Microstructural Evolution in Ti-5111 Friction Stir Welds
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ICTION stir welding (FSW) is a solid-state welding technique that uses a rotating tool to ‘‘stir’’ together the two sides of a joint. FSW was developed by The Welding Institute (TWI, Cambridge, United Kingdom) in 1991[1] and has rapidly developed into a commercially important process for the joining of aluminum alloys. In FSW, the rotating tool heats the surrounding material through both frictional and adiabatic heating, softening that material and facilitating its flow around the rotating tool. As the tool is traversed along the joint line, material is continually transferred around the tool and deposited on the trailing side, forming a solid-state bond between the two pieces. The FSW process is depicted schematically in Figure 1. Several studies investigated the evolution of grain structure and crystallographic texture after FSW, focusing primarily on aluminum alloys.[2–10] There has been comparatively little work, however, on the microstructural evolution that occurs during the FSW of steels[11– 13] and titanium alloys.[14–18] FSW of hexagonal- closepacked (hcp) a titanium is particularly interesting because slip is primarily restricted to close-packed h11 20i directions.[19–22] Because of this limited number of slip systems, plastic deformation of hcp a titanium K.E. KNIPLING, Metallurgist, is with the Naval Research Laboratory, Code 6355, 4555 Overlook Avenue SW, Washington, DC 20375. Contact e-mail: [email protected] R.W. FONDA, Metallurgist, is with the Naval Research Laboratory, Code 6356. Manuscript submitted January 29, 2011. Article published online April 2, 2011 2312—VOLUME 42A, AUGUST 2011
usually requires an additional combination of hc þ ai slip or deformation twinning.[19,20,23–26] The present study investigates the deformation mechanisms that occur during FSW of Ti-5111 (Ti-5Al-1Sn-1Zr-1V0.8Mo, wt pct), a near-alpha titanium alloy developed for marine applications requiring excellent toughness and corrosion resistance.[27]
II.
EXPERIMENTAL PROCEDURES
The weld examined in this study is a bead-on-plate (no seam) weld that was prepared at the Edison Welding Institute with a tungsten-based alloy tool. The 12.7 mm (0.5-in) thick Ti-5111 plate was prepared by Timet (Henderson, NV). The FSW tool had a simple geometry with a narrow shoulder and a truncated conical shape without threads, flats, or other features (further details on the tool geometry are provided in Reference[18]). Welding was performed at 140 rpm and 51 mm min1 (2 in. min1), corresponding to a 360-lm tool advance per revolution. The tool was extracted from the plate immediately upon completion of the weld, and the weld terminus was quenched with cold water to preserve the microstructure surrounding the welding tool. The weld end was sectioned in plan view through the plate mid thickness, as shown in Figure 1, and prepared for detailed examination by optical microscopy, scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). Metallographic preparation of the specimens was carried out using conventional te
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