I've been experimenting with cutting Aluminium on my mdf router and have broken quite a few bits when cutting at lower rpm, and I don't understand why?
What tends to happen especially if the feed rate is too high and coolant/lubrication is inadequate is that chips get packed into the flutes then you get a mess because they rub, melt and the cutter breaks.Always use two flute if possible to get better chip clearance. Keep the chipload below 0.0005" per tooth (0.01mm). With carbide you should be able to go at full rpm because the surface feet per minute is just over 600; carbide can cut aluminum at 3 times that. At 20000rpm your feed will be not more than 400mm/min.But really the most important thing is adequate lubrication. You could try automatic transmission fluid (ATF) it works very well. Brush it on ahead of where the cut will be. Kerosene works well but is more of a fire hazard. Or see if your local Shell petrol station has ever heard of Shell Dromus B. This is a cutting fluid made by Shell for the last 80 or a 100 years ro so. It is mixed with water normally but works very well just brushed on neat.
Ok, my depth of cut is 0.3mm and my feedrate is 450-600mm per minute. (That's 0.012" and ~24ipm, I think.)
I think your depth of cut maybe slightly to much for such a small bit. If it was a 4 flute end mill, maybe. I also think your feed may be a bit fast.