Observations of Tool-Workpiece Interactions during Friction Stir Processing of Ti-6Al-4V
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Significant tool wear manifested as submicron tungstenrich particles was observed in the workpiece after friction stir processing (FSP) of investment cast Ti-6Al-4V on both the surface and in the bulk of the stir zone. The tool was manufactured from a tungsten alloy, which stabilizes the b phase and can locally suppress the b transus. A postprocessing a/b heat treatment was performed to demonstrate the microstructure effects of tungsten dissolution. DOI: 10.1007/s11661-006-9059-4 The Minerals, Metals & Materials Society and ASM International 2007
FRICTION stir welding (FSW) was developed at The Welding Institute by Thomas et al.[1] In this solidstate process, a rotating cylindrical and nonconsumable tool is plunged into the seam between two plates and traversed along the seam. The tool rotation and features on the tool, such as flutes or threads, provide sufficient frictional heat and plastic deformation to stir the material together. The microstructure in the stir zone is refined compared to that of the base material owing to a recrystallization process.[2] Mishra et al.[3] applied the concepts of FSW to a monolithic plate of 7075-T651 aluminum. The process was used with the intention of microstructural modification to produce fine grains for superplastic forming rather than creating a joint. Various tool materials are currently used for FSW and friction stir processing (FSP) and can be broadly divided into those used for joining or processing lowtemperature and high-temperature materials. The lowtemperature tools are those used for aluminum alloys including tool steels[4,5] and carbon steels.[6] Joining steel 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: [email protected] 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, USA, is currently on leave at the National Science Foundation. Manuscript submitted August 10, 2006. METALLURGICAL AND MATERIALS TRANSACTIONS A
and titanium alloys, which have higher flow stress at elevated temperatures, required new tool materials to be investigated. Tools manufactured from tungsten,[7] tungsten-based alloys,[8] polycrystalline cubic boron nitride,[9] and molybdenum-tungsten based alloys[10] have been used to join steel and titanium. This article describes the interactions between a tungsten-25 pct rhenium tool and an investment cast and hot isostatically pressed Ti-6Al-4V workpiece. The base material consisted of 1- to 1.5-mm-diameter prior b grains with a coarse lamellar colony structure created by slow cooling from above the b transus. The tool travel speed was 1.7 cm s-1 (4 in. min)1), the tool rotation was 100 rev min)1, and the down force was 38.70 kN (8700 lbf). Processing was done under argon atmosphere to minimize oxidation of both the workpiece and the tool
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