Texture Development and Material Flow Behavior During Refill Friction Stir Spot Welding of AlMgSc
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ALUMINUM alloys have been increasingly used in the automotive industry in an attempt to reduce vehicle weight and improve fuel economy.[1,2] The welding technique for assembling aluminum sheets is one of the key technology drivers for the realization of such objectives.[3] Thus, friction stir spot welding (FSSW), a variant of friction stir welding (FSW), is considered an alternative welding technique to replace the widely used resistance spot welding and other similar processes due to its advantages such as less energy consumption, longer tool life, and no use of consumables.[1,2,4] Conventional FSSW leaves a keyhole behind, which may compromise the weld integrity. To avoid this, a refill process was integrated into FSSW by Helmholtz-Zentrum Geesthacht[5] (HZG, former GKSS), which was named refill friction stir spot welding (refill FSSW) or friction spot welding (FSpW) internally in HZG and Germany.
JUNJUN SHEN, UCEU F.H. SUHUDDIN, and JORGE F. DOS SANTOS are with the Department of Solid-State Joining Processes, Materials Mechanics, Institute of Materials Research, HelmholtzZentrum Geesthacht, Max-Planck-Str. 1, 21502, Geesthacht, Germany. Contact e-mail: [email protected] SARA B.M. LAGE is with the Department of Solid-State Joining Processes, Materials Mechanics, Institute of Materials Research and also with the Department of Materials Engineering, Federal University of Sa˜o Carlos, Rod. Washington Luiz km. 235, 13565-905, Sa˜o Carlos, Brazil. CLAUDEMIRO BOLFARINI is with the Department of Materials Engineering, Federal University of Sa˜o Carlos. Manuscript submitted November 30, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS A
The basic concept of refill FSSW via sleeve plunging is intuitive, as shown in Figure 1. In the beginning, the clamping ring holds the materials against the backing anvil. Then, the sleeve rotates and plunges downward into the material to the desired depth, while the rotating pin simultaneously moves upward. In the meantime, a softened plasticized material rod and a local softened keyhole are formed by frictional heating and plastic deformation. After a certain time (not mandatory), the rotating sleeve and pin return to their initial positions, refilling the keyhole with the softened rod, resulting in a keyhole-free joint. Unlike FSW, where a traversing motion is involved in addition to the rotating motion, refill FSSW is similar to a special torsion process where the length of the rod (or shaft) varies all the time. At any moment, the central axis of the rod is subjected to minimal shear strain and the edge to the maximal. In general, the material undergoes intense plastic deformation at elevated temperatures, similar to FSW, resulting in the generation of fine and equiaxed recrystallized grains. Such a fine microstructure is usually favorable for joint mechanical properties. Since its invention, refill FSSW has been used for joining similar metals or materials[6–10] as well as joining dissimilar materials.[11–14] HZG proved that refill FSSW creates joints with superior mechanical properties t
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