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Com-stir: Compound motion for friction stir welding and machining

Connect, no. 124, May/June 2003, p.8

A new friction stir welding (FSW) variant has been developed - Com-stir TM - which involves the application of a rotary motion in combination with an orbital motion, ( Fig.1). This technique has the following potential advantages over conventional friction stir welding:

  • It can form wider welds, advantageous for lap and T-joint welds and materials processing
  • It should give higher quality welds as surface oxide fragmentation is more efficient and heat generation is more uniform across the weld
  • Less workpiece jigging is necessary because of the reduced resultant torque applied during welding or machining
  • Welding of dissimilar materials and spot welding should also be easier.
Fig.1. The Com-stir TM principle
Fig.1. The Com-stir TM principle

In conventional rotary FSW, the relative velocity of the tool increases from zero at the probe centre to maximum velocity at the outer diameter of the shoulder. The combined orbital/rotary Com-stir TM technique can significantly modify the differential in velocity between the probe centre and the shoulder diameter. This technique provides a range of options between an almost complete orbital motion to an almost complete rotary motion by selectively changing the relative rotational speed. This feature can be pre-selected or varied automatically by in-process control to suit the conditions. Although opposed rotations are shown in Fig. 1, rotations that are in the same direction could also be used.

Orbital motion results in a relative interfacial velocity of constant magnitude but of continuously varying direction. Reported work for bar-to-bar orbital friction welding indicates that higher heating rates and increased metal flow can be expected.

The Com-stir TM technique which also uses both rotational and orbital motions, benefits from the higher heating rate and greater metal flow.

Fig. 2. Macrosection showing a wide weld region produce by Com-stir TM
Fig. 2. Macrosection showing a wide weld region produce by Com-stir TM

When Com-stir TM is used for material processing, the increase in the width of the processed zone (and hence the volume of material processed) is particularly advantageous. Figure 2 shows a wide weld in a lap joint made by Com-stir TM . In this example the weld width measures 230% of the top plate thickness (cf 110% for conventional FSW). Much wider weld regions are believed to be possible and this is being further investigated.

Further work is also being undertaken to investigate and develop combined motion techniques for machining. Such a technique could eliminate the need to use 'free-machining' materials where certain less desirable materials are added to a metal to improve machinability, often at the expense of weldability such as lead additions to steel and selenium to stainless steel.

Further details may be obtained from Wayne Thomas, Ian Norris, David Staines and Edward Watts. Please contact us.

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