The present study has proposed the noble laser welding method of dissimilar metals joint, where the solid/liquid state diffusion process is effectively adopted to avoid unfavorable intermetallic reaction layer produced in the conventional liquid/liquid process. The suitable laser welding conditions for realizing the solid/liquid state diffusion process were predicted based on the unsteady thermal convection Boundary Element Method (BEM) analysis. Laser welding experiments were carried out by using Nd:YAG laser based on the results of BEM analysis. Dissimilar joints between steel and aluminum alloy could be successfully welded by using the solid/liquid state diffusion process. The welding conditions obtained by the experiments were in good agreement with those predicted by the BEM analysis. Thin intermetallic layer at the interface was detected. The maximum failure load of the joint was 69% of that of the base metal (A5052-O), while it would depend upon the bonded width.
Joule heating of welding wire is very important for stabilizing arc discharge. Joule heating of bulk wire is simple, has been estimated in many papers. On the other hand, Joule heating at sliding contacts during welding has not been estimated exactly. In this paper, current branch and contact resistance were measured during welding, and the temperature of wire above molten droplet was calculated using the measured values. Contact resistance of the main sliding point effected the average temperature of wire, and current branch effected the deviation temperature of wire. Joule heating power at sliding contacts of non-Cu-coated and Cu-coated solid wire is 108W and 59W respectively. Joule heating power of bulk wire is 829 W and 902 W respectively, so Joule heating power of the sliding contacts can change the average and deviation of melting speed during welding.
High speed video image analysis, optical measurement, and sound pressure analysis were conducted to reveal the difference between Cu-coated solid wire's and non-Cu-coated solid wire's molten droplet formation and transfer. It was obvious that the molten droplet size of non-Cu-coated solid wire was smaller than that of Cu-coated solid wire in any welding conditions except for very slow droplet formation. As Cu coating diffusion time was similar to molten droplet formation and transfer time, Cu coating on solid wire surface influenced the formation and the transfer of molten droplet. Cu coating prevented oxidization on solid state wire surface, and reduced the amount of oxygen on the molten droplet surface. Non-Cu-coated solid wire was the ideal material for the formation of small molten droplets of the short-circuit and the spray transfer.
This paper deals with the sufficient conditions under which an instantaneous source is able to be used instead of a moving source being a welding source. The error calculated from between the temperature of the instantaneous source and one of the moving source was investigated. The applicable conditions less than 1% and 5% error, respectively, were shown and was able to use for judging useful source from the informations such as welding speed, thermal diffusivity and the distance from between the aimed position of temperature and the weld center line. The approximate equations for maximum temperature rise of moving source, which are an accuracy of about 99%, were proposed.
Changes in arc voltage do not always mean fluctuations of arc length in aluminium alloy welding because cathode fall voltage increases with the absence of oxide. In addition, with the absence of oxide cathode spots become concentrated and the current density near the cathode surface becomes high so that the potential gradient there increases. After the breaking of a short circuit at electrode positive polarity cathode spots are formed on the centre of weld pool surface where little oxide exists. As a result, arc voltage becomes abnormally high in spite of the short arc length. Moreover, when polarity is switched from electrode positive to electrode negative right after the detachment of a droplet, cathode spots are formed on the surface of remained melt metal at the wire tip where little oxide exists so that arc voltage also becomes abnormally high. The arc becomes unstable when those abnormal arc voltages are included in the feedback signal for arc length control. Therefore, a digital filter is developed to obtain correct arc length information by eliminating those abnormal arc voltages from the feedback arc voltage. The algorism of arc length control system is another important factor that affects arc stability. A new algorism is developed to monitor continually the feedback arc voltage of the present pulse cycle and to determine the right timing to terminate the base duration of the present pulse cycle. By using this algorism the average current and average voltage of every pulse cycle plot a line with a decreasing characteristic called pulse V-I characteristic. The inclination of the pulse V-I characteristic can be set freely and controlled accurately. Arc stability is improved by optimisation of the inclination in AC or DC pulsed MIG welding. Furthermore, low frequency modulated AC pulsed MIG welding process is developed by setting two pulse V-I characteristics and switching between them periodically at a low frequency.
The formation of stable back beads in the first layer weld during one side multilayer welding is important to achieve high quality welded metal joints. The authors thus employed the switch back welding method for V groove joints without backing plates. In this method, the power source, wire feed motor, and robot manipulators were controlled by the computer based cooperative control. The torch is moved backward and forward like switch back. First, the torch is moved forward with high speed. The arc is discharged to the each root edge of the groove. The droplet is deposited to each root edge, too. After that, the torch is moved backward with the same high speed. During the backward motion, the deposited metal makes the bridge between the both edges due to the surface tension. Next, the torch is moved forward with slow speed to form the bead. The mentioned torch motion is repeated. When constant welding conditions are used, the back bead becomes unstable by affection of the disturbances such as gap variations due to the welding distortion. In this method, it is important to measure the gap. The authors discuss a method to detect the gap and to get the stable back bead by using a visual sensor, the CCD camera. During the slow forward torch motion, the weld pool can be observed. The shape in the front part of the weld pool depends on the gap. It can be detected by processing the weld pool images. The authors investigate the method to take the gap from the weld pool image. The use of feed-forward control of back beads to compensate for variations of root gaps was also tested. In order to verify the performance of the feed forward controller, the gap is changing from 4.6 mm to 2.5 mm without the tack welding.
A feasibility study has been conducted to determine whether mathematical models can be used for the numerical simulation of MAG (Metal Active Gas) arc welding. In the present work, a three-dimensional (3-D), non-stationary thermal model for butt joint is developed. The transient temperature distribution in the base metal is numerically analyzed to estimate the molten pool size using a finite differential model based on the heat flow equation, and the theoretical configuration of the molten pool is calculated, taking account of the balance of gravity, surface tension and arc pressure. To evaluate the validity of the model analysis, the calculated results are compared with experimental results for MAG welding. Good correspondence is demonstrated between experiment and calculation. According to the software, the weld profile and temperature history in the work-piece for a set of process parameters is easily estimated. To predict the MAG arc welding phenomenon comprehensively, the developed welding process model has been linked to a deformation model. Accordingly, there is the possibility that the software is useful as an engineering tool in a manufacturing environment.
PWR's core internals have aging due to IASCC (Irradiation Assisted Stress Corrosion Cracking). Although stress influences the occurrence of IASCC greatly, we have little data about PWR's irradiation creep which reduces stress. In this report, we assessed irradiation creep by using the removal torque of the baffle former bolts at the time of replacement. Then, we concluded that irradiation creep in PWR is approximately 50% of the irradiation creep in the fast breeder reactor. Moreover, we used this result to check the method for IASCC prediction method proposed in the last report which is applicable also to barrel former bolt and weld metal of barrel, and we confirmed the applicability of the IASCC prediction method for the actual plant.
PWR's core internals have aging due to IASCC (Irradiation Assisted Stress Corrosion Cracking). Using IASCC data and the results of an inspection of an actual plant's baffle former bolts, we have devised a formula for predicting IASCC. We have also applied this method to barrel former bolts and core barrel which have aging of IASCC. We divided these test objects into several groups based on a number of similar structure in order to conduct inspections efficiently. Our findings show that that at the fastest, baffle former bolts should be inspected by about 250,000 hours (30 years) after a plant begins operation. Barrel former bolts develop IASCC at a slower rate, and there is little possibility of the core barrels being affected by IASCC..
Active metal brazing of ceramic to metal has been applied to the fabrication of advanced airtight seals for LPRMs(Local Power Range Monitors) used in BWRs(Boiling Water Reactors) to improve their life and reliability. Silicon nitride was selected as the ceramic part of the joint because of its superior swelling resistance. A cone-shaped silicon nitride and a copper interlayer were employed to join them to stainless steel parts. The effect of the copper-interlayer shape on the residual stress was studied. The relation between mechanical properties of the silicon nitride and tensile strength of the joints was also evaluated. It turned out that the copper interlayer design had significant effects on the residual stress of the joints. It was also found that the tensile strength of the joints was remarkably dependent on fracture toughness of the silicon nitride. Based on these results, the tensile strength change of the joints during neutron irradiation was studied and a life of the seal parts was discussed. Fifty years are obtained for the initial fracture toughness 5.4 MPa·m1/2 silicon nitride and 25 years for that of 4.4 MPa·m1/2 silicon nitride. Therefore the life period of the seal parts using silicon nitride that has initial fracture toughness 4.6-5.9 MPa·m1/2 is predicted to be between 25 and 50 years. It is more reliable seal parts comparison to the conventional seal parts used currently applying alumina ceramics as the ceramic part of the joint.
There has been growing interest in high strength steel with a tensile strength higher than 800 MPa, which would be ideal for weight reduction and better performance of steel structures. However, there are two major problems in the application of high strength steel to structures. One is that the fatigue strength of welded joints is far lower than that of the base metal, and the other is that the weld metal and the heat-affected zone (HAZ) are likely to have cold cracking. These problems must be solved before high strength steel can be widely used in steel structures. As a solution to the problem of cold cracking, a reduced preheating type of high strength steel has recently been developed in which cold cracking in the HAZ is reduced by reduction of alloy components (lower Ceq, Pcm) without loss of strength. However, since the resistance to cold cracking of the weld metal has not been improved, preheating of the weld metal is still necessary, and the problem remains toward its wider use. Recently, Ohta et al. developed a low transformation-temperature welding consumable, in which compressive residual stress is induced by transformation expansion of martensitic transformation of weld metal at a low temperature near room temperature, and demonstrated that this consumable improves the fatigue strength of welded joints. We considered that the reduction of tensile residual stress in the low transformation-temperature welding consumable might also be effective in decreasing the cold cracking and that this consumable would be useful in manufacturing high performance welded joints with improved fatigue strength and cold-cracking resistance. In the present study, we examined the effects of low temperature-transformation weld materials on the prevention of cold cracking in high strength steel. Firstly, the y-shaped weld cracking tests demonstrated that reduction of residual tensile stress induced by martensitic transformation expansion of weld metal was effective in reducing cold cracking. Secondly, the effects of the degree of joint constraint on cold cracking in low transformation-temperature weld materials were examined by the y-shaped and H-shaped weld cracking tests. The cracking ratio was high at low degrees of joint constraint and low at high degrees of joint constraint. Two causes were considered: one was that the reduction of tensile residual stress by transformation expansion is higher at a higher degree of joint constraint, and the other was that the weld metal of martensitic structure alone is sensitive to cold cracking. According to the above investigation, we attempted to develop a weld material with high resistance to cold cracking at different degrees of constraint. To maintain the effects of reduction of tensile residual stress by transformation expansion to reduce the amount of diffusible hydrogen and the sensitivity of cracking, which are other causes of cracking, we designed and developed a 2-microstructure phase weld metal (martensite + retained austenite) by modifying the low temperature-transformation weld material to obtain a lower Ms point. We examined the effects of the degree of joint constraint on cold cracking, and confirmed that the cracking rate of the modified rod was almost 0% at all degrees of constraint, and its resistance to cold cracking was high.
Characteristics of a welded joint should be evaluated considering not only mechanical but also metallurgical properties, and weld residual stress is possible to be controlled by considering and changing mechanical / metallurgical properties of the materials. History of thermal stress due to phase transformation and residual stress during welding heat cycles are studied in order to clarify the generating mechanism of residual stress and the effects of material properties such as phase transformation on stress generation. Two materials of high-tensile strength steels are used in the numerical simulation and experiment. Material property of each microstructural phase is used and the time- and temperature-dependant proportion of microstructure are considered by using CCT-diagram in the analysis. Thermal stress history obtained by the numerical simulation agrees well with the experimental result during welding heat cycles. The effects of welding heat cycles on thermal stress histories and residual stress are then investigated.
Residual stress in welded joints by using a new weld metal with the property of low-temperature phase transformation is numerically analyzed. 10 % Nickel and 10 % Chromium are involved in the developed welding material for producing the property of martensitic phase transformation at a low temperature and for generating compressive residual stress. A fillet-welded joint between plate and stiffener is used for the numerical simulation of the thermal elastic-plastic finite-element analysis with coupling phase transformation effect. Moving heat source is considered by using the gradual deposition of the finite-element during welding process. Distribution of the computed residual stress mostly agrees with the measuring values by strain gauge. Compressive residual stress distributes in the weld metal for both longitudinal and transverse directions with weld line. The effects of the material of weld metal and welding pass sequence on residual stress are then investigated. Residual stresses on the weld toe and root are improved lower by using the low-temperature transformation weld wire than the conventional one, regardless of the sequence of welding deposition. It is found that the weld metal with property of low-temperature phase transformation is effective to reduce residual stress near weld metal.
The Weibull stress criterion has been accepted as a fracture criterion for cleavage fracture of steels and has been applied to the analysis of the fracture toughness. However, the effect of the temperature and/or the yielding scale on the critical Weibull stress has not yet clarified. In the present work, the dependency of the critical Weibull stress on the yielding scale was investigated on a low carbon steel. The Weibull parameters of the critical Weibull stress were statistically evaluated with notched round bar tensile specimens. The 28 specimens for both 1mm radius notched round bar and 50mm radius hourglass bar were repeatedly fractured at -196°C and the Weibull stress analyses were carried out based on the finite element stress solution. Although the constant shape parameter was successfully obtained independent on the specimen configurations, the scale parameter for the critical Weibull stress which was defined with the original Beremin's formulation was higher in the sharp notch specimen. The locations of the trigger points for cleavage in all fractured specimens were quantified by SEM observation. The local stress and strain conditions at the trigger points were also estimated by means of the FE solutions. The local tensile stress at trigger was not constant and showed the dependency on the local plastic strain. In the original formulation of the Weibull stress proposed by Beremin, enough numbers of the nucleated micro-cracks are assumed and the fracture probability from micro-cracks is defined by only the maximum principal stress. However, the present test results suggest that the straining history for the micro-crack nucleation should be also considered for the cleavage fracture criterion. Modified Beremin model, in which the probability of the microcrack nucleation is also considered, was applied to the present results. The variation of the critical Weibull stress with the plastic constraint was almost disappeared in the modified Beremin model. The contribution of plastic strain on the cleavage fracture was discussed.
The Weibull stress criterion has been accepted as a fracture criterion for cleavage fracture of steels and has been applied to the analysis of the fracture toughness. However, the effect of the temperature and/or the yielding scale on the critical Weibull stress is not yet clarified. In the present work, the dependency of the critical Weibull stress on temperature was investigated on a low carbon steel. The Weibull parameters of the critical Weibull stress were statistically evaluated with notched round bar tensile specimens. Three test temperatures of -196, -130 and -100 °C were selected at which fully cleavage fracture surfaces could be obtained. The 28 specimens were repeatedly fractured at each test temperature and the Weibull stress analyses were carried out based on the finite element stress solution. Both the shape parameter and the scale parameter for the critical Weibull stress which was defined with the original Beremin's formulation rarely depended on test temperature. The locations of the trigger points for cleavage in all fractured specimens were quantified by SEM observation. The triggers of the specimens at -130 °C and -100 °C located at the positions of the peak principal stress. But, the triggers at -196 °C were deflected from the peak stress position to the larger strained region. In the original formulation of the Weibull stress proposed by Beremin, enough numbers of the nucleated micro-cracks are assumed and the fracture probability from micro-cracks is defined by only the stress. However, the present results suggest that the strain history for the micro-crack nucleation should be also considered for the cleavage fracture criterion. The formulation of the fracture probability, which covers both the micro-crack nucleation process and the propagation process, are also discussed in this work.
Laser welding systems have been installed in many assembly lines because of small welding distortions. Nonetheless, desires to produce earth-friendly automotive components such as fuel injectors with low volume of residual hydrocarbons in exhaust emissions require higher accuracy of microns or sub-microns order to maintain their function. For examples, circularity deviation of such an injector after welding is requested to be less than 1 micrometer. Laser welding is a processing with small distortion, however, making the distortion smaller is critical to meet such a high level objective. In this paper, we consider the deformation mechanism by using numerical analyses and experiments, and propose a new method, twin beams method, to decrease the deviation of circularity. Twin beams, which is separated by 90 degrees, obtain a smaller deviation of circularity because of canceling expansion and shrinkage during the welding process. This method is brought into production facilities.
Aluminum nitride (AlN) is one of the attractive ceramics applicable to the surface modification because of its excellent properties in chemical stability and thermal conductivity. Generally, thin AlN coatings are fabricated by CVD or PVD process. However, these processes cannot produce thick coatings, which might contribute to the structural parts, because of quite low deposition rate in these processes. Thick coatings can be easily given by thermal spray process. However, fabrication of AlN coating by conventional thermal spraying is fairly difficult due to decomposition of AlN at 2273K. In this research, AlN coating was fabricated by reactive Radio Frequency (RF) plasma spraying. Reactive plasma spray process, in which metal element reacts with surrounding active species in plasma, is effective for the spraying of non-oxide ceramics. RF plasma can offer the enough reaction chance to the elemental metal because both of its low flowing velocity and high energy. By increasing nitrogen content in plasma gas, AlN coating without pure Al phase was attained while the coating microstructure was heterogeneous, brittle and quite porous. By decreasing nitrogen content in plasma gas, on the other hand, Al/AlN composite coating with more homogeneous, less porous microstructure could be attained. Changing nitrogen fraction in plasma gas may be effective for controlling AlN content in Al/AlN composite coating.