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INTRODUCTION

FRICTION-STIR processing (FSP) is a new, solidstate processing technique for microstructural modification,[1,2] which was developed based on the principle of friction-stir welding (FSW).[3] A rotating, nonconsuming tool with a pin and shoulder is inserted into the material and traversed along the desired path. Because of the frictional heat generated, the material undergoes severe plastic deformation, resulting in significant microstructural changes in the processed zone. FSP creates a region called the ‘‘stir zone’’ or ‘‘nugget zone,’’ where the microstructural refinement takes place producing equiaxed ultrafine grains with high-angle grain boundaries.[4,5] A single-pass FSP with a pin diameter of 6 mm may produce a processed zone 6 to 8 mm wide and a depth depending on the length of the tool pin. Such a narrow processed zone is not suitable for practical engineering applications. The extension of FSP to multipass FSP addresses this issue, whereby the overlapping of passes can be used to produce bulkprocessed material.[6] The study on the possibility for fabricating large bulk ultrafine-grained materials through multipass FSP in commercial 7075 aluminum reported that the grains of the multipass FSP regions exhibited the same characteristics as the grains in the center region of single-pass FSP sample.[6] Johannes and Mishra[7] investigated the effect of multipass FSP on the superplasticity of 7075 Al rolled plate by using mini tensile specimens cut from the center of each stir zone on the staggered pass samples. The result show good superplastic ductility in each stir zone of the multipass FSP 7075 Al samples. The study also indicated that overlapping FSP passes is a feasible K.N. RAMESH, M. Tec Scholar, S. PRADEEP, Research Scholar, and VIVEK PANCHOLI, Assistant Professor, are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Roorkee (IITR), Roorkee 247 667, Uttarakhand, India. Contact e-mail: [email protected] Manuscript submitted September 28, 2011. Article published online May 26, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

method to create a large, fine-grained aluminum alloy plate. Ma et al.[8] studied the effect of multipass FSP on the microstructure and tensile properties of a cast aluminum-silicon alloy A356. The work reported that in the multipass processed material, the strength of the previously processed zones was lower than that of the subsequent processed zones because of an overaging from the thermal cycles. Nakata et al.[9] reported an improvement in mechanical properties of an aluminum die casting alloy subjected to 14 pass FSP. The average tensile strength in the parallel direction was higher than in the perpendicular direction. However, ductility was almost the same in both directions. It is clear from the available literature that the multipass FSP heating cycle of one pass affected microstructure and mechanical properties of other passes. In this work, multipass FSP of Al 5086-O alloy was done using two different variants of the proces