To improve the quality of a joint composed of metallic micro-structural parts, a novel resistance brazing method called the "two-step resistance brazing method" has been developed. In this method, filler metals plated on the surface of base metals are butted and alloyed due to the Joule's heat by performing two-step energizing. In this study, the numerical simulation of the joining process for the two-step resistance brazing method has been performed. A mathematical formulation and corresponding calculation scheme, which is derived from using the control-volume method, are developed for a model based on contact resistance between the filler metals plated on the surface of the base metals, and transient two-dimensional heat conduction with the Joule's heat generation. The calculated result, based on a base metal of type 304 stainless steel and plated filler metals of gold and copper, is discussed and compared with experimental data.
Arc welding phenomena have been theoretically investigated by using various numerical models, which have been proposed and developed together with rapid progress of the computer technologies. However, most of the numerical models are two-dimensional axial symmetric model that are available only for stationary arc on the flat plate. Actual welding processes applied to manufacturing field are performed with various joint geometries and the welding arc moves on the base metal. Therefore, they should be non-axial symmetric phenomena and it is required to be discussed with three-dimensional model. In this research, TIG arc characteristics in V-groove welding are numerically investigated by using our three-dimensional numerical model. It is shown that both of heat input and arc pressure on the groove surface are significantly different from the flat plate surface. In the groove heat input and arc pressure are varied sensitively with location and aiming of the TIG torch. Experimental results show the validity of calculation results of TIG arc heat input distribution on the groove surface.
Metals and plastics have been widely used in industrial applications, and joining of a metal to a plastic is necessary and important from a manufacturing viewpoint. Therefore, we have developed laser-assisted metal and plastic joining (LAMP joining) as an innovative rapid laser direct joining without adhesives or glues. In this research, the joining between a Type 304 stainless steel plate of 3 mm thickness and a polyethylene terephthalate (PET) plastic sheet of 2 mm thickness was exploited at several traveling speeds with a diode laser beam of line profile at 170 W power. The joints of 30 mm in width possessed extremely strong tensile shear loads of approximately 3,000 N at the maximum. Transmission electron microscope photographs (TEM) of the joints demonstrated that Type 304 and PET were bonded together on the atomic or molecular level through a Cr oxide film of stainless steel. X-ray Photoelectron Spectroscopy (XPS) and Time-of-Flight Secondary Ion Mass Spectrometry (TOE-SIMS) analysis indicated the possibility of chemical bonding between hydrocarbon and metal through oxygen. Concerning reliability evaluation for the LAMP joints, some joints could have high shear tensile loads of approximately 3,000 N even after heat cycle test and show superior airtightness in helium leak test. Consequently, it was confirmed that the LAMP joining of SUS304 and PET plates could produce a sound joint of high strength, nanostructural binding through the oxide film and fundamental reliability for practical use.
In the previous reports, one of the authors showed that a long specimen's in-plane strain geodetically estimated from the final shape after laser forming has sufficient agreement with the in-plane strain measured from a distance change of landmarks. For short specimens, these two type in-plane strains are not compatible because of the in-plane strain parallel to laser heating line. To form bowl shape thin plate by laser forming, accurate in-plane strain should be introduced into the plate. So, both parallel and perpendicular in-plane strains should be presented accurately for objects in various size and shape. In this report, effects of both specimen length and initial curvature on in-plane strains in two directions, parallel and perpendicular, are investigated experimentally.
FEM-MD combined method is proposed for the microscopic stress analysis of steels. In this numerical method, Finite Element Method (FEM) is applied to the stress analysis inside grains, and Molecular Dynamics (MD) is applied to the calculation of the atomic configuration near the grain boundary in order to consider the microscopic heterogeneity and the deformation near the grain boundary that influences the stress distribution. Slip length between two grains caused by the mismatch of the displacement near the grain boundary is calculated by FEM. Slip resistance, which is necessary to calculate slip length, is obtained by calculating the atomic configuration near the grain boundary by MD. The combination of FEM and MD is realized by using slip resistance in FEM and slip length in MD. The validity of modeling of the deformation near the grain boundary is investigated by comparing the deformation near the grain boundary calculated by FEM-MD combined method to that observed in the experiment in the case of a load applied to the specimen. Calculated slip length coincides with measured one. FEM-MD combined method is applied to the investigation of the influence of change in the grain shape caused by the thermal history such as the weld zone upon the strength characteristic. The high stress region tends to increase in the case of the grain diameter larger and it is indicated that grain coarsening due to the weld thermal history increases the possibility of the crack initiation. FEM-MD combined method is expected to be helpful in investigating the mechanism of fracture or the strength characteristic of the complicated microstructure such as the weld zone by evaluating the microscopic stress distribution.
As for steel frames of buildings, the welding of single bevel T-joints with steel backing is usually used. Recently, there have been some experimental reports concerning 25 degree groove welding by semi-automatic arc welding. However it is not yet carried out in the actual field. The shortening of welding time leads to a reduction of emissions of greenhouse gas. Therefore, as this is an earth environment improvement, we expect this welding process in the steel frames of buildings to become attractive. In the same way as semi-automatic arc welding, the welding of the first layer is one of the biggest problems for the narrow groove welding in steel frames of buildings by welding robots. In this study, we researched actual assembling precision of T-joints, and we found the relationship between welding condition and crack occurrence. Our results demonstrated the probability of narrow groove welding of T-joints between square steel tubes and diaphragms where robot welding was used.
Iron based high carbon - high chromium hard overlay is used in order to give a high wear resistance for abrasive parts. As weld, this hard overlay has a microstructure with primary precipitated carbide M7C3, secondary precipitated carbide and matrix gamma-phase, and has a hardness of 800—900 HV. For this hard overlay, the influences of heat treatment to the wear resistance were investigated. The wear resistances of the hard overlay did not change at the heat treatment conditions of 773 K (500 degree C) x 1 hour or 823 K (550 degree C) x 1 hour. But these dropped down at the heat treatment conditions over 823 K. Hardness of the hard overlay changed like as the wear resistance. These phenomenon were affected by the microstructure change that gamma - phase in the matrix phase of the weld metal transformed to alpha - phase by heat treatment at the conditions over 823 K.
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 for getting the excellent joint strength and have found that the formation of this brittle IMC is regionally prevented by using the advanced hot-dip aluminized steel sheet and by adopting the suitable joining conditions. In particular this paper focuses the mechanism of creating this IMC free region in case of MIG-braze welding and the results obtained are as follows. (1) The creation of IMC-free region is initiated as the first process by the dissolution of the τ5 phase (Fe-Al-Si) in the aluminized layer into the weld metal, and the temperature more than 886K for dissolution during MIG-braze welding and the use of filler metal for dilution of Fe and Si in τ5 phase, have significant effects. (2) As the second process, the diffusion between aluminum-alloy weld metal and base steel is restricted by AlN on the surface thin layer of the base steel existed under 908K temperature during MIG-braze welding.
Microcracking behaviours in the multipass welds of alloy 690 were investigated. The effect of impurity elements such as P and S on ductility-dip cracking susceptibility in the reheated weld metal was evaluated by the spot-Varestraint test using different filler metals varying the contents of P and S. The ductility-dip cracking susceptibility was reduced with a decrease in the content of impurity elements in the filler metal, and the amount of (P+1.2S) in the weld metal should be limited to 30ppm in order to prevent the microcracking in the multipass weld metal. Numerical calculation of microsegregation and molecular orbital analysis have suggested that ductility-dip cracking was attributed to the grain boundary embrittlement due to the grain boundary segregation of P and S. FEM analysis of thermal strain predicted that ductility-dip cracks would occur during multipass welding when the plastic strain-temperature curve intersected the ductility-dip temperature range (DTR). The effect of the addition of rare earth metals (REM) to the weld metal on the microcracking susceptibility was examined by using the La or Ce containing filler metals. The ductility-dip cracking susceptibility could be significantly improved by adding 0.01-0.025mass%REM to the weld metal. Microstructural analyses revealed that the ductility-dip cracking susceptibility decreased as a result of lowering the grain boundary segregation P and S due to the scavenging effect of REM. The multipass welding test confirmed that microcracks in the multipass welds of alloy 690 were completely prevented by using the filler metals containing approx. 0.03mass%REM.
It is well known that weld distortion, which has a negative influence on material properties, structural fabricability, and structural integrity, should be controlled appropriately. There are many methods to control or reduce weld distortion, but most of them involve some costly process in addition to welding. In-process control of weld distortion becomes more preferable than post-welding process or other methods, when manufacturing efficiency is considered. In recent years, in-process control welding by additional cooling has been proposed as one of techniques for reducing weld distortion and partially applied for thin-plate structure in industries. However, the effectiveness of additional cooling method has not yet been fully clarified. In this study, the effectiveness of additional cooling method and appropriate cooling condition for effective reduction of weld distortion are investigated by three-dimensional thermal elastic-plastic analysis. In addition, the effect of locally cooled temperature distribution on generation behavior of plastic strain is discussed. As the result, it is concluded that the effectiveness of additional cooling and appropriate cooling condition for reduction of weld distortion are dependent on weld distortion under consideration and welding conditions. Especially, it is necessary for reduction of weld distortion to set the cooling torch at the appropriate position. For example, in order to reduce angular distortion effectively, the appropriate cooling position is dependent on the mechanical melting length during welding.
Authors tried to braze magnesium alloy in air using no flux with the aid of ultrasonic vibration, and investigated the effect of brazing conditions on the joint properties. The main results obtained in this study are as follows. Applying ultrasonic vibration made it possible to braze the magnesium alloy in air without flux and the joint strength was so high that the joint fractured partially in the base metal. The brazing temperature at which solid and liquid phases coexisted in filler metal could provide the brazed joint with the maximum tensile strength. This seemed to be because the liquid phase in the filler metal infiltrated into the cracks occurred in the oxide film on the faying surface during heating and the solid phase would rub against the oxide film to detach, resulted in removing the oxide film from the faying surface. The optimal time for applying ultrasonic vibration could effectually detach and remove the oxide film from the faying surface. The excessive applying time of ultrasonic vibration caused defects such as cavity in the brazed layer and led to decrease the joint strength.
This study investigated ductile crack initiation limit of pipeline girth welded joints with different strength mis-match. The ductile crack initiation limit for the girth welded joints was evaluated by conducting three point bending fracture toughness tests and wide plate tensile tests with a surface notch. In addition, effect of heat input on the ductile crack initiation limit of weld metal was evaluated on the assumption that a welding condition would be varied in the field in the actual pipeline construction. As the results, the equivalent plastic strain at the notch tip for the ductile crack initiation of the three point bending tests was consistent with those of the wide plate tests, and the heat input hardly affected the ductile crack initiation limit within the range of this study. This meant that the ductile crack initiation limit of the pipeline girth welded joints with strength mis-match was able to be estimated using the equivalent plastic strain obtained from the three point bending tests. Based on these results, we proposed a procedure to determine the rational fracture toughness requirements which took into account the difference in the plastic constraint between standard fracture toughness test and pipeline girth welded joints. This procedure was also possible to determine the required strength matching level for a strain-based design for girth welded joint containing surface notch in the center of the weld metal in terms of preventing the ductile crack initiation.
It is well known that weld residual stress and distortion should be controlled appropriately for structural integrity. Recently, it has become much more necessary to control weld distortion for highly improving manufacturing efficiency. Various studies on control of weld distortion had been conducted based on clarification of influential dominant factors for that. The influential dominant factors had been studied from a viewpoint of temperature distribution in plate thickness section. Without considering moving of weld heat source, the temperature distribution is controlled by weld heat input (Qnet) per weld length. Angular distortion, which is controlled by temperature distribution along direction of plate thickness (h), is controlled by heat input parameter (Qnet/h2). However, it has been recently known that the conventional results cannot be applied to all welding processes because such processes are becoming more diversified. It is significant for more accurate control of angular distortion to investigate once again the relationship between the heat input parameter and angular distortion. In this study, series experiments on the relationship between heat input parameter and angular distortion is performed. The effects of welding current and welding speed are investigated individually in both TIG and MAG welding. The difference between these welding methods is also investigated. Based on the result, the effects of them are discussed in relation to temperature distribution during welding. It is considered that angular distortion is affected by temperature distribution not only in plate thickness section but also along welding direction. So, angular distortion is not always controlled by only the conventional heat input parameter because the heat input parameter is proposed as the influential dominant factor for temperature distribution in plate thickness section. It is concluded that generation characteristics of inherent strain should be considered in relation to three dimensional temperature distributions during welding for more accurate control of angular distortion.
Due to the rapid improvement of digital cameras, especially the pixel resolution, digital image correlation (DIC) has been introduced to measure the deformation and strain of structures. Using digital cameras for the DIC technique is an easy and fast method for obtaining structural information, represented as all the pixel points in a photo. Because a wide range of structural deformation can be obtained with high accuracy, this method has the potential to be very useful. Currently, DIC can execute a measurement with high accuracy only when the out-of-plane displacement is small. When the out-of-plane displacement is large, the deformation causes the measurement error. Therefore, a stereo imaging method using two digital cameras is proposed in the present study. The proposed method can measure not only in-plane deformation but also out-of-plane deformation with high accuracy without calibration of the errors caused by the out-of-plane displacement. In this paper, the measurement accuracy of the proposed method for in-plane and out-of-plane deformation is discussed through the application of a bead-on-plate welding test. The proposed method can measure transverse shrinkage and angular distortion with high accuracy. In contrast to the vernier caliper and laser distance meter measurement methods, which can measure only a few points at a time, the proposed method using two digital cameras can measure the full field in a short time. These results confirm that the proposed method is more advantageous than other methods.
Ferritic stainless steel SUS430 sheets were friction stir welded by using a Ni-base dual two-phase intermetallic alloy tool. After friction stir welding, the SUS430 work and the tools were evaluated in terms of microstructure and mechanical properties. The tensile specimens cut from the welded joints fractured in the base metal portion and their fracture strength was equal to that of the base metal. The stir zone comprised of recrystallized fine microstructure was observed, and also the thermo-mechanically affected zone was observed in an advanced side. Hardness in the upper one third layer of the welded cross section was higher than the base metal. The admixture matter from work to tool surface occurred while that from tool to work surface did not take place in the SEM-EPMA resolution level. The amount of wear of tool was negligibly small, suggesting that the Ni-base dual two-phase intermetallic alloy is promising as a new type of friction stir welding tool used for high melting materials such as steel.
In the pulsed MAG welding with highly precise periodic drop transfer, welding current was analyzed by FFT (Fast Fourier Transform). By this analysis peculiar frequency of the molten pool was detected by resonance phenomena with the base plate vibration and it was confirmed that peculiar frequency falls according to growth of the molten pool. Furthermore natural frequency was detected at natural welding state that is without vibration of the base plate and confirmed by resonance method with the base plate vibration. To amplify natural vibration of the molten pool, drop transfer pulse was modulated consisting of regular pulse peak current and higher pulse peak current. The higher pulse peak current makes arc pressure change and molten pool is pushed down in the period of higher pulse. Because short circuit is apt to occur at the time that the molten pool is at the top of front side, so it is effective for amplify the molten pool oscillation that higher pulse phase begins at this time instantaneously or somewhat later. According this way higher peak was gained at peculiar frequency by FFT analysis. It means that amplify of the molten pool oscillation was successful. Appropriate period of higher pulse for maintain stable oscillation is from about 1/4 to 1/2 of the oscillation cycle.
LNG storage tanks made of 9%Ni steel plate have safely been operated at the many LNG export and import terminals in the world for the last half-century. Over the years extensive improvement and enhancement have been achieved on the 9%Ni steel plate in terms of toughness in steel to withstand the cryogenic temperatures under an operating condition with LNG. This paper reports research and development of the 7%Ni - TMCP steel plate. As nickel is an expensive and valuable rare metal, it was aimed to reduce nickel content to 7% to save investment costs for constructions of LNG storage tanks. Inferiority of the lower nickel content was compensated by adjusting chemical contents of Mn, Si, Cr etc. and by Thermo-Mechanical Controlled Process (TMCP) with an intermediate heat treatment. In order to evaluate fitness of the 7%Ni - TMCP steel plate and its weld for LNG storage tanks a series of testing was conducted. Three different kinds of plate thicknesses of 40, 25 and 10 mm were chosen to run large scale fracture toughness tests such as duplex ESSO tests, cruciform wide plate tests as well as small scale tests. Those mechanical tests showed excellent quality of the steel and demonstrated safety of the steel which was considered the same safety level as the conventional 9%Ni steel plate. Hence, it is considered that the 7%Ni - TMCP steel plate warrants serious consideration for use in LNG storage tanks.
In resistance spot welding of thin sheet-thick sheet-thick sheet joint, when the sheet thickness ratio is large (sheet thickness ratio = total thickness of sheet joint/thickness of the thin sheet positioned on the out side of the joint), how to stably secure the nugget between the thin sheet and the adjoining thick sheet is a key issue. If the sheet thickness ratio is so large, nugget formation between the thin sheet and thick sheet is extremely difficult. In order to control of the nugget (position of formation, shape, etc. of the nugget) during welding for three sheets joint with a high sheet thickness ratio, optimum welding process was investigated. The developed "2-step force, 2-step current" welding process was suitable for high sheet thickness ratio joint and relaxed the constraints on the sheet thickness ratio. In Step 1 (first part of welding period) of the welding process, a nugget is reliably formed between the thin sheet and thick sheet by applying conditions of low electrode force, short welding time, and high current. In the subsequent Step 2 (second part of welding period), a nugget is formed between the two thick sheets by apply high welding force and a long welding time. In the weld results of a three sheet joint (0.7mm+2.3mm+2.3mm; sheet thickness ratio: 7.6) using mild steel GA (0.7mm) as the thin sheet and 780MPa high strength GA (2.3mm) in the two thick sheets, "2-step force, 2-step current" spot welding process showed the wide available welding current range.
Fatigue properties of cast aluminum welded joints by friction stir welding (FSW) and MIG welding were investigated, comparing with that of the base plate. Fatigue crack propagation tests for the da/dN-ΔK relation and bending fatigue tests for the S-N relation were performed. Fatigue cracks in both FSW and MIG specimens were accelerated, when the fatigue crack tip reached the stir zone or the weld metal. This behavior was discussed based on the crack closure induced by the crack surface roughness and the residual stress. In the S-N properties, the influence of specimen surface finishing on fatigue life was also examined. Fatigue lives of the FSW and MIG specimens in the "as weld" condition were in the range of the largely scattered base plate fatigue lives, in spite of the different fatigue crack initiation site in each specimen such as the porosity in the base plate, the tool mark bottom in the FSW and the weld toe in the MIG. The FSW specimens with the polished surface showed the particular improvement in fatigue strength for finite fatigue life.
In recent years, in order to reduce the costs of transportation and construction of pipelines, which are often constructed using multiple-electrode submerged arc welding (SAW), higher joint performance is required. Therefore, there has arisen the need to understand theoretically and control appropriately metallurgical and mechanical characteristics in Heat Affected Zone (HAZ), which has a significant influence on the strength and toughness of welded joints. Commonly, metallurgical phenomena in HAZ are evaluated based on the highest temperature and the cooling rate. Therefore, in order to control metallurgical and mechanical characteristics in HAZ by means of the welding conditions, evaluating the temperature distribution and the temperature history near the melted zone is essential. However, a detailed investigation of the temperature distribution for multiple-electrode submerged arc welding has not yet been performed enough. In present study, in order to investigate the temperature distribution and histories during multiple-electrode submerged arc welding, the experimental results are compared with the theoretical results. In the theoretical analysis, the temperature rise equation in multiple heat sources welding is developed using the method of summation. Furthermore, on temperature distribution during welding, the effects of multiple heat sources, such as the number of heat sources and the distance between each electrodes, are considered quantitatively through the thermal conduction theoretical analysis. As the result, the distance between lead heat source and final heat source primally influences the area with the difference between a single heat source welding and multiple heat sources welding. Based on the results, it is expected to control temperature distribution near melted zone by more appropriate heat input characteristics, which is depended on heat source arrangement.