The tungsten itself becomes oxidized after putting down a certain amount of weld beads. You'll notice the tungsten becoming light grey or off white. This means it's time to change the tungsten. The strength of the arc will deminish from the oxidation on the tungsten tip. Allowing the tungsten to stick out too far or removing the tungsten from the completed weld before post purge cooling of the weld will decrease the effectiveness of the tungsten which will have to be reground and balled up using DC-Positive as mentioned on my previous post.
The oxidation on the aluminum requires alot more heat than the aluminum itself because aluminum oxide on the surface melts at 3,700 F while the base material melts at around 1,200 F so that is why it is critical to remove the oxidation before welding. Also degreasers work best to prepare the weld, but make sure the degreaser doesn't contain hydrocarbons. Cleaning aluminum can be done simply with soap and water. I've done this many times with great success. Etching soultion is also an option for cleaning the material. Before beginning to weld use a stainless steel brush and brush ONLY in one direction. DO NOT brush heavly on the area to be welded or you could add contaminates to the weld.
And again I'd like to remind that a GAS LENS is critical for the cleanist welds possible regardless of the material being welded. The standard is a collet body but the gas lens body consists of a series of coarse and fine screens inside the collet body which gives better shielding of argon on the weld and provides optimum laminar gas flow to allow your welds to look their best. The gas lens also allows the tungsten to be extended out further for improved weld joint access.