QUARTERLY JOURNAL OF THE JAPAN WELDING SOCIETY
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PRINT ISSN: 0288-4771

QUARTERLY JOURNAL OF THE JAPAN WELDING SOCIETY Vol. 25 (2007), No. 3

  • Hot Cracking in Laser Braze Joint of High Strength Stainless Steel using Gold Brazing Filler Metal

    pp. 403-409

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    DOI:10.2207/qjjws.25.403

    Hot cracking behavior in laser braze joint of 13Cr-4Ni stainless steel has been investigated using BAu-4 (Au-18%Ni) filler metal. The diode laser brazing was carried out for the T-fillet joint as well as the L-fillet joint which simulates the second bead in the T-fillet joint. The tandem beam brazing with a post-heating beam was also carried out in order to prevent the hot cracking. The laser powers of main and trailing beams were varied. The center line crack occurred in the second braze bead of the T-fillet joint, and was characterized as ductility-dip crack. Hot cracking susceptibility in Au-Ni braze metal was evaluated by spot-Varestraint test. The ductility-dip cracking temperature range (DTR) was about 1000-1250K. The FEM analysis of thermal stress/strain revealed that the rise curve of plastic strain intersected the DTR in the single beam brazing, however, it would not intersect the DTR by post-heating during laser brazing. The tandem beam brazing confirmed that no cracks occurred in the L-fillet braze bead made with trailing beam at the laser power of 200-300W.
  • Influence of Minor Elements in Electrode Wire on Spattering Phenomena in CO2 Gas Shielded Arc Welding

    pp. 410-415

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    DOI:10.2207/qjjws.25.410

    Although CO2 gas shielded arc welding is the mainstream arc welding method, a large amount of spatter is generated during welding. To solve this problem, the mode of spatter generation was elucidated and the influence of minor elements in the electrode wire on the amount of spatter generation was investigated, clarifying the following facts. In CO2 gas shielded arc welding, because the arc generated at the suspended droplet is constricted and the arc point is violently displaced on the droplet surface, the droplet undergoes repeated irregular shaking as it is transferred to the molten pool. As a result, a large amount of spatter is generated by fragmentation of the droplet due to short-circuiting/re-arcing during droplet transfer, or by scattering of the droplet by the arc force. As countermeasures for this unstable droplet transfer, addition of Ti to the wire has the effect of suppressing short-circuiting by stabilizing the molten pool, while addition of K reduces the amount of spatter by stabilizing the arc. Conversely, addition of REM or Ca causes an increase in large-sized spatter due to extension of the arc and accompanying scattering of the droplet.
  • Microscopic Observation of Inclusions related to Acicular Ferrite Formation in Steel Weld Metal

    pp. 416-420

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    DOI:10.2207/qjjws.25.416

    To clarify the microstructural evolution in low carbon low alloy steel weld metal, the inclusions related to acicular ferrite formation were investigated from the crystallographic points of view.
    The inclusions of each sample were prepared by focused ion beam and observed by transmission electron microscope. Further more, compositions of inclusions were analyzed by energy dispersive spectroscopy.
    The inclusions related to acicular ferrite formation were multi-phase and they consisted of MnS, MnAl2O4 and amorphous. It was observed that these inclusions were enclosed by Ti concentrated area.
    Thus it could be concluded that the Ti concentrated area of the inclusion surface would play a role for the heterogeneous nucleation of acicular ferrite.
  • Modeling of Molten Drop Oscillation in Gas shielded Metal Arc Welding

    pp. 421-425

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    DOI:10.2207/qjjws.25.421

    In Gas shielded Metal Arc Welding process, the molten drop at the electrode wire tip is detached and transferred into the weld pool by various driving forces, which have been discussed mainly focusing on gravity force, electro-magnetic force, surface tension force and plasma drag force. In this paper modeling of the oscillation phenomena of molten drop at the wire tip are described. An analytical model evaluating natural frequency of liquid pendant drop was proposed on the basis of mass-spring model. The numerical model developed is an axial symmetrical 2D model which enables to calculate and visualize time-change of the drop shape and the flow in the drop using VOF-CSF method. It is shown that the natural frequency of the pendant drop decreases with increase of the drop size by both analytical and numerical models. And the natural frequency evaluated by analytical model is shown to be little higher than that by numerical model because of neglecting viscous resistance of liquid flow.
  • Microstructure Observation of Weld Interface between 6063 Al alloy and S45C Carbon Steel by Means of Friction Stirring

    pp. 426-430

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    DOI:10.2207/qjjws.25.426

    Microstructure of weld interface between 6063 and S45C by means of friction stirring was observed by TEM. The reaction layer was formed along welding direction and depth direction of tool insert. The layers were composed of various intermetallic compounds such as Fe2Al5 and FeAl. The constitution differed with welding conditions. At 500 mm/min of welding speed, the interface had a thin reaction layer. At 1000 mm/min of welding speed, the interface had layered structure. However, both of the weld interfaces had a reaction layer under 1 μm and achieved high tensile strength of weld joint.
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  • Numerical Modeling of Lap-Joint MAG Welding

    pp. 431-435

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    DOI:10.2207/qjjws.25.431

    One of the important problems in the welding engineering is to construct a numerical model for the computer simulation of actual welding processes. Lap joints have been widely used for the welding of thin plates in the manufacturing processes in motor industry. In the present paper, a numerical model for lap-joint MAG (Metal Active Gas) welding has been developed to make clear the influence of various parameters on the welding phenomena. In the model, the transient temperature distribution in the base metal is numerically calculated, based on the heat flow equation, and the theoretical configuration of the molten pool is analyzed on the basis of the calculus of variations, where a finite difference model with non-uniform mesh is applied to reduce CPU time. As a result, the calculating time in numerical simulations with the present model is shorter than 1/10 of the one by the model with uniform mesh. To evaluate the validity of the present model, the numerical results are compared with the experimental ones and good agreement between calculations and experiments are demonstrated. Accordingly it is concluded that the model developed in the present work is useful in the welding engineering.
  • Micro Observation of Dissimilar Joint Interface between Low Carbon Steel and Pure Aluminum Produced by Laser Pressure Welding

    pp. 436-442

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    DOI:10.2207/qjjws.25.436

    Dissimilar metals joints of low carbon steel (SPCC) and pure aluminum (A1050) sheets were produced using the laser pressure welding method by changing the laser power and the roller pressure. The tensile shear strength and peel strength of welded joints were measured. The fractured interfaces of the joints were observed by SEM and analyzed by EDX. Moreover, detailed phases formed in the welded interface were investigated by TEM.
    In the tensile shear test and peel test, the strengths of the joints yielded under the optimum welding conditions were so high that the fracture occurred in the aluminum base metal. The thicknesses of intermetallic compound layers were about 50 nm to 100 nm. In the TEM observation and electron diffraction pattern results, the amorphous phase and the intermetallic compounds consisting of Fe2Al5 and Fe4Al13 were observed. The welded zone was cooled rapidly due to the heat transfer from the welded zone toward respective base metals and rolls. Therefore, the amorphous phase was formed in the most rapidly cooled part. Moreover, the Fe2Al5 and Fe4Al13 phases were formed in the welded interface.
  • Numerical Simulation of Metal Vapor Behavior in Gas Tungsten Arc Welding

    pp. 443-449

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    DOI:10.2207/qjjws.25.443

    The present modeling of a gas tungsten arc in helium or argon accounts for metal vapor contamination from the weld pool as an anode. The whole region of gas tungsten arc welding, namely, tungsten cathode, arc plasma and weld pool is treated in a unified numerical model. A viscosity approximation is used to express the diffusion coefficient in terms of the viscosities of shielding gas and iron vapor. The time dependent two-dimensional distributions of temperature, velocity and iron vapor concentration are predicted, together with the weld penetration as function of time for a 150 A arc at atmospheric pressure. It is shown that thermal plasma in gas tungsten arc is markedly influenced by iron vapor from the weld pool surface and concentration of the iron vapor into the plasma is dependent on temperature of weld pool surface.
  • Verification of Validity and Generality of Dominant Factors in High Accurate Prediction of Welding Distortion

    pp. 450-454

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    DOI:10.2207/qjjws.25.450

    When out-of-plane distortion was measured objecting to butt welding, if tack welding was easily performed, the position of a neutral axis was variously changed by the linear misalignment. Out-of-plane distortion was generated in the unexpected direction because the neutral axis changed. From such as bitter experience, a test specimen to measure out-of-plane distortion was proposed so as to solve this problem. Using the proposed specimen, out-of-plane distortion was measured for butt welding.
    In the high accurate prediction of welding distortion, the dominant factors, which were satisfying the yield condition (especially at high temperature above 700 degree Celsius) and regarding the weld metal as welds, had been specified in fillet welding previously. So as to verify the validity of the dominant factors, the experiment and thermal elastic plastic analysis by FEM were carried out using newly developed specimen for butt welding. The validity of the dominant factors in the high accurate prediction of welding distortion objecting to fillet welding was verified by the results of butt welding and its generality was confirmed. It was confirmed that residual stress was also highly predicted because welding distortion was highly predicted.
  • High-Speed Observation and Spectroscopic Analysis of Laser-Induced Plume in High-Power Fiber Laser Welding of Stainless Steel

    pp. 455-460

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    DOI:10.2207/qjjws.25.455

    This study was undertaken to obtain a fundamental knowledge of the generation behavior and internal state of a plume induced during bead-on-plate welding of a 20 mm thick Type 304 stainless steel with a 10 kW fiber laser beam of a 130 μm spot diameter, on the basis of 40,000 f/s high-speed video observation and spectroscopic analysis. This 0.9 MW/mm2 ultra-high power density laser produced a 12 mm deep partial-penetration weld with 1.3 mm narrow bead width at 3 m/min welding speed. According to the high-speed observation pictures, laser-induced plumes were repeatedly generated from a keyhole at the interval of about 0.5 ms period to reach 12 mm height at the maximum. The plume was also characterized by bright blue emission. The spectroscopy indicated that the plume was emitted from the line spectra of neutral atoms such as iron (Fe), chromium (Cr) and manganese (Mn), which were alloying elements of Type 304. However, ionized spectra of alloying elements and line spectra of argon (Ar) neutral atom were not apparently detected under these welding conditions. Furthermore, the temperature and ionization degree of the laser-induced plumes were calculated to be 6,000 K and 0.02, respectively, by the Bolzman plots and Saha's equation. Consequently, the plume induced with a 10 kW fiber laser of extremely high power density was judged to be in a state of weakly ionized plasma.
  • Interaction between Laser Beam and Plasma/Plume Induced in Welding of Stainless Steel with Ultra-High Power Density Fiber Laser

    pp. 461-467

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    DOI:10.2207/qjjws.25.461

    Recently 10 kW fiber lasers with high beam quality have been developed to achieve the power density of about 1 MW/mm2. A plume induced during welding with such an ultra-high power density laser was in a weakly ionized state. The objectives of this research are to obtain a fundamental knowledge of interaction between a laser beam and an induced plume and to evaluate the effect of this ultra-high power density laser on the weld penetration. The observation result of the plume induced during bead-on-plate welding of a 20 mm thick Type 304 plate with 10 kW laser showed the growth of about 12 mm height at the repetition cycle of around 0.5 ms. The interaction was investigated by the power meter measurement or the high-speed behavior observation of a fiber laser probe beam (wavelength : 1.09 μm), which passed vertically through the plume. The attenuation of the probe laser beam was measured to be about 4%, which was not mainly caused by Inverse Bremsstrahlung but by Rayleigh scattering due to ultrafine particles and partly by absorption and reflection due to spatters. The probe beam observed was refracted at 0.6 mrad angle in average, which was much lower than the 90 mrad divergence of the focused fiber laser beam. Moreover, a stable laser welding process could be produced at such ultra-high power density that 12 mm deep penetration was obtained even if the laser peak power was decreased 1 ms periodically from 10 kW to 8.5 kW. It was consequently considered that the interaction between a focused 10 kW fiber laser beam and a weakly ionized plume was too small to exert the reduction in weld penetration.
  • Discussion on HAZ Toughness Evaluation by One-Bead Weld Method

    pp. 468-472

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    DOI:10.2207/qjjws.25.468

    The engineering method for fracture toughness evaluation of beam-to-column weld HAZ using one-bead welds, called one-bead weld method, has been examined in this study. Materials used are 26 structural steels of 400 to 490 MPa strength class produced by different milling conditions. One-bead welds are made by CO2 arc welding with single bevel groove under heat inputs of 4.0 to 5.0 kJ/mm. From these welded joints, the Charpy specimens and 3-point bend CTOD specimens with the same thickness of 10mm are extracted. The notch is located at the weld HAZ on a straight bond side perpendicular to the plate surface. Charpy tests and CTOD tests are carried out at 0°C and -40°C, respectively, where the CTOD test temperature is determined on the basis of WES2808 correlation formula. It has been found that the Charpy absorbed energy is affected by the chemical composition of the base steel, such as C, Mn, P, S, N, and Ti. Based on similar test results, an engineering parameter, fHAZ, is proposed by the Building Center of Japan for estimating the HAZ toughness. However, the present study indicates that the total amount of N is not a proper measure, but the free N controls the HAZ toughness. From the fracture mechanical point of view, the correlation between the Charpy energy and the critical CTOD is useful. It is noted that the CTOD test results are sensitive to the length of CGHAZ along the crack front. The Charpy-CTOD correlation with respect to the HAZ toughness should be made with attention to the base-metal chemical compositions as well as the CGHAZ size along the crack front.

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