Theres a few things that could be going on here. Id say you are to cold rather then too hot. Alum turns dead soft at around 800 and will have a yellowish tint when u over heat it. If its real cloudy and white it needs more amps. The size of the piece u are welding also has alot to do with amp settings. Smaller pieces will heat up quick and have nowhere to go and continue to heat up and require less heat. I also recomend using a stainless steal brush to knock the oxide off before you weld. If you do not do this you will never get penatration because aluminum oxide melts at like 3 times the temp of aluminum.