Materials Science Forum Vols. 580-582

Abstract: To improve the joining efficiency of Bi-Sr-Ca-Cu-O ( BSCCO) superconducting tapes, a new diffusion bonding technology with a direct uniaxial pressing at high temperature was developed to join 61-filament tapes. It was observed that bonding parameters such as bonding pressure and holding time, significantly affected the critical current ratio (CCRo). A peak CCRo value of 89 % for the lap-joined tapes was achieved at 3 MPa for 2 h when bonding temperature was 800 °C. Compared with the conventional diffusion bonding technology, this new technology remarkably shortened the fabrication period and improved the superconductivity of the joints. The bonding interface and microstructures of the joints were evaluated and correlated to the CCRo. An uniaxial pressing at high temperature was beneficial to interface bonding, and there was an optimal pressure value for the CCRo.

Abstract: Friction stir welding (FSW) makes the stir zone with fine recrystallized grain structure. The recrystallized grains would be formed through dynamic recrystallization at high temperatures and high strain-rate. The present study experimentally simulated the dynamically recrystallized microstructure of a friction stir welded Al alloy 1050 produced at 600 rpm rotation and 100 mm/min travel speed, using combination of the plane-strain compression at various strain rates and the subsequent cooling along the cooling cycle of FSW. The equiaxed grain structures similar to the microstructure of the stir zone were produced at strain rates between 0.1 and 32 s-1; the grain size decreased with increasing strain rate. Strain rate during the FSW could be estimated to be about 1.8 s-1. The present study suggests that plane-strain compression test can simulate the recrystallized grain structure of the friction stir welds.

Abstract: In non-pulsed GMA welding, spatter can be reduced by controlling the short-circuit current to a low level just before the re-arcing. The reduction of spatter requires improving the accuracy of predicting the re-arcing by stabilizing the molten metal transfer, and improving the consistency of accuracy against disturbances. The Controlled Bridge Transfer (CBT) process, which optimizes the accuracy of predicting the re-arcing in real time in response to the molten metal transfer, realizes stable, low spatter level GMA welding.

Abstract: Utilizing alternating welding process parameters, deposition practices, and welding consumables, particularly during multiple pass welding, it is possible to improve a variety of weld metal properties. There are available a number of phenomena occurring during welding that allow weld metal designers the ability to generate macro- and micro-structural features amenable to implementation of composite theory. These phenomena include solidification microsegregation during dendrite growth, gas-metal reactions between the selected alternating shielding gas composition and weld pool, and solidification microstructural orientation during welding. Additional methods of producing composite welds including specially designed weld compositions, weld metal solidification modification by arc pulsing, and dual wire deposition may be utilized to achieve single pass and multipass composite weld metal deposition. Composite welds are a potential method to solve challenging demands such as high-toughness at low temperature, creep strength at high temperature, and customized design for corrosion, wear, or cracking resistance.

Abstract: It is important to consider the interaction between arc plasma and electrodes because melting of electrodes strongly affects arc plasma. Therefore, a GMA model will be developed, based on the unified model of TIG arc. As a first step, a TIG arc model with a calculation for molten cathode shape has been proposed. This model is calculated in two cases; molten W cathode and Calculation result of W cathode. In the case of W cathode, cathode shape change was found to affect the arc plasma property strongly. Calculated results of radial temperature distributions on electrode surface and arc pressure distributions at the anode surface are very similar to the experimental results.

Abstract: Underground PE double wall pipes have been supplying waste water to sewage disposal plant. Many processes have been introduced to join PE pipes, but most of these methods have lots of disadvantages such as costs, lack of reliability and difficulties in joining, etc. Recently butt welding has been paid much attention to joint PE pipes as this process has many advantages such as cost, safety and reliability. In this study, a newly developed heat plate, a patent-pending heat plate with a groove, was used to butt-weld PE double wall pipes with different misalignment gaps to simulate real underground conditions. The resulting joining characteristics of double wall pipes were compared with those from a conventional heat plate, in terms of stiffness, flattening and leakage tests. The results from the stiffness and flattening test showed that there were no big differences between the butt-welded joints made from these two plates. From the leakage test, although PE pipes welded with a conventional heat plate did leak below the required test conditions (10 min. at 0.75kgf/cm2), the pipes welded with a patent-pending grooved heat plate did not show any leakage even at a pressure 1.3 times higher than the required conditions. It was noted that by utilizing a grooved heat plate more complete fusion at the pipe joints could be obtained, which in turn induced a high quality joints.

Abstract: Study of current attachment at thermionic cathode for TIG arc at atmospheric pressure is attempted from numerical calculations of arc-electrodes unified model. The calculations show that the maximum temperature of arc plasma close to the cathode tip for W-2% ThO2 reaches 19,000 K and it is the highest value in comparison with the other temperatures for W-2% La2O3 and W-2% CeO2, because the current attachment at the cathode tip is constricted by a centralized limitation of liquid area of ThO2 due to its higher melting point. The calculations also show that, in cases of W- 2% La2O3 and W-2% CeO2, the liquid areas of La2O3 and Ce2O3 are widely expanded at the cathode tip due to their lower melting points and then produce uniform current attachments at the cathode. It is concluded that the current attachment at thermionic cathode is strongly dependent on work function, melting point and Richardson constant of emitter materials.

Abstract: Study of current attachment at thermionic cathode for TIG arc at atmospheric pressure is attempted from numerical calculations of arc-electrodes unified model. The calculations show that the maximum temperature of arc plasma close to the cathode tip for W-2% ThO2 reaches 19,000 K and it is the highest value in comparison with the other temperatures for W-2% La2O3 and W-2% CeO2, because the current attachment at the cathode tip is constricted by a centralized limitation of liquid area of ThO2 due to its higher melting point. The calculations also show that, in cases of W- 2% La2O3 and W-2% CeO2, the liquid areas of La2O3 and Ce2O3 are widely expanded at the cathode tip due to their lower melting points and then produce uniform current attachments at the cathode. It is concluded that the current attachment at thermionic cathode is strongly dependent on work function, melting point and Richardson constant of emitter materials.

Abstract: Finite element simulations of the experimental tests to explosively weld three layers of Al5056, Al6061 and SS304L tube composites, using various stand-off distances and explosive ratios, are presented in this study. The Williamsburg equations of state and Johnson-Cook constitutive equations were used to describe the behaviors of the explosive and the tubes, respectively. In this paper, the external parameters (dynamic angle and collision velocity) are related to the physical parameters (shear stress and plastic strain). The numerical results showed that very high localized plastic deformation was produced at the bond interface. Moreover, it was found that the shear stress magnitude and signs can provide the necessary criteria for bonding. In addition, the sufficient criterion for bonding was found to be the magnitude of the plastic strain produced at the collision point. The new welding window based on the internal parameters is proposed.

Abstract: Residual stresses are produced in weldments due to mismatching and non-uniform distributions of plastic and thermal strains. Attempts were made to analyze the residual stresses distributions produced in the TIG welding process using 2d and 3d finite element analyses. No attempts were made to find the optimum shape of grooves and gas flow rate to minimize the tensile residual stresses in the weldments, yet. In this paper, the effects of geometry configurations and inert gas flow rate on the residual stress distributions are presented using the thermo-elastoplastic constitutive equations and compared with the x-ray diffraction method. In this study, convection due to both air and inert gas flow rate along with conduction and radiation are considered.