New welders often have the same question: How is welding aluminum different from welding carbon steel? Similarly, veteran welders who have spent the bulk of their careers welding steel might wonder the same thing. No matter who it’s coming from, it’s a valid question.
One of the first things to understand is that while carbon steel is often more tolerant of certain welding practices, that doesn’t mean welding aluminum has to be more difficult. Equally important to understand is that while there are differences in the techniques used for welding the two materials, there are also differences in the ways to prepare for welding each.
To help dispel misconceptions, it’s key to learn about the important differences between aluminum and carbon steel. And, of course, any proven tips that can be implemented into the welding process will ultimately optimize results.
While some carbon steel applications allow a welder to weld over rust or dirt on the base material, aluminum must be cleaned prior to welding to produce good results. To do so, make sure the aluminum base material is dry. From there, degrease it with a clean towel and remove the oxide layer on the material’s surface with a stainless steel brush.
If these steps are not taken and the welder attempts to weld over an oxide layer, porosity will result. Additionally, be sure to clean the material where the weld joints come together to further avoid porosity and a lack of fusion.
Weld prep is clearly an important part of the process to address, but proper storage also plays a central role when it comes to aluminum materials and filler metals.
To properly store materials and filler metals, monitor the humidity levels where they are stored and keep the material dry. Store aluminum material vertically, so humidity doesn’t trap moisture between pieces.
The more a welder follows proper storage procedures with aluminum, the less work is required to clean and prep it for welding when the time comes. While proper storage is important with carbon steel, it’s even more critical for aluminum.
The causes of porosity with aluminum are quite different than with carbon steel. In regard to aluminum, the material’s oxide layer is the main cause for porosity. Aluminum’s 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 the presence of 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.
The physical properties of aluminum and carbon steel dictate how the two materials handle heat. 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.
A common mistake with aluminum is using undersized filler metal wire. The wire needs to be large enough to carry the higher amperages to overcome heat sinking. While the welder may be able to use 0.035-in. wire on 1/4-in. 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 welding 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 metal, which increases costs and potentially increases distortion. The American Welding Society (AWS) D1.2 Structural Aluminum Welding Code provides some recommended joint designs for aluminum.
Feeding aluminum wire through the welding gun is often compared to pushing a wet noodle up a hill. Aluminum is a softer material, so it requires the right combination of gun, wire feeder and consumables to achieve success.
For best results, use a push-pull aluminum feeder system or a spool gun, both of which allow the aluminum wire to be fed over a long distance without buckling. Also, operators typically use U-groove drive rolls with aluminum.
Because aluminum is very susceptible to centerline 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 wire. Using either type 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.
Successfully welding aluminum requires understanding how the material differs from steel. Following some common tips and best practices can help produce the best results.