Tool technology is the most important factor in the friction stir welding process. The concept and recent development of the MX Triflute TM FSW tool represents a major improvement. Using the MX Triflute TM type tools encouraging results and better performance have been achieved. The team at TWI has achieved these results over a range of materials, from six to 50 mm thickness in a single pass.
Early in the development of FSW, it was realised that the form of the welding tool was critical in achieving sound welds with good mechanical properties. In general terms, the tool as shown in the figure comprises a shoulder and a probe. The shoulder compresses the surface of the workpiece and contains the plasticised weld region. Heat is generated on the surface by friction between the rotating shoulder and the workpiece surface and, when welding thin sheets, this is the main source of heat.
As the workpiece thickness increases, more heat must be supplied by friction between the rotating probe and the workpiece. Also, the main function of the probe is to ensure sufficient working of the material at the weld line and to control the flow of the material around the tool to form a satisfactory weld. The probe generally has a profiled or threaded surface to facilitate a downward augering effect.
Preferably, the probe has an odd number of equally spaced flutes to maintain maximum cross-section opposite to any re-entrant feature. It should also be noted that the change in section between the shoulder and the probe is well radiused in order to reduce stress concentration. In essence, the probe is tapered to maintain approximately a uniform stress distribution owing to torsion and the forward thrust. The helical flutes are comparatively steeply angled and this, together with a coarse outer thread provides a significant augering effect and flow path.
The MX Triflute TM tools were designed so that the probe was not parallel sided but frustum shaped. Together with re-entrant features, the MX Triflute TM probe displaces substantially less material during welding (approximately 70%) than the cylindrical pin type probe. It provides a more uniformly stressed tool and allows for a more efficient flow path. It also incorporates a coarse helical ridge around the triflute lands, This is used to reduce the tool volume further, (and therefore aid material flow), and help break up and disperse surface oxides. Moreover, the re-entrant helical flutes and thread features used on these probes increase the surface area of the probe. This means that the interface between the probe and the plasticised material is also increased.
It is believed that the major factor determining the high performance and the superiority of the MX Triflute TM probe over the conventional cylindrical pin type probe, (especially for thick plate welding), is the ratio of the volume of the probe swept during rotation to the volume of the probe itself. It is the ratio between 'dynamic volume' as opposed to the static volume that is important in providing a sufficient flow path. Typically, this ratio for similar root diameter and length probes was 1.1:1 for conventional pin probes and 2.6:1 for MX Triflute TM probes.
MX Triflute TM is a trademark of TWI
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