Page 40 - Welding of Aluminium and its Alloys

32 The welding of aluminium and its alloys

beads and a higher resistance to hot cracking. In the root pass, however, a
small cross-section weld bead may increase the risk as it will be required to
carry the contractional stresses and restraint.
There is very little that can be done to improve the properties of the weld
metal. Solid solution strengthening can be useful and the selection of the
appropriate filler metal can significantly contribute to a high weld metal
strength.As a general rule the weld metal will match the parent metal prop-
erties only when the parent metal itself is in either the as-cast or annealed
condition. Where cold work has been used to increase the strength of the
parent metal it is not practicable to match these by cold working the weld.
The lower strength in the weld metal must therefore be accepted and com-
pensated for in the design. With some of the precipitation-hardening alloys
a post-weld ageing treatment can be carried out to increase the strength of
the weld metal, provided that the weld metal contains those alloying ele-
ments which will give precipitation hardening as mentioned above. The
effectiveness of this heat treatment will depend upon the filler metal com-
position and dilution. For example, a single pass AC-TIG weld in a 6061
series alloy made with a 4043 filler metal will give an ultimate tensile
2
strength of around 300N/mm in the post-weld aged condition, a multi-pass
2
MIG weld made with a 4043 filler will give approximately 230N/mm .
Changing the 4043 filler to a 4643, which contains only 0.2% of magnesium,
will improve the strength after post-weld ageing to match that of the auto-
genous AC-TIG weld. This is a further example of the importance of the
correct selection of filler metals and the control of consistency during
welding of the aluminium alloys.

2.4.2 Heat affected zone

As mentioned earlier, alloys in the as-cast or annealed condition may be
welded without any significant loss of strength in the HAZ, the strength of
the weldment matching that of the parent metal. Where the alloy has had
its strength enhanced by cold work or precipitation hardening then there
may be a substantial loss of strength in the HAZ.
The cold worked alloys will experience a loss of strength due to recrys-
tallisation in the HAZ. Recrystallisation begins to take place when the tem-
perature in the HAZ exceeds 200°C and progressively increases with full
annealing taking place over 300°C as illustrated in Fig. 2.17. This shows a
1XXX alloy cold worked to different amounts and heat treated at a range
of temperatures, showing how the annealing heat treatment results in a
major loss of strength.The result of this in practice is illustrated in Fig. 2.18
which shows a 5XXX alloy TIG welded.
A similar picture can be seen in the heat-treatable alloys. The situation
here is somewhat more complex than with the work-hardened alloys but