We performed ultrasonic bonding on sheets of 1050 and 5052 with different oxide thickness of 4, 15 and 30 μm. The oxide films macroscopically bent and distributed like whirl-shaped near bond interfaces. Microscopically, we observed two kinds of features of the oxide films, dot-shaped and line-shaped. We evaluated the bonding situation by an immersion method of ultrasonic testing (C-scope mode). The ratio of good bond area was ultrasonically obtained and depended on the thickness of the oxide films and input energy in bonding. The maximum tensile shear load decreased with the increase in the thickness of the oxide films. By using the results of both a tensile shear test and the good bond area obtained ultrasonically we could estimate the tensile shear strength of bonds.
Aluminum alloy has been used in various industrial sectors because of such a desirable mechanical or metallurgical property as low density, high tolerance to corrosion and high recycling efficiency. On the welding of aluminum alloys, it is required to remove the oxide layer from the surface which prevents from making a proper weld joint. AC method is used in the TIG arc welding of aluminum alloys where the cathode spot breaks down the oxide layer covering the material surface during the period of the electrode positive (EP). The object of this study is to make clear the influence of AC current waveform on AC TIG welding of aluminum alloys. Especially, the influence of frequency and EP time ratio on the bead profile and cleaning width has been discussed in the present paper. As a result, it is made clear that the power consumption in arc space is an important factor governing the bead size, and the power consumption in EP alternation is strongly related to the cleaning action.
In the construction sites of chemical and electric power plants etc., many pipes are welded with conventional MIG/TIG arc welding methods in these days. Nevertheless, much time should be necessary to weld thick pipes with these welding methods because of their slow welding speed and multiple paths welding to fill up their joints. On the other hand, laser welding which can weld thick plate with low distortion at high speed needs small tolerance of gap. From these backgrounds, the development of a new welding process with a YAG laser where the accurate adjustment of a joint before welding is not necessary has been expected. In this research, therefore, we developed an all position filler addition YAG laser welding process for stainless steel butt joint with large gap. As a result of the observation of the droplets from the filler wire during welding and the compositional analysis of the distribution of the filler wire in the welded bead, it was revealed that the droplets were scattered to the opposite direction of an irradiated laser beam and transported to the back side of the work piece due to the melted metal flow along a keyhole by evaporation recoil force of plasma/plume even if the welding was performed in overhead position. As the result of applying this welding process to a SUS304 stainless steel pipe joint, which was 267.4 mm in diameter and 5 mm in thickness and 1 mm in gap tolerance, class 1 weld was attained in the radiographic test of JIS.
Large amounts of welding fumes are produced in joining aluminium-magnesium alloys with DC pulsed MIG process. Welding fumes consist of magnesium oxide, aluminium oxide and pure aluminium. Magnesium element evaporates from both the surface and inside of droplets that are superheated. Evaporation inside of droplets produces magnesium bubbles, leading to violent explosions of droplets and emissions of magnesium vapour and liquid spatters. Part of magnesium vapour escaped from the shielded arc area is oxidised and then becomes super fine fume particulates that their shapes can not be recognized even with transmission electron microscope. Part of spatters ejected from the shielded arc area are partially or completely oxidised and then become fume particulates. In AC pulsed MIG welding the average heat content of droplets is low, and besides, the temperature distribution inside a droplet is deduced to be more uniform compared with that of DC pulsed MIG welding. In consequence, the evaporation of magnesium is restrained so that both frequency and violence of droplet explosions decrease clearly. AC pulsed MIG welding process can effectively control welding fumes at source in joining aluminium-magnesium alloys.
Mechanisms of high melting coefficient and bead formation in welding aluminium-magnesium alloy with AC pulsed MIG welding process are investigated. In electrode negative polarity cathode spots clime up to the solid part of wire tip for the existence of oxide and arc root covers uniformly a large area on the surface of wire tip. This polarity prevents a droplet on wire tip from being overheated in part so that the heat transferred from arc plasma to wire tip is effectively used for wire melting. Moreover, as electrode negative ratio increases at a certain welding current, arc plasma transfers more heat to wire tip at the cost of reducing the heat transferred to base metal. As a result, wire melting coefficient becomes greater at a higher electrode negative ratio. Since the heat input to base metal is a sum of the heat transferred from arc plasma to base metal and the heat brought into weld pool by droplet transfer, it does not change obviously as electrode negative ratio changes drastically at a certain welding current. At a constant wire feed rate, heat input to base metal and penetration depth decrease as electrode negative ratio increases mainly due to the decrease of welding current. Besides, reinforcement becomes high and bead width becomes narrow as electrode negative ratio increases because of the lower heat input to base metal and reduction of heat content of droplets. Accordingly, AC pulsed MIG welding process can successfully solve the burn through problem in welding thin sheet joints and greatly improve the gap bridging ability. Moreover, thin sheet joints can be welded with high speed and small distortion.
In the thermal spraying of practical powder materials, we have confirmed that the splat shape changes to a disk type from a splash type with increasing substrate temperature. Substrate temperature increasing may accompany the change in something on the substrate surface, because the changing effect was maintained till the substrate was cooled down to the room temperature range. Thus, we already pointed out that this non-reversible change in substrate surface due to the heating might be the possible domination for the transition phenomenon of the thermal sprayed particles. However, the essence of the substrate surface change due to the heating has not been clearly understood yet. In this study, SUS304 substrate surface once heated to 673 K was analyzed precisely by atomic force microscope (AFM), X-ray photoelectron spectroscopy (XPS) and auger electron spectroscopy (AES). The results obtained revealed that the change of the substrate surface occurred not in the chemical composition but mainly in the surface roughness, especially surface morphology in nano-meter order. Hence, the substrate heating may bring about the change in the physical way on the substrate surface and this change induces the transition phenomenon.
The present paper describes the quality and its improvement of friction welded joints between a rectangular section bar and a circular section bar. Three kinds of rectangular section bars with different aspect ratio were used for the experiment. The rectangular section bars and circular section bars were joined by using a continuous drive (brake type) friction welding machine with an electromagnetic clutch in order to prevent a braking deformation during rotation stop. The low heat input friction welding method (LHI method) was used for joining. The following conclusions were given. (1)The first phase of the friction stage was composed of wear and seizure stages. This result was similar to the join of circular section bars of same diameter. (2)When the areas of the weld faying surfaces were same each other, the initial torque and the elapsed time for initial torque hardly changed in spite of the aspect ratios of rectangular section bars. (3)The welded joints obtained only the first stage (up to initial torque) had 100% joint efficiency, but fractures occurred between the welded interface and base metal. When small aspect ratios of rectangular section bars and a circular section bar were welded, several joints fractured at the base metal although they had a little defect in the welded interface. (4)When the joints were made by the LHI method in Ar atmosphere, the welded joints had 100% joint efficiency, but fractures occurred between the welded interface and base metal. (5)The welded joints had 100% joint efficiency and fractured at the base metal when the joints were made by the LHI method with adding forge pressure (improving LHI method). On the other hand, most of the welded joints made by the conventional method with forge pressure had fractures occurred between the welded interface and base metal. (6)The improving LHI method has more advantages than those welded by the conventional method, i.e., less burn-off, less burr and less deformation of welded interface area. As a conclusion, it was clarified that the improving LHI method is the most effective friction welding method for rectangular section bars and circular section bars.
On maintenance of structures, there is a strong need to know stress conditions of steel structure in use. Stress measuring technique using the magnetostriction effect utilizes 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 an experimental study on dynamic stress measurement of steel bridges and calibration of magnetostriction sensitivity of the steel used for bridges. Following results were obtained. (1)By revised magnetostriction stress measuring equipment, dynamic stress fluctuations of steel bridges under real traffic were measured within the accuracy of 95%. (2)Calibration procedures of magnetostriction sensitivity of steel used for bridges were proposed.
It is well known that it is important to keep flashing continuous just before upsetting for high quality of weld joints in flash welding. To achieve continuous flashing, mechanical control has been applied. As mechanical flashing control has a limit of response for more precise control, an application of power electronics especially PWM inverter is discussed for flash welding. Since no study has been reported regarding flashing phenomena using square wave alternating current as well as application of PWM inverter for flashing phenomena control, flashing phenomena in square wave alternating current by use of PWM inverter power supply are discussed for flashing control in this paper. It is concluded that PWM inverter is suitable to control flashing phenomena such as number of flashing pulses or degree of heating.
In previous study, flashing phenomena in square wave alternating current have been discussed for flashing control. The results give a suggestion that PWM inverter has possibility to control flashing phenomena in half a cycle. In this study, flashing phenomena control strategy for flash welding is discussed by use of PWM inverter power supply. The results are summarized as follows : 1 PWM inverter enables to control flashing phenomena in half a cycle. 2 PWM inverter control for continuous flashing is proposed.
In previous study, flashing phenomena control strategy for flash welding has been discussed by use of PWM inverter power supply. It is concluded that PWM inverter has possibility to control flashing phenomena in half a cycle. And PWM inverter control for continuous flashing is proposed. In this study, PWM inverter control for resistance heating with a few flashing is discussed. The results are summarized as follows : 1 PWM inverter control for resistance heating is proposed. 2 Combined control of PWM inverter control and mechanical platen control for resistance heating is proposed.
The hot dip galvanizing for corrosion protection is often uesd for structural steel architecture in recent years. However, it is estimated to occur galvanizing crack at a intersection of many welding lines in column connection joints. In this study, galvanizing tests with partial column joint models and hot elasto-plastic FEM analysis were carried out in order to clarify mechanism of galvanizing crack. As a result, a dip speed is not so relative to galvanizing crack, neither is a presence of backing metal. It became also obvious that the higher a thickness ratio is, the higher a stress at the scallap is. We, furthermore, propose to reduce a thickness ratio and to put a hole in order to prevent galvanizing crack.
Biocorrosion was suspected in the corrosion failure of a cooling system of a stainless piping welded joint, carrying marine water. Marine water, which is used for cooling water in a plant was transferred to the laboratory and used for experiments. In the first experiment, weld metal samples were exposed to the test solution for 56 days (marine water and sterilized marine water (control)). The surface condition of experimental coupons was observed using a Scanning Electron Microscope (SEM). In another experiment, the free corrosion potential of these materials was monitored for 56 days. Pitting corrosion was found in the coupons exposed to marine water. Free corrosion potential ennoblement was found to be significant compared to the controls. It was suspected that this corrosion case was biocorrosion. In the second experiment, coupons were exposed to diluted nutrient medium containing single cultures of microbes isolated from the biocorrosion causing marine water sample used for the first experiment. After the exposure test, surface condition of the experimental coupon was observed using SEM. Pitting corrosion sites was found in coupons exposed to some of the isolates. The results indicate that they contribute to the corrosive effect of the SUS316L welds.
We have investigated an effect of bonding parameters on the joint strength of diffusion bonds between tantalum and copper (Case A), and between tantalum and Cu-Cr binary alloys (Case B). Main results obtained are as follows. In the case of A, it was made clear that intermetallic compounds were not formed within the bonded zone. The joint strength was increased with increasing of bonding temperature, on the bonding time of 1.8 ks and the bonding pressure of 5.6 MPa. The maximum joint strength was 275 MPa, which is equal to the tensile strength of the tantalum base metal, at bonding temperature of 1323 K on the same bonding time and pressure. The reasons why the joint strength was increased with increasing of bonding temperature or bonding pressure are decreasing of the non-bonded areas at the bonded zone. In the case of B, the joint strength was decreased when the Cr content was 0.56 at% or more in the Cu-Cr binary alloys under the bonding conditions of the bonding temperature of 1173 K, bonding time of 1.8 ks and bonding pressure of 5.6 MPa. Cr2Ta of intermetallic compound was formed within the bonded zone of the tantalum and the Cu-Cr alloy. Cr2Ta has increased controlling by diffusion rate. When the area of Cr2Ta formed within the bonded zone was 60% or less, the joint strength was equal to the tensile strength of the tantalum base metal. However, the joint strength was decreased remarkably the area of Cr2Ta was 80% on more within the bonded zone.
800 MPa high strength steel with a low carbon equivalent has been developed by means of refining ferrite grains. In the welded joints of the 800 MPa steel with ultra-fine ferrite grains (less than 1 μm) and dispersed fine cementite particles, fine ferrite grains coarsen due to the welding heat input, and then remarkable softening occurs in the heat-affected zone. For the ultra-fine grained steel (UFG steel), ultra-narrow gap gas metal arc (UNG) welding was utilized as a low heat input welding process. By the UNG welding process with the cooling time from 1073 K to 773 K of about 4.5 sec, the width of the softened region in the heat-affected zone (HAZ) of the ultra-fine grained steel can be minimized to below 2 mm. In order to clarify the softening behavior in the HAZ of UFG steel, the effect of welding thermal cycle on the microstructures of UFG steel was discussed on both the HAZ of the welded joint of UNG welding and its synthetic HAZ, and the results were also compared with the HAZ of SM490 steel. The peak temperature at each point of HAZ was estimated from thermal conduction theory and the measured data. Slightly coarsened ultra-fine ferrites (fine grain A) and larger polygonal ferrites (fine grain B) can be observed in the regions of the peak temperatures from 920 K to Ac3 (1150 K). It was found that the softening in the HAZ of UFG steel occurs within the peak temperature range of about 920 K (below Ac1=980 K) to 1300 K, and the softening in the regions of the peak temperatures from 920 K to 990 K is dependent on the grain size of the larger polygonal ferrite grains (fine grain B). M-A constituents begin to form when the peak temperature exceeds Ac1, and its volume fraction increases with the peak temperature. As second hard phase, M-A constituents restrain the softening of the HAZ in the peak temperature range from about 1000 K to 1250 K. It is known from the results that the hardness in the HAZ has the lowest value at the peak temperature of 1100 K.
This study demonstrates the applicability of advanced criterion to ductile cracking evaluation of practical steel structures under large scale cyclic loading. This advanced ductile cracking criterion is based on the effective damage concept for ductile cracking proposed by authors in the previous report 1, of which applicability has been fundamentally verified by means of small scale specimens extracted from steel plate. The main concern is an application of this proposed criterion to welded structures with practical scale. The specimens modeled beam-to-column connection of steel peer, in which connection is strength overmatched joint, were tested in cyclic loading, and analyzed by FE-method employed combined hardening material models of both base steel and weld metal. The ductile cracking occurred from the surface of stress-strain concentration area, that is of weld metal on web plate thickness around connection. The critical value for ductile cracking in beam-to-column specimen evaluated with the accumulation of effective plastic strain at the position where ductile cracking occurred was found to be well agreement with that obtained by using small scale tensile specimen. Accordingly, the advanced criterion could be applied to evaluation of structural integrity with respect to ductile cracking under large scale seismic loading, and to structural design to prevent ductile cracking in welded structures as well.
An investigation has been made of friction welding of Cr-Zr copper alloy, which is very difficult to join by conventional fusion welding processes because of its high thermal conductivity and susceptibility to solidification cracking and blowhole formation. It was revealed that brake timing and stopping period had significant effect on the formation of weld interface in friction welding of Cr-Zr copper alloy. The weld wes established only in the center area of the forging sufrace during the friction stage. Severe metal flows were required to expand the weld interface to the whole forging surface to obtain the sound joint from those experiments. When the brake wes operated after applying an upset pressure, referred to as brake timing “after”, the adequate metal flow was generated in the weld. Optimized parameters were brake timing “after” and stopping period more than 0.3 s to join Cr-Zr copper alloy without defects. The joint efficitncy estimated form tensile strength wes more than 92% at the optimum bonding paremeters. All specimens failed in the soften area apart from weld interface on tensile test.