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Skew-stir, variation on a theme


Connect, no. 113, July/August 2001, p.3

Friction stir welding is in commercial use and has become the preferred welding technique for certain materials and applications. The process can achieve high quality welds in a growing number of non-ferrous and ferrous materials. Invariably manufacturers and fabricators want to weld as fast as possible; increasing the welding speed while maintaining or improving weld integrity and strength will reduce production costs and increase profitability.

A major factor in achieving weld integrity and process efficiency is the design of the FSW tool to provide a suitable ratio between the volume of the probe swept during rotation to the volume of the probe itself. With conventional tools, the dynamic to static volume ratio is achieved by the design of the probe geometry.

During friction stir welding the FSW tool needs to prevent material from escaping, augur material downwards and allow plasticised material to flow from the front to the back of the tool. TWI is carrying out investigations to improve further the dynamic to static volume ratio of the tool by use of a skew motion.

Skew-stir TM

The Skew-stir TM variant of friction stir welding differs from the conventional method in that the axis of the tool is given a slight inclination (skew) from that of the machine spindle. As shown in Fig. 1, the face of the shoulder is produced such that it is at 90◦ to the axis of the machine spindle.

The intersection of the two axes can occur above the plate, through the plate thickness, and to a position below the plate being welded. This intersection, or focal point, can be varied to suit the material, the process parameters, and the tool geometry.

Fig. 1. Basic principle of Skew-Stir TM showing different focal points
Fig. 1. Basic principle of Skew-Stir TM showing different focal points

Tilting the plate substrate or machine spindle with respect to the shoulder can then additionally produce a plate to tool tilt, similar to that often used in conventional FSW.

When the focal point is positioned slightly above the top surface of the plate, at any position through the thickness of the plate, or slightly below the plate, the shoulder contact face makes a nominally orbital movement.

This orbital motion of the shoulder is dependent on the angle of skew and the focal point. The greater the skew angle and the greater the distance the focal point from the top of the plate, the greater is the shoulder orbit.

Friction Skew-stir TM welding increases the extent of the plasticised material surrounding the probe. The Skew-stir TM technique, therefore, provides a method of increasing the 'dynamic to static volume ratio' of the probe by the skew motion of the tool.

Fig. 2. Prototype asymmetric Skew-stir TM tool
Fig. 2. Prototype asymmetric Skew-stir TM tool

Traditionally, the 'dynamic to static volume ratio' is provided by the geometry of the probe because of its re-entrant features. Skew-stir TM, therefore, can be used to advantage where complex shaped tools cannot be used. Moreover, the skew action results in a greater volume of plasticised material within which the disrupted surface oxide layer can be dispersed. This should minimise the risk of undesirable joint remnant features.

Further work at TWI is being undertaken to investigate the following:

  • the advantage of the Skew-stir TM technique for lap welds
  • the development of the technique for crack repair, so as to encompass existing solid-phase or fusion welds.
  • the effect of the inherent forging action associated with Skew-stir TM in improving the weld quality at the root.
  • reprocessing bulk material

To find out more contact Wayne Thomas, Peter Fielding, Phil Threadgill, & Dave Staines. Acknowledgements to Peter-Temple Smith (retired) for his much valued help and support. Please contact us.

For more information please email:


contactus@twi.co.uk