In article . com,
wrote:
A titanium structure would weigh less...

No, substantially more -- NASA estimated an extra 2t -- although that
would be more than made up by lighter thermal protection.

NASA did study this option. Why did they pick aluminum? Was it just
the cost of titanium fabrication.

They balanced the extra costs of working with titanium against lighter and
lower-maintenance thermal protection, and decided that there was very
little overall difference. The decisive factor, in the end, was that the
USAF was worried about the limits of US titanium production and processing
facilities, especially with production starting on the F-15 (which used a
fair amount of it), and didn't want to see NASA using titanium in
quantity.

...It would also seem to me that the reduction in weight would be
nice, and help with either added payload, higher orbits, larger return
payload, and safer aborts.

At the point when this decision was being made, orbiter size was still
vague, so it wouldn't have made a difference in payload -- aluminum just
required a slightly larger orbiter.

Anyone know .. is it some exotic alloy of aluminum, or just standard
aircraft grade aluminum?

Pretty standard alloys -- mostly 2024, if I recall correctly. There's
some titanium, in particular in the engine thrust structure, and some
composites, notably the cargo-bay doors.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |