An experimental and CFD model based evaluation of shoulder-less, conical tools for FSW

A friction stir welding (FSW) tool geometry, consisting of only a conical probe, is being 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 dissengagement with minimal surface defect in applications which do not permit a weld termination defect (e.g. pipe, pressure vessel, fuselage, nosecone). In addition, because the vertical position of the tool relative to the material surface is less critical with a conical tool, it can be used in a closed loop fashion (i.e. without force control) and on materials whose thickness is highly variable. The use of a conical probe without a shoulder is not documented in published literature and it is the aim of this work to estabilish 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) model. It is concluded that this manner of tooling is capable of producing acceptable welds when applied to butted alluminum plates and that similar methods would likely be effective in the applications described previously.