Friction stir welding using a PCBN-based tool

1. A friction stir welding tool insert comprising a polycrystalline cubic boron nitride, PCBN, composite material with a textured surface layer, as determined by optical microscopy or scanning electron microscopy, characterised in that the textured surface layer has a depth of between 0.5 and 50 μm, and wherein the textured surface layer comprises a pre-defined repeating pattern, wherein the PCBN composite material comprises cBN particles dispersed in a matrix material comprising a tungsten-rhenium matrix.

2. The tool insert as claimed in claim 1 , in which the tool insert comprises a stirring pin and a shoulder, the stirring pin extending from a rounded apex to the shoulder.

3. The tool insert as claimed in claim 1 , in which the textured surface layer comprises a plurality of surface features selected from any one of the following: truncated pyramids, cuboids, crevices, ridges, grooves.

4. The tool insert as claimed in claim 3 , in which the plurality of surface features show a characteristic repeat distance locally.

5. The tool insert as claimed in claim 3 , in which the plurality of surface features are uniform in size.

6. The tool insert as claimed in claim 1 , in which the textured surface layer is homogeneous with a core of the tool insert.

7. The tool insert as claimed in claim 1 , in which coverage of the textured surface layer extends across an entire surface of the tool insert.

8. The tool insert as claimed in claim 1 , in which the textured surface layer is provided only in pre-determined zones on the tool insert.

9. The tool insert as claimed in claim 8 , in which the textured surface layer is provided on a working face of the tool insert.

10. The tool insert as claimed in claim 8 , in which the textured surface layer is provided in a region where the stirring pin merges with the shoulder.

11. The tool insert as claimed in claim 8 , in which the textured surface layer is provided in a region extending between a first bound located radially at a third of a radius of the shoulder and a second bound located radially at two thirds of the radius of the shoulder.

12. The tool insert as claimed in claim 11 , in which the textured surface layer extends at least partially across the surface between the first and second bounds.

13. The tool insert as claimed in claim 11 , in which the textured surface layer extends across the whole of the surface between the first and second bounds.

14. The tool insert as claimed in claim 1 , the textured surface layer having a surface roughness, Sa, of 0.5 to 5 μm.

15. The tool insert as claimed in claim 14 , the textured surface layer having a surface roughness, Sa, of 1 to 2 μm.

16. The tool insert as claimed in claim 1 , in which the PCBN composite material comprises an aluminium compound at least partially coating the cBN particles within the matrix material.