Friction — Dissimilar Metals

As indicated above, dissimilar metals with low frictional characteristics are most suitable for plain bearing applications. With steel shafts, plain bearings made of bronze or Babbitt are commonly used. Bronze is one of the harder bearing materials and is generally used for low speeds and heavy loads. 

Transition joints are used to join dissimilar metals where flanged, screwed, or threaded connections are not practical. They are used when fusion welding of two dissimilar metals forms interfaces that are deficient in mechanical strength and the ability to keep the system leak-tight. Transition joints consist of a bimetallic composite, a stainless steel, and a particular kind of aluminum bonded together by some proprietary process. Some of the types in use throughout the cryogenic industry are friction- or inertia-welded bond, roll-bonded joint, explosion-bonded joint, and braze-bonded joint. 

Inertia (friction) welding in the solid state, which is useful for joining dissimilar metals, i.e., aluminum to steel and stainless steel. At least one of the pieces to be welded must be circular and capable of being rotated at high velocities before being brought into contact with pressure to the second member. 

The energy required to shear these asperity welds accounts, in part, for metal to metal friction there is a further component contributing to the overall friction, ploughing . This is the force Fp required to displace the softer metal of the two dissimilar metals in sliding contact. The total frictional force F is given by Equation (3.1) as ... 

M. Fukumoto, T. Yasui, Y. Shimoda, M. Tsubaki, and T. Shinoda, Butt Welding Between Dissimilar Metals by Friction Stirring, Proceedings of the Fifth International Conference on Friction Stir Welding, Sept 14-16, 2004 (Metz, France), TWl, paper on CD... 

E.E. Murr, Y. Li, R.D. Rores, E.A. Trillo, and J.C. McClure, Intercalation Vortices and Related Microstructural Features in the Friction-Stir Welding of Dissimilar Metals, Mater. Res. Innovat., Vol 2,1998, p150-163... 

For truly clean metal surfaces that have, for example, been treated by electron bombardment at elevated temperatures and ultrahigh vacuum, coefficients of friction in the range of 3-6 have been recorded. Sometimes, when good contact is achieved between clean surfaces of the same metal, the two surfaces will in fact weld or seize, to the point that the union is as strong as the bulk metal. For dissimilar metals, a similar result may be encountered if there exists a degree of mutual miscibility. If the two metals are mutually immiscible, there may or may not be such seizure. 

Cold Welding (CW) Ductile metals such as carbon steels, aluminum, copper and precious metals. Friction Welding (FRW) can weld many material types and dissimilar metals effectively, including aluminum to steel. Also thermoplastics and refractory metals. 

M.J. Peel, A. Steuwer, and P.J. Withers, Dissimilar Friction Stir Welds in AA5083-AA6082, Part II Process Parameter— Effects on Microstructure, Metall. Mater. Trans. B, Vol. 37, July 2006, p 2195-2206... 

Following are coefficients of kinetic sliding friction for dissimilar pure metals in contact with each other. 

Press-fit, also known as interference-fit, force-fit, shrink-fit, or friction-fit, relies on friction to hold the parts together [6,8]. As shown in Fig. 26.11, for a press-fit the outer diameter of the shaft is slightly larger than the inner diameter of the hub resulting in interference between the two. It is simple, inexpensive, and it can be used to join similar and dissimilar plastics as well as plastic to metal. To reduce the insertion force, it is possible to cool the shaft and/or heat the hub, thereby reducing the interference. 

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