Aluminum Welding 101

January 2025

In industrial applications and construction, the bulk of aluminum fabrication utilizes either gas metal arc welding (GMAW) or gas tungsten arc welding (GTAW). However, choosing the best welding process to meet your application and code requirements can be challenging. While GMAW is often the best choice for heat control, GTAW offers incredible control of the deposited weld size, shape, joint penetration, and root reinforcement. 

 

Advances and Challenges 

Common procedure when welding aluminum with GTAW is to use a water-cooled torch with a zirconiated, ceriated, or pure tungsten electrode. Aluminum welding has seen great advances in recent years, with inverter technology becoming commonplace, thus increasing the ability to use standard-size electrodes over a much wider range of base material thicknesses. The ability to change settings such as AC frequency and balance (the relation of direct current electrode negative [DCEN] penetration to direct current electrode positive cleaning) offers greater influence over the welding conditions. 

 

WD Jan 25 - Aluminum Welding 10 - Photo 1.jpeg
Common procedure when welding aluminum with GTAW is to use a water-cooled torch with a zirconiated, ceriated, or pure tungsten electrode. 

 

While inverters have their benefits, older technologies such as DCEN with 100% helium shielding gas should not be overlooked. An increase in the use of magnetic resonance imaging machines, for example, has caused the cost of helium to skyrocket. Despite the higher price tag, helium has a higher ionization potential, which increases the energy of the arc, resulting in better fusion, increased penetration, and faster travel speeds. With the increased arc energy, it is also possible to make smaller welds with less heat input and penetration, limiting the incorporation of discontinuities already present in the base metal into the welds. We often see this particular application in vacuum-quality welds on aluminum vessels.  

Meeting the requirements of vacuum service, which will be inspected with a helium spectrometer (helium leak detection test), is highly complicated. Helium is the second smallest atom and will leak in what would normally be a high-quality GTAW weld. Minor porosity, too small for an air molecule to fit through, will seep helium at a high rate.  

With porosity a major problem, proper preparation of the base material and filler is essential. A stainless-steel wire brush, used specifically for aluminum to avoid depositing unwanted particles, should be used to clean the weld zone and at least ½ in. on both sides of the area to be welded. Brushing should be followed by wiping the area with acetone. Filler metal should also be scrubbed with an abrasive, such as a Scotch-Brite pad, then acetone wiped. Using a larger diameter filler reduces the ratio of surface area to deposited aluminum, which reduces porosity as well. Using high-quality filler metal cut to size rather than extruded is another way to lessen oxides in the weld area. 

Aluminum welding also has unique safety concerns. In addition to all the normal arc welding requirements, consider the following when choosing personal protection equipment: 

  • With aluminum welding, the UV is increased due to the reflectivity of the arc off the base metal. 
  • Fillers containing silica will increase UV as well. 
  • Extra care should be taken to minimize radiation burns and protect exposed skin.  

Making the average-quality, partial-penetration, decorative-type aluminum welds with GTAW is easy and fun. Meeting code requirements and making complete joint penetration welds requires skill, attention to detail, and patience.  

Heat-treated aluminum alloys require specific heat control, especially lower interpass temperatures of not over 400°F. If these alloys are allowed to cool from liquidus too slowly and dwell at elevated temperatures, they lose the benefits of their heat treatment, such as their tensile strength. This can cause significant issues when trying to qualify for a welding procedure. Due to GTAW’s slower travel speeds, joint design, weld bead size, and travel speed, torch and filler manipulation must be taken into careful consideration to control heat input. 

In repairs of the weld surface or defect removals, mechanical means are most often used. Excavations must be carefully controlled to preserve the original joint design, including bevel angles, according to the welding procedure. Grinding with abrasive wheels often leaves grit deposits in the soft aluminum weld zone. The weld area can be shaped using air tools with various carbide burrs or bits. While this method prevents the grit deposits, oil becomes a problem. The air tool must be oiled to keep it working, and the grinding bits clog up when the flutes are filled with aluminum. To prevent this, the bits are dipped in WD-40 or a similar lubricant. Post-excavation cleaning must be very carefully done to ensure all oils have been removed. Remember, when the welder says, “I will burn the small problem out,” this is translated to mean, “I will fail this radiograph.”

 

In conclusion 

To accomplish code-quality welds, especially welds in high-vacuum applications being checked by a helium mass spectrometer, attention to proper welding quality details must be carefully adhered to. Small, low-heat input welds, limited penetration, good bead shape, and especially adequate gas shielding of the liquid weld pool and filler rod must be assured. 

 

This article was written by James E. Greer (a welding professor who has taught the American Welding Society workshop, Arc Welding of Aluminum for Code Related Applications, is a Senior Certified Welding Inspector, and past president of the American Welding Society) for the American Welding Society. 

 

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