Friction stir welding tool having a scroll-free concentric region

1. A tool for forming a friction stir weld joint or friction stir processing a workpiece, the tool comprising:

a support body including a shoulder and a pin that are concentric and rotatable about a first axis, wherein said pin extends from said shoulder parallel with the first axis, and wherein said shoulder comprises a flat planar face that is concentric about the first axis, and extends from said pin and terminates at a peripheral boundary of said flat planar face; and

a scroll pattern in said flat planar face of said shoulder, said scroll pattern extending from said pin and terminating prior to said peripheral boundary of said flat planar face to define a peripheral scroll-free concentric region on said flat planar face of said shoulder, said scroll-free concentric region being concentric about the first axis, and extending from said scroll pattern and terminating at said peripheral boundary of said flat planar face, said scroll-free concentric region comprising at least 5% of the total surface area of said flat planar face of said shoulder, and

wherein said scroll pattern comprises a plurality of spiral formed grooves each of which spirals outward for less than a full revolution around said flat planar face.

2. The tool of claim 1 wherein said scroll-free concentric region comprises about 45% to 65% of the total surface area of said flat planar face of said shoulder.

3. The tool of claim 1 wherein said scroll-free concentric region comprises about 5% to 40% of the total surface area of said flat planar face of said shoulder.

4. The tool of claim 1 wherein said scroll pattern has a uniform depth.

5. The tool of claim 1 wherein said shoulder further comprises a chamfer that extends from said peripheral boundary of said flat planar face and terminates at a peripheral boundary of said shoulder.