Page 54 - Welding of Aluminium and its Alloys

46 The welding of aluminium and its alloys

3.4.2.4 Unassigned (or other alloys) (8XXX series)

The 8XXX series is used to identify those alloys that do not fit conveniently
into any of the other groups, such as 8001 (Al-Ni-Fe) and 8020 (Al-Sn).
However, contained within this 8XXX group are the aluminium–lithium
(Al-Li) alloys, a relatively new family that gives substantial weight savings
of up to 15% and a higher Young’s modulus compared with some of the
other high-strength alloys. Each 1% of lithium added results in an approx-
imate 3% reduction in weight. These advantages mean that significant
weight savings can be achieved in the design of aerospace structures and
that the very high-strength unweldable alloys, such as those in the 2XXX
series, may be replaced by the weldable, lighter Al-Li alloys.
The Al-Li alloys generally contain some 2–3% of lithium and small
amounts of copper and magnesium. They are fully heat treatable, with a
number of different precipitates, the principal one being Al 3Li. Typical of
these alloys are 8090 (AlLi2.5Cu1.5Mg0.7Zr) and 8091 (AlLi2.6Cu1.9Mg
0.8Zr). Lithium has a great affinity for oxygen and this reactivity requires
great care to be taken during any process that involves heating the alloy.
These processes comprise melting, casting, high-temperature heat treat-
ment and welding. Failure to remove the oxidised layer will result in gross
porosity – some 0.2mm should be machined off to be certain of complete
removal. It may also be necessary to purge the back face of the weld with
an inert gas to prevent oxidation and porosity.As with the 7XXX alloys the
Al-Li alloys have a similar response to the heat of welding, losing strength
in the HAZ, although a post-weld artificial ageing treatment can restore a
large proportion of this strength.
A further family of alloys that may fall into this group once they have
been assigned a designation are those containing scandium. These are new
alloys, still to a great extent in the development phase. Scandium is a rare
earth element that has been found to be highly effective in increasing
strength by age hardening and by grain refinement, the latter being particu-
larly useful in weld metal. Scandium is likely to be used in conjunction with
other alloying elements such as zirconium, magnesium, zinc or lithium
2
where tensile strengths of over 600N/mm have been achieved in labora-
tory trials.

3.5 Filler metal selection

Filler metal specifications are to be found in BS 2019 Part 4, although this
will be replaced in the near future by a CEN specification. The BS specifi-
cation lists 11 filler metal types in the 1XXX, 3XXX, 4XXX and 5XXX
series and details the delivery conditions. BS 2901 does not include any filler
metals capable of being age hardened. The American Welding Society has