Material Flow Modification in a FSW Through Introduction of Flats
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MOST FSW tool studies in the literature focus on the resulting structural integrity of the FSW joint since that ultimately affects the cost and robustness of the process. Recent reviews of tool design have been published by Rai et al.[1] with special regard to material selection and wear. A tool design is considered good if it improves the quality of the FSW within an acceptable parameter processing window.[2] Usually the acceptable processing window is determined by the capability of the FSW machine being used. Generally the tool is made from a material that retains sufficient strength at the FSW temperature and does not chemically react with the work pieces. The tool dimensions are ultimately designed to resist breakage during the FSW process. Specific design features of the FSW tool are often based on intuitive concepts,[3] details of which can often be closely guarded. Different FSW tools, especially those with flats or flutes have been compared on the basis of the ratio of swept to static volume with a higher swept volume reported to promote better mixing.[4,5] While various studies in the literature target tool design, the main published focus is on the overall geometric relationships. A typical tool is illustrated in Figure 1 and is comprised a shoulder and a pin. The pin shape can range from tapered and smooth, shown in JUDY SCHNEIDER, Professor, is with the University of Alabama in Huntsville, Huntsville, AL Contact e-mail: Judith.schneider@ uah.edu SHANE BROOKE, Welding Engineer, and ARTHUR C. NUNES, Jr., Welding Theoretician, are with the NASA Marshall Space Flight Center, Huntsville, AL S. Brooke and A.C. Nunes, Jr., are employed by NASA Marshall Space Flight Center. U.S. Government work is not protected by U.S. Copyright. Manuscript submitted September 13, 2015. Article published online November 30, 2015. 720—VOLUME 47B, FEBRUARY 2016
Figure 1(a), to cylindrical and threaded, shown in Figure 1(b). In low-melting temperature alloys, such as Al and Mg, the ratio of the shoulder diameter to the pin diameter is nominally 2:1,[2] whereas in higher melting temperature alloys with low thermal conductivity, such as Ti,[6,7] the ratio may decrease to 1.2:1. The interaction of the tool motion with the work piece generates the temperature needed to plasticize the material so that it can flow around the tool in the solid state. The heating has been attributed to friction between the shoulder[8–10] and the workpiece and to deformation around the weld tool[11–16] as the movement of the pin shears the material in thin layers forming a banded structure referred to as ‘‘onion rings[17].’’ In spite of the name ‘‘friction stir welding,’’ the heating actually appears to be mainly a result of deformation in the vicinity of the tool. Schemes to optimize shoulder dimensions relate to containing the material within the flow zone.[18–20] Most studies concur that shoulder contact is needed to contain the plasticized material to prevent wormhole formation.[11] Whether this is simply a geometrical constraint or a source of heat input is
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