In the friction stir welding (FSW) process, heat is generated by friction between the tool and the workpiece. This heat flows into the workpiece as well as the tool. The amount of heat conducted into the workpiece determines the quality of the weld, residual stress and distortion of the workpiece. The amount of the heat that flows to the tool dictates the life of the tool and the capability of the tool for the joining process. In this paper, we formulate the heat transfer of the FSW process into two boundary value problems (BVP)—a steady state BVP for the tool and a transient BVP for the workpiece. To quantify the physical values of the process the temperatures in the workpiece and the tool are measured during FSW. Using the measured transient temperature fields finite element numerical analyses were performed to determine the heat flux generated from the friction to the workpiece and the tool. Detailed temperature distributions in the workpiece and the tool are presented. Discussions relative to the FSW process are then given. In particular, the results show that (1) the majority of the heat generated from the friction, i.e., about 95%, is transferred into the workpiece and only 5% flows into the tool and (2) the fraction of the rate of plastic work dissipated as heat is about 80%.

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Hot rolled DH36 carbon steel, 6.4 mm in thickness, was friction stir welded at speeds of 3.4 mm s^-1 (8 in min^-1), 5.1 mm s^-1 (12 in min^-1), and 7.6 mm s^-1 (18 in min^-1). Single pass welds free of volumetric defects were produced at each speed. The relationships between welding parameters and weld properties are discussed. Optical microscopy, microhardness testing, and transverse and longitudinal tensile tests have been performed. Bainite and martensite are found in the nugget region of the friction stir welds whereas the base material is comprised of ferrite and pearlite. The maximum hardness is observed in the weld nugget, and the hardness decreases gradually from the weld nugget, through the heat affected zone, to the base metal. Tensile testing also indicates overmatching of the weld metal relative to the base metal. Maximum hardness and longitudinal (all weld metal) tensile strengths increase with increasing welding speeds. Weld transverse tensile strengths are governed by the base metal properties, as all transverse tensile bars fail in the base metal.