D. K. Aidun

Three-dimensional transient model for arc welding process

A direct computer simulation technique, discrete element analysis (DEA), was utilized in the development of a transient multidimensional (2-D and 3-D) mathematical model for investi-gating coupled conduction and convection heat transfer problems associated with stationary and moving arc welding processes. The mathematical formulation considers buoyancy, electro-magnetic, and surface tension driving forces in the solution of the overall heat transfer conditions in the specimen. Furthermore, the formulation of the model allows realistic consideration of the geometrical variations in the workpiece. The model treats the -weld pool surface as a truly deformable free surface, allowing for the prediction of the weld surface deformations such as the “weld crown.≓ A marked element formulation was employed to monitor the transient de-velopment of the weld pool as determined by the latent heat considerations and the calculated velocities in the weld pool. The model was utilized to simulate the heat and fluid flows in the weld pool that occur during stationary (spot) and moving (linear) gas tungsten-arc welding. Also, the present analysis considers a simple rectangular specimen and a geometrically complex specimen to demonstrate the capability of the model to simulate realistic 3-D arc welding prob-lems. The results of the present investigation clearly demonstrate the significant influence of the heat and fluid flows and the specimen geometry on the development of the weld. Comparison of the predicted and the experimentally observed fusion zone and heat-affected zone (HAZ) geometries indicate good agreement.

Characterization of precipitates in CdTe and Cd1−xZnxTe grown by vertical Bridgman-Stockbarger technique

Abstract Different polishing solutions were tested for exposing precipitates in CdTe and Cd 0.96 Zn 0.04 single crystals grown by vertical Bridgman-Stockbarger technique. A solution of 5–7% Br 2 in methanol and E-solution were both effective. High resolution scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS) was employed to characterize those exposed precipitates. Most of the polyhedral-shaped Te precipitates with a size range from 3 to 20 μm had voids inside. Partially dissolved Te precipitates were observed in CdTe samples that had been annealed in Cd vapor at 700°C for 10 min. Isolated areas mis-oriented from the matrix were observed in CdTe and Cd 0.96 Zn 0.04 Te that had been annealed in Cd vapor at 700°C for 20 and 50 h, respectively. Te precipitate images were recorded with EDS. By SEM/EDS, Cd-rich precipitates were observed in some Cd-annealed CdTe. C and Na impurities were detected in some Te precipitates.

Penetration in spot GTA welds during centrifugation

Convective flow during arc welding processes mainly depends on electromagnetic force, Marangoni force, and buoyancy force. The Marangoni flow (caused by surface tension gradient,dγ/dT)and the buoyancy driven flow are the major factors in controlling weld penetration in austenitic stainless steels, such as types 304 and 316. Alloys 304 and 316 were subjected to a 7 s spot gas-tungsten arc (SGTA) welding at 1 g (g = 9.8 m/s2)and 5 g accelerations. The welds at 5 g were performed on Clarkson University’s multigravity research welding system (MGRWS). The cross sections of the fusion zones were polished/etched, and their depth (D)and width (W)were measured to ± 0.025 mm. It was determined that the depth/width ratio (D/W)of the welds decreased as the acceleration increased from 1 to 5 g. This result indicates that increase in buoyancy driven flow will produce wider but shallower welds during SGTA welding.

Numerical Simulation of the Effect of Gravity on Weld Pool Shape

Understanding the physical phenomena involved in the welding process is of substantial value to improving the weldability of materials. The intense heat and the arc inherent in fusion welding make direct experimental observation of the weld pool behavior rather difficult. Thus numerical models that can predict the processes involved have become an invaluable tool for studying welding.

Influence of simulated high-G on the weld size of aL–lI alloy

Abstract Whether welding processes are used on earth or in space, they have the same objective: to obtain defect-free welds. To fully understand the effect of gravity on the weld pool geometry and solidification one should perform experiments within a broad range of gravitational acceleration. High-gravity arc welding experiments were done on Al–Li alloy using the centrifuge called Multi-Gravity Research Welding System (MGRWS). At a high “ g ” level, buoyancy-driven flow is the dominant force in the weld pool over the Marangoni and the electromagnetic forces. Preliminary results show that the average grain size in the fusion zone at 1 g is smaller that at 5 g .

Fracture and mechanical properties of P100 Gr/6061 Al composite

The effects of heat treatments on the mechanical and fracture properties of continuous fiber Pitch 100 graphite/6061 aluminum (P100 Gr/6061 Al) composite were evaluated by varying solution temperature and aging time. All heat treatments resulted in decreased mechanical properties due to the reaction at the fiber/matrix interface during solution treatments. SEM analysis of the fractured samples showed a large degree of fiber pullout, interface failure, and matrix cracking.

Integrated physics-based approach for the reliability prediction of thermal systems

A broad reliability prediction method that can deal with complex thermo-fluidic systems is introduced. The procedure provides an engineering tool by integrating multiple computational packages that enable the simulation of a wide array of systems, especially those involving physics interactions such as fluid flow and solid medium. Computational Fluid Dynamics, Finite Element Method, Monte Carlo Simulation and fatigue analysis tools are integrated within this physics-based reliability prediction approach. The complete procedure is demonstrated using a simple example, and then validated using boiler pipes experimental data. CFD simulations are used to determine the convective terms necessary for the transient FEM thermal analysis. The thermal analysis provides maximum thermal stress whereby the fatigue life of the component is evaluated. As a result of input parameters uncertainty, the expected life will be in the form of a Probability Density Function, which enables the calculation of the reliability of the component.

Deformation Behavior of Cadmium Telluride Stressed at Elevated Temperatures

CdTe crystals were uniaxially compressed along several crystallographic axes at temperatures from 773 to 1353 K. The applied stress ranged from 14 to 74% of the critical resolved shear stress (CRSS) measured in the authors’ laboratory. The deformed specimens were annealed without applying stress at temperatures from 573 to 1073 K. No twins were observed after the above operations. Dense slip bands were observed on most of the compressed specimens. Secondary slip systems were activated in some experiments. CdTe crystals were sheared along {111}<112> at 1073 K with a load of 40% CRSS. Slip bands, but no twins, were observed. Synchrotron x-ray topography was used to study in situ the effect of stress on crystal deformation. CdTe specimens were uniaxially stressed in tension along <112> at 293 to 673 K. When the load reached ~50% of the CRSS, the topograph began to distort, indicating the beginning of plastic deformation. No twins were observed on the stressed specimens.

Etch pits originating from precipitates in CdTe and Cd1−xZnxTe grown by the vertical Bridgman-Stockbarger method

Abstract Infrared microscopy and scanning electron microscopy with energy dispersive spectroscopy were employed to characterize etch pits formed on CdTe and (Cd,Zn)Te surfaces with E-solution, EAg, Bagai and Nakagawa etchants. Some of the larger flat-bottomed triangular etch pits on the (111)A or (111)B surfaces originated from precipitates. Thus, the etch pit density may differ from the true dislocation density by the presence of etch pits from precipitates.

Ground-based experiments and theory in preparation for floating zone melting and directional solidification of cadmium telluride in space

Abstract The objective of this program is to apply theoretical and experimental methods to optimize the equipment and procedures for floating zone melting and Bridgman-Stockbarger growth of cadmium telluride crystals in space. Computer codes were developed for the computation of heat transfer in the furnace and thermal stress in the resulting crystal. The predictions for the temperature field are being compared with experimental measurements. It was found that if the crystal sticks to the ampoule wall, differential thermal expansion between crystal and ampoule contributes much more to the stress than does the temperature field in the crystal. Thus, one goal of solidification of cadmium telluride in space is to reduce or eliminate contact of the crystal with the ampoule wall. Another goal is to find coatings and linings which reduce sticking of the grown crystal onto the ampoule. We developed techniques for measuring the surface tension and contact angle of molten cadmium telluride vs temperature and stoichiometry. The surface tension decreased with increasing temperature and with decreasing cadmium concentration. Wetting increased in the following order: pyrolytic boron nitride, carbon-coated quartz, sandblasted quartz, HF-etched quartz, and plain quartz. Additional coatings and potential ampoule lining materials are being developed and will be tested both for wetting by the melt and for sticking by the solid. Techniques are being developed for measuring sticking. We are also developing techniques for measuring the mechanical properties of cadmium telluride and for the direct observation of defect formation and evolution vs temperature. X-ray topography will be done in real time using the National Synchrotron Light Source at Brookhaven National Laboratory, in collaboration with the National Institute of Standards and Technology. Techniques are being developed for floating zone melting of cadmium telluride in space. We have successfully float zoned 5 mm rods on Earth. Evaporation of cadmium and tellurium from the molten zone is prevented by adding excess cadmium combined with heating of the entire ampoule. Still needed is a suitable automatic method to assure that the proper zone length is achieved in larger diameter rods in space.