Page 179 - Welding of Aluminium and its Alloys

162 The welding of aluminium and its alloys

8.12 Launch of a Boeing Delta II Rocket in August 1999 containing
friction stir welded joints. Courtesy of TWI Ltd.

II rocket (Fig. 8.12). It will soon be used for non-structural components in
conventional commercial aircraft and is being actively considered for struc-
tural use. Friction stir welding has also been introduced into shipyards with
great success and is being actively investigated for applications in the
railway rolling stock and automotive industries.
The process utilises a bar-like tool in a wear-resistant material, for alu-
minium generally tool steel, a tool lasting in the region of 1–2km of welding
before requiring replacement. The end of the bar is machined to form a
central probe and a shoulder, the probe length being slightly less than the
depth of the weld required. The bar is rotated and the probe plunged into
the weld line until the shoulder contacts the surface. The rotating probe
within the workpiece heats and plasticises the surrounding metal. Moving
the tool along the joint line results in the metal flowing from the front to
the back of the probe, being prevented from extruding from the joint by
the shoulders (Fig. 8.13). This also applies a substantial forging force which
consolidates the plasticised metal to form a high-quality weld.