Here's a close-up of one of the flutes in the part I formed yesterday. Apparently I had fluted this spot slightly too much, then tried to un-flute it a bit and ended up with a crack. After thinking about it some more, I decided that forming these flanges with tempered aluminum just wasn't going to work. I considered ordering some 2024-O material, to form the flanges from. However, this is less than ideal...my parts will undoubtedly be subject to minimum lot charges at whatever heat treatment shop I use. And it's probably going to take several attempts to get them just right. And, because some pretty severe forming will be required, it wouldn't surprise me if the parts need an intermediate annealing step to counter the effects of work hardening. And, I know from prior experience that heat treating results in some warpage, so even if I get the parts perfect, they almost certainly will not fit as well after heat treatment...and then, I'll sort of be back in the same boat I am now in that I will need to do some (minor) forming in tempered material.
For all of those reasons and others, I decided...