Page 178 - Welding of Aluminium and its Alloys

Other welding processes 161

carrying with it any contaminants, oxides, etc. Thus two atomically clean
metal surfaces are brought together under pressure and an intermetallic
bond is formed. The heat generated is confined to the interface, heat input
is low and the hot work applied to the weld area results in grain refinement.
This rapid, easily controlled and easily mechanised process has been used
extensively in the automotive industry for items such as differential casings,
half shafts and bi-metallic valves.Since the introduction of this conventional
rotating method of friction welding many developments have taken place
such as stud welding, friction surfacing, linear and radial friction welding,
taper plug welding and friction stir welding.
One very important characteristic of friction welding is its ability to weld
alloys and combinations of alloys previously regarded as unweldable. It is
possible to make dissimilar metal joints,joining steel,copper and aluminium
to themselves and to each other and to successfully weld alloys such as the
2.5% copper–Al 2618 and the AlZnMgCu alloy 7075 without hot cracking.
The primary reason for this is that no melting takes place and thus no brittle
intermetallic phases are formed.

8.5.1 Rotary/relative motion friction welding

The rotary/relative motion friction welding process (Fig. 8.11) is suited to
the joining of fairly regular shaped components, one of them ideally being
circular in cross-section. Equal diameter tubes or bars are the best example
since equal heating can take place over the whole contact area. There are
a couple of disadvantages to this process. The first is that one of the com-
ponents must be rotated and this places a restriction on the shape and size
of the items to be welded, the second is that items to be welded cannot be
presented to the mating part at an angle.
The welding parameters comprise the rotational speed which determines
the peripheral speed, the pressure applied during the welding process and
the duration of the weld cycle. The metal extruded from the joint forms a
flash on the outside of the weld and this is generally machined off to give
a flat surface.

8.5.2 Friction stir welding

The most significant process for the welding of aluminium to be developed
within the last decade of the twentieth century was the friction stir process,
an adaptation of the friction welding process. This process was invented at
TWI in the UK in 1991 and, unlike the conventional rotary or linear motion
processes, is capable of welding longitudinal seams in flat plate. Despite
being such a new process friction stir welds have already been launched
into space in 1999 in the form of seams in the fuel tanks of a Boeing Delta