The Weldability of Aluminum

Aluminum and its alloys are popular metals because of their low weight, good corrosion resistance, and weldability. Although they typically possess low strength, certain alloys can have mechanical properties similar to steels. Aluminum alloys can be joined by a wide range of methods. But they also possess several properties that require understanding depending on which method you are using. Therefore, the first step to successful aluminum welding is to familiarize yourself with the various aluminum alloys, their properties, and the considerations in choosing filler metal for each.

Heat-Treatable vs Non Heat-Treatable Aluminum Alloys Charts

Here is a helpful breakdown of the weldability of aluminum alloys in heat-treatable and non-heat-treatable variants and which fillers the base metals are best paired with:

Non Heat-Treatable Aluminum Alloys

For non-heat-treatable alloys, the material strength of alloys depends on the effect of work hardening and solid solution hardening of alloy elements such as magnesium and manganese. These are mainly found in the 1000, 3000 and 5000 series aluminum alloys. When welded, these alloys may lose the effects of work hardening and cause softening of the heat affected zone adjacent to the weld.

Heat-Treatable Aluminum Alloys

The material hardness and strength of heat-treatable alloys depend on their composition and the heat treatment. The main alloying elements of these materials are defined in the 2000, 6000 and 7000 series aluminum alloys. Note that when fusion welding heat-treatable alloys, the hardening constituents in the heat affected zone (HAZ) is redistributed and results in a reduction in material strength in the local area.

Processes

TIG (Tungsten Inert Gas), MIG (Metal Inert Gas), and oxyfuel processes are suitable processes for fusion welding most of the wrought grades in the 1XXX, 3XXX,5XXX, and 6XXX series; the 5XXX alloys in particular have excellent weldability. These processes are also well suited for medium strength 7XXX series alloys. We don’t recommend fusion welding high strength alloys, such as 7010, 7050, and a majority of the 2XXX alloys, because they are prone to liquation and solidification (see Common Aluminum Welding Defects below). The Friction Stir Welding technique is particularly suited for producing sound welds in aluminum alloys. This technique is an excellent choice for heat-treatable alloys which are prone to hot cracking.

Common Aluminum Welding Defects

The weldability of Aluminum and its alloys is good if you take appropriate precautions. However, it is important to know the defects that may occur and how to avoid them. The most common defects in fusion welds are:

Porosity

Aluminum is one of the metals most susceptible to porosity. Porosity is caused when hydrogen gas gets trapped in the weld pool as the metal cools. Hydrogen becomes present from either water vapor or hydrocarbon contamination through oils, greases, lubricants, and solvents. While the weld metal is in the molten state, it absorbs a high amount of hydrogen. Then as it solidifies, it tries to expel the hydrogen. However, if the weld is solidifying even moderately quickly, the hydrogen doesn’t have a chance to escape and instead stays behind and forms small pores within the weld. To minimize the likelihood of porosity from hydrocarbon contamination, thorough cleaning of material surface and filler wire should be performed. Here are some appropriate techniques:

Mechanical cleaning: After degreasing, use wire brushing, scraping, to remove impurities on the surface of the metal. Solvents: Organic solvents can be used to remove grease, oil, dirt, and loose particles. Dipping, spraying, or wiping the metals with solvents before welding can help remove any contaminants and impurities. Chemical etching: Use a solution of 5% sodium hydroxide to clean the metals, followed by a rise with NO3 and water to remove reaction products on the surface.

Solidification Cracking

Because aluminum alloys experience high thermal expansion and substantial contraction upon solidification, they are also susceptible to cracking. Typically, these cracks occur along the center line of the weld. It’s mainly caused due to an incorrect filler and parent metal combination, incorrect weld geometry, or when welding under high restraint conditions. Furthermore, impurities like sulfur and phosphorus are a major factor since these elements separate during solidification. Therefore, it is important to remove the oil or grease contamination from the weld area before welding. Also, metals with a low melting point, such as copper, tin, lead, and zinc, should also be avoided. Risk of solidification cracking can be further reduced by using a non-matching, crack-resistant filler such as those from the 4XXX and 5XXX series alloys.

Liquidation Cracking

Heat treatable alloys, particularly the 6XXX and 7XXX alloys, are more prone to liquation cracking. This type of cracking results from localized melting at grain boundaries of the heat affected zone, combined with the inability to withstand the contraction strains as the weld metal cools. But the risk of liquation cracking can be reduced by using a filler metal with a lower melting temperature than the parent metal.

Weld Bead Profile Imperfections

Aluminum’s high thermal conductivity and the quick solidifying weld pool makes its alloys particularly susceptible to profile imperfections on the weld bead. To combat the risk of inconsistency, undercut, and insufficient fusion or penetration, ensure you are using the correct parameters and technique for the weld.