Friction welding of S15CK carbon steel was carried out in order to examine the relationship between heat input for welding and joint strength. The joint strength is evaluated from tensile strength. The heat input is a heat source for welding solid materials and was classified into 6 inputs: friction heat input, deformation heat input and total heat input in the friction stage and upset stage. The most important heat input for evaluating the joint strength among these heat inputs was experimentally investigated through the tensile strength of joints welded under various welding conditions. As a result, it was made clear that the deformation heat input in the upset stage is much strongly related with the joint strength, and sound weld joints can be obtained deformation heat inputs more than about 100J/s. Moreover, it was recognized that the joint strength is possible to evaluate by the upset loss, therefore, sound welded joints can be obtained with about 1.2 mm upset burn-off length or more also.
Utilization of magnesium alloy has increased in recent years as structural metals, such as automotive industry and mobile information tools. Hence welding techniques are required to be developed for these applications. Lasers are known to be an excellent tool for them. This paper presents the laser weldability of wrought type of AZ31 magnesium alloy with high power continuous wave (cw) Nd: YAG and CO2 lasers. The effects of center and back shielding conditions were examined. In the case of CO2 laser welding without back shielding, backside of the bead formed groove resulted from the reaction of the laser-induced plasma and low surface tension of the magnesium. In the case of Nd: YAG laser without back shielding, a welded bead was significantly better than that with CO2 laser. Because the metal vapor did not change to plasma plume due to the short wave length, it played the role of shielding the backside of the bead. From the tensile tests of the butt joints fabricated with CO2 and Nd: YAG laser, tensile strength of the joints which were made with Nd: YAG laser in both case with and without back shielding was sufficiently high.
A large amount of porosity is liable to be formed in high power CO2 laser welds of stainless steels. The formation mechanisms of porosity have not been satisfactorily understood up to the present. In this study, therefore, microfocused X-ray transmission in-situ observation system was utilized to observe the behavior of a keyhole and the formation situations of bubbles and pores during high power CO2 laser welding. It was observed that a keyhole fluctuated in geometry unstably and vibrated up and down dynamically, and accordingly many bubbles were frequently generated in the bottom part of a molten pool from the tip of the deep keyhole. Almost all bubbles were captured into spherical pores by solidifying solid-liquid interface during floating up near the bottom part of the weld pool. Moreover, it was revealed that partial penetration welding with a forward inclination of a laser beam or in N2 shielding gas, and full penetration welding in any shielding gas were beneficial to the reduction and prevention of porosity because the formation of bubbles was suppressed or prevented.
The effects of chromium and silicon contents on the roughness of root bead surface in gas tungsten inert gas arc welding without inert gas backing was investigated from a point of view of the oxide layer formed on root bead surface. Test welding was carried out on plates containing various contents of chromium and silicon. The roughness of root bead surface was measured by observation of a cross section of weld metal. The thickness of the oxide layer was also measured and chemical compositions were analyzed by EPMA. The roughness of root bead surface increased in weld metals with higher chromium content as a result of the random formation of thick and porous oxide on the root bead surface. The increase of silicon content improved the roughness of root bead surface. The thickness of oxide on root bead surface was thinned by increasing the silicon content and the high silicon concentration was detected at the interface of oxide and the weld metal. From these results, the improvement of roughness of root bead surface by silicon was considered to be due by preventing the random growth of oxide by the formation of a dense silicon oxide layer as a barrier of oxygen diffusion at the interface of oxide and weld metal. The increase of roughness by increasing the chromium content was also considered to be caused by the formation of thick and porous oxide as a result of the prevention of the formation of uniform and dense oxide layer of silicon.
In the first report, stabilizing effects of a CO2 laser on a TIG arc behavior were discussed from the point of the arc voltage and of the interaction between the arc and the laser plume. In this report, the TIG arc behavior was investigated using high speed video images when the traveling direction of the base metal was changed. In this case, the backward direction meant that the moving direction of the base metal was same as the direction of a tungsten electrode, and in the case of the forward direction, the metal moved to the opposite direction. The main experimental conditions were as follows. The TIG arc current, the laser output, the traveling speed of a SUS304 base metal, Ar gas flow rate and the distance between the base metal and a tungsten electrode were 100 A (DCEN), 2 kw, 150 mm/s, 1×10-3 m3/s and 15 mm, respectively. In the case of the backward direction, the bead width was smaller than one which was formed under the same experimental conditions except the traveling direction of the base metal. In this case, the temperature of the base metal remained normal in front of the combined heat source of the laser and the arc, and rose rapidly just before the heat source. In the case of the forward direction, the temperature rose slowly at the far point from the heat source and rapidly at the near point of it. Furthermore, two or three bonds were observed in the beads. On the other hand, it was appeared that anode spots were observed mainly in the area of the laser spot, and sometimes on the bead at the same time using high speed video images. As a result, the bead width under the backward direction was narrower than the forward one, and the TIG arc melted not only the area arround the laser spot on the base metal but also the solidified bead.
In the present work, a three-dimensional (3-D), non-stationary thermal model for MAG (Metal Active Gas) welding is developed. The transient temperature distribution on the base metal is numerically analyzed in order to estimate the molten pool size by using a finite difference model based on the heat flow equation, and the theoretical configuration of molten pool is calculated, taking account of the balance of gravity, surface tension and arc pressure. The developed model can be applied to various welding processes such as multi-pass welding, lap welding and welding with torch weaving. To evaluate the validity of the model analysis, the calculated results are compared with the experimental results in MAG welding. Good correspondence is demonstrated between experiment and calculation. Accordingly, it is concluded that the model is capable of predicting MAG arc welding process.
The effect of friction welding condition on the friction welding phenomena of high density polyethylene was investigated with a brake type friction welding machine which was built as a trial for polyethylene. The experimental results are as follows; 1) The initial torque, steady torque and total loss were increased by increase of friction pressure. These tendencies were the same manner as carbon steel. 2) The rate of total length of rotated side and fixed side changed with friction welding condition. These phenomena were different from the those of carbon steel. 3) The friction welding condition range of polyethylene for good appearance became narrower than that of carbon steel. 4) In the case of polyethylene, the environment in workshop became worse by occurrence of cotton dust, scatter of liquefied polyethylene and bad smell, when friction welding was investigated except pertinent friction welding condition. Therefore, this material required sufficient care, when friction welding condition was selected.
An ultrashort-pulse laser is expected as a useful tool for precision microfabrication. The ablation phenomena due to an ultrashort-pulse laser irradiation occur in the period of picosecond order. Therefore, it is very difficult to observe the phenomena insitu. The authors have been conducting the molecular dynamics simulation for many years. In this paper, picosecond-pulse laser ablation phenomena of aluminum were analyzed by the modified molecular dynamics methods, which was developed in last report. Mainly obtained results are summarized as follows: (1) Evaporation does not occur immediately from the beginning of laser irradiation, but starts several picosecond later. (2) There are two types in laser ablation process. One is explosive evaporation which occurs when pulse width is extremely short, and the other is calm evaporation which occurs when pulse width is comparatively long. (3) In the former process, relatively large particles also scatter, and in the latter process, the size of scattering particles is relatively small. Scattering velocity of evaporated particles is several kilometer per second. (4) A shock wave induced by the Gaussian beam irradiation propagates radially from the irradiated center of the prior surface to the interior of the material. It was shown that its propagation velocity is equal to one of elastic wave.
Rapid fusion and evaporation phenomena of silicon with ultrashort-pulse laser irradiation were simulated by a threedimensional molecular dynamics model. Evaporation process was elucidated, and both propagation velocity of shock wave and scattering velocity of evaporated atoms were analyzed quantitatively. Crystal orientation dependence of laser shock phenomena and fusion process was also examined with the models of Si (100) and Si (111) surfaces. Laser absorption in the material, that is, absorption coefficient is dominant in fusion process with subpicosecond pulse laser irradiation.
In the weld metal of A5083 welded with high energy density such as electron beam and laser beam, the amount of solute (mainly magnesium) of the primary crystals tends to increase with the increase in the welding speed. The solute concentration of solid phase tends to increase generally with the increase in the solid fraction and increases rapidly near the last stage of solidification, that is, the dendrite boundary. Though the dissolved amount of magnesium in the weld metal estimated with Xray diffraction technique becomes lower than that of the base metal, this tends to approach to that of the base metal after the weld metal was homogenized. In this paper, we examined these phenomena more precisely. First, we calculated the distributions of magnesium by applying the rapid solidification theory when the dendrite arm spacings (DAS) were changed to several steps. Then, the dissolved amount of magnesium in the dendrite was calculated by considering the distributions of magnesium and we made clear the influence of DAS on the dissolved amount of magnesium. Next, we also calculated the distributions of magnesium when the weld metal was homogenized and made clear the influence of DAS on the dissolved amount of magnesium after the homogenization. Moreover, the actual weld metals, in which DAS were changed, were obtained by changing the welding conditions with electron beam and we could experimentally confirm the same tendency as mentioned above which were simulated by the calculation. It was made clear that the Charpy impact energy of the weld metals linearly increased with the increase in the dissolved amount of magnesium.
The weldability of Fe-36%Ni alloy (Invar alloy) was investigated to clarify the main factor influential for weld cracking in multi-pass weld metal. The tests were carried out using the Fe-36%Ni plates containing various contents of sulfur. The weld cracking susceptibility was evaluated by double bead longitudinal Varestraint test and restraint weld cracking test using gas tungsten arc welding (GTAW). The ductility of weld metal at an elevated temperature was evaluated by hot ductility test. The crack in multi-pass weld metal occurred in the grain boundary away from the fusion boundary of the subsequent weld pass. From the result of hot ductility test of weld metal, the reduction of area was lower at temperature range from 1073 to 1173K. This range was consistent within the temperature range where cracking portion in the multi-pass weld metal was heated by subsequent weld thermal cycles. Consequently, the crack in the multi-pass weld metal was specified as ductility dip cracking. The ductility dip cracking susceptibility was increased in the higher sulfur content weld metals and sulfur concentration was detected on that crack surface. Further, the oxides and sulfides, such as Mn2SiO4 and α-MnS, were very slightly observed on the grain boundary in weld metal. From these results, the ductility dip cracking in weld metal was concluded to be mainly caused by grain boundary embrittlement by segregation of sulfur.
The effects of alloyed elements on ductility dip cracking susceptibility in multi-pass weld metal of Fe-36%Ni alloy (Invar alloy) was investigated to obtain the proper chemical compositions for preventing the ductility dip cracking and to clarify its mechanism. The tests were carried out using the Fe-36%Ni plates containing various contents of manganese, titanium, calcium, carbon and niobium. The ductility dip cracking susceptibility was evaluated by double bead longitudinal Varestraint test and restraint weld cracking test using gas tungsten arc welding (GTAW). The ductility dip cracking susceptibility decreased with the addition of both carbon and niobium. By the addition of these elements, the micro structure of weld metal was changed from the cellular structure with straight grain boundary to the cellular dendritic structure with grain boundary like saw teeth. Additionally, in the weld metal containing both niobium and carbon, the eutectic microstructure, with the niobium carbide, was produced. From these results, the prevention of ductility dip cracking in weld metal, with the addition of both niobium and carbon, is thought to be due to: (1) the decrease of sulfur concentration in the grain boundary by formation of eutectic structure with niobium carbide; and (2) the locking of the intergranular slip by the anchoring effect of the cellular dendritic structure with grain boundary like saw teeth.
The effects of carbon and niobium on solidification cracking susceptibility of Fe-36%Ni alloy (Invar alloy) were investigated to prove the validity of the addition of both carbon and niobium for preventing the solidification cracking as well as the ductility dip cracking. The tests were carried out using the Fe-36%Ni plates containing various contents of carbon and niobium. The solidification cracking susceptibility was evaluated by transverse Varestraint test using gas tungsten arc welding (GTAW). The microstructure of weld metal quenched by liquid tin was also observed. The solidification cracking susceptibility decreased with the increase of niobium in a range less than 1.5 mass% in weld metal containing 0.2 mass% carbon. In weld metal with 0.2% carbon, the eutectic microstructure of niobium carbide and austenite was produced in the dendrite space of primary austenite. Additionally, the residual liquid at the final stage of solidification disappeared at a higher temperature in weld metal containing 0.2% carbon and 1% niobium compared with the weld with 0.02% carbon and 1% niobium. From these results, the decrease of solidification cracking susceptibility by the addition of both carbon and niobium is considered to be due to reducing the temperature range, which both solid and liquid exist together, as a result of the decreasing niobium content in residual liquid by the formation of niobium carbide.
High temperature tensile tests were conducted on butt brazed joints of type 316 stainless steel. Two kinds of newly developed Ni-cu based filler metals, two kinds of standard nickel filler metals, BNi-2 and BNi-5, and a standard silver filler metal, BAg-21, were tested. Each of the joints was made to have a constant joint clearance of 50 micrometers. Tests were carried out at 298K, 423K, 573K, 723K, 873K and 973K, in air atmosphere. Strengths of the nickel brazed joints were less than 300MPa at all the test temperatures. The relatively low strength were attributed to formation of brittle eutectic phase at the center of the joints. This indicates that the joint clearance for nickel filler metals should be controlled to be narrower than 50 micrometers to obtain practical strength in industrial applications. In the joints with BAg-21, the strength decreased drastically above 573K. This filler metal therefore would not be applicable for the joints used at high temperatures. For the joint with Ni-Cu-Sn filler metal, one of the developed Ni-Cu filler metals, the strength was about 340MPa at 298K. The strength decreased gradually with the increase in temperature. At 973K, the strength was less than 100MPa. This filler metal is not applicable for the use at high temperatures over 600K, consequently. On the contrary, the joints with Ni-Cu-Mn filler metal showed about 400MPa of tensile strength from 298K to 723K. The result indicates that this filler metal can braze wide gap joints with better mechanical property compared to nickel filler metals, and can be used in high temperatures around 700K.
The stress wave propagation and the stress distribution in adhesive scarf joints with dissimilar adherends subjected to impact bending moments are analyzed in an elastic deformation using three-dimensional finite-element method (FEM). An impact load is applied to the joint by dropping weight. The one side of the adherend is fixed and the other side of the adherend is subjected to an impact load. FEM code employed is DYNA3D. The effects of scarf angles of the adherends, the adhesive thickness and Young's modulus of the adhesive, the combinations of materials of the adherends on the stress propagation and distributions at the interfaces are examined. The scarf angles are changed from 15 to 90 degrees. It is found that the maximum value of the Mises' equivalent stress σM appears at the interface between the adhesive layer and the adherend made of mild steel, and it is independent of the combinations of the adherend materials. In the case where the scarf angle is 45 degree, the maximum value of the Mises' equivalent stress σM is minimal. The stress σX is the greatest in all of stress components. The maximum value of the Mises' equivalent stress σM increases as the adhesive layer thickness and Young's modulus of the adhesive increase. It is seen that the position where the maximum value of the Mises' equivalent stress σM occurs moves toward edge of the adhesive interface as the thickness and Young's modulus of the adhesive increase. In addition, experiments were carried out to measure the strain response of the adhesive scarf joints subjected to impact bending moments using strain gauges. A fairly good agreement is seen between the analytical and the experimental results.
Welding deformation is complexly affected by different factors, and that is predicted to depend on the skills of welding operatives, being affected by welding parameters such as the welding current, arc voltage and welding speed. It is therefore desirable to treat welding deformations statically. This paper describes an investigation into the statistical properties of the welding parameters, transverse shrinkage and angular distortion when welding operatives having specified skills are engaged in semi-automatic MAG welding of butt welding joint with backing plate. The results obtained may be summarized as follows. (1)The distribution characteristics of the welding heat input, product of welding current and voltage, and the reciprocal of welding speed are clarified, besides and they conform to a normal distribution. (2)The distribution characteristics of the transverse shrinkage and angular distortion have been made clear. And they conform to a normal distribution are clarified. (3)95% confidential interval of mean valus and standard deviation of population in the transverse shrinkage and angular distortion when welding operatives obtaining license SN-2F in JIS Z 3841 are engaged in semi-automatic MAG welding of butt welding joint with backing plate are estimated.
Girth welded joints of pipelines may possibly contain defects which could reduce the fatigue strength of the joints under earthquake conditions. Hence it is necessary to identify the fatigue properties of the joints containing various defects and clarify the acceptable size of weld defects. In this study, very low cycle fatigue tests and fatigue life estimation by fracture mechanics analysis of butt welded joints containing defects were carried out. The influence of plate thickness ranging from 9mm to 19mm on fatigue properties and the acceptable defect size in the welded joints were investigated. It was recognized that the fatigue life for the thickness of 19mm is shorter than that for the thickness of 9mm when the ratio of defect size to thickness is constant. The acceptable defect size in the welds under the earthquake conditions prescribed by the Japan Gas Association, namely, the strain amplitude of 0.5% for 50cycles of cyclic displacement, is proposed as a result of the analysis.
The development of the joining method of titanium with stainless steel is important to major applications in the field of tableware, automobile and sports equipments. Therefore, this study was performed with the objective of producing a nonleak and high strength lap joint between pure titanium and stainless steel. A pulsed Nd:YAG laser was utilized as a heat source because of its low total heat input and easy combination of pulse width and energy. Using flat sheets of titanium and stainless steel, the properties of lap joints were evaluated by shearing strength as well as metallurgical observation and chemical analyses of their cross sections. Vacuum tubes of dissimilar laser lap joints between titanium and stainless steel were manufactured experimentally, and were subjected to the leakage test. Dissimilar high strength lap joints could be produced, and the shear load of 4200N per 40mm width and shear stress of 190MPa were achieved for the specimens made under optimum conditions. Then melting of the bottom sheet was confined in a small overlap area to lessen the formation amount and zone of intermetallic compounds by controlling the pulse energy and the power density. In addition, in the case of laser shot at the top titanium sheet, high strength joints were produced under more wide irradiation conditions, probably because of the formation of Ti solid solution containing larger contents of alloying elements such as Fe, Cr, Ni, etc. in the bottom sheet. Moreover, the tubes of nonleak joints could be produced only when a pulsed Nd:YAG laser was irradiated on the titanium inner sheet. This is attributed to the difference in the expansion coefficient between titanium and stainless steel in addition to the above-mentioned easier welding.