Aluminum shrinkage at solidification: 6%

In your case it was probably half-inch or greater thickness, okay. And the reason is residual stresses. You're getting fatigue cracks — it's not that it's inherently so bad, but when you start making relatively heavy structures — by heavy, I mean thick-wall — aluminum shrinks six percent, and so you get very severe distortion or residual stresses if you're more than half an inch thick. You're getting more residual stress and less distortion. Remember my general rule: three-eighths is kind of the worst of both — the worst distortion. Very thin is not a lot of stress or distortion. Three-eighths is the worst distortion. Thicker than that, you've got lots of residual stress.