Welding Tips and Tricks

I have a scrap yard about 25 minutes from my house. They buy and sell all types of scrap steel and aluminum. I wanted to weld up some storage racks/boxes for my aluminum boat. The part will be painted afterwards.
So my question is, when I get to the scrap yard to get some aluminum, 1) is there any way to tell what type of aluminum it is? 2) does it really matter all that much?
I will be welding it to the extrusion pieces that I bought from the boat manufacturer (and I don't even know what that type is to be honest).
This will be my first welding attempt at aluminum. I do plan on doing a bunch of practicing first. I just didn't know if all aluminum could be welded to each other, without issue. Or if there was a type of aluminum I should avoid (and if so, how do I tell). And if that's the case, perhaps I should just order new aluminum, in a specific alloy, for my boat project.
I hope this post made sense. I just didn't want to spend money on scrap aluminum plate that wouldn't work, or be compatible, with the aluminum used in my boat.
Thanks guys. I'm totally new to this and trying my best to figure it out.

The cost of scrap yard aluminum is enough advantage to make it worth a gamble. There are quite a few varieties, some are difficult to weld reliably, others might not bend well. My local supplier traffics mostly in 6061 extruded material, and sheet. A company I work at occasionally prefers 6061 for casting. Many times it'll be stamped what it is. Electrical conduit is 6063, and has no where near the tensile strength of 6061, but bends nicely.

I use 4943 Maxal from Hobart for most purposes, 5356 is a better choice in filler for certain varieties.

As filler rod is a valuable tool for sucking excess heat out of molten weld, I use 3/32, and occasionally 1/8". I have ten pounds of 1/16" but rarely want it.

Easy way to find out if it’s going to weld ok is get some pieces big enough to cut some test strips from. Then weld up some 6-12 inch pieces. Let it cool and then do some bends to see if it’s going to crack along the weld.

Just about all bar stock and sheet grades are weldable. Ductility and toughness will vary.
The unweldable grades are usually ordered specifically by manufacturers and can be extruded or pressed like ladders. If these items end up at the scrapers it will be recognisable like a ladder and parts will be riveted. So go for bar stock drops or items previously welded well. Avoid previously riveted parts.


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Be aware of a property Jody calls "hot short" Aluminum is incredibly weak until it cools to a given temperature. The act of welding, introducing filler to be melted sucks heat from the weld, bringing the temp below the weakest state.
If you want to see this demonstrated, but two pieces tightly together, and run a weld joining them. If you have the experience I have, it'll crack as it cools. Always use filler, even if you will grind flush afterward.

Willie