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Novel developments in friction stir welding

Connect, no. 118, May/June 2002, p.3

Flared tools for lap welding

Early in the development of friction stir welding (FSW), it was realised that the form of the welding tool was critical in achieving sound butt and lap welds with good mechanical properties. For lap welds, the width of the weld interface and shape of the notches at the edge of the weld are of fundamental importance, especially for those applications that are subject to fatigue. Figure 1 shows a Triflute flared probe, with straight flutes designed for lap welds.
Fig.1. FSW tool with Flared-Triflute TM probe and tip profile for lap welding
Fig.1. FSW tool with Flared-Triflute TM probe and tip profile for lap welding

In the flared probe, the core remains as a taper frustum but the flute lands are flared out at an inverted angle so as to increase the tip diameter. In addition, a tip profile has been included in the shape of a three pronged whisk. These features collectively increase the difference between the swept volume and the static volume of the probe, thereby further improving the flow path around and underneath the probe. The whisk type tip profile provides an improved mixing action for oxide fragmentation and dispersal at the weld interface (see Fig.2).

Fig.2. FSW tool with Flared-Triflute TM probe with tip profile positioned across the lap weld interface
Fig.2. FSW tool with Flared-Triflute TM probe with tip profile positioned across the lap weld interface

A lap joint made with a Flared-Triflute TM probe is shown in Fig.3. In this example the width of the weld region is 190% of that of the plate thickness and little upper plate thinning is apparent at the weld interface.

Fig.3. Macrosection showing a wide weld region produced using a Flared-Triflute TM probe. Lap weld produced in 6mm thick 5083-0 condition aluminium alloy at a weld travel speed of 4mm/sec (240mm/min)
Fig.3. Macrosection showing a wide weld region produced using a Flared-Triflute TM probe. Lap weld produced in 6mm thick 5083-0 condition aluminium alloy at a weld travel speed of 4mm/sec (240mm/min)

The outer regions of the weld at the overlapping plate/weld interface show a slight upturn (see Fig.4a and b). A greater presence of oxide interface remnant is revealed at the retreating side (see Fig.4a) compared with the advancing side (see Fig.4b).

Fig.4. Detail at the extremes of the weld region for Flared-Triflute TM type weld Fig.4a) Retreating Side
Fig.4. Detail at the extremes of the weld region for Flared-Triflute TM type weld Fig.4a) Retreating Side
Fig.4b) Advancing Side
Fig.4b) Advancing Side

Weld quality

Mechanical testing of lap welds was undertaken using a hammer 'S' bend test. Bend testing was carried out with the weld region unrestrained. This lap 'hammer 'S' bend test' proved a discerning method of establishing basic weld integrity and any problems caused by plate thinning. Figure 5 shows the typical results achieved from welds produced with a Flared-Triflute TM probe.
Fig.5. Hammer 'S' bend tested lap welds using a Flared-Triflute TM probe
Fig.5. Hammer 'S' bend tested lap welds using a Flared-Triflute TM probe

Flared-Triflute TM probes are well suited for lap and 'T' and similar welds where the interface is 90◦ to the machine axis. Welds made with this tool, using the same process conditions, showed improvement over conventional pin type probes. Moreover, tool and technique used provide an effective method of increasing the width of the weld region; particularly advantageous for material processing. Repeatability trials achieved consistent and good results and when compared with results from a conventional threaded pin type probe the following was established.

  • Flared-Triflute TM probe gave >100% improvement in travel speed.
  • It also allowed a 20% reduction in axial force.
  • Upper plate thinning was also reduced by a factor of 4.

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