by Galen White
Challenging material needs the right filler for effective welds
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.
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 per cent 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 per cent), 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 per cent 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 per cent 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 per cent 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.
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. SMT
Galen White is senior welding engineer, Hobart Brothers Co., Troy, OH