Try Tig tacking a butt joint on 16 gauge aluminium without adding any filler rod if you are in any doubt about the stresses inherent in Tig butt welding aluminium sheet. I will guarantee you at least 50% of your tacks will crack. This exercise will substantiate my point with a lot more clarity. I've been involved in restoring aluminium race cars since 1984 (D-Types, C-Types and Lightweight E's etc) and never, ever did myself or any of the guys I worked with use the Tig setup for butt fusing aluminium sheet. It was common knowledge that Tig = cracks somewhere down the line on bodywork. I'm sure someone with metallurgy experience will be able to explain the 'how's and why's', but for us it was just fact based on experience.
As a side bonus, gas fusing aluminium is so much faster, so win-win.
Matt
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