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Aluminum Workshop: Guidelines for preheating T6

Q: We are welding 6061-T6 that is ¼ in. to 1 in. thick. To weld the thicker materials we are preheating to 300 to 600 degrees F. We are not happy with the strength of the welds. Is there something we can do, such as heat treating after welding, to increase the strength of the weld?

A: You can completely reheat-treat after welding (i.e., resolution-treat at 1,060 degrees F and quench and age at 350 degrees F) to restore the T6 properties. However, this is difficult to do and it is limited to small weldments—a motorcycle frame may be the biggest component you can practically do. It is generally too difficult to quench large weldments uniformly.

Unfortunately, however, I think you have completely missed the point and you are actually causing the problem. Why are you preheating at all, much less preheating to such a high temperature? Remember, the final heat treatment to establish the T6 temper is done at 350 to 400 degrees F. If you preheat the aluminum in the range of 300 to 600 degrees F, you have destroyed the T6 properties before you have even struck an arc.

In general, preheat is never required to weld aluminum alloys. If you want to preheat, the temperature should be limited to 200 degrees F maximum. This sort of preheat often is useful to drive off condensation and moisture.

Many fabricators use excessive preheat to compensate for the fact that their welding equipment does not have enough capacity. I can understand this approach—a fabricator has one aluminum project and can’t incur the expense of buying new equipment for one project. However, if you are in the business of fabricating aluminum, you need heavy-duty equipment. For gas metal arc welding (GMAW), you need a power source that will supply at least 300 amps at 60 percent duty cycle. For gas tungsten arc welding (GTAW) ¼-in. material, you need a 300-amp power supply minimum. GTAW on material thicker than ¾ in. requires a 500-amp power supply and 3⁄16-in.-diameter tungsten electrodes. You will find out that you don’t need to preheat and that your welds will be stronger.

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.