Best thing to do is put it on the highest speed possible, on hammer setting and push as hard as you can..... hahaha just kidding..
whenever I thread metal I always put the tap in my drill... very slowly though.. and definitely not on hammer haha. was tapping 10mm steel the other day with a 6mm thread just fine. Only time I have snapped one like that is when my battery ran out and I replaced it with fully charged one and wasn't expecting the speed increase so much...... snap! I would have thought you would be fine in alu.