Aluminum Workshop: Busting the universal filler myth

Q: My friend told me that he never worries about what grade of aluminum alloy he is welding on. He always uses 1100 filler. He says since 1100 is essentially pure aluminum and doesn’t contain any alloying elements, that you can use it to weld any aluminum alloy. Is this true? It would eliminate a lot of confusion.

A: It would certainly make life easier if you could use 1100 filler to weld any aluminum alloy. Unfortunately, your friend is absolutely wrong. You should use 1100 aluminum filler to weld only 1100 and other 1XXX aluminum base materials. It makes a very poor filler metal for any other aluminum alloy and can cause cracking in some alloys.

The first reason you shouldn’t use it as a universal filler metal is strength. The ultimate tensile strength of 1100 is only 13 KSI. This will not give acceptable mechanical properties if it is used to weld alloys like 6061-T6 or 5083-O, which require welded tensile properties of 24 KSI and 40 KSI, respectively.

The second reason is to avoid weld cracking. If you remember, we learned previously that alloys such as 6061 and the rest of the 6XXX alloys have a base metal chemistry that is nominally 1 percent magnesium and 1 percent silicon, which is very crack-sensitive. Yet we still weld these alloys every day with fillers like 4043 (5 percent silicon) and 5356 (4.5 to 5.5 percent magnesium). The idea is that by using a filler with a chemistry very different from the base material, we create an alloy of base material and filler material in the weld that isn’t crack-sensitive as it solidifies.

The danger in using 1100 as a universal filler alloy is that, because it has no alloy content, it is diluted with the base material in the weld and ends up with a weld chemistry that is very crack-sensitive. That is what will occur with many 2XXX, 5XXX, and 6XXX alloys if you weld them with 1100 filler.

So, by all means, keep your filler metal selector chart handy and just know that there is no universal filler metal, 1100 or otherwise.