initial peak torque immediately after both weld faying surfaces contacted each other. (2) The friction torque have included wear and seizure stages during the first phase when the substrates were welded under the welding conditions of (a) high friction pressure and low friction speed, i.e., 90MPa and 8.8s-1, and (b) low friction pressure and high friction speed, i.e., 30MPa and 27.5s-1. (3) The wear of both surfaces started at the periphery portion (outer surface), and it extended toward the center portion (center axis) of the welded interface. When the substrates were welded under high friction pressure and high friction speed, the initial seizure and the joining began at the periphery portion, and they extended toward the center portion. The initial seizure and the joining randomly began on the weld faying surface and extended toward whole surface with high friction pressure and low friction speed. On the other hands, they began at center portion and extended toward periphery portion with low friction pressure and high friction speed. (4) The initial seizure portion was described by using the relationship between the temperature and the relative speed of the welded interface, and seizure temperature of them. (5) The modeling was presented for the phenomena of the first phase of the friction stage.