WELDING WORKBOOK Advantages and Limitations of Friction Welding Friction welding (FRW) is a solid-state process that produces a weld when two or more workpieces, rotating or moving relative to one another, are brought into contact under pressure to produce heat and plastically displace material from the faying surface (weld interface). The main variations of friction welding are direct drive friction welding (FRW-DD), inertia friction welding (FRW-I), and friction stir welding (FSW). However, FSW features substantial differences in mechanics from the other two processes and is not covered here. In direct drive friction welding, the welding machine supplies the energy required to make the weld through a direct motor connection for a preset period of the welding cycle. The stored rotational kinetic energy of the welding machine supplies the energy required to make an inertia friction weld. Following are some of the terms and definitions related to friction welding: Friction speed. The relative velocity of the workpieces at the time of initial contact. Friction force. The compressive force applied to faying surfaces during the time there is relative movement between the workpieces from the start of welding until the application of the forge force. Friction time. The duration of time from the application of friction force until the application of forge force. Friction upset distance. The decrease in length of workpieces during the time of friction welding force application. Forge (upset) force. The compressive force applied to the weld after the heating portion (friction stage) if the welding cycle is essentially complete. Forge (upset) distance. The total reduction in the axial length of the workpieces from the initial contact to the completion of the weld. Figure 1 shows the basic steps in the friction welding process. As shown in Fig. 1A, one workpiece is rotated and the other held stationary. When the appropriate rotational speed is reached, the two workpieces are brought together (B) under axial force. Abrasion at the weld interface heats the workpiece locally and upsetting (axial shortening) starts, as shown in (C). These two steps occur during the friction stage. Finally, rotation of the workpiece ceases and upset force (D) is applied to consolidate the joint. This occurs during the forging stage. Advantages Following are some operational and economic advantages of friction welding: • No filler metal is required for all similar and most dissimilar material joints • Flux and shielding gas are not normally required • Solidification defects and porosity are normally not a concern • The process is environmentally clean due to the minimization of sparks, smoke, or fumes • Surface cleanliness is not as critical compared to other welding processes • Offers narrow heat-affected zones • Well suited for joining most engineering materials and dissimilar metal combinations • In most cases, the weld is at least as strong as the weaker of the two materials being joined (high joint efficiency) • Operators are not required to have manual welding skills • Easily automated for mass production • Short cycle times • Requires minimal plant requirements such as space, electric power, and special foundations. Limitations Following are some limitations of friction welding: • In general, one workpiece must have an axis of symmetry and be capable of rotation about that axis • Alignment of the workpieces may be critical to developing uniform frictional heat • Preparation of the interface geometry may be critical to achieving proper heat balance • Capital equipment and tooling costs are high, but payback periods typically are short for high-volume production.♦ 70 JANUARY 2013 Datasheet 337 Excerpted from the Welding Handbook, Vol. 3, ninth edition. A B C D Fig. 1 — Basic sequence of friction welding.
Welding Journal | January 2013
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