Friction Stir Welding

Friction stir welding is a relatively new joining technique where a rapidly spinning pin warms then blends together material across a joint line.  It has quickly become a preferred technique for welding non-ferrous metals in certain engineering applications. However, when friction stir welding titanium, some alloys are easier than others. It is particularly difficult to produce sound welds in commercially pure (CP) titanium.

Recently, it has been found that placing a pure nickel metal foil in the joint prior to friction stir welding, lowers welding forces, improves weld surface finish, and reduces weld defects [Ref.]. Originally, the foils were placed in the weld as fiducial markers [Ref.]. While many foil compositions have been tried, the exact mechanism by which the nickel produces these effects is unknown at this time.

We have examined the role of nickel foil thickness on the mechanical properties of the resulting weldment. Friction stir welded plates were fabricated using two different nickel foil thicknesses; 0.051 and 0.102 mm thick (0.002 and 0.004 inches thick). Macroscale mechanical testing of the welds and microtensile testing of samples from various regions in the weld were performed to compare mechanical properties. Microstructural analysis, x-ray computed microtomography, and SEM of the failure surfaces was performed to examine the role of the foils on resulting weld properties.