Aluminum freezes at about 1100 - 1400* (depending on alloys). That's a dull red at best. Overheat it and it will be a much brighter red of course. I would hope you never see that bright red color in a weld pool, no need to be much over melt point.
Kind of like water, which as it melts, it likes to stay at 32* while all the water thaws before you see a rise in temperature. It doesn't like to be have dissimilar temperatures until it finishes it's phase change, always trying to maintain thermal uniformity. Aluminum is very thermally conductive like water is.
Thus: a skilled welder that is doing it right has never seen bright red aluminum.
A long time ago I saw a video of mixed scrap being processed. It was an aluminum and steel mix, stuff like transmissions and engine blocks. They brought the whole pile of scrap up to about 1500* and then tipped it to dump out the now-liquid aluminum, magnesium, and zinc. The rest could now be processed as scrap steel. The liquid aluminum was then re-processed as mixed aluminum scrap to get the other alloys out. In that video, I never saw the initial pile glow bright red, just kind of dull red while the liquid aluminum was in there.
Cool videos, I wonder if that is part of their alloy separation process.
