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19

Technical Services

Heat-Treatable and Non Heat-Treatable Alloys & the Heat Affected Zone (HAZ)

Aluminum alloys can be separated into one of two groups,

heat-treatable

and

non heat-treatable

. 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

group differently.

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

distinct areas:

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.

Heat Treatable

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.

Non Heat-Treatable

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.

Non-Weldable Alloys

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.

Summary

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.