Aluminum Etc.: How to seal aluminum joint welds

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"Is it always necessary to drill a hole for gas expansion when sealing or capping an aluminum weld like we do when welding steel?"

The short answer is no. But, and there is always a but, knowing what is happening with your weld will help you to make good decisions about your project.

It's important to remember that cosmetic aluminum welds need to be visually appealing and have structural integrity but are not coded as structural or high-pressure vessels. Structural, high-pressure, and nuclear welds follow a strict weld procedure specification; you must be qualified on those welds by a CWI; and the welds will need to undergo nondestructive testing to ensure they are sound.

When sealing your aluminum weld, be sure to avoid the following:

The gummy weld at the end of a bead is caused by gas expansion while it is still inside your weldment. If you are in a hurry, drill a small hole into the weldment to allow for the gas to escape. You can come back and seal it up once everything has cooled down and the gas has dissipated. You should be able to seal without a problem, as long as doing so doesn't raise the temperature of your weld significantly.

Another way to prevent gas expansion is to deliberately stop your weld short by ½ in. on the closing weld. Next, give the weldment a chance to cool down and the gas to escape. Walk away and start your next project, or maybe have lunch. Once everything has cooled down, clean the area to be welded to reduce or eliminate the oxide layer that most likely has formed during the cooling process. Finally, complete your weld bead.