Aluminum Workshop: Sizing up feedability and conductivity in contact tips

Q: There seems to be a lot of controversy about what size contact tip to use for GMAW aluminum. Can you give me any advice?

A: The contact tip has two functions. First, it provides guidance for the welding wire. Second, it provides a means of conducting welding current from the welding torch into the filler wire. Both of these functions are very important.

Guiding the Welding Wire. Obviously, you can’t smoothly feed a 0.062-in.-diameter wire through a contact tip with a 0.062-in.- diameter hole bored into it. The wire just won’t fit and feed smoothly. To remedy this, manufacturers make the hole in the contact tip a bit bigger. How big? Well, if the only purpose of the tip was wire guidance, you would make it a lot bigger, say, 0.090 in.

Conducting the Welding Current. But guiding the wire isn’t the only function of the contact tip. If the hole in the contact tip is too big, it is difficult to transfer current uniformly to the filler wire because it bounces around as it goes through the tip. To achieve the appropriate current transfer, the hole diameter should be as small as possible while still allowing smooth feeding.

So what’s the right size? In my experience, the diameter of the hole in the tip should be about 0.008 in. bigger than the wire diameter. This lets the wire feed smoothly and still provides good electrical contact.

One of the most common mistakes welders make is using contact tips with holes that are too big. I’ve seen many people GMAW aluminum using 3⁄64-in. wire and a 1⁄16-in. contact tip. This is not recommended. While it may make wire feeding easy, it also makes for erratic current transfer in the tip, which causes arcing in the tip and frequent burnbacks. This, by the way, is the most common cause of excessive burnbacks.

So what’s the solution? Use the correct contact tip size. Also, the cast of the GMAW wire acts like a spring and forces the wire into good contact with the inside diameter of the hole in the tip, giving more consistent electrical contact. Don’t remove all the wire cast with a wire straightener. Leave some of it in; it’s good for you.