July 10, 2007
Q: I have some aluminum sheet that I would like to use to fabricate a project. The only marking on it is "Alclad." Other than that, I have no idea what alloy it is. Can you tell me what filler metal to use to weld it?
A: Whether you realize it or not, you have seen Alclad sheet hundreds of times. The fuselage skin and wings of every commercial airplane you have ever seen are made from Alclad sheet. Alclad is a thin layer of pure aluminum that is roll-bonded onto a thicker sheet of one of the structural aluminum alloys. The purpose of the pure aluminum layer is to give improved atmospheric corrosion resistance, which is obviously important in airplanes. However, the pure aluminum isn't strong enough to use in structural fabrication, so it is bonded to a higher-strength structural alloy.
Can you weld Alclad? Well, it depends. You haven't given a complete description of the sheet, which would normally include a designation showing which structural alloy is used under the pure aluminum. Alclad is made using many structural alloys. Some of them, such as 6061, are weldable. However, many of them, such as 2024 and 7075, are not. (Remember, airplanes are riveted, not welded. There's a good reason for that.) So without knowing which structural alloy has been used, I can't tell you whether you can weld it or not. Try to find the alloy designation on the sheet. If it is 6061, then you can weld it just like you weld 6061. If it is 2024, 7075, or one of the other 2XXX or 7XXX alloys, you should not weld it. Unfortunately, if I had to guess, I would say the structural alloy is probably 2024, since this is the most common alloy used to make Alclad sheet. So I would be very careful about using this sheet for any structural fabrication.
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