In order to investigate the effect of gravity on the arc shape in gas tungsten arc (GTA) welding, GTA welding was carried out using the drop-shaft type microgravity system at the Japan Microgravity Center (JAMIC) . The materials used were an aluminum alloy and pure silver. It has became clear that the effect of gravity on the arc shape is not observed under general welding conditions (over 70 A) regardless of the observed region and the shielding gas flow rate. The displacement of the arc gap during the transition from the terrestrial environment to the microgravity environment can change the arc shape.
The thermal cycle during friction welding is an important factor that significantly affects on the mechanical properties and microstructures in welded joints. The weld heat input has been normally estimated using the torque during friction welding, however most of industrial friction welding machines are not able to measure the torque which cannot be available to use process control. In this paper, the calculation of heat input rate is proposed for friction welding under a new concept for the dissimilar welded joints. This heat input concept, which is solved governing equations combined with high temperature strength, and thermal conduction is successfully expanded into joints by dissimilar materials with large differences of thermal behaviors. It is revealed that mechanical properties are correlated with the proposed heat input rate to dissimilar joints by pure aluminum and plain low carbon steel. Mechanical properties are evaluated by notch tensile strength. This weld heat input rate is also related with intermetallic phase formation that is confirmed in the transmission electron microscope observations. As a conclusion, the optimum welding condition is obtained with increased the heat input rate. 1) By using the simple assumptions made in this study, it is possible to obtain the heat input rate, to a certain extent, to predict the mechanical properties of the friction welded joints to dissimilar materials. It is also clarified this proposed heat input rate predicts formation of intermetallic compounds at the weld interface. 2) The calculated heat input rate correlates with the tensile strength of the welds. High joint strength is obtained in the case of welds of higher heat input rate. 3) Intermetallic phase does not form when friction time is less than 1 second. This is also confirmed from the results of calculation. Intermetallic phase at weld interface is determined as Fe4A13 using diffraction pattern of TEM observation.
Laser texturing is a technique to make texture on a hard disk for a computer with small bumps generated on NiP film on aluminum substrate by single shots of short pulsed Nd : YLF (Yttrium Lithium Fluoride) laser. It has been already used practically, but the mechanism of bump formation has not been elucidated yet. The purpose of this study is thermohy-drodynamics analysis on the mechanism of bump formation. The vaporization recoil pressure, Marangoni force that depends on temperature gradient and the surface tension are considered at the free surface. The VOF (Volume of Fluid) method is used for the analysis with the free surface. The mechanism of bump formation is presented from the calculated results.
In the first report, stabilizing effects of a CO2 laser on a TIG arc were discussed from the arc voltage. It was made evident that the unstable arc which formed discontinuous weld beads was stabilized and good beads were made with the laser beam. In such a case, the laser plume played the role as a path of electrons, and the anode spots of the arc were formed around the hot spot by the laser beam. In this report, the effect of Ar gas flow rate on shapes of weld bead sections will be investigated. In the case of the conditions that the TIG arc current, the quasi-single mode laser output, the traveling speed of a base metal and the distance between a SUS304 base metal and a tungsten electrode were 100 A (DCEN), 2 kW, 150 mm/s and 15 mm, respectively, the beads width increased with the Ar shielding gas whose flow rate was from 0.67 × 10-3 to 1.33 × 10-3 m3/s. When the gas flow rate was 1.33 × 10-3 m3/s, the value of the ratio of beads width by the arc with the laser to ones by the laser only was 2.15. On the other hand, the penetration depth increased 22% under the same conditions. The arc voltage became high when the gas flow rate was large. The same phenomena were recognized under the multi mode laser. When the base metal-electrode distance and the gas flow rate was large, the arc path became long and the arc column was bent easily. Therefore, the anode spot rolled from side to side of the laser spot and the bead width was increased by the arc. In such a case, the penetration depth was scarcely increased compared with the one by the laser only. When the gas flow rate was large, the absorption coefficient of the laser increased, because the melted surface was irradiated with the laser beam. As the result, it would be considered that the role of the arc in this surface treatment process is extension of the beads width, and the penetration depth is increased by the laser.
A transition phenomenon in a flattening of thermal sprayed particle onto a flat substrate surface has been investigated. Our previous results showed that a splat morphology change transitionally from irregular splash shape to disk shape at certain temperature range with increasing a substrate temperature. The most interesting feature of the phenomenon lies in a drastic change at a certain narrow temperature range. The reason for this drastic change of the splat morphology is not always clear yet, because an in-situ observation of flattening is still difficult. In this study, in-flight measurement of velocity and temperature of the particle was conducted and the transition temperature of the splat was measured. The transition phenomenon of the splat was investigated based on the measurement results of the particles. The transition temperature increases with increasing the velocity, temperature and diameter of the particles. From the result of a breakup phenomenon of a liquid film, Splashing parameter: K=We0.5 Re0.25 was introduced as a determinant of the splashing. The transition temperature increases with increasing Splashing parameter based on the measured data of the particle. However, from the relation with the transition temperature, it was found that only Splashing parameter could not determine the transition phenomenon and some new determinant based on the data of the flattening particle was required. Then, Flattening K value : Kf=0.5a1.25Re-0.3K was defined and seemed to correspond to the boundary line of the splat morphology. From this result, it was found that the flattening velocity increases with decreasing the substrate temperature.
In this study, the influence of impurity elements to the weld metal crack susceptibility in the electron beam welding of a pipeline steel was investigated. 19 mm thick API 5L-X65 steel plates with different sulfur, silicon and phosphorus content levels were welded and the extent of the weld crack occurrence was compared. As the result of the variance analysis of the experimental data, sulfur was found to be the most harmful element to the crack occurrence. The Auger electron spectroscopy analysis also showed the segregation of sulfur to the fracture surface of the weld cracking. To clarify the upper limit of the sulfur content for the prevention of the weld cracking, commercially available test pipes with different sulfur contents were welded. As the result, it was confirmed that the sulfur content should be kept no more than 0.002% to prevent the weld cracking in the all position electron beam welding process.
For the building of ships, bridges, and other welded structures where welding equipment must be transported to the site, it is desirable from the standpoint of increased work efficiency to decrease the weight and size of the equipment to be thus transported. Accordingly, a lightweight, compact, high frequency oscillation device with an electromagnetic force have been developed. This work discusses the capabilities of the new oscillation method that has been developed, and offers consideration of its application to the actual bead formation phenomenon in GMAW process. The results can be summarized as follows: 1) The oscillating path of the current method stops at both oscillation terminals, and is characterized by high speed movement of 400 mm/sec. Maximum oscillation frequency is 40 Hz. 2) Arc heat and force can be dispersed in GMAW process with the current method, such that the penetration depth can be limited and the bead width expanded. 3) In addition, an flat bead surface can be obtained in horizontal fillet welding, where problems of overlap and undercut are prevented.
In this report, the effect of microstructural change during aging at around 1000 K on the susceptibility of reheat cracking in the FCAW weld metal of Type 308 stainless steel was investigated. Precipitates in the weld metal sampled from actual equipment, which was damaged during operation, were identified and the change in the microstructures were observed with aged weld metals which were made from the flux cored wires with various Bi contents. Ferrite contents were depressed compared with the as-welded condition and it was observed that most of the ferrite phases were transformed to σ phases and carbides in the weld metal sampled from the actual equipment. As for the aged specimen, σ phases and carbides precipitation were observed under specific aging conditions, however, Bi had no effect on incubation time for precipitation of the σ phase. Therefore, Bi does not promote the precipitation of the brittle phase. On the other hand, oxygen contents of the weld metal and hardness of the austenite phases increased with increasing Bi contents in the weld metal. This phenomenon is considered to be the dispersion hardening by oxide particles including Bi oxide and can be the secondary factor that increases the susceptibility of the reheat cracking in the FCAW weld metal.
Al-Mg-Si alloys are widely used for industrial components because of those excellent extrudability, high corrosion resistance, and relatively high strength within the various aluminium alloys. However, welds of Al-Mg-Si alloys are known to have high crack susceptibility and to be softened due to weld heat input. Laser welding process has significant advantages in terms of low heat input, low distortion, high welding speed, and ease to automation compared with the conventional welding process. In particular, there is considerable interest in welding heat treated alloy, such as Al-Mg-Si alloy, since low heat input makes softened region of welds minimize. This paper describes the results of welding efforts of Al-Mg-Si alloys, mainly A6063-T5 with 5 kW CO2 laser. Wel-dabilities, solidification crack susceptibilities were studied in bead-on plate tests. Solidification crack susceptibilities were estimated by using a crater cracking test and a self-restraint cracking test, a modified form of Houldcroft test. After that, welding with filler materials were performed in order to prevent the solidification cracking in weld metal. Microstructure, alloying element distributions, and mechanical properties were studied. Effects of groove geometry and Si contents of weld metals on properties have been established. Solidification cracking in weld metal has been prevented by using high Si content Al-Si alloy, such as A4047, and getting Si content of all the weld metal more than 2%.
Laser welding process has significant advantages in terms of low heat input, low distortion, high welding speed, and ease to automation compared with the conventional welding process. In particular, there is considerable interest in welding heat treated alloys, such as Al-Mg-Si alloy, since low heat input makes the softened region of welds minimize. In the previous study, weldabilities and solidification crack susceptibilities of Al-Mg-Si alloys have been reported using 5kW CO2 laser. Solidification cracking in the weld metal has been prevented by using Al-Si alloy filler wire enriched with Si. However, tensile strengths of welds were lower compared to the base metal because of softening of welds. In the present study, the behaviors of hardness values of welds of Al-Mg-Si alloy, 6N01-T5, by natural and artificial aging treatment were investigated. Hardness values of the softened region and weld metal were fully recovered to those of the base metal after an artificial aging treatment at 463 K for 28.8 ks without solution annealing. It was caused by the precipitation of Al-Si eutectic in weld metals and the precipitation of G.P. zones in weld metals and heat-affected zones Tensile strengths of welds after the postweld artificial aging treatment had equivalent tensile strengths to those of base metal.
The hot cracking susceptibilities of HP-modified heat-resisting cast alloys exposed at the elevated temperature for the long term were evaluated by the miniature spot-Varestraint testing. The as-cast, aging and/or used (service exposed) specimens of seven types of the HP-modified alloys were employed for the spot-Varestraint testing. Ductility-dip cracking as well as liquation cracking was occurred in HAZ of each specimen after the spot-Varestraint testing. The ductility-dip cracking susceptibilities of alloys except for HP43AZ much increased in aged and used conditions compared with as-cast condition, while the liquation cracking susceptibilities did not change so much during long term aging. The ductility-dip cracking susceptibility of HP43AZ containing zirconium was remarkably restrained even in used condition. It was deduced that the improvement in the ductility-dip cracking susceptibility of HP43AZ was attributed to reducing the occupied ratio of microconstituents on cellular dendrite boundary by the zirconium addition which possessed refinement effect of dendritic cell.
Changes in microstructures of modified HP-type heat-resistant cast alloys with long term aging were investigated using the as cast, artificially aged and practically used alloys at the elevated temperature. The microscopic observation and the X -ray diffraction analysis for microconstituents in heat-treated alloys were conducted in order to clarify the changes in morphologies and phases of microconstituents with heat treatment. In the niobium containing alloys, chromium-rich carbide of M7C3 and niobium carbide (NbC) changed to M23C6 and nickel-niobium silicide (7 phase) with long term aging, respectively. On the other hand, only chromium-rich carbide (M7C3 or M23C6) precipitated in as cast condition in the niobium-free alloys and the transformation of M7C3→ M23C6 or M23C6+nickel-rich silicide (G phase) occurred in the long term aging situation. The lamellar structure of eutectic microconstituents in the as cast alloy tended to agglomerate and coarsen with aging, and finally linked forming a continuous network surrounding the cellular dendrite boundaries. Zr addition to the HP cast alloy made it possible to reduce the microconstituents on the dendrite boundary due to the refinement of dendritic cell.
We investigate the development and the growth feature of columnar crystals in weld metals near the fusion boundary in 99.99% Al, A1050 and A5052 welded using laser. In the case of 99.99% Al, the columnar crystals which are developed epitaxially grow toward the center of the weld metals when shielded in argon. However, stray crystals are often observed in the weld metals shielded in nitrogen gas. This results from the development of cell structure and the competitive growth tends to occur due to the absorption of the trace of nitrogen gas in Al. The weld metals shielded in nitrogen gas are harder than the weld metals shielded in argon. This is caused by the development of AIN. The development of the strayy crystals becomes easier in A5052 than in A1050. In the case of A5052, it is very rare that epitaxial growth is clearly recognized and many stray crystals begin to grow from very near the fusion boundary. The growth rate of the columnar crystals increases as welding speed increases. The growth rate of columnar crystals when welded with pulse laser is faster than that welded with continuous laser. The maximum value obtained is 1.9 m/min and this is faster than that obtained in the weld metal welded by GTA.
There are few reports on the bonding process for Bi-2223 multifilament wires in Bi-family superconducting wires because of the difficulty of bonding them directly. Using diffusion bonding process with interlayer materials, Bi-2223 multifilament wires are expected to be bonded at no resistance of the joint. The aim of this study is to develop the interlayer materials that have the better superconductivity characteristics. The superconducting powder with a cation ratio of Bi : Pb : Sr :Ca:Cu=1.8:0.4:2:2:3 was mixed with various amounts of Ag2 O powder. Then the powder was cold-pressed with various pressures into disk-shaped pellets. The compounds were sintered at various temperatures in air for 100 h. The most suitable treatment conditions were investigated from the viewpoint of superconductivity, composition and structure. The most suitable interlayer material was cold-pressed using the compounds with 2 mass% Ag2O powder and heated under heating conditions of 847°C, 100 h in air subsequently. Critical current density increased with an increase in the ratio of 2223 Phase and the density of the interlayer materials. There was a suitable heating temperature to increase the ratio of 2223 Phase. The density of the interlayer materials was increased by the addition of Ag2O filling up the voids of interlayer materials.
In order to make a p-n junction without epitaxial growth method, a diffusion bonding of p-type silicon single crystal to n-type one was tried. The effect of bonding conditions such as bonding temperature, surface treatment and misorientation angle at bonding interface on the electric characteristics of the joint was investigated. Joints were made using samples prepared with a (001) crystal plane at bonding surface. All the samples were joined in a vacuum and an air using radio frequency heating. The electric device with a rectifying action was made by the diffusion bonding. The joints was formed above bonding temperature 800°C and the current voltage characteristics depended on the temperature, the thickness of oxide film and the misorientation angle at interface. The rectifying action deteriorated with an increase in the bonding temperature and the oxide film on bonded silicon crystal. The electric characteristics of the joints depended on the misorientation angle at bonding interface. It was found that low-angle boundaries and coincidence boundaries were better for the rectifying action than random high-angle boundaries.
This paper deals with a tendon force, which was proposed by White, generating in welded joint by welding. Tendon force is an integral value of inherent stress which is equal to a restraint stress produced by an inherent strain. The inherent strain is connected with the maximum of temperature rise induced by an arc energy. Equations for the maximum temperature rise are classified by the non-dimensional welding speed parameter composed with a welding speed, a plate thickness and a thermal diffusivity. Predictive equations of the tendon force are proposed on the basic of inherent strain. It is shown that predictive equations are useful by comparing with experimental data.
An explosion clad plate composed of copper and mild steel is dealt with as a typical example of the bonded dissimilar plate. Tensile tests are carried out using rectangular plate specimens extracted from the clad plate. An artificial through-the-thickness edge crack or parallel cracks are made in each specimen. The cracks are close and perpendicular to the explosive interface. When a tensile load is applied perpendicularly to the crack plane, stress field near cracks, stress intensity factor, crack opening displacement and fracture strength are examined by experiment as well as elasto-plastic finite element analysis. The stress field and the stress intensity factor are evaluated by an infrared stress imaging system. The effects of the material inhomogeneity, the residual stress, the change of material characteristics and the interaction of parallel cracks on the stress field, the stress intensity factor, the deformation behavior and the fracture strength are revealed. The lower strength material ahead of the bonded interface increases the stress intensity factor and the crack opening displacement. The higher strength material ahead of the bonded interface decreases the stress intensity factor and the crack opening displacement. The effects of the material inhomogeneity and the interaction of parallel cracks on the crack opening displacement can be explained from the square of stress intensity factor at the low applied stress level. The brittle fracture strength of the inhomogeneous specimen can be evaluated from the stress intensity factor and linear elastic fracture mechanics criterion.
The effects of both microstructure and oxygen content on the toughness of weld metals of a 950 MPa high strength steel have been investigated. The weld metals were prepared by SMAW (shielded metal arc welding), SAW (submerged arc welding), GMAW (gas metal arc welding) and GTAW (gas tungsten arc welding) processes. The microstructures of the weld metals were characterized with optical microscopy, SEM (scanning electron microscopy) and TEM (transmission electron microscopy) as well as continuous cooling transformation (CCT) diagram. The oxygen content was measured with a non-dispersive infrared absorption analyzer. The microstructures of all weld metals in as-welded state prepared by SMAW, SAW, GMAW and GTAW processes were martensitic structures, although slight differences in their morphologies were observed. The microstructures of the reheated weld metals in multi-pass weldment depended on both reheating temperatures TP and welding process. When TP≥Ac3, the reheated microstructures of SAW and GMAW weld metals consisted of fine martensitic structures, while that of SMAW was martensite with coarsened laths. The microstructure of the reheated weld metal of GTAW was lath martensite with a large amount of carbide precipitates. When TP≥Ac3, especially when Tp-Ac1, a large amount of precipitation were observed in the reheated weld metals of SAW, GMAW and GTAW, while the precipitation in that of SMAW was only slight. The toughness of the weld metals was closely related to their microstructures and oxygen contents. In particular, superior toughness could be associated with the formation of fine martensitic structures. Furthermore, the toughness of the weld metal was increased with the decrease in oxygen content, when the microstructure consisted primarily of martensite.