Seminar Report On Diffusion Welding

Diffusion bonding or diffusion welding is a solid-state welding technique used in metalworking, capable of joining similar and dissimilar metals. It operates on the principle of solid-state diffusion, wherein the atoms of two solid, metallic surfaces intersperse themselves over time. This is typically accomplished at an elevated temperature, approximately 50-75% of the absolute melting temperature of the materials.[1][2] Diffusion bonding is usually implemented by applying high pressure, in conjunction with necessarily high temperature, to the materials to be welded; the technique is most commonly used to weld "sandwiches" of alternating layers of thin metal foil, and metal wires or filaments.[3] Currently, the diffusion bonding method is widely used in the joining of high-strength and refractory metals within the aerospace[1] and nuclear industries.[citation needed]

The act of diffusion welding is centuries old. This can be found in the form of "gold-filled," a technique used to bond gold and copper for use in jewelry and other applications. In order to create filled gold, smiths would begin by hammering out an amount of solid gold into a thin sheet of gold foil. This film was then placed on top of a copper substrate and weighted down. Finally, using a process known as "hot-pressure welding" or HPW, the weight/copper/gold-film assembly was placed inside an oven and heated until the gold film was sufficiently bonded to the copper substrate.[4]

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Due to its relatively high cost, diffusion bonding is most often used for jobs either difficult or impossible to weld by other means. Examples include welding materials normally impossible to join via liquid fusion, such as zirconium and beryllium; materials with very high melting points such as tungsten; alternating layers of different metals which must retain strength at high temperatures; and very thin, honeycombed metal foil structures.[6][7][8] Titanium alloys will often be diffusion bonded despite as the thin oxide layer can be dissolved and diffused away from the bonding surfaces at temperatures over 850 ÂC.

When joining two materials of similar crystalline structure, diffusion bonding is performed by clamping the two pieces to be welded with their surfaces abutting each other. Prior to welding, these surfaces must be machined to as smooth a finish as economically viable, and kept as free from chemical contaminants or other detritus as possible. Any intervening material between the two metallic surfaces may prevent adequate diffusion of material. Specific tooling is made for each welding application to mate the welder to the workpieces.[10] Once clamped, pressure and heat are applied to the components, usually for many hours. The surfaces are heated either in a furnace, or via electrical resistance. Pressure can be applied using a hydraulic press at temperature; this method allows for exact measurements of load on the parts. In cases where the parts must have no temperature gradient, differential thermal expansion can be used to apply load. By fixturing parts using a low-expansion metal (i.e. molybdenum) the parts will supply their own load by expanding more than the fixture metal at temperature. Alternative methods for applying pressure include the use of dead weights, differential gas pressure between the two surfaces, and high-pressure autoclaves. Diffusion bonding must be done in a vacuum or inert gas environment when using metals that have strong oxide layers (i.e. copper). Surface treatment including polishing, etching, and cleaning as well as diffusion pressure and temperature are important factors regarding the process of diffusion bounding.[6][7][8]

Diffusion bonding is primarily used to create intricate forms for the electronics, aerospace, and nuclear industries. Since this form of bonding takes a considerable amount of time compared to other joining techniques such as explosion welding, parts are made in small quantities, and often fabrication is mostly automated. However, due to different requirements, the required time could be reduced. In an attempt to reduce fastener count, labor costs, and part count, diffusion bonding, in conjunction with superplastic forming, is also used when creating complex sheet metal forms. Multiple sheets are stacked atop one another and bonded in specific sections. The stack is then placed into a mold and gas pressure expands the sheets to fill the mold. This is often done using titanium or aluminum alloys for parts needed in the aerospace industry.[14]

In this study, hydrogen absorption and diffusion were investigated for various high-alloyed ferritic-austenitic duplex steels. On account of the specific transformation and solidification behaviour, respectively, of duplex steels as compared to single-phase ferritic and austenitic steels, special conditions have to be considered concerning hydrogen absorption which may ultimately lead to microstructure-dependent hydrogen-assisted weld metal cracking. Hydrogen absorption during welding may occur via the shielding gas, moisture from Qthe surroundings or via the welding filler material. As a contribution to the interpretation and prediction of hydrogen-induced cracking in welded duplex steels, the actual hydrogen absorption via the arc as well as the weld metal hydrogen diffusion was investigated for the first time in a duplex steel DS (1.4462), a super duplex steel SDS (1.4501) and in a lean duplex steel LDS (1.4162). Isothermal heat treatment using carrier gas hot extraction enabled quantification of the amounts of hydrogen trapped in the respective microstructure areas. The hydrogen diffusion coefficients were determined by analytical and numerical calculation. The total hydrogen concentrations and the diffusion coefficients were found to be nearly identical. Trapped hydrogen was however observed to be dependent on the material and on the microstructure condition. The influence of hydrogen on the mechano-technological properties of the weld metal was characterized with the help of tensile tests. In addition, the hydrogen embrittlement effect was detected in scanning electron microscopic analyses

Dr. Jill J. Bauman earned a B.S. in Physics at the University of Florida while working in the Micro-Kelvin Laboratory, one of the premier low-temperature centers in the world. There she developed and tested radiation shielding materials and diffusion welding techniques for nuclear demagnetization refrigerators. She earned an M.S. degree in Physical Oceanography from the State University of New York (SUNY), Stony Brook, in the Marine Sciences Research Center. During this time, she held a Research Associate position at Brookhaven National Laboratory where she conducted her thesis to understand the correlation between oceanic phytoplankton biomass and global cloud albedo. Dr. Bauman earned a Ph.D. in Atmospheric Physics from the Institute for Terrestrial and Planetary Atmospheres, SUNY. During this time she conducted her dissertation research in the Atmospheric Physics Branch at NASA Ames Research Center. During her residency at Ames she became interested in space flight mission implementation and attended the International Space University in Barcelona.

Dr. Yiyu Wang joined ORNL since 2018. He earned his Ph.D. degree from the University of Alberta in 2017. His research focuses on physical metallurgy and welding metallurgy of structural materials, involving study of non-equilibrium phase transformations, materials characterization, testing and modeling of mechanical performance of welds. Dr. Wang is an active member of many professional associations, including the CWA, CIM, and AWS, by presenting technical papers in conferences and seminars. Dr. Wang has authored more than 30 peer-reviewed journal papers and 20 conference papers in topics of creep-resistant steel welding, pipeline integrity, failure analysis of welded pressure vessels, and diffusion bonding of dissimilar metals. He is a co-recipient of the 2017 W. H. Hobart Memorial Award and the 2018 Warren F. Savage Memorial Award from the AWS, for the best Welding Journal papers in pipe welding and welding metallurgy.

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