Influence of the Tool Shoulder Contact Conditions on the Material Flow During Friction Stir Welding
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TRODUCTION
FRICTION stir welding (FSWing) uses a non-consumable, rotating tool which is plunged into the workpiece and translated along the weld seam to join pieces of metal.[1] Deformational and frictional heating resulting from the interaction of the rotating tool and the workpiece soften the metal to produce a joint by stirring material from two pieces of metal together. Figure 1 summarizes the terminology associated with the FSW process. On the advancing side (AS) of the FSW, the tool feed and the tool rotation directions coincide. The tool feed direction and tool rotation direction are opposite on the retreating side (RS) of the FSW. The differences in the RS and AS movement result in an asymmetric flow field around the weld tool. A cross section of the FSW is referred to as the transverse view, while the top surface is referred to as the plan view. The cross section (transverse view) of the resulting joint, shown in Figure 2, consists of three distinct metallographic regions outside of the parent material (PM): a stir zone (SZ), a thermo-mechanically affected zone (TMAZ), and a heat-affected zone (HAZ). In the TMAZ, the PM grains show evidence of mechanical deformation as they elongate from the HAZ and bend toward the nugget region which consists of refined grains. The HAZ contains grains that have been heated but not mechanically deformed by the welding process. HALEY R. DOUDE, Postdoctoral Associate, is with the Center for Advanced Vehicular Systems, Mississippi State University, Mississippi State, MS. Contact e-mail: [email protected] JUDY A. SCHNEIDER, Professor, is with the Mechanical Engineering Department, Mississippi State University, Mississippi State, MS. ARTHUR C. NUNES, Jr., Aerospace Engineer, is with the Marshall Space Flight Center (MSFC), National Aeronautics and Space Administration (NASA), Hunstville, AL. Manuscript submitted September 7, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
The flow of material is reported to take place both in straight through flow, similar to slip line theory, as well as vertical or through material thickness flow.[2–10] As the material moves through the thickness of the workpiece, this gives rise to the observed marker flow occurring multiple times around the pin tool.[11] As the rotating tool moves along the weld seam, new material is drawn into the SZ and deposited in the wake of the weld in a layer-by-layer pattern giving rise to the onion ring structure observed.[2,12–15] The spacing of the onion rings has been correlated with the distance traveled by the tool during a single rotation.[2,7,8,10,12,13,16–20] Both the shoulder and the pin are reported to influence the flow patterns of the weld metal.[3,6,7,9,14,20–24] Since the mechanical properties of the resulting weld are affected by the thermo-mechanical processing conditions, much effort has been concentrated on various numerical modeling approaches to define the material flow and temperature profile.[2,3,5,25–32] The ability to link the process parameters with the resulting temperature profile relies on understanding h
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