A microtexture investigation of recrystallization during friction stir processing of as-cast NiAl bronze
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tir processing (FSP) is an adaptation of friction stir welding (FSW), a solid-state joining process originally developed at The Welding Institute.[1] Friction stir processing enables localized modification and control of microstructures in near-surface layers of metallic components.[2,3,4] In the process, a cylindrical, wear-resistant tool consisting of a smaller diameter pin with a concentric, larger-diameter shoulder is rotated and forced into the surface of the work piece. As the tool penetrates, a combination of frictional and adiabatic heating softens the material so that tool rotation induces a stirring action and flow of material about the pin. The severe, but localized, plastic deformation results in formation of a stir zone (SZ), while adjacent regions that experience only moderate straining comprise the thermomechanically affected zone (TMAZ). Large areas may be processed by traversing the tool in a pattern on the work piece surface. Friction stir processing has been employed to homogenize and refine microstructures in both cast and wrought metals, including alloys of Al[5–10] and Mg[11,12] and higher melting alloys of Cu,[13] Fe,[14] and Ti.[15] Benefits of FSP in cast metals include elimination of porosity and local conversion of cast microstructures to a wrought condition, with enhanced near-surface properties. Significantly improved strength/ductility combinations[16,17] and high-strain-rate superplasticity[3–7,18] have been achieved by FSP of wrought materials. Depending on alloy constitution and prior history, the FSP thermomechanical cycle will induce various KEIICHIRO OH-ISHI, Postdoctoral Associate, is with the National Institute for Materials Science, Ibaraki 305-0047, Japan. ALEXANDER P. ZHILYAEV, NRC Senior Research Associate, and TERRY R. McNELLEY, Distinguished Professor, are with the Department of Mechanical and Astronautical Engineering, Naval Postgraduate School, Monterey, CA 939435146. Contact e-mail: [email protected] Manuscript submitted October 14, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
restoration and transformation processes that will be reflected in SZ and TMAZ microstructures. However, the direct measurement of temperatures, strains, and strain rates in these regions is difficult due to steep gradients and transients in these quantities, and this is an impediment in the assessment of processing-induced microstructures. Quantitative microstructure analysis following FSP of the cast NAB materials of interest here has been used to develop estimates of local peak SZ temperatures. The estimates have relied mainly on assessment of the transformation products of the b phase observed in the microstructure of processed material. Details of the estimation procedure have been given in previous reports.[19,20] Briefly, during cooling at rates ;10#3 °C s#1 after casting, the NAB solidifies as a single-phase b bcc solid solution. At about 1030 °C, the primary a begins to form with Widmansta¨tten morphology in the b. At 930 °C, the globular precipitate, kii, which is nominally Fe3Al having a DO3
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