If you know welding, then you know, one of the more complicated types of materials to work with is aluminum. Preparation, wire feeding, and shielding gas play a big role in successfully welding with aluminum.
In this blog, we will go through common issues that occur when welding aluminum and how you can overcome them to get the best welds possible.
The most important aspect of welding with aluminum is preparation of the metal. Aluminum oxidizes very quickly and can cause issues with your weld; which is why keeping your base metal clean is crucial.
Before even getting near the base metal with a wire brush, you need to soak and wipe it with solvents to remove any grease and oil. Wire brushing prematurely causes scratches and pits in the base metal where the grease gets trapped and becomes much harder to remove. Start off with a degreasing solvent such as methyl ethyl ketone, acetone, or lacquer thinner. Even using a mild degreasing soap would help in a pinch, although it’s not the best at removing oils. Alcohol and other vapor degreasers produce volatile compounds into the air and most have been prohibited for this use. Using a micro-fiber cloth, thoroughly clean the base metal until you’re sure all of the grease is removed.
Now, use your stainless-steel wire brush to remove the remaining contaminants that may be stuck to the base metal. Continually brush the area where you plan to weld until all the surface oxides are gone. This takes multiple passes, and there’s no such thing as too much wire brush in this case. Any oxidation on the base metal can cause porosity and prevent proper fusion of the weld. Once you’ve wire brushed the faying surfaces clean, you are ready to immediately weld.
Since aluminum oxidizes so quickly, if the time between cleaning, and welding, exceeds a few minutes (varies based on environmental conditions) then you need to perform cleaning procedures again to ensure it’s free of contaminants. If there is black, sooty residue on the base metal after welding, this indicates that there are contaminants either from the gas, base metal, or filler wire.
Equally as important is the preparation of your torch. Make sure that the consumables that you are using are compatible with aluminum welding or you run the risk of wire feed issues or contamination of the weld pool. A common mistake is using a steel wire liner when feeding aluminum wire. As the wire travels through the liner, it will rub and bump into the liner. During these brief encounters the wire could easily pick up small amounts of the steel liner and contaminate the wire.
Aluminum wire comes in many options; the most common ones being 4043 and 5356 filler alloys. Each filler alloy is different, we won’t be getting into those differences here, but typically the 4xxx series aluminum wire is more difficult to feed than 5xxx series wire. Aluminum has a few differences from standard steel that you need to be aware of.
First, is the difference in the coefficient of thermal expansion (CTE) between aluminum and steel. CTE is essentially the amount a material expands as temperature is increased. Aluminum has a much higher CTE at about 24 hundred-thousandths, per 1 degree Celsius while most steel has a CTE of about 13 hundred-thousandths of a millimeter, per 1 degree Celsius. These numbers might seem small and insignificant, but once you heat the metal to welding temperatures, it makes all the difference. To combat this expansion, some manufacturers have specific consumables and liners for aluminum welding. Slightly oversized contact tips, for instance, to leave enough room for the wire to expand without binding up. If the wire gets caught in the contact tip, this could result in burnback.
You’ll see burnback in just about every welding application, but with aluminum it’s especially common due to the oxides on the surface of the base metal. These oxides act as an insulator that stops the flow of electricity from the wire to the base metal. If the wire is still feeding through the torch and happens to makes contact with the base metal, an arc is created. Burnback can destroy your consumables, and possibly your entire torch which is why it is extremely important to make sure your base metal is free of oxides.
Aluminum’s columnar strength is also much lower than steels. It is a weaker, softer metal; which means in most cases you have to use special equipment such as a push-pull torch. These torches have miniature drive rolls in the handle that, in congruence with the wire feeder’s drive rolls, provide slight tension on the wire to keep it from bending or snapping.
The most common, and recommended shielding gas to use when welding aluminum is pure argon gas for materials up to ½” thickness. Once a material exceeds ½” thickness, helium is added to the argon to increase heat and penetration of the arc. The amount of helium varies from 25% all the way up to 75% based on the recommendations.
An important variable to keep track of is the dew point of your shielding gas. Argon that is up to the AWS (American Welding Society) A5.32/5.32M-97 specification, has a dew point of -60 degrees Celsius. As long as your gas supplier follows those specifications, you shouldn’t have problems with the gas straight out of the cylinder. Although, moisture can still find its way into the weld through the gas lines. Inspect your lines to make sure there is no condensation. High moisture levels could allow hydro carbons into the weld, which will create porosity and other imperfections.
Once you set up a welding station capable of welding with aluminum, my advice is to just leave it set up for aluminum and use a separate station for steel (if possible). It will prevent inconsistencies and will allow you to easily switch from steel to aluminum in a matter of a few minutes. Welding with aluminum is a bit more complicated than with steel, but if you follow the guidelines and tips outlined above, you’ll be on your way to aluminum welding perfection.