Connect, no. 119, July/August 2002, p.3
The skew-stir TM variant of FSW differs from the conventional method in that the axis of the tool is given a slight inclination (skew) to that of the machine spindle (See Connect - July/August 2001).
The skew-stir TM technique enables the ratio between the 'dynamic' (swept) volume and the static volume to be increased by the skew motion of the tool. This can be additional to that provided by the use of re-entrant features machined into the probe. It is this ratio that is a significant factor in enabling a reduction or elimination of void formation thereby improving process efficiency.
The skew action results in only the outer surface of the probe making contact with the extremities of the weld region. The FSW tool does not rotate on its own axis, and therefore only a specific part of the face of the probe surface is directly involved in working the substrate component material. Consequently, the inner part of the tool can be cut away to improve the flow path of material during welding, (see Fig.1a, b, and c).
The skew-stir TM technique provides a flow path and a weld nugget region of width greater than the diameter of the probe. In addition the skew action provides an orbital forging action at the root of the weld. A weld made with the skew-stir TM technique is shown in Fig.2.
In this Skew-stir TM example, the width of the weld region is 195% of the plate thickness and no upper plate thinning at the weld interface is apparent. The welds produced with an A-Skew TM probe revealed a nominal downturn at the outer regions of the overlapping plate/weld interface and a greater presence of oxide interface remnant on the retreating side, Fig.3a, compared with the advancing side, Fig.3b.
Weld quality Mechanical testing of lap welds was undertaken using a hammer 'S' bend test carried out with the weld region unrestrained. This lap hammer 'S' bend test proved a discerning method of establishing basic weld integrity and freedom from weakness caused by plate thinning. Fig.4 shows the typical results achieved from a weld produced with A-Skew TM probes.
A-skew TM probes are well suited for lap and 'T' and similar welds where the interface is 90◦ to the machine axis, ie parallel with the work piece surface. Welds made with these tools, using the same process conditions, showed improvement over conventional pin type probes. Moreover, the 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.
- A-Skew TM probes gave >100% improvement in travel speed.
- When compared to a conventional threaded pin type probe it also allowed a 20% reduction in axial force.
- Upper plate thinning was also reduced by a factor of >4.
See Connect - May/June 2002 for a comparison with the Flared-Triflute TM tool.
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