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Aluminum Workshop: Mg content in your aluminum 5XXX alloy

This month’s topic doesn’t deal specifically with welding aluminum, but I think you will agree it has important implications in the design and fabrication of aluminum structures. This topic and its associated problems have come up in conversations twice in the past two months. It deals with the postweld cracking of 5083 after long periods of service at elevated temperatures above 150 degrees F. One of the failures I am not allowed to discuss. However, the other one I can discuss freely.

I had a phone call recently from the owner of a large aluminum hulled yacht. The hull plates in most large boats or ships are fabricated using 5083 AlMg alloy because of its high strength and excellent salt water corrosion properties. However, the owner reported long longitudinal cracks in the hull at the stern near the transom. It didn’t take long to find the cause.

It seems that the designer of the ship had decided to run the exhaust pipes from the diesel engines longitudinally along the hull to the transom. The pipes were actually touching the interior surface of the 5083 hull. The 5083 was reaching local service temperatures of around 300 degrees F in those areas. Why should that be a problem?

The answer is that 5083 contains 5 percent Mg. It is a well-known fact that AlMg alloys containing more than 3 percent Mg can be sensitized (i.e., made susceptible) to stress corrosion cracking if they are exposed to service temperatures over 150 degrees F—yes Fahrenheit, not Celsius—for long periods of time. In other words, this application was a perfect recipe for stress corrosion cracking.

To fix the problem we patched the hull and moved the exhaust pipes so they were about 12 in. from the hull.

Welding and Aluminum 5XXX Alloys

What does this have to do with welding? It’s important to remember that all of the 5XXX alloys containing more than 3 percent Mg are susceptible to stress corrosion cracking, including 5356, 5556, and 5183, which are the three most common 5XXX fillers. If the welds are subjected to elevated temperature service for long periods of time, the welds can be subject to stress corrosion cracking, even if the parent material—6061, for example—isn’t .

Be careful when choosing both the base material and filler metal in such cases. Don’t just assume that 5356 is always the best choice for a filler metal. It often is, but stop to consider the application before you jump to conclusions.

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.