A thermal and microstructure evolution model of direct-drive friction welding of plain carbon steel
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I. INTRODUCTION
DIRECT-DRIVE friction welding is a well-established solid-state joining process, which can be used to join a wide range of conventional steel alloys, as well as more metallurgically challenging systems such as dissimilar metal combinations and superalloys. Figure 1 illustrates the four basic stages in direct-drive friction welding. In the start-up stage, one work piece is clamped in a spindle and a variable speed DC motor is used to rotate it at a predetermined speed relative to a stationary work piece. To begin the heat-up stage, the two parts are brought together and an axial compressive force, F1, is applied. Initially, heat is generated by friction at the faying surfaces. This raises the temperature of the metal at the weld interface and causes a decrease in the flow stress of the metal. When the flow stress of the heated metal at the weld interface becomes less than the applied axial compressive stress, the metal begins to plastically deform at a high strain rate. This plastic deformation now generates the heat at the interface. At the same time, the plastically deforming metal at the interface flows radially outward to create the flash, carrying with it any oxides and contaminants at the faying surfaces. This plastic flow of metal and formation of the flash occurs during the burn-off stage and results in axial displacement of both work pieces toward each other and shortening of the overall weldment. Finally, the welding process is completed during the forging stage by stopping all rotation and applying a high compressive force, F3. Friction welding has several advantages over conventional fusion welding processes. Since friction welding is a solidstate joining process, all defects associated with melting and T.C. NGUYEN, formerly Postgraduate Student, with the Department of Mechanical Engineering, University of Waterloo, Waterloo, ON N2L 3G1 Canada, is now Professor with the School of Engineering and Information Technology, Conestoga College, Kitchener, ON N2G 4M4 Canada. Contact e-mail: [email protected] D.C. WECKMAN, Professor, is with the Department of Mechanical Engineering, University of Waterloo, Waterloo, ON N2L 3G1 Canada. Contact e-mail: [email protected] Manuscript submitted August 10, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS B
solidification in a typical fusion weld are absent in a friction weld. During friction welding, the heat is highly concentrated at the weld interface. As a result, a friction weld has a very narrow heat-affected zone (HAZ), which limits the variations in mechanical properties of the base metal to a small region. Friction welding has the additional advantages that filler metal, flux, and shielding gas are not required and the cycle time of the process is very short; small parts take only a few seconds to weld. Finally, once a suitable welding procedure has been established, the resultant weld dimensions and high weld quality are very consistent and reproducible. During friction welding of plain carbon and low-alloy steels, the metal within th
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