A Comparison of the Inertia Friction Welding Behavior of Similar and Dissimilar Ni-Based Superalloys
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INERTIA friction welding (IFW) is a widely-used method for solid-state joining of similar and dissimilar materials.[1–3] Three independent parameters, the flywheel moment of inertia I, the initial flywheel kinetic energy Eo, and the axial compression force P, control the welding process. Proper selection of these parameters is important in achieving good weld quality. Each of the parameters contributes to different physical processes controlling IFW. During IFW, the joining surfaces rotating relative to each other (with an angular velocity controlled by I and Eo) are brought into contact by the axial compression force P. Interface friction under the force P transforms the kinetic energy of the flywheel into heat, and the temperature at the joining surfaces increases rapidly resulting in a decrease in the yield stress ry of one or both of the joining materials below the applied flow stress r = P/A, where A is the contact area of the joining surfaces. When this condition (ry < P/A) is achieved, the surface layer begins to flow and is extruded laterally outside the contact area, thus forming a characteristic flash. Thereby, colder material is then brought to the interface under the action of the axial force P. This self-limiting process results in a near-constant temperature at the interface during the
O.N. SENKOV, D.W. MAHAFFEY, and S.L. SEMIATIN are with the Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, OH 45433. Contact e-mail: [email protected] Manuscript submitted May 3, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
upset portion of the welding process and continuous flow of plasticized material into flash. Friction between the welding surfaces also produces a friction torque and, respectively, shear stresses which may cause plastic shear in the azimuthal direction, if the friction-induced shear stress s exceeds the yield shear stress sy of the welded material. For a solid cylinder of radius r experiencing friction torque M, the shear stress is calculated as s = 1.5M/(Ar). Achieving a defect-free weld interface between similar materials requires gross plastic flow at the temperatures produced during IFW. Thus, the conditions ry < P/A and sy < 1.5M/(Ar) should be met at temperatures at which the ductility of the workpiece materials is sufficient to sustain severe plastic deformation without forming cracks or porosity. When identical materials are welded together, both parts experience similar levels of softening and plastic flow. Therefore, optimal welding conditions are achieved when the oxidized and/or contaminated surface layers are ejected into the flash, and fresh, nascent surfaces are brought into contact. However, the situation is more complex when materials with dissimilar mechanical and/or thermal properties are joined. In this case, one of the materials may start to deform before the mating workpiece is softened. The continuous movement of the deforming material into the flash may not allow the interface temperature to increase further and thus facil
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