Page 51 - Welding of Aluminium and its Alloys

Material standards, designations and alloys 43

cracking. A continuous network of Mg 2Al 3 along the grain boundaries may
make the alloy sensitive to stress corrosion in the form of intergranular cor-
rosion. The alloy can be sensitised by prolonged exposure to temperatures
above 80°C. In service at or above this temperature in mildly corrosive
environments the magnesium content should be limited to a maximum of
3%. Alloys for service in these conditions are generally of the 5251 or 5454
type, welded with a 5554 (AlMg3) filler metal. In multi-pass double-sided
welds a 5% Mg filler may be used for the root passes to reduce the risk of
hot cracking, followed by 5554 filler for the filling and capping passes.
The 5XXX alloys containing between 1% and 2.5% magnesium may be
susceptible to hot cracking if welded autogenously or with filler metal of
a matching composition. The solution is to use more highly alloyed filler
metal containing more than 3.5% magnesium.

3.4.2 Heat-treatable alloys

3.4.2.1 Aluminium–copper alloys (2XXX series)

The aluminium–copper alloys are composed of a solid solution of copper
in aluminium which gives an increase in strength, but the bulk of the
strength increase is caused by the formation of a precipitate of copper alu-
minide CuAl 2. To gain the full benefits of this precipitate it should be
present as a finely and evenly distributed submicroscopic precipitate within
the grains,achieved by solution treatment followed by a carefully controlled
ageing heat treatment. In the annealed condition a coarse precipitate forms
along the grain boundaries and in the overaged condition the submicro-
scopic precipitates coarsen. In both cases the strength of the alloy is less
than that of the correctly aged condition.
The early aluminium–copper alloys contained some 2–4% of copper.This
composition resulted in the alloys being extremely sensitive to hot short-
ness, so much so that for many years the 2XXX were said to be unweld-
able. Increasing the amount of copper, however, to 6% or more, markedly
improved weldability owing to the large amounts of eutectic available to
back-fill hot cracks as they formed. The limit of solid solubility of copper
in aluminium is 5.8% at 548°C; at ambient this copper is present as a
saturated solid solution with particles of the hardening phase copper alu-
minide, CuAl 2, within the grains as a fine or coarse precipitate or at the
grain boundaries.
The effect of welding on the age-hardened structure is to re-dissolve the
precipitates, giving up to a 50% loss in ultimate tensile strength in a T6
condition alloy. The weldable alloy 2219 (AlCu6) can recover some of this
strength loss by artificial ageing but this is usually accompanied by a reduc-
tion in ductility. The best results in this alloy are obtained by a full solution