Getting under the “skin”
Welding aluminum is the coveted art form of TIG welding. Aluminum welding is the type of welding relatively few welders do on a daily basis, if ever. It’s been raised to a mystical black art in the eyes of the untrained, and in the trained eye, it is a delicate balance of many variables that allow the weld to properly fuse.
One of the issues that must be dealt with is the oxidation layer. While welding oxidation appears as a dull skin that floats on top of the weld puddle. The weld puddle is vaguely visible and it may look like a scummy appearance on top. When the filler rod is dipped in, the skin absorbs the rod, but no visible weld is made. This is the frustrating part of welding aluminum. Even if the oxide layer is…well, let’s just put this welding truth out there from the start: All aluminum is oxidized when you get it. It doesn't matter how shiny it appears when you get it. Aluminum rapidly begins to form an oxide layer even after it is completely cleaned off due to interaction with oxygen in the atmosphere. It must be thoroughly removed.
Of course the AC cleaning action on most welders can take care of a lot of it, but even occasionally when it is particularly heavy and thick; spots of oxide will remain in and on the weld and it will begin to form an oxide layer. There are several ways to remove the oxide layer, but the best is good old fashioned brushing. Brush the metal thoroughly with a stainless steel brush only. A hand held brush is all that is needed in to get the job done, along with plenty of effort.
Carbon steel brushes will introduce carbon into the weld, another contaminant. A high speed wheel brush can cause smearing of the aluminum down further into the metal. A variable speed grinder can be run slow enough to get the job done, but be careful for flying aluminum particles that can get into the nose and eye.
Once you completed welding oxidation and the oxide layer is removed, the fine dust like particles remaining should be thoroughly removed as well. Along with these particles oil from the manufacturing process is also present, so a thorough wash of surface is in order with a cleaner such as acetone or aluminum cleaner. A lint free rag should be used to finish the removal of contaminants. Don’t forget that if the metal is anodized, removal of the surface layer may be particularly difficult. In this case plenty of cleaning action in the AC balance should be put into the weld.