Shaped welding

1. A method for friction welding a first workpiece to a second workpiece, the first workpiece having sides that are curved and/or define multiple interior and exterior corners, the method comprising:

locating the first workpiece in a recess or aperture of a first tool, the recess or aperture being shaped in a complementary manner to at least part of the sides of the first workpiece and being sized to leave a space of 0.01 mm to 1 mm between an inner wall of the recess or aperture and the at least part of the sides of the first workpiece, wherein the first tool holds the first workpiece for friction welding;

bringing a weld face of the first workpiece while located in and held by the first tool and a weld face of a second workpiece to be friction welded to the first workpiece into frictional engagement; and

moving the first tool and the second workpiece relative to one another to heat the weld faces of the first and second workpieces wherein the first workpiece expands to bridge the space to provide an interference fit and thereby lock the first workpiece to the first tool and to form a weld that joins the first and second workpieces together.

2. The method as claimed in claim 1 , wherein the first tool has outer edges that are clamped to a friction welding apparatus capable of bringing the weld faces of the first and second workpieces into frictional engagement.

3. The method as claimed in claim 1 , wherein the second workpiece is located within a recess or aperture of a second tool, wherein the recess or aperture of the second tool is shaped in a complementary manner to at least part of the second workpiece, and wherein the aperture or recess of the second tool is larger than the second workpiece.

4. The method as claimed in claim 1 , wherein the method comprises applying a forge force to the first and/or second workpiece to join the first and second workpieces together, the forge force being applied until cooling has taken place so that the weld has become firm and the first workpiece has re-contracted and is removable from the first tool.

5. The method as claimed in claim 4 , wherein the first and/or the second workpiece is oscillated in a direction substantially perpendicular to the forge force applied to the first and/or second workpieces.

6. The method as claimed in claim 5 , wherein the first and/or the second workpiece is oscillated at about 10 to 100 oscillations per second.

7. The method as claimed in claim 1 , wherein the weld comprises a multiple part weld, wherein the first tool is used to hold at least some of the workpieces to be welded in a single operation.

8. The method as claimed in claim 1 , further comprising initially shaping the first tool by laser cutting or water-jet cutting before locating the first workpiece in the recess or aperture of the first tool.