Mechanical Properties, Microstructure and Crystallographic Texture of Magnesium AZ91-D Alloy Welded by Friction Stir Wel

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INTRODUCTION

FRICTION stir welding (FSW) is a solid-state joining process that was invented by The Welding Institute (TWI), UK in 1991.[1] The use of FSW is now more prevalent because it is rapid, precise, and easy to perform.[2] It is increasingly being used in the shipbuilding, rail transport, automotive, small-component manufacturing, and aerospace industries.[2,3] The greatest advantage of this method, however, is the ability to weld light structural materials, such as certain aluminum alloys and magnesium alloys that used to be considered unweldable or difficult to weld by conventional fusion-welding techniques.[4] Indeed, this technique utilizes a rotating tool with a shoulder and a profiled probe that is plunged into work pieces and traversed along the weld centerline.[2,5] No melting takes place during the process, thus maintaining relatively low temperatures and producing good-quality welds with significantly low residual stresses.[6] The motion of the tool generates frictional heat within the work pieces, extruding the softened plasticized material around it and forging the same material in place so as to form a solid-state seamless joint. In this process, every parameter plays a particular role, which leads to several phenomena: the material undergoes intense shearing and dynamic recrystallization simultaneously. A. KOUADRI-HENNI, Associate Professor, is with the PSM Team, European University of Brittany, France, INSA of Rennes, LGCGM, EA 3913, 20 Avenue des Buttes des Coesmes, 35708 Rennes, France. Contact e-mail: afi[email protected] L. BARRALLIER, Professor, is with the MecaSurf Team, Arts et Me´tiers Paris Tech, France, ENSAM, Mecasurf, JE2504, 2 cours des arts, 13617 Aix en Provence, France. Manuscript submitted April 10, 2013. Article published online July 25, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A

Several authors have confirmed this point and indicate that it is important to separate out the effect of final shoulder deformation through the forging action after the pin has passed.[7–10] Indeed, the top layer undergoes shoulder deformation after the pin has passed through. This adds a shear deformation component at lower temperature to the recrystallized volume processed by the pin.[8] In this process, the FSW material consists of four distinct microstructural zones: nugget or FSW, the thermo-mechanically affected zone (TMAZ), the heataffected zone (HAZ), and the base material (BM). Each zone has a different thermo-mechanical history.[9,10] In addition, depending on the tool rotation rate and traverse speed, the nugget region can contain a ring pattern or other microstructural variations.[5,6,10,11] What makes FSW even more complex is that the nugget region consists of sub-domains. The literature indicates that it is difficult to understand how the nucleation of new grains and continuous deformation influence the final texture. Indeed this FSW process leads to strong crystallographic texture. The deformation under the shoulder regions is likely to influence the final texture significantly. This

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