Page 50 - Welding of Aluminium and its Alloys

42 The welding of aluminium and its alloys

3.4.1.4 Aluminium–magnesium alloys (5XXX series)

Up to about 5% magnesium can be dissolved in aluminium to provide a
substantial amount of solid solution strengthening: the higher the magne-
sium content, the higher the strength. The amount of magnesium that can
be dissolved under equilibrium conditions at ambient temperature is only
some 1.4%, meaning that there is always a tendency for the magnesium to
come out of solution when the higher magnesium content alloys are heated
and slowly cooled. This reaction is very sluggish and welding processes do
not cause any appreciable change in the microstructure except in the cold
worked alloys where mechanical strength will be reduced.
The standard aluminium–magnesium alloys have iron and silicon as
impurities and deliberate additions of around 0.4–0.7% of manganese to
increase strength further, mainly by dispersion hardening. Chromium may
be added in place of or in addition to manganese to achieve the same
strength increase, 0.2% chromium being equivalent to 0.4% manganese.
The iron forms FeMnAl 6; the silicon combines with magnesium to form
magnesium silicide, Mg 2Si, most of which is insoluble.
The magnesium alloys may all have their microstructure changed by the
heat of welding.The microstructure of a butt weld in 5083 (AlMg4.5Mn0.7)
in the annealed condition, welded with a 5356 filler shows the following fea-
tures. The parent metal will have a fine-grained structure composed of a
matrix of a solid solution of magnesium in aluminium, dispersion strength-
ened with a fine precipitate of the compound Mg 2Al 3 together with coarser
particles of Al-Fe-Si-Mn. In the HAZ where the temperature has been
raised to around 250°C further Mg 2Al 3 will be formed which may begin to
coalesce and coarsen. Where temperatures begin to approach 400°C some
of the Mg 2Al 3 will be redissolved and closer to the weld, where tempera-
tures are above 560°C, partial melting occurs, causing some shrinkage
cavitation. The weld metal is an as-cast structure of a supersaturated solu-
tion of magnesium in aluminium with particles of the insoluble inter-
metallics such as Mg 2Si. The cooling rates of the weld metal are generally
fast enough to prevent the precipitation of Mg 2Al 3.
The strength of aluminium–magnesium weld metal is generally close
to that of the annealed wrought parent metal of the same composition and
it is not difficult to achieve joint strengths at least equal to the annealed
condition. Butt joints in parent metal with more than 4% magnesium
sometimes show joint strengths less than that of the annealed parent alloy.
In MIG welding this may be due to the loss of magnesium in the arc and
it may be advisable to use a more highly alloyed filler such as 5556
(AlMg5.2Cr).
5083 is normally welded with a filler metal of similar composition because
the higher magnesium contents increase the risk of stress corrosion