Deconvoluting the link between weld tool geometry and process parameters
Querin, Joseph Andrew
CommitteeNunes Jr., Arthur
Advancing the process of friction stir welding (FSW) currently relies on a trial and error approach toward selecting tool geometries and determining process parameters. Before a more formalized design process can be established, a methodology for process parameter selection must be developed. In FSW, various flow paths, as influenced by the tool design, have been proposed to explain the formation of a high quality weld. Central to the discussion is whether the metal is extruded around the tool or whether the metal crosses a shear zone which differentiates the stir zone from the parent material. Probing the interaction between the weld tool and workpiece can shed light on the effectiveness of smooth tapered pins in preventing slippage at the tool interface. Creation of a shear surface a finite distance from the weld tool requires that the metal stick to the tool surface so that it is deformed by shearing in the workpiece. Tool slippage would then be limited by pressure generated from either threads or a critical taper angle. In contrast, if the metal were extruded around the tool, a slipping condition would occur at the interface. These variations in the material flow should be observed in the resulting weld structure and force/torque data acquired during the weld. In the present study, a family of weld tools was designed and used to produce FSW panels for comparison. The tools included smooth surface pins with different taper angles and a standard threaded pin. From each weld, specimens were removed for metallographic analysis and tensile testing. Variations in the tool geometry were observed to affect the mechanical power input and tool heat dissipation. The initial process parameters were selected based on a constant “pseudo heat index (PHI)” taken from FSW literature, but for each corresponding tool, variations were seen in the process parameter window that resulted in defect-free welds and in their weld properties. Using a one-dimensional heat transfer model, a method is proposed for calculating process parameters that takes into account the specific tool design.