How is welding aluminum different than steel?
How is welding aluminum different from welding carbon steel? The two materials differ in many ways that affect how you prepare for welding and the techniques used during welding.
Carbon steel is often more tolerant of certain practices, but that doesn’t mean welding aluminum has to be more difficult. Learn more about important differences between aluminum and carbon steel — and get tips to optimize results.
1. Cleaning, weld prep and storage
While some carbon steel applications will allow you to weld over rust or dirt on the material, aluminum must be cleaned before welding to produce good results.
Make sure the aluminum base material is dry, degrease it with a clean towel and remove the oxide layer on the material’s surface with a stainless steel brush. Attempting to weld over the oxide layer will result in porosity. Also be sure to clean the material where the weld joints come together to avoid porosity or lack of fusion.
Proper storage plays an important role with aluminum base materials and filler metals. Monitor the humidity levels where aluminum is stored and keep the material dry. Store aluminum base material vertically, so the humidity doesn’t trap moisture between pieces. The more you follow proper storage procedures with aluminum, the less work you have to put into cleaning it when you want to weld it. While proper storage is important with carbon steel, it’s even more critical for aluminum.
2. Issues with porosity
The causes of porosity are very different with aluminum than with carbon steel.
The material’s oxide layer is the main reason for porosity on aluminum. That hydrated oxide layer absorbs moisture, which holds hydrogen that results in porosity. A weld puddle that freezes faster with aluminum — trapping hydrogen in the weld — is another main cause of porosity in the weld.
When welding aluminum, set the heat high enough so the puddle stays fluid longer, allowing the hydrogen to escape. Using a helium-blend shielding gas can also help reduce hydrogen.
With carbon steel, porosity occurs because of the presence of carbon monoxide and nitrogen in the weld. To combat this, choose steel filler metals that include deoxidizers, which help tie up the oxygen and keep it from forming porosity.
3. Heat sinking
The physical properties of aluminum and carbon steel mean the two materials handle heat much differently.
Aluminum pulls heat faster and wants to distribute it throughout the part, while carbon steel doesn’t distribute heat as fast. This makes aluminum less forgiving to errors (such as using too much heat) during welding, especially on very thin materials.
One common mistake with aluminum is using undersized wire. You need a wire that’s large enough to carry the higher amperages to overcome heat sinking. While you may be able to use .035-inch wire on 1/4-inch steel, that size of wire may not be large enough for the same thickness of aluminum.
Proper joint angles are also critical with aluminum to overcome heat sinking issues. Included angles on joints for groove welds are typically larger than steel. Groove weld joint angles should be large enough to accommodate the needed amount of filler metal required to reach the needed amperage for proper fusion in the MIG process. Groove angles that are too tight can cause bead profile problems as well as lack of fusion defects. Groove angles that are too large can require excessive amounts of filler, which increases costs and potentially increases distortion. American Welding Society (AWS) D1.2 Structural Aluminum Welding Code provides some recommended joint designs for aluminum.
4. Feeding differences
Feeding aluminum filler metal wire through the welding gun is often compared to pushing a wet noodle. Aluminum is a softer material, so it requires the right combination of gun, feeder and consumables to achieve success.
Use a push-pull aluminum feeder system or a spool gun, both of which allow you to feed aluminum wire over a long distance without it buckling. Also, operators typically use U-groove drive rolls with aluminum.
Because aluminum is very susceptible to center-line cracking from crater cracks, look for a feeder that offers a hot start and crater finish. These features help prevent cracking in aluminum welds.
It’s typically recommended to use smooth V and knurled V drive rolls (depending on the type of wire) when welding steel. Smooth V drive rolls are often used for solid wire, while knurled V drive rolls are used for metal-cored or flux-cored wires. Using either of those types of drive rolls on aluminum can result in excessive deformation and shaving of the wire.
Over-pressuring the drive rolls can cause shaving and feed problems in steel as well. Drive roll pressure should be just tight enough to curl the wire against a nonconductive surface into circles about the size of a cantaloupe without the wire slipping in the drive rolls.