News Sections

Sep 12, 2014

Aluminum Filler Metals: Selection, Characteristics and More

From its strength and versatility to its corrosion resistance, aluminum offers benefits that make it an appealing material for a variety of industries. The material, however, is not without its challenges when it comes to welding. Because of its low melting temperature and high thermal conductivity, extra care must be taken to prevent burn through on thinner material and to ensure adequate fusion or penetration on thicker material. Using the appropriate equipment— machines that offer Pulsed MIG capabilities or TIG capabilities — is important. So too is selecting the right filler metal and understanding the weld characteristics each type provides.

From its strength and versatility to its corrosion
resistance, aluminum offers benefits that make
it an appealing material for a variety of industries.

There are many aluminum filler metals available in the marketplace, including those for applications found in aerospace or architectural structures to filler metals for heat exchangers, trailer fabrication and more. The filler metals range from pure aluminum to varieties with added copper, silicon or magnesium.

Among the variety of aluminum filler metals, 4043 and 5356 alloys are the most common and the least expensive. Together, those filler metals are used for welding about 80 percent of the time and are available in wires for MIG welding or cut-lengths (often called filler rods or straight lengths) for TIG welding. Knowing how to choose between the two alloys, as well as the characteristics each provide is important to producing good weld quality.

Aluminum filler metal characteristics
As with any type of filler metal, 4043 and 5356 aluminum alloys each have unique characteristics.
4043 filler metals tend to have a more fluid weld pool due to the addition of silicon (5 percent), which allows the bead to ‘wet out’ or flow into the base metal more easily. This characteristic produces a more aesthetically pleasing weld requiring less clean up, reduces leaks and minimizes cracking. 

5356 filler metals contain 5 percent magnesium. As with the silicon in a 4043 product, the addition of this alloying element affects the performance of the  filler metal. In this case, the magnesium increases the strength and toughness of the weld. When welding with 5356 aluminum filler metals, the resulting weld tends to be rippled rather than smooth and due to the 5 percent magnesium, more smut (black soot) could be present at the edges of the weld that the welding operator will need to clean.

Both 4043 and 5356 filler metals operate with 100 percent argon shielding gas, as it provides good arc initiation and stability. Thicker aluminum applications sometimes require the addition of helium, which improves heat transfer to the base metal and helps increase weld penetration. Helium can be expensive, though, and the application often sacrifices arc stability when it is added.

How to make the selection    
The goal when selecting aluminum filler metals is to choose an alloy that produces a weld that best meets the requirements of the product and its intended use.  Sometimes the strongest weld is desirable — other times, leak resistance, ductility, corrosion resistance or toughness are more important.

The most common of aluminum filler metals are 4043
and 5356 alloys, both of which are available in wires
for gas metal arc welding (GMAW) or cut-lengths (often
called filler rods) for gas tungsten arc welding (GTAW).

When trying to choose between the common 4043 and 5356 aluminum filler metals, there is a series of five questions to ask to narrow down the selection.

1.    What is the aluminum base designation?
2.    Will the welded component be exposed to sustained elevated temperature?
3.    Will the weldment be subjected to post-weld anodizing?
4.    Will shear strength, ductility and toughness be prime considerations?
5.    Will post-weld heat treatment be performed?

Providing the base metal is compatible, a 4043 aluminum filler metal will be best for the following:

•    Applications that will be subject to long-term elevated temperature exposure (above 150 degrees Fahrenheit).
•    Applications where it is necessary to reduce the risk of termination and shrinkage cracking.
•    Applications where aesthetics are particularly important, since these filler metals provide bright, clean welds.
•    Applications where distortion is of particular concern, as it minimizes this defect.

A 5356 aluminum filler metal would be most appropriate for:

•    Welding on 5XXX or 6XXX series of aluminum when tensile strength is most important.
•    Achieving good anodized color matching on 5XXX and 6XXX base metals.
•    Applications requiring higher ductility and toughness (such as those subject to dynamic loading).
•    Applications requiring higher shear strength.

When welding 6XXX series aluminum or 5XXX series aluminum containing less than 3 percent magnesium, it is possible to use either a 4043 or a 5356 filler metal to complete the job. The same holds true for higher shear strength applications; however, it typically takes three fillet passes with a 4043 product to equal the shear strength of one pass using a 5356 filler metal.

Proper storage and handling

Proper storage and handling of aluminum filler metals is essential to ensuring the highest quality welds. It is important to keep the filler metals in a clean, dry area that is of a similar ambient temperature as the weld cell. Moving them from a cold area to a warmer area could cause condensation to form on the surface of filler metals and lead to poor weld quality.

If a spool of aluminum wire isn’t going to be used cover the spool with the plastic bag the wire shipped in or with another protective cover. TIG cut-lengths are best stored in their original box to protect against dirt and debris.

Getting the best results
In any welding application, the goal is to complete the job with high quality and efficient results. Speed and weld integrity are just as important when welding aluminum as any other material. Knowing the basic filler metal characteristics and selection criteria for applications employing this material is a good first step in getting the best welding performance. Additional practice with welding this material — especially for welding operators new to the material — is also a good idea. Consider welding scrap materials as a starting point to get a firsthand experience of the different operating characteristics of aluminum filler metals, including the 4043 and 5356 alloys discussed here. And when additional information about the aluminum welding process is necessary, look for resources with a local welding distributor or filler metal manufacturer to help. They often have technical support specialists to offer advice.

Additional information: equipment considerations

In addition to proper selection, storage and handling with 4043 and 5356 filler metals — or any aluminum filler metals for that matter — it is important to have the right welding equipment for the job. This equipment includes:

A spool gun or push-pull gun for the MIG welding process. These guns provide consistent feeding of the aluminum filler metals, minimize wire feeding issues and allow for welding further distances from the wire feeder (often as far as 30 feet away).

U-groove drive rolls. These drive rolls prevent the wire from being pinched or compressed, and help ensure proper wire feeding.

Nylon inlet guides and liners. Unlike metal components, nylon components won’t “shave” the wire as it feeds through from the wire feeder to the contact tip, lessening the opportunity for build-up of debris inside the liner.

Contact tips for aluminum wire. Consumables for aluminum wires typically have an “A” (for aluminum) stamped on them and are sized accordingly for these type of wires. For example, 4043 and 5356 wires are available in 0.047 inch (3/64 inch) diameters (as opposed to the standard 0.045-inch diameter of steel wires), so the corresponding bore on the contact tip for these wires is slightly oversized, allowing for smooth wire feeding and good arc stability.