Page 53 - Welding of Aluminium and its Alloys

Material standards, designations and alloys 45

will not drop to that of an annealed structure but will be close to that of
the T4 condition.

3.4.2.3 Aluminium–zinc–magnesium alloys (7XXX series)
7XXX series alloys may, from a welding point of view, be conveniently
divided into two groups. The first group is the high-strength alloys contain-
ing more than 1% copper, normally used in the aerospace industry and
joined by non-welding methods. The second group is the medium strength
alloys which have been developed for welding.
Aluminium and zinc form a eutectic containing solid solutions of 83%
zinc in aluminium and 1.14% aluminium in zinc. The addition of magne-
sium complicates the situation with additional ternary eutectics and
complex intermetallics being formed, these intermetallics providing dis-
persion hardening and precipitates of composition MgZn 2. Copper provides
further precipitation hardening, forming CuAl 2 and an intermetallic of the
copper–zinc system.
Welding of the hardened high-strength alloys results in a major loss of
strength, the high-strength alloys such as 7022 (AlZn5Mg3Cu) or 7075
(AlZn5.5MgCu1.6) in particular suffering a considerable reduction in
strength.Although almost all of this strength loss can be recovered by a full
heat treatment, the loss in ductility is so great that brittle failure is a real
possibility. The alloys are also very prone to hot cracking and the combi-
nation of these adverse features is such that the high-strength alloys are
rarely welded. Joining techniques such as riveting or adhesive bonding are
often used to avoid these problems.
The lower-strength non-copper-containing alloys such as 7017
(AlZn5Mg2.5Mn0.7), 7020 (AlZn4.5Mg1) and 7039 (AlZn4Mg2.5Mn0.7)
are more readily weldable. The response of these alloys is very similar to
that of the 6XXX series, with a loss of strength in the heat affected zones,
some of which can be recovered by suitable heat treatment. The alloys will
age naturally but it may take up to 30 days for ageing to proceed to com-
pletion. The strength loss in the 7XXX alloys is less than that in the 6XXX
series and this, coupled with the natural ageing characteristic, makes this
alloy a popular choice for structural applications where on-site repair and
maintenance work may be required.
One problem peculiar to the 7XXX series is that the zinc rapidly forms
an oxide during welding, affecting the surface tension of the weld pool and
increasing the risk of lack of fusion defects.This requires the use of welding
procedures in which the welding current is some 10–15% higher than would
be used for a 5XXX alloy. It has also been found to be beneficial to use a
shorter arc than normal so that metal transfer is almost in the globular
range.