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INTRODUCTION

FRICTION stir welding (FSW) is a relatively new welding process developed in the early 1990s and has drawn much attention as an innovative industrial joining process.[1–5] FSW is a low-temperature joining process that uses the friction of a rotating tool to heat and join work pieces through plastic deformation. It has a multitude of advantages over typical fusion joining techniques including improved mechanical and metallurgical weld properties, rapid joining speeds, elimination of weld cracking and porosity, and no need for shielding gas or filler metal.[1–3] In addition, as the work piece is never heated above melting point, FSW eliminates the dendrite formation associated with other hightemperature fusion joining processes.[3,5] FSW also has the ability to join materials including 7xxx series aluminum alloys, metal matrix composites, and magnesium alloys that are otherwise considered to be ‘‘hardto-weld’’ alloys.[2–7] The ability of FSW to join these otherwise ‘‘hard-toweld’’ materials and the rapid growth of FSW in the aluminum joining industry has led to much investigation into the mechanical, metallurgical, and textual effects of FSW on various aluminum alloys. These studies and others have revealed that while the texture of FSWs is often complex, different areas throughout the weld are characterized as having one or more shear components. This has led to generalized observations that material flow during FSW follows a tortuous path of shear deformation along various planes and directions depending on the crystal structure of the material.[3,4,8–12] JOHN YOUNG, Graduate Student, and DAVID FIELD, Professor and Interim Director, are with the School of Mechanical and Materials Engineering, Washington State University, Pullman, WA. Contact e-mail: [email protected] TRACY NELSON, Professor, is with the School of Mechanical Engineering, Brigham Young Universiy, Provo, UT. Manuscript submitted August 13, 2012. Article published online March 13, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A

Possibly as a result of the attention paid to otherwise ‘‘un-weldable’’ alloys and because tool materials that hold up under conditions required for joining steels are not as readily available, significantly less academic attention has been focused on the FSW of ferrous alloys. Those studies that concentrate on ferrous alloys have mainly developed an understanding of the mechanical, metallurgical, and tool effects of FSW, rather than focusing on the textural evolution.[3,13–15] With the exception of very few,[16] those studies that do focus on the texture development of ferrous alloys seem to only consider alloys in which FSW does not induce a phase transformation or the transformation is neglected.[17–19] The purpose of this study is to use the observed crystallographic texture as a forensic tool to investigate material flow under the tool shoulder and at the weld mid-plane during FSW on ferrous alloys, specifically on HSLA-65 steel.

II.

TRANSFORMATION TEXTURES

For FSW, it is expected that the steel will form a s