Microstructural Changes Due to Friction Stir Processing of Investment-Cast Ti-6Al-4V
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FRICTION stir welding (FSW) is a relatively new solid-state joining process developed and patented in 1991 by Thomas et al.[1] at The Welding Institute (Cambridge, United Kingdom). The process involves plunging a cylindrical rotating tool with an extended pin into the interface between two plates and traversing the tool along this seam. The friction between the tool and the work piece generates heat, which reduces the flow stress thereby enhancing plasticity. The tool rotation moves material from the leading edge of the tool to the trailing edge. The process has been applied extensively to aluminum alloys, but recently there has been interest in FSW of steel,[2] nickel-aluminum-bronze alloys,[3,4] and titanium alloys.[5–8] New tool materials were needed to accommodate the higher temperatures associated with joining these materials. Tool materials reported to be used so far include polycrystalline cubic boron nitride,[2] molybdenum-tungsten–based alloys,[9] and tungsten-based alloys.[8] A FSW tool[10] to locally modify the microstructure of a monolithic plate of 7075–T651 aluminum, a technique A.L. PILCHAK, Graduate Research Associate, and J.C. WILLIAMS, Professor and Honda Chair, are with the Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210, USA. Contact e-mail: pilchak.1@osu. edu M.C. JUHAS, Senior Assistant Dean for Diversity and Outreach, College of Engineering, and Research Scientist, Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210, USA, is currently on leave as Program Director for Diversity and Outreach, Directorate for Engineering, National Science Foundation, Arlington, VA 22230, USA. Manuscript submitted August 30, 2006. METALLURGICAL AND MATERIALS TRANSACTIONS A
referred to as friction stir processing (FSP). The prior rolled plate microstructure was converted to equiaxed grains with a mean diameter of 3.3 lm. Furthermore, they demonstrated that the structure obtained by FSP was able to be superplastically formed at high strain rates. As the interest in FSW or FSP of titanium alloys increases, we believe there is merit in attempting to introduce standard microstructural nomenclature that will, hopefully, reduce confusion in the literature. This is especially important for titanium alloys because the microstructures are complex, mainly due to the allotropic phase change that occurs during processing. In the present study, we define the microstructural features in FSW and FSP titanium and present data on FSP of investment-cast and hot isostatically pressed (HIP) Ti-6Al-4V plate. Titanium alloy castings have become increasingly popular because large, complex, one-piece shapes can be made to replace components that were previously assembled by mechanically fastening several pieces together. Eliminating the stress concentrations due to fasteners and fastener holes is desirable for improved fatigue life because they are prime sites for fatigue crack initiation. Further, fine equiaxed microstructures enhance fatigue
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