Page 39 - Welding of Aluminium and its Alloys

Welding metallurgy 31

also reduce the size of the HAZ and consequently the shrinkage stresses
across the joint.
• Use high-speed, small-volume multi-run procedures instead of large
volume, single run deposits.
• Select welding and assembly sequences that minimise restraint and
residual stresses.
• Apply an external force to maintain the weld in compression while it is
in the hot short range.
• Select a filler metal with a melting point close to that of the parent metal,
see Appendixes C and D.

2.4 Strength loss due to welding

In order to effect a weld the components to be joined are heated to a high
temperature, in the case of fusion welding above the melting point of the
parent metals, and brought together to enable the components to coalesce.
The heat of the welding operation is conducted into the parent metal such
that in any welded joint there are three distinct areas – the weld metal in
a fusion welded joint, the HAZ in the parent material and the unaffected
parent metal. The HAZ may be further subdivided into areas with partic-
ular properties depending upon the alloy system involved. Since the HAZ
will have experienced one or more cycles of heating and cooling the prop-
erties may be radically different from those of the unaffected parent metal.
This is particularly the case with those aluminium alloys that have been
strengthened by either cold working or precipitation hardening. One aspect
of this is the width of the HAZ, a function of the high thermal conductiv-
ity of aluminium and the consequent size of the area where there has been
a substantial loss of strength.Only when the alloy is in the as-cast or annealed
condition will the properties of the HAZ match those of the parent metal.

2.4.1 Weld metal
In a fusion weld the weld metal is an as-cast structure consisting of a
mixture of the filler metal, if added, and the parent metal(s). The proper-
ties of this weld depend upon the composition, the quality and the grain
size of the deposit.These in their turn depend on the parent and filler metal
compositions, the amount of dilution, the quality of the welding process and
the welder and, lastly, the rate of solidification. With the exception of a
couple of 2XXX filler wires most filler metals available are not capable of
being age hardened, although dilution with parent metal may enable some
age hardening to take place. Fast solidification rates will give a finer grain
size and hence better mechanical properties than slow solidification rates.
Small weld beads therefore generally have better properties than large weld