Non-heat-treatable aluminum alloys constitute a group of alloys that rely solely upon cold work and solid solution strengthening for their strength properties. They differ from heat-treatable alloys in that they are incapable of forming second-phase precipitates for improved strength. Therefore, non-heat-treatable alloys cannot achieve the high strengths characteristics of precipitation-hardened alloys.
The absence of precipitate-forming elements in these low- to moderate-strength non-heat-treatable alloys becomes a positive attribute when considering weldability, because many of the alloy additions needed for precipitation hardening (for example, copper plus magnesium, or magnesium plus silicon) can lead to liquation or hot cracking during welding.
Non-heat-treatable aluminum alloys constitute a group of alloys that rely
solely upon cold work and solid solution strengthening for their strength
properties. They differ from heat-treatable alloys in that they are
incapable of forming second-phase precipitates for improved strength.
Therefore, non-heat-treatable alloys cannot achieve the high strengths
characteristics of precipitation-hardened alloys.
The absence of precipitate-forming elements in these low- to
moderate-strength non-heat-treatable alloys becomes a positive attribute
when considering weldability, because many of the alloy additions needed
for precipitation hardening (for example, copper plus magnesium, or
magnesium plus silicon) can lead to liquation or hot cracking during
welding. In addition, joint efficiencies are higher in not-heat treatable
alloys because the heat-affected zone (HAZ) is not compromised by the
coarsening or dissolution of precipitates. This obviates the need for
thick joint lands or postweld heat treatment and favors the use of welded
structures in the as-welded condition.
Alloy Classification and Typical Applications
Non-heat-treatable wrought aluminum alloys can be placed into one of four
groups using standard Aluminum Association designations:
Al (99% minimum purity)
Al - Mn
Al - Si
Al - Mg
The 1xxx-series alloys are of commercial purity (>99% Al) and are used
where thermal/electrical conduction or corrosion resistance becomes
paramount over strength in design considerations (for example, alloy 1100
is used for sheet metal work, fine stock and chemical equipment). Alloys
with purity levels greater than 99,5% are used for electrical conductors
(for example alloy 1350).
The 3xxx-series alloys are used in applications where added additional
strength and formability are needed, in addition to maintaining excellent
corrosion resistance (for example alloy 3004 is used for sheet metal work,
storage tanks, and beverage containers). Typical applications include
cooking utensils, pressure vessels, and building products (siding, gutters
and so on). These alloys get their strength from cold work and fine
(Mn, Fe)Al6 dispersoids that pin grain and subgrain boundaries. There is
also a small degree of solid solution strengthening from both manganese and
4xxx Alloys. Apart from their use as welding filler material, the
4xxx-series alloys have limited industrial application in wrought form.
The 5xxx-series alloys are used in cases where still higher
strengths are required. This strength is achieved from large quantities
of magnesium in solid solution. More importantly, magnesium promotes work
hardening by lowering the stacking fault energy, thus reducing the
tendency for dynamic recovery. Applications for 5xxx-series alloys include
automobile and appliance trim, pressure vessels, armor plate, and
components for marine and cryogenic service.
While these alloys normally exhibit good corrosion resistance, care must be
taken during processing to avoid formation of continuos b-Mg3Al2
precipitates at grain boundaries, which can lead to intergranular
corrosion. This can occur in heavily cold-worked, high-magnesium alloys
exposed to temperatures from 120 to 200oC. Alloy 5454 possesses the highest
magnesium content suitable for sustained elevated temperatures and has
become the standard alloy used for truck bodies for hot oil or asphalt
applications, and storage tanks for heated products.
Filler Alloy Selection
Filler alloys used to join non-heat-treatable alloys can be selected from
one of three alloy groups:
Al (99% minimum purity)
Al - Si
Al - Mg
Commonly used filler alloys include 1100, 1188, 4043, 4047, 5554, 5654,
5183, 5356 and 5556. Selecting the best filler alloy for a given
application depends on the desired performance related to weldability,
strength, ductility and corrosion resistance.
In general, the filler alloy selected should be similar in composition to
the base metal alloy. Thus, a 1xxx filler alloy is recommended for joining
1xxx- or 3xxx-series base metal alloys. An exception to this rule is
encountered when weldability becomes an issue. Weldability of
non-heat-treatable aluminum alloys can be measured in terms of resistance to
hot cracking and porosity.
Hot cracking. Problems with hot cracking are encountered when
welding under highly constrained conditions or when welding certain alloys
that are highly susceptible to cracking. Similar problems may be
encountered when 1xxx fillers are used to join 5xxx alloys (or vice versa)
or when welding dissimilar metal alloys such as alloys 1100 and 5083, where
mutual dilution may result in low magnesium levels. Electron-beam welding
or laser-beam welding can also result in cracking when magnesium, a
high-vapor-pressure alloying element, is boiled off. The problem is
aggravated when welding in a vacuum environment.
Another approach to be taken when hot cracking persists is to use 4xxx
fillers. These aluminum-silicon alloys have exceptional resistance to
cracking, due in part to their abundance of liquid eutectic available for
Porosity. Non-heat treatable aluminum alloys are susceptible to
hydrogen-induced weld metal porosity, as are all aluminum alloys in general.
This porosity forms during solidification due to the abrupt drop in hydrogen
solubility when going from liquid to solid. Porosity can best be avoided by
minimizing hydrogen pickup during welding.
When non-heat-treatable alloys are welded, microstructural damage is
incurred in the HAZ. Unlike the case of heat-treatable alloys, whose
strengthening precipitates may dissolve or coarsen, the HAZ damage in
non-heat-treatable alloys is limited to recovery, recrystallization and
grain growth. Thus, loss in strength in the HAZ is not nearly as severe as
that experienced in heat-treatable alloys. For this reason, 5xxx-series
alloys are popular for use in welded pressure vessels where reasonable
joint strengths can be obtained in the as-welded condition without the
need for post-weld heat treatment.
The weld metal of non-heat-treatable aluminum alloys is typically the
weakest part of the joint and is the location of failure when the joint is
loaded. This is in contrast to most heat-treatable aluminum alloys, where
the heat-affected zone often is the weakest link. The weld metal
microstructure of the non-heat-treatable alloys consists of
columnar-dendritic substructure that has interdendritic eutectic
constituents - primarily (Fe, Mn)Al6, for 1xxx and 3xxx alloys; and Mg3Al2
for 5xxx alloys.
An important application for alloy 5083 is the construction of tactical
military vehicles. The hulls and turrets of vehicles such as the M113
armored personnel carrier, the M2/M3 infantry and cavalry fighting vehicles,
the M109 self-propelled howitzer, and AAV7A amphibians all consists of
welded 5083 aluminum structures. There are also a myriad of brackets, clips
and so on, welded to the hulls and turrets, although not normally
fabricated to ballistic requirements.