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Aluminum Workshop: What not to do when excessive contact tip burnback occurs

Q: Recently I’ve had a lot of problems with my contact tip repeatedly burning back during welding. Sometimes I go through eight to 10 contact tips per day. I’m feeding 3/64 -n. 5356 wire, and my thinking is that if I use an oversized contact tip, I could reduce the occurrence of burnbacks. Do you think this is a good idea?

A: This is a very bad idea. You must remember that a contact has two different and competing purposes. First, we must be able to feed wire of the proper diameter smoothly through the hole in the contact tip. Of course, you can’t feed 0.0475-in.-dia. wire through a 0.0475-in.-dia. hole, so manufacturers do bore the diameter of the hole slightly larger than the wire diameter. While it is true that an even larger hole would allow for smoother wire feeding, it would make it difficult for the contact to achieve its second purpose, which is to transfer the welding current reliably and consistently to the weld filler wire. That requires us to have good electrical contact between the tip and the wire, which means we want the hole to be as small as possible.

If you use oversize tips, the contact between the tip and the wire is sporadic and nonuniform, resulting in more burnback, not less. So what size should the bore in a contact tip be? To be honest, you can find a lot of differing opinions on this subject. In my opinion, the contact tip should be 0.007 in. to 0.008 in. larger than the nominal wire diameter. Another school of thought dictates that tips for 5356 should have a slightly larger bore diameter than those for 4043, since the coefficient of thermal expansion is slightly higher for 5356 (13.4) than for 4043 (12.3). In other words, there is no real hard and fast number. My own recommendation is that there be no more than 0.010-in. clearance between the tip bore diameter and the wire diameter.

A lot of other factors can lead to excessive burnback. I don’t have space to address them in this column, but I will plan to address them in the next issue.

About the Author
Aluminum Consulting Inc.

Frank Armao

President

Aluminum Consulting Inc.

440-479-0239

Frank Armao was an active member of the AWS D1 Committee, chairman of the AWS D1 Aluminum Subcommittee, and member of the Aluminum Association Committee on Welding and Joining. He also was the author of The WELDER's "Aluminum Workshop" column from 2001 to 2020.