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Aluminum Workshop: Don’t know the alloy? Make an educated guess

Q: I need to repair an aluminum casting, but I have no idea what specific alloy it is. What filler metal should I choose?

A: You can’t tell what an aluminum alloy is just by looking at it. Realistically, you’ll probably have to choose the filler alloy without being sure which casting alloy you have. How do you do that? I’ll explain how to make an educated guess, which should be right most of the time.

If you remember, the casting alloys have a three-digit designation system, such as A356. Casting alloys fall into one of six families—1XX (pure Al), 2XX (AlCu), 3XX (AlMgSi), 4XX (AlSi), 5XX (AlMg), and 7XX (AlZn). How can we pick a filler alloy? Well, if they were equally common, it would be very difficult, but they aren’t. Let’s see how and where they are used.

  • 1XX. These castings are not widely used because their pure aluminum content makes them too weak to be useful structurally.
  • 2XX and 7XX. Both of these castings are very strong alloys and are fairly uncommon. For the same reason that the 2XXX and 7XXX wrought alloys are mostly unweldable, these too are unweldable because their chemistry makes them prone to cracking. The alloys that are weldable, such as A222 and A240, should be welded with 4043. So if you try 4043 filler and the casting is one of these alloys, you may get lucky and produce a good weld. If you are trying to weld an unweldable alloy, the weld will crack regardless of what filler alloy you use.
  • 3XX. These are the most common casting alloys and are among the easiest to weld. A356-T6 is the most common, followed closely by A319-T6. If you have an automotive block or cylinder head casting, chances are it is one of these two alloys. All 3XX alloys should be welded using 4043.
  • 4XX. These are not very common. However, they should be welded using 4043.
  • 5XX. Although not as common as 3XX, 5XX castings should be welded with 5356.

Have you figured it out yet? Using 4043 is the best recommendation for welding aluminum castings of an unknown alloy. If the casting cracks, you probably have an unweldable alloy and there’s nothing you can do about it. Even if you use 4043 on a 5XX casting such as B535, you won’t do any real harm. The weld will contain excess magnesium silicide, which could reduce the toughness, but it will not fail catastrophically. However, in the great majority of cases, 4043 will be the best filler metal choice for your application.

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.