Heat-Treatable and Non Heat-Treatable Alloys & the Heat Affected Zone (HAZ)
Aluminum alloys can be separated into one of two groups,
. All aluminum
alloys can be strengthened through cold working operations,
but only the heat-treatable alloys can be strengthened
through thermal processes. Because of this characteristic,
the heat from arc welding processes affects each
Most aluminum alloys are annealed to their softest
condition when they reach approximately 650°F (350°C).
Considering that aluminum melts somewhere in the area
of 1100-1200°F (600-650°C) it can be expected that
portions of the Heat Affected Zone (HAZ) will reach the
annealing temperature during an arc welding process.
This results in the weld assembly enduring a thermal cycle
that can greatly change the properties of the weldment.
The completed weld assembly can be broken up into three
1. Weld Metal
2. Heat Affected Zone (HAZ) adjacent to the weld
3. Base material beyond the HAZ which has been
unaffected by the welding operation.
All of the 2XXX, 6XXX, 7XXX, and a few of the 4XXX series
alloys are heat-treatable. This means that these alloys can
be strengthened, not only through cold working, but also
through thermal processes.
In the case of the heat-treatable alloys, the heat affected
zone presents some interesting dynamics. Although the
HAZ will see the annealing temperature during the arc
welding process, it is often not fully annealed. As a result
of the rate at which the assembly cools, a partial heat
treatment typically occurs, which generally results in
tensile strengths slightly higher than the annealed
strength of the alloy.
However, the strength of the HAZ can also be negatively
affected by the amount of time it spends at the elevated
temperatures encountered during arc welding. This is the
“time at temperature” relationship, which drives the resultant
strength of the weldment. The longer the weldment is at
elevated temperatures, the weaker the HAZ is typically
expected to be. This ”overheating” phenomenon can
actually result in a groove weld tensile strength below
the minimum required.
This group includes all of the 1XXX, 3XXX, 5XXX, and the
remainder of the 4XXX series alloys. These alloys can only
be strengthened through strain hardening. Thermal treatments
cannot be used to strengthen these aluminum alloys.
The heating of the HAZ, which takes place during the arc
welding operation, is sufficient to anneal the base material
within the HAZ area, and return it to the alloy’s softest condition.
Unlike the heat-treatable alloys, non heat-treatable alloys do
not generally require extended amounts of time at elevated
temperatures to become annealed.
Annealing eliminates the strain hardening as well as the
microstructure that is developed as a result of cold working.
For this reason, the minimum tensile strength requirements
for as-welded non heat-treatable alloys, regardless of the
original work hardened temper, is based on the annealed
strength of the base alloy.
There are also a few heat-treatable alloys, which are
considered to be “non-weldable”. It is not that they cannot
be welded, but that they should not be welded if there is
ANY LIABILITY involved with the finished product. Due to
the chemistries of these alloys, arc welding temperatures
produce a microstructure that will lead to grain boundary
cracking and there is nothing that can be done to prevent it.
The two most common alloys in this group are 2024 and
7075, but there are many others.
When designed and welded properly, the tensile strength
of a groove weld will be determined by the HAZ. While the
heat from the arc welding process will affect both groups
of alloys, the reduction in strength in the HAZ will be fairly
predictable in the non heat-treatable alloys. Since the
heat-treatable alloys respond uniquely to the heat, the
resulting HAZ strength can be less predictable. In order
to obtain consistent results, the use of qualified weld
procedures is highly recommended.