Recently, lightening, speed-up and decreasing vibration of the transport vehicles have been discussed for improving of environmental problems. As one of the solution, the material hybrid concept using aluminum alloys and high strength steels has been proposed. Therefore, new welding processes by which these dissimilar materials can be joined in high reliability and productivity are demanded. Laser Roll Welding was developed for joining of dissimilar metals by M. Kutsuna, M. Rathod and A. Tsuboi in 2002. Up to now, a CO2 laser has been used as a heat source. In the present work, Laser Roll Welding of low carbon steel and aluminum alloy using a 2kW fiber laser was investigated to improve the joint properties due to the effective heating characteristics. Effects of the process parameters were studied. Otherwise, the influences of process parameters on the weldability, the formation of intermetallic compound layer and the mechanical properties have been investigated. As a result, various types of intermetallic compound layer were confirmed at the Laser Roll Welded joint interfaces. When intermetallic compound layer thickness was less than 10μm, specimen was failure in the base metal of low carbon steel in the tensile shear test.
In CO2 arc welding of solid wire, metal transfer phenomena and spatter generation are investigated by with rectangular pulse current, and low spatter CO2 arc welding process with high frequency pulse current is developed. The optimal conditions of high frequency pulse CO2 arc welding are in the range of peak current : 450-550A and pulse frequency: 450-750Hz. These high frequency pulse currents have the effect of the droplet oscillation due to resonance between applied pulse frequency and the natural frequency of the droplet. The droplet is transferred consistently every 9-11 pulses and the average interval of metal transfer is about 16ms which is reduced to half than that of conventional CO2 arc welding. This average droplet weight is 34mg, showing a large reduction in comparison with that of the conventional method. As a result, total spatter weight is reduced by 70% in comparison with the conventional method, and especially, large spatters more than 0.5mm in diameter are reduced from 1.5g/min to 0.2g/min.
The soiling of the slag, spatter and the fume etc which come in contact with the steel sheet surface with welding cleaned making use of the steel sphere shot material of large particle diameter, high projection pressure with peening processing strongly (below, calling this strong peening cleaning). In this research, the cleaning state of the soiling with welding and improvement of fatigue strength of the hot galvanized welded joint was inspected, when surface of SM490A welded joint was cleaned with strong peening cleaning. The following experimental results were obtained: 1) The fatigue limit of smooth base metal which did strong peening cleaning with about 320MPa was high value remarkably in comparison with smooth base metal with about 245MPa. 2) The fatigue limit of welded joint which did strong peening cleaning with about 300MPa was high value remarkably in comparison with as welded joint with about 170MPa. 3) The strong peening cleaning was high efficiency and cleaning state was satisfactory remarkably. 4) The cause of remarkable rise of fatigue limit (300MPa) of welded joint which did strong peening cleaning was because in fatigue limit (about 170MPa, 57%) of as welded joint, the improvement effect (about 130MPa, 43%) with peening cleaning was joined. It was considered that improvement effects were rise (about 68MPa, 23%) of hardness of the weld toe, relief (about 43MPa, 14%) of stress concentration, increase (about 136MP, 45%) of compressive residual stress and the decrease (about—96MPa, —32%) by increase of surface roughness. 5) The fatigue strength of hot galvanized welded joint decreased remarkably. This was thought as the thing due to the decrease (about HV40) of hardness of the surface, the decrease (about—188MPa) of compressive residual stress and the influence of many factors which was accompanied hot galvanizing.
The Gas Hollow Tungsten Arc (GHTA) welding experiments on aluminum pipe were carried out in a simulated space environment using an aircraft. A vacuum chamber and welding machine for GHTA welding test were placed in the cabin of aircraft and the 10-2 G gravity environment was produced by a parabolic flight of the aircraft. The square butt welding joints with non root gap on aluminum pipe were made by orbital welding in the vacuum chamber without wire filler metal using DC or DC-pulsed power supply under the 10-2 G and 1 G gravity conditions. The welding phenomenon during the aluminum GHTA welding recorded in the high-speed video image was analyzed and also the macrostructure and mechanical properties of butt weld joints were investigated. The welding experiments under simulated space environment showed that the DC-pulsed GHTA process could make the welding joints without the weld defects such as a lack of fusion, oxide film inclusion and spattering, though throat thickness decreased by the impulsive arc pressure of pulsed current. It was also clarified that the arc discharge phenomenon and melting characteristic at the molten pool surface during the DC-pulsed GHTA welding were insensitive to the gravity condition. However, the sagging weld metal made at 1 G gravity condition increases a little than that welded under the 10-2 G gravity condition.
Cavitation bubble activity caused by ultrasonic irradiation in water was applied to the hard surface such as carburized steel and TiN-coated on WC-super hard alloy to investigate whether it is possible or not to introduce residual compressive stress on the hard surface. The main results obtained in this study are as follows. In the case of carburized steel with hardness of 700HV, the large residual compressive stress of approximately 820MPa was successfully introduced on the surface by the ultrasonic irradiation in water. Furthermore, it was revealed that the ultrasonic irradiation in water successfully introduced the large residual compressive stress of approximately 2400MPa on the surface of TiN-ceramic coated on WC-super hard alloy of which the initial surface residual compressive stress was about 1800MPa. The redundant application of the ultrasonic irradiation on the TiN ceramic surface after the residual stress reached the maximum caused not the variation in the surface roughness and micro-hardness but a little damage such as very small pits on the surface.
In the control of maintenance of steel structures, it is important to measure quantitatively the wall-thinning and the actual stress of damaged and depleted materials by a non-destructive method. The main purpose of this study is to establish the thickness and the stress measurement technology for steel structures by using the electromagnetic acoustic method which does not require a coupling medium. In this paper, an experiment using specimens that imitated the effects of localized corrosion was performed by an electromagnetic acoustic transducer, in order to examine a method for a wall-thinning measurement using resonance methods. As a result, by focusing on the amount of peak amplitude of the resonance spectrum equivalent to each thickness it was confirmed that measurement of the wall-thinning depth and the wall-thinning area was possible. In addition, by the examination using a numerical simulation of the resonance spectrum, this method's detection principle was clarified and the new wall-thinning evaluation was established.
Microstructure development in weld material containing Ti and B has been in-situ observed by using laser scanning conforcal microscopy, with various cooling rate. The samples with different aluminum contents were prepared for the observation. In the sample with low Al/O ratio (0.48–0.73), acicular ferrite microstructure transforming from intragranular nucleation site was in-situ observed. On the other hand, with high Al/O ratio, the bainite microstructure originating from grain boundary of austenite was in-situ observed. The transformation temperature decreased with increasing aluminum content. It was cleared that the microstructure difference was due to the different inclusion in the matrix. Based on those observations, the chemical composition, cooling rate and the microstructure were systematically related and the microstructure developments were summarized as a CCT diagram.
The core technology of underwater TIG welding process has been developed and welding equipment system has been manufactured, for application to the maintenance of the spent fuel storage pool of Rokkasho reprocessing plant. Basic experiments for understanding the conditions of dry area and the range of welding conditions was performed, and mock examination for simulation of real environment by using the developed welding equipment was also carried out to judge the applicability of the system. For the purpose that can be selected water removing method for different spatial conditions of the parts to be maintained in underwater, two kinds of welding equipment systems of Chamber type and Partition type were developed and manufactured. On the basis of fundamental experiments, the conditions of dry area formation and welding parameters range for high-reliability weld were discussed. Thus the proper condition in this process was able to be established. With the welding equipment systems of the Chamber type and Partition type, the practical use examination of underwater TIG welding process was executed by mock examination for simulating the real environment. As a result, it was confirmed that the underwater TIG welding could obtain the same reliability as a usual in-air TIG welding, and the operation and the control at remote distance were also possible. And the reliability of the patch-plate fillet weld could be evaluated by remote inspection with the expansion visual test.
A study was made on the relation between CTOD value and M-A constituent for the single-heat-cycled weld HAZ of YS 320 to 360 N/mm2 class TMCP steels with increased heat input. It was found that the M-A constituent disappeard and lost its deterioration effect on the HAZ CTOD toughness when the heat input exceeded about 15kJ/mm, although this boundary heat input depended on the steel chemistry. On the other hand, the austenite grain size increased monotonically with increasing the heat input. But the austenite grain size could not be the controlling factor of the HAZ toughness, and its effect deferred between base metals. However, the HAZ toughness was related to the fracture facet size for the large heat input conditions. This fracture facet size, which represents the fracture toughness, is considered to be a measure of the uniformity of the transformed microstructure from austenite in the HAZ.
Application of cold spray process is expected in the various industrial fields such as automobile, aircraft and electronics. So many investigations for practical application have been actively performed in many research organizations. However, as the process has just started in these days, fundamental aspect like deposition mechanism itself has not been well understood up to today. To establish the higher reliability or controllability of the cold spray process, fundamental research, especially on the deposition mechanism, may become a key for the process development. We tried the clarification of particle deposition mechanism in cold spray process by experimental approach. In this study, deposition behavior of sprayed individual particle onto substrate surface was investigated precisely. Self designed cold spray equipment was installed in the laboratory and deposition behavior of sprayed individual metallic particles on the substrate surface was fundamentally investigated. As a preliminary experiment, pure Cu particles were sprayed on mirror polished stainless steel substrate surface. Particles diameter, process gas, gas pressure, gas temperature and substrate temperature were systematically changed as the process parameters, and effect of these parameters on the flattening or deposition behavior of an individual particle was investigated.
Fatigue test for the steel plate deck was carried out by using the wheel load traveling test machine. The position where fatigue crack initiation was specified by FSM (Field Signature Method) could be confirmed by visual inspection (9-11)×104 cycles later. The time of initiation of small crack (less than 150mm), which cannot be confirmed by visual inspection, could be specified by FCs value that is the deviation of FC value. The direction of propagation and the length of fatigue crack could be specified. Being conscious of the site, it was investigated whether initiation and propagation of fatigue crack can be detected with high accuracy and extensively or not by impressing pulse current in the wide range (1200×1600 (in mm)) and by extending the distance (80, 160 and 240 (in mm)) between pairs. Although FC value became small as the distance extended, FC values increased with the similar tendency. So, specification of fatigue cracks was not difficult.
Cold spray has attracted a great deal of attention for fabrication of thick, high purity metallic coatings. In cold spray, metal particles are accelerated by process gas with solid phase and deposited onto substrate or previously deposited coating. In order to increase the particle velocity to deposit harder metal materials, the process gas pressure need to be higher. However, using high gas pressure needs large facility and much gas consumption. In this study, the availability of low pressure cold spray upon fabrication of copper coatings was examined. Copper particles were sprayed with changing process gas temperature and pressure which was less than 1 MPa. It was effective to raise the gas temperature to enhance the deposition efficiency. It was possible to fabricate copper coatings with high deposition efficiency with controlling the gas temperature and pressure by low pressure cold spraying. The optimal particle size of copper powder was determined empirically for low pressure cold spraying. The hardness of the coatings did not depend on the porosity of the coatings, but depended on the process gas temperature. Process gas temperature affected not only deposition efficiency but also the characteristics of the coatings such as electrical conductivity.
The present study applied Friction Spot Joining (FSJ), which was recently developed as a lap joining technique of Al alloys, to two sheets of Al alloy 6061, 1mm in thickness, and then examined the microstructural feature in the weld. The weld had the nugget-shaped stir zone around the exit hole of the probe, and the stir zone exhibited the equiaxed grain structure having finer grain size than that of the base material. The crystallographic texture analysis using electron backscattered diffraction method suggested that the material movement occurred along the rotating direction of the welding tool in the wide region including the stir zone. In the periphery of the nugget-shaped stir zone, which was characterized as region having the finer grain size than that of the stir zone interior, any inclusions and precipitates were not found in the SEM scale. The weld was softened around the weld center. The softening could be explained by dissolution and/or growth of the strengthening precipitates due to thermal cycle of FSJ.
In order to improve fatigue strength in welded joints, low transformation-temperature welding wire has been developed in which residual tensile stress can be reduced. In application of the low transformation-temperature welding wire, the prevention of cold cracking without pre-heating in high strength steel welded joints is expected and examined from the control of residual tensile stress. However, it is expected that residual stress distribution in welded joint can be suggested by numerical analysis, because the residual stress cannot be measured simply and non-distractively. In this report, martensite transformation behavior such as Ms point, transformation expansion and so on is measured firstly by Formaster test. And temperature dependence of several mechanical properties was measured in full-austenite and full martensite microstructures, and temperature dependence of mechanical properties was estimated in dual phase microstructure of austenite and martensite. By these data, numerical analysis was carried out and martensite transformation behavior was compared with measured and calculated results in side rigid model test. From the comparison, it was suggested that transformation superplasticity had to be considered in numerical analysis. Next, the increase of Ms point due to transformation induced plasticity was guessed from the comparison with measured data by laser speckle measurement and calculated data under transformation superplasticity consideration. From the all results, it was found that the measured transformation behavior and residual stress had the good agreement with the calculated results under transformation superplasticity and transformation induced plasticity considerations.
About the thesis, we made the welding experiment by using three kinds of test pieces, the actual size and the diaphragm and the butt joint. Then, we examined the influence to strength, cooling time and carbon equivalent of weld metal by welding conditions about the difference of test pieces. We calculated estimate equation that estimate cooling time and chemical component. And we came out strength of weld metal is estimated by those numerically values. Consequently, we got strength of weld metal is estimated by heat input, interpass temperature, carbon equivarent of welding wire and shape of test piece.
In order to clarify the effect of tip velocity on the weld solidification process of hot-work tool steel (SKD61) during the welding, the information about microstructure evolution was obtained by the combination of liquid tin quenching and Time Resolved X-ray Diffraction (TRXRD) technique using intense synchrotron radiation. From the experimental results, it was found that the solidification mode was changed from FA mode (L→L+δ→L+δ+γ→L+γ→γ) to A mode (L→L+γ→γ) at high tip velocity. Moreover, the effect of tip velocity on the microstructure selection during solidification between the primary δ — ferrite and the primary γ — austenite was theoretically proven by the KGT model. Therefore, it was understood that the solidification cracking susceptibility of hot-work tool steel (SKD61) weld metal was increased due to the δ to γ transition of the primary phase.
This study aims to ensure the safety of the nuclear power plants. The accidents of leak from the welded zones at the pipe penetration part of reactor vessel or at the coolant pipe are reported at home or abroad. One of the main causes is the welding residual stress. So, it is important to know the welding residual stress for the keeping of high safety of the plants, the estimation of plants life cycle and the plan of maintenance. The welded joints of the nuclear power plants have complex shapes, and the welding residual stresses also have complex distributions three-dimensionally. In this study, the inherent strain method combined with Finite Element Method (FEM) is used to measure the welding residual stresses accurately. The mock-up is idealized for the welded joint at the pipe penetration part of actual reactor vessel. The inherent strain method is applied to measure the residual stresses. In this method, the inherent strains are unknowns. When the residual stresses are distributed complexly in 3-dimensional stress-state, the number of unknowns becomes very large. So, the inherent strains are expressed with some functions to decrease the number largely. The theory, the experiment process and the analyzed results are explained. The characteristics of the distributions of residual stresses and their production mechanisms are discussed. The inherent strain method gives the most probable values and the deviations of the residual stresses. The deviations are small enough for the most probable values. It assures the high reliability of the estimated results.