The Effect of Concurrent Straining on Phase Transformations in NiAl Bronze During the Friction Stir Processing Thermomec
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TRODUCTION
FRICTION stir processing (FSP) is a solid-state technique involving the use of a nonconsumable rotating tool that consists of a cylindrical shoulder region and a projecting, concentric pin of smaller diameter. Initially, the pin is forced against the surface of a deformable work piece so that frictional heating results in softening of the work piece material. Subsequently, additional heating occurs due to adiabatic deformation induced in a column of material that forms around the tool pin. Softening due to the combined effect of these heat sources allows tool penetration until the shoulder comes into contact with the work piece surface. The shoulder acts to constrain upward flow of the deforming material, and additional work piece deformation is induced by the action of the tool shoulder on the work piece surface. When the resulting stir zone attains a sufficient temperature, the tool may be traversed in a predetermined pattern over the surface to process a volume of material defined by the pin tool profile and the processing pattern. FSP is an allied process of friction stir welding (FSW),[1] and both have been conducted on cast as well as wrought metals including alloys of Al and Mg as well JIANQING SU, Senior Postdoctoral Associate, is with the Missouri University of Science and Technology, Rolla, MO 65409. SRINIVASAN SWAMINATHAN, Research Scientist, is with General Electric–Global Research, Bangalore 560066, India. SARATH K. MENON, Research Professor, and TERRY R. McNELLEY, Distinguished Professor, are with the Department of Mechanical and Aerospace Engineering, Naval Postgraduate School, Monterey, CA 93943-5146. Contact e-mail: [email protected] Manuscript submitted July 23, 2010. Article published online February 24, 2011 2420—VOLUME 42A, AUGUST 2011
as higher melting alloys of Cu, Fe, and Ti. When applied to an as-cast metal such as NiAl bronze, FSP converts the as-cast microstructure to a wrought condition in the absence of macroscopic shape change.[2,3] The FSP thermomechanical cycle involves rapid transients and steep gradients in temperature, strain, and strain rate that, together, depend on tool geometry, work piece constitutive behavior, and processing parameters such as tool rotational speed (rpm) and traversing rate.[4] Direct measurement of the temperature cycle by thermocouples embedded along the tool path within the tool profile in thick plates of as-cast NiAl bronze showed that peak stir zone temperatures are 0.92 to 0.97 Tmelt during FSP of this material.[5] These direct measurements were conducted during single-pass FSP runs using a Densimet 176 tool with a shoulder diameter of 28.6 mm. The pin was in the shape of a truncated cone 12.7 mm in length, having a base diameter of 15 mm, a tip diameter of 6.3 mm, and a stepped-spiral feature on its surface. The FSP runs were typically conducted at 1000 to 1200 rpm and at a traversing rate of either 50.8 or 152.4 mm min–1 on plates initially at room temperature, although one run was conducted after the plate was preheated to a temperature T = 400
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