Page 181 - Welding of Aluminium and its Alloys

164 The welding of aluminium and its alloys

8.15 2 metre long friction stir weld in 10mm thick A6082 alloy.
Courtesy of TWI Ltd.

incompatible alloys that cannot be fusion welded together can be success-
fully joined; thirdly, the stirring and forging action produces a fine-grain
structure with properties better than can be achieved in a fusion weld and,
lastly, low boiling point alloying elements are not lost by evaporation. Other
advantages are low distortion, no edge preparation, no porosity, no weld
consumables such as shield gas or filler metal and some tolerance to the
presence of an oxide layer.
One disadvantage to the process is that the ‘keyhole’ remains when the
tool is retracted at the end of the joint. While this may not be a problem
with longitudinal seams where the weld may be ended in a run-off tab that
can be removed, it restricts the use of the process for circumferential seams.
This disadvantage has been overcome by the use of friction taper plug
welding. Tools with a retractable pin are also being investigated and have
given some promising results.
Alloys that have been welded include the easily weldable alloys 5083,
5454, 6061 and 6082 and the less weldable alloys 2014, 2219 and 7075. In
the case of alloys in the ‘O’ condition tensile failures occur in the parent
material away from the weld. As far as the effect on the HAZ is concerned
heat input is less than that of a conventional arc fusion weld. This results in
narrow heat affected zones and a smaller loss of strength in those alloys
that have been hardened by cold-working or ageing. Table 8.3 lists the