Showing papers by "Tarasankar Debroy published in 1994"

Phenomenological Modeling of Fusion Welding Processes

Abstract: Welding is utilized in 50% of the industrial, commercial, and consumer products that make up the U.S. gross national product. In the construction of buildings, bridges, ships, and submarines, and in the aerospace, automotive, and electronic industries, welding is an essential activity. In the last few decades, welding has evolved from an empirical art to a more scientifically based activity requiring synthesis of knowledge from various disciplines. Defects in welds, or poor performance of welds, can lead to catastrophic failures with costly consequences, including loss of property and life.Figure 1 is a schematic diagram of the welding process showing the interaction between the heat source and the base metal. During the interaction of the heat source with the material, several critical events occur: melting, vaporization, solidification, and solid-state transformations. The weldment is divided into three distinct regions: the fusion zone (FZ), which undergoes melting and solidification; the heat-affected zone (HAZ) adjacent to the FZ, that may experience solid-state phase changes but no melting; and the unaffected base metal (BM).Creating the extensive experimental data base required to adequately characterize the highly complex fusion welding process is expensive and time consuming, if not impractical. One recourse is to simulate welding processes either mathematically or physically in order to develop a phenomenological understanding of the process. In mathematical modeling, a set of algebraic or differential equations are solved to obtain detailed insight of the process. In physical modeling, understanding of a component of the welding process is achieved through experiments designed to avoid complexities that are unrelated to the component investigated.In recent years, process modeling has grown to be a powerful tool for understanding the welding process. Using computational modeling, significant progress has been made in evaluating how the physical processes in the weld pool influence the development of the weld pool and the macrostructures and microstructures of the weld.

Oxide Matrix Composite by Directional Oxidation of a Commercial Aluminum‐Magnesium Alloy

Abstract: Oxidation rates of a commercial Al-Mg alloy, Al-5083, were investigated to understand the synthesis of an oxide-based composite. In some experiments, the oxide layer was grown with several platinum wires embedded in the matrix to facilitate transport of electrons. The oxidation rate was not controlled by the rate of transport of the electrons and positive holes through the ceramic oxide layer. In the initial phase of the composite growth, an incubation period was observed due to formation of MgO on the alloy surface. At 1273 K or higher, the incubation period was followed by a period of sustained oxidation after MgO was converted to a spinel layer. It is demonstrated that, after the initial incubation period, the rate of oxidation is influenced by the rate of transport of ions through the oxidematrix composite.

Electrical conductivity of alumina/aluminum composites synthesized by directed metal oxidation

Abstract: Electrical conductivities of Al[sub 2]O[sub 3]/Al composite synthesized by directed oxidation of an aluminum alloy, and sintered Al[sub 2]O[sub 3]-4% MgO are measured. The high conductivity of the Al[sub 2]O[sub 3]/Al composite compared to sintered Al[sub 2]O[sub 3]-4% MgO is shown as proof of the presence of continuous metal channels in the composite. Furthermore, the conductivity data are used to determine the activation energy for the diffusion of the dominant charge carrier in the oxide matrix.

Weld pool phenomena

Abstract: During welding, the composition, structure and properties of the welded structure are affected by the interaction of the heat source with the metal. The interaction affects the fluid flow, heat transfer and mass transfer in the weld pool, and the solidification behavior of the weld metal. In recent years, there has been a growing recognition of the importance of the weld pool transport processes and the solid state transformation reactions in determining the composition, structure and properties of the welded structure. The relation between the weld pool transport processes and the composition and structure is reviewed. Recent applications of various solidification theories to welding are examined to understand the special problems of weld metal solidification. The discussion is focussed on the important problems and issues related to weld pool transport phenomena and solidification. Resolution of these problems would be an important step towards a science based control of composition, structure and properties of the weld metal.

Effect of Pressure on Plasma‐Assisted Chemical Vapor. Deposition of Silicon Oxide(s)

Abstract: In most chemical vapor deposition processes, the film growth rate increases with the concentrations of the reactant gases. However, in the plasma-assisted chemical vapor deposition of silicon oxide films, the deposition rate decreases when the concentrations of silane and nitrous oxide are increased by enhancing the reactor pressure from 0.5 to 2 torr (66 to 270 Pa). The deposition rate and the plasma properties have been examined for various reactor pressures to seek an improved understanding of the deposition process. Photo emissions from the plasma were monitored to determine the species present in the plasma and to calculate electron energy and density. With the increase in pressure, both the electron temperature and density decreased, and, consequently, the concentration of active species decreased. Although the concentrations of both silane and nitrous oxide increased with total pressure, the deposition rate decreased. The results emphasize the crucial importance of electron energy and density to generate sufficient concentration of active species responsible for film growth.