Re: External stiffeners, paint, 'exotic' metals?

[John Brownlee <jonnie@chronic.lpl.arizona.edu>]

	I grew up in what may be one of the only titanium dealer
households in America, so I can actually weigh in here. Titanium is great. 
It is a love and joy when it is finished, but a crotchety old woman to get
out of the shop. Material costs are high, usually on the order of tens of
dollars a pound for bar or plate in small quantities. The real hidden cost
is the cost of working the metal, in particular welding or casting it.
Titanium must be worked in an inert environment when above the
beta-crystalline transition, which is different for different alloys
(alloying is yet another big question). Bottom line: if a hot (let alone
wet) weld touches oxygen or water vapor, it cannot be trusted. The cost of
working with Ti is always underestimated in every project I saw dear old
dad take on. Once saw a 3/8 plate weld crack like glass as a demonstration
of how incorporated oxygen and hydrogen can destroy structural integrity.
	You -CAN- use some of the various flavors of *IG welding for Ti,
but it takes a lot more careful preparation than for any other metal, and
the inert gas shielding has to be really thorough. A good trailing shield
and flushing out behind the strike is essential. The old TIG setup alone
is not going to cut it. 
	We always shopped out casts. I don't recall to whom, or what they
did. I would presume that most hull manufacturing would be done by rolling
plates and making casts. It was always easier to hot-roll a plate than
make a casting, I do recall that. Machining the metal wasn't too bad, but
you HAVE to use carbide tools and keep the work piece cool. It is heats up
above the beta transition, again, galling can occur and ruin your cut. 
Plenty of cool, water-based lubricant worked for us on all but the largest
machining tasks. 
	I'm digging pretty far back in the memory banks. Somewhere I have
some pamphlets in PDF about working Ti and the various constants for the
myriad alloys available from the mills. We used to work a lot with
TI-Al4-6V, an aerospace alloy with wonderful high-temperature
characteristics. It was the easiest material to machine from a temperature
standpoint, but it gobbled up carbine tooling!
	Wow. A question I actually can chime in on. Cool.

							John

John Brownlee
Lunar and Planetary Lab
University of Arizona
jonnie @ lpl . arizona . edu