Connect, no. 124, May/June 2003, p.1
TWI and the University of Cambridge are conducting a project on the modelling of Friction Stir Welding (FSW). Modelling is important to increase understanding of the process enabling more effective tools to be designed.
The modelling work has used the commercial Computational Fluid Dynamics (CFD) package FLUENT to model both 2 and 3 dimensional flow. The model is unique in that it allows the material to slip as well as stick to the tool surface. This technique has enabled the comparison of tool profiles, which was not possible with the conventional 'stick' model.
The work showed that the 'Trivex TM ' tool ( Fig. 1) minimised the traversing force. The tool consists of three convex sides that prevent entrapment of material. The traversing force was least when the tool area to swept area ratio was between 70-80%. Two tools were produced, based on this concept. The first ( Fig. 1a) used no threads while the second ( Fig. 1b) was threaded along the length.
These two tools were compared against one version of the MX-Triflute TM family of tools, which had the same pin and shoulder dimensions. Butt welds were made using 6.35mm 7075-T7351 aluminium alloy on the ESAB SuperStir TM machine. This machine permitted the measurement of traversing and down forces, which were reduced by 18-25% and 12% respectively with the new tools. The tensile strength of the welds made with the new tool matched those made with the MX-Triflute TM .
This work has demonstrated the potential benefits of the Trivex TM tool over one version of the MX-Triflute TM tool in lowering both the traversing and down forces. In particular, the Trivex TM tool without threads shows remarkable promise, because it is inherently much easier to manufacture than either the MX-Triflute TM or MX-Trivex TM tools and the weld strength is comparable. Avoiding the use of threads is also beneficial for fatigue resistance of the tool, as the threads act as crack initiation sites.
Further details of this work will be presented at the upcoming Friction Stir Welding Symposium in Salt Lake City in May. Future investigations of the Trivex TM tool concept will focus on thick section butt welds, lap welds and welds in other materials such as steel.
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