one problem i've come up with on my own is that apparently all the metal shavings evidence is considered shaky because people don't know how carefully the tie was handled, and apparently people aren't sure the tie was cooper's in the first place. then there is the possibility the shavings were transferred to the tie by someone other than cooper, etc.
but still, if the high-antimony titanium alloy is as rare as this guy says (not just the bits of high-purity titanium, which were rare-ish at the time but not exceptional), that creates a relatively small pool of people who had contact with the tie or could at least have been a transfer source for it. (dropping the a priori assumption that the tie was cooper's). an obvious cluster within that pool were the 450 people at the henderson, NV TIMET plant, including vordahl. these people were all laid off when the SST project was cancelled, a few months before the hijacking. so all of them had both a grievance with the industry and a need for money, common elements of cooper profiles. what's special about vordahl in particular? as a scientist, and very bright one, he's a better fit to the profile than most of the workers. he seems particularly associated with the special alloy, since he has several patents on it -- when almost no one else had a patent involving high-antimony titanium alloy at the time.
and, IMO, he resembles the composite sketches. same "pouty" lower lip, same face shape, same wide-set eyes, same huge forehead.
it's not conclusive, but IMO, he's the most compelling suspect i've seen.