Hi,
does the broken endmills show Built-up-Edge (BUE), that is to say aluminum stuck in the flutes?
Aluminum is 'sticky' and prone to BUE, and this is especially so if the chips are not cleared from the cutzone and are recut.
Many people use a continuous air-blast, either with oil mist or without, to blow the chips out of the cutzone. I use flood cooling, and it does cool but really
its all about removing the chips so they don't get recut.
Craig