Sindo Kou

Bio: Sindo Kou is an academic researcher from University of Wisconsin-Madison. The author has contributed to research in topics: Welding & Crystal growth. The author has an hindex of 44, co-authored 188 publications receiving 5229 citations. Previous affiliations of Sindo Kou include Carnegie Mellon University & University of Wisconsin–Milwaukee.

Fluid flow and weld penetration in stationary arc welds

Abstract: Weld pool fluid flow can affect the penetration of the resultant weld significantly. In this work, the computer simulation of weld pool fluid flow and its effect on weld penetration was carried out. Steady-state, 2-dimensional heat and fluid flow in stationary arc welds were computed, with three driving forces for fluid flow being considered: the buoyancy force, the electromagnetic force, and the surface tension gradient at the weld pool surface. The computer model developed agreed well with available analytical solutions and was consistent with weld convection phenomena experimentally observed by previous investigators and the authors. The relative importance of the influence of the three driving forces on fluid flow and weld penetration was evaluated, and the role of surface active agents was discussed. The effects of the thermal expansion coefficient of the liquid metal, the current density distribution in the workpiece, and the surface tension temperature coefficient of the liquid metal on weld pool fluid flow were demonstrated. Meanwhile, a new approach to free boundary problems involving simultaneous heat and fluid flow was developed, and the effort of computation was reduced significantly.

Solidification and liquation cracking issues in welding

Abstract: Solidification cracking can occur in the fusion zone during the solidification of the weld metal Liquation cracking, on the other hand, can occur in the partially melted zone during the solidification of the liquated material These two types of cracking are reviewed in this article, including the factors that affect cracking and the remedies

Al-to-Mg Friction Stir Welding: Effect of Material Position, Travel Speed, and Rotation Speed

Abstract: Because joining dissimilar metals is often difficult by fusion joining, interest has been growing rapidly in using friction stir welding (FSW), which is considered a revolutionary solid-state welding process, as a new way to join dissimilar metals such as Al alloys to Mg alloys, Cu, and steels. Butt FSW of Al to Mg alloys has been studied frequently recently, but the basic issue of how the welding conditions affect the resultant joint strength still is not well understood. Using the widely used alloys 6061 Al and AZ31 Mg, the current study investigated the effect of the welding conditions, including the positions of Al and Mg with respect to the welding tool, the tool travel speed, and the tool rotation speed on the weld strength. Unlike previous studies, the current study (1) determined the heat input by both torque and temperature measurements during FSW, (2) used color metallography with Al, Mg, Al3Mg2, and Al12Mg17 all shown in different colors to reveal clearly the formation of intermetallic compounds and material flow in the stir zone, which are known to affect the joint strength significantly, and (3) determined the windows for travel and rotation speeds to optimize the joint strength for various material positions. The current study demonstrated clearly that the welding conditions affect the heat input, which in turn affects (1) the formation of intermetallics and even liquid and (2) material flow. Thus, the effect of welding conditions in Al-to-Mg butt FSW on the joint strength now can be explained.

Formation of Liquid and Intermetallics in Al-to-Mg Friction Stir Welding

Abstract: In dissimilar-metal friction stir welding (FSW), intermetallic compounds can form in the stir zone and significantly reduce the joint strength. The formation of intermetallic compounds in Al-to-Mg FSW was investigated in lap and butt FSW of the widely used 6061 Al and AZ31B Mg and discussed using the binary Al-Mg phase diagram as an approximation. Temperature measurements during lap FSW indicated a 703 K (430 °C) peak temperature, slightly below the eutectic reaction (Mg) + Al12Mg17 → L at 710 K (437 °C), because the thermocouples were pushed downward during welding. The intermetallic compounds in the stir zone were revealed by color etching and identified by X-ray diffraction (XRD), electron probe microanalysis (EPMA), and transmission electron microscopy (TEM) as Al3Mg2 and Al12Mg17. Additional FSW was conducted near the edge of the upper sheet, and the liquid droplets squeezed out during welding solidified along the edge. Optical microscopy of the solidified droplets and EPMA revealed dendrites of Al3Mg2 and Al12Mg17 and interdendritic eutectics, thus indicating eutectic reactions (Mg) + Al12Mg17 → L (710 K (437 °C)) and (Al) + Al3Mg2 → L (723 K (450 °C)). Differential scanning calorimetry (DSC) confirmed that the solidified droplets melted at 709 K (436 °C) and 722 K (449 °C), nearly identical to the eutectic temperatures. Formation of intermetallic compounds on the order of 1 mm in size suggests they form upon solidification of the liquated material instead of solid-state diffusion.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

Cadmium zinc telluride and its use as a nuclear radiation detector material

Abstract: We present a comprehensive review of the material properties of cadmium zinc telluride (CZT, Cd1ˇxZnxTe) with zinc content xa 0:1‐0.2. Particular emphasis is placed on those aspects of this material related to room temperature nuclear detectors. A review of the structural properties, charge transport, and contacting issues and how these are related to detector and spectrometer performance is presented. A comprehensive literature survey and bibliography are also included. # 2001 Elsevier Science B.V. All rights reserved.

Stretched exponential relaxation in molecular and electronic glasses

Abstract: Stretched exponential relaxation, , fits many relaxation processes in disordered and quenched electronic and molecular systems, but it is widely believed that this function has no microscopic basis, especially in the case of molecular relaxation. For electronic relaxation the appearance of the stretched exponential is often described in the context of dispersive transport, where is treated as an adjustable parameter, but in almost all cases it is generally assumed that no microscopic meaning can be assigned to even at , a glass transition temperature. We show that for molecular relaxation can be understood, providing that one separates extrinsic and intrinsic effects, and that the intrinsic effects are dominated by two magic numbers, for short-range forces, and for long-range Coulomb forces, as originally observed by Kohlrausch for the decay of residual charge on a Leyden jar. Our mathematical model treats relaxation kinetics using the Lifshitz - Kac - Luttinger diffusion to traps depletion model in a configuration space of effective dimensionality, the latter being determined using axiomatic set theory and Phillips - Thorpe constraint theory. The experiments discussed include ns neutron scattering experiments, particularly those based on neutron spin echoes which measure S( Q,t) directly, and the traditional linear response measurements which span the range from to s, as collected and analysed phenomenologically by Angell, Ngai, Bohmer and others. The electronic materials discussed include a-Si:H, granular , semiconductor nanocrystallites, charge density waves in , spin glasses, and vortex glasses in high-temperature semiconductors. The molecular materials discussed include polymers, network glasses, electrolytes and alcohols, Van der Waals supercooled liquids and glasses, orientational glasses, water, fused salts, and heme proteins. In the intrinsic cases the theory of is often accurate to 2%, which is often better than the quoted experimental accuracies . The extrinsic cases are identified by explicit structural signatures which are discussed at length. The discussion also includes recent molecular dynamical simulations for metallic glasses, spin glasses, quasicrystals and polymers which have achieved the intermediate relaxed Kohlrausch state and which have obtained values of in excellent agreement with the prediction of the microscopic theory.