A Semianalytical Thermal Model for Fiction Stir Welding
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INTRODUCTION
FRICTION stir welding (FSW) is an emerging solidstate joining process developed by The Welding Institute of Cambridge (Cambridge, United Kingdom). In FSW, a cylindrical, shouldered tool with a profiled probe is rotated and slowly plunged into the joint line between two pieces of sheet or plate material, which are butted together (Figure 1). The probe may have a diameter one-third of the cylindrical tool and typically has a length slightly less than the thickness of the workpiece. The parts have to be clamped onto a backing bar in a manner that prevents the abutting joint faces from being forced apart. Frictional heat is generated between the wear-resistant welding tool and the material of the workpieces. This heat causes the latter to soften without reaching the melting point and allows traversing of the tool along the weld line. The plasticized material is transferred from the leading edge of the tool to the trailing edge of the tool probe and is forged by the intimate contact of the tool shoulder and the pin profile. It leaves a solid phase bond between the two pieces. The process can be regarded as a solid phase keyhole welding technique, since a hole to accommodate the probe is generated and then filled during the welding sequence. The process advantages result from the fact that the FSW process takes place in the solid phase below the melting point of the materials to be joined. The benefits therefore include the ability to join materials that are difficult to fusion weld, low distortions, excellent mechanical properties as proven by fatigue, tensile and P. FERRO, Ph.D. Researcher, and F. BONOLLO, Professor, are with the Department of Management and Engineering, University of Padova, Stradella San Nicola 3 - 36100 Vicenza, Italy. Contact e-mail: [email protected] Manuscript submitted July 7, 2009. Article published online December 18, 2009 440—VOLUME 41A, FEBRUARY 2010
bend tests, no loss of alloying elements, no arc, no fume, no porosity, no cracking, and no filler wire.[1–5] The plastic deformation and temperature profile during FSW produce a microstructure characterized by a central weld nugget surrounded by a thermomechanically affected zone (TMAZ) and heat-affected zone (HAZ) (Figure 1). Both experimental studies and modeling of the FSW process were carried out in order to better understand and foresee the effect of process parameters on welded materials. In particular, numerical and analytical models were developed. The numerical method permits a high grade of coupling between thermometallurgical and mechanical effects but with a great effort in the calculation time. On the other hand, even if the solution of the analytical models is almost instantaneous, they are not able to couple thermal and mechanical effects and, thus, foresee the thermal input of the process. For this reason, these last models are not really predictable. Different coupled thermomechanical numerical models are present in the literature. Dong et al. developed a work[6] in which they reported a series of general findings based on a set
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