Selection and Weldability of Non-Heat-Treatable Aluminum Alloys

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:

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 magnesium.

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:

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 back-filling.

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.

Weld Properties

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.