On Mon, 03 Nov 2003 21:41:35 GMT, "tony"
wrote:

wondering what kind of parts
are typically made of magnesium.

Not many. Magnesium is a lousy metal in service, because it corrodes
like crazy and also has cracking problems. If you meet a "magnesium"
casting, then the chances are that it's actually Elektron - an alloy
of aluminium and magnesium. Aluminium's self-passivating oxide film
protects against the worst of the corrosion.

If you find old copies of Flight magazine from the late WW2 period
you'll see _lots_ of adverts for magnesium "The wonder metal that will
rebuild our country when peace comes". Although it was developed
enormously for wartime aircraft, its poor long-term service made it
fall from favour by the early '50s.

In the '60s, Lotus racing cars were designed by Chapman on his
"simplificate and add lightness principle". The "wobbly web" wheels of
the type 23
http://www.motorcities.com/contents/00JTH504579522.html
are so infamously unsafe today that they're no longer allowed on a
track, and are replaced for competition
http://www.srgtc.org.uk/images/Lotus_23B.jpg

The "Kirk" bicycle of the late '80's was a cast magnesium frame. Not a
bad road frame (until it cracked or turned to dust) but absolute
rubbish as a mountain bike. There are many stories of them returning
from rides in two halves.

Magnesium is a useful stiffening addition to aluminium alloys.
Although the density of the casting isn't much less than another
aluminium alloy, the extra stiffness may allow thinner walls. These
are obviously lighter, but thin castings are also easier castings to
make. This is why Volkswagen used it (intermittently and not
universally).

Pure magnesium is mainly used as diecastings, because it has suitable
properties for easy casting. AZ91 is the usual alloy (9% Al), either
AZ91B or AZ91HP - trace elements (mainly iron) are reduced for the
High Purity form, which improves corrosion resistance.

These days it's cropping up increasingly often, but it's mainly for
its easy casting and light weight in relatively low-stressed consumer
goods. The NeXT computer (the cube) had one of the first magnesium
cases and they're now commonplace for cellphones.

--
Die Gotterspammerung - Junkmail of the Gods