This is a common misconception. While it is much more common to encounter arc blow while welding steel, it isn’t uncommon to see it while welding aluminum. If the aluminum isn’t magnetic, how does this happen? Arc blow occurs when a magnetic field interacts with flowing weld current, which causes the arc to deflect. During steel welding, the steel itself can be made magnetic when it comes in contact with magnetic clamps and high magnetic fields. Welding current causes another magnetic field, and any current-carrying conductor is surrounded by a magnetic field. In this case, the current being carried through the arc causes a local magnetic field. We still have the magnetic field caused by the welding current when welding aluminum. But since aluminum isn’t magnetic, what causes the arc to deflect? The answer often is that the welding fixture, which is usually made from steel, has become magnetized over time. The local fields the welding arc sets up can cause this. This magnetic field interacts with the welding current and its associated field, which causes arc blow. To put it into perspective, the naturally occurring magnetic field of the Earth is about 0.5 gauss. A magnetic field of 20 gauss in the vicinity is enough to disturb the welding arc. By the time it gets to about 50 gauss, there is so much arc blow that welding is virtually impossible. On a number of occasions I have seen fixtures that had become so magnetized that the magnetic field at the weld preparation was 150 gauss. Can you remove the magnetic field from the fixture? Maybe. Some companies specialize in degaussing (demagnetizing). However, this might not be practical for large fixtures, so you may need to degauss locally the heavily magnetized areas instead. One tool that is worth having is a hand-held gaussmeter. These are relatively cheap ($100 or so) and will tell you immediately if you are having arc blow problems caused by high magnetization.