I've never heard of one cracking, and the one on mine is solid as a rock. But manufacturing defects do happen. The aluminum may have been poured wrong. It would be covered under Ego's five-year warranty. Give Customer Service a call (and make sure your tool is registered if it isn't already, to help speed along the process).
CS can be reached at 1-855-EGO-5656 (1-855-346-5656) from 8:30AM - 8:00PM EST (Monday - Friday) & 10:00AM - 6:00PM EST (Saturday & Sunday).
I'm sorry to hear that! I'll be purchasing my Ego trimmer tomorrow and you've convinced me to go with the carbon shaft. Plus, I'll be carrying the trimmer around in the back of my Kubota RTV so for me, the aluminum shaft would be more likely to break.