The application of shoulderless conical tools in friction stir welding: An experimental and theoretical study

A friction stir welding (FSW) tool geometry, consisting of a shoulderless conical probe, is investigated for application to closed contour welding, variable thickness welding, and open-loop control welding. By use of a tapered retraction procedure and a ramped rotational velocity, a conical tool may facilitate material disengagement with minimal surface defects in applications which do not permit weld termination defects (e.g. pipes, pressure vessels, fuselages, nosecones). In addition, because the vertical position of the tool relative to the material surface is less critical with a conical tool than with other tool designs, it can be used in a open-loop fashion (i.e. without process force feedback control) and on materials whose thicknesses are highly variable. The use of a conical probe without a shoulder is not documented in the literature and it is the aim of this work to establish the conditions for mechanically sound welds. Effective tool geometries and process variables are found via experimental analysis. Thermal, tensile, macrosection, and process force data are presented along with a computational fluid dynamics (CFD) process model. It is concluded that this type of tooling is capable of producing acceptable welds when applied to butted aluminum plates and that similar methods would likely be effective in the applications described previously.