Recrystallization Phenomena During Friction Stir Processing of Hypereutectic Aluminum-Silicon Alloy
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INTRODUCTION
HYPEREUTECTIC aluminum-silicon alloy is known for its light weight, high thermal conductivity, low coefficient of thermal expansion, superior wear resistance, high elastic modulus, and strength, which makes it a promising candidate for the defense, aerospace, and automobile industries.[1–6] However, with the increase in the silicon content, the mechanical properties, particularly ductility and toughness, are known to deteriorate. Further, due to coarse primary silicon particles, the poor machinability of the alloy further limits its applicability.[7] These shortcomings can be overcome by various microstructural refinement techniques, for example, melt modifiers,[8,9] severe plastic deformation,[10–12] and rapid solidification.[13–17] Based on the principle of friction stir welding process, friction stir processing (FSP) is a relatively new processing technique developed by Mishra and co-workers.[18–20] During FSP, a cylindrical rotating tool with a shoulder A.G. RAO, Research Scholar, is with the Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology, Bombay, Powai, Mumbai 400 076, India and also working as Scientist in Marine Materials Division, Naval Materials Research Laboratory, Shil-Badlapur, Ambernath (East) 421 506, India. Contact e-mail: [email protected] K.R. RAVI, Assistant Professor, is with the Nanotech Research Facility, PSG Institute of Advanced Studies, Coimbatore 641 004, India. B. RAMAKRISHNARAO, Scientist, and V.P. DESHMUKH, Scientist and Head, are with the Marine Materials Division, Naval Materials Research Laboratory. A. SHARMA, Associate Professor, is with the Department of Mechanical Engineering, Indian Institute of Technology, Bombay. N. PRABHU and B.P. KASHYAP, Professors, are with the Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology, Bombay. B. Ramakrishnarao—Deceased Manuscript submitted January 30, 2012. Article published online November 13, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
and pin is plunged and traversed along the length of the material. The tool serves the two purposes of heating the work piece and the movement of the material, thus causing intense plastic deformation, material mixing, elimination of porosities, and homogenization of the stirred zone.[21] Owing to its simple concept, FSP is now used for producing fine-grained materials,[22–25] synthesis of functionally gradient material,[26] creation of surface and hybrid composite,[27–31] and refinement of cast microstructure.[32–35] FSP of hypoeutectic Al-Si alloy was reported[36–40] for refinement of silicon particles and homogenization of microstructure. The refinement of the microstructure by FSP resulted in increase of mechanical properties, particularly the ductility and fatigue life in hypoeutectic Al-Si alloy. However, only few investigations were reported on the effect of FSP on hypereutectic Al-Si alloy, in particular by Rao et al.[41] and Jiang et al.[42] Although many researchers have investigated the mechanisms res
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