Page 52 - Welding of Aluminium and its Alloys

44 The welding of aluminium and its alloys

treatment and ageing after welding, not often possible in a fully fabricated
structure. The less weldable alloy 2014 (AlZnMgCu) may also be heat
treated to recover some tensile strength but the improvement is not as
great as in 2219 (AlCu6) and may exhibit an even greater reduction in
ductility.
Filler metals of similar composition such as 2319 (AlCu6) are available
and weld metal strengths can therefore be matched with the properties in
the HAZ.

3.4.2.2 Aluminium–magnesium–silicon alloys (6XXX series)
The hardening constituent in 6XXX series alloys is magnesium silicide
Mg 2Si. These alloys contain small amounts of silicon and magnesium, typi-
cally less than 1% each, and may be further alloyed with equally small
amounts of manganese, copper, zinc and chromium.The alloys are sensitive
to weld metal cracking, particularly when the weld metal is rich in parent
metal such as in the root pass of the weld. Fortunately the cracking can be
readily prevented by the use of filler metals containing higher proportions
of silicon such as 4043 or, with a slightly increased risk of hot cracking, the
higher magnesium alloys such as 5356.
With these heat-treatable alloys the changes in the structure and mechani-
cal properties, briefly discussed in Chapter 2, are complex and strongly
dependent on the welding conditions employed. Welding without filler
metal or with filler metal of parent metal composition is rarely practised
because of the risk of weld metal hot cracking. A weld metal with a com-
position close to that of the parent metal may age-harden naturally or may
be artificially aged to achieve a strength approaching, but never matching,
that of the aged parent metal.
In the overheated zone in the HAZ closest to the fusion line, partial
melting of the grain boundaries will have taken place. Temperatures have
been high enough and cooling rates sufficiently fast that solution treatment
has taken place, enabling some ageing to occur after welding. Adjacent to
this is the partially solution-treated zone where some of the precipitates
have been taken into solution, enabling some post-weld hardening to occur,
but those not dissolved will have been coarsened. Outside this will be the
overaged zone where precipitate coarsening has taken place and there has
been a large drop in strength.
The strength losses in the 6000 alloys are less in the naturally aged metal
than in the artificially aged alloys.The strength of the weld and HAZ in the
artificially aged condition generally drop to match that of the naturally aged
alloy with a narrow solution-treated zone either side of the weld and an
overaged zone beyond this, which is weaker than the T6 condition.With
controlled low-heat input welding procedures the strength of the weldment