Date:         Wed, 30 Jun 2004 22:23:56 +1200
Reply-To:     Andrew Grebneff <andrew.grebneff@STONEBOW.OTAGO.AC.NZ>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Andrew Grebneff <andrew.grebneff@STONEBOW.OTAGO.AC.NZ>
Subject:      Re: Curious Question (NVC)
In-Reply-To:  <40E1C655.1050703@bellsouth.net>
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>Cast aluminum has a granular crystal quality in it's structure. Milled
>Aluminum (hammered, rolled) does not.
>
>Of two identical parts, one a machnined aluminum casting, and the other
>machined in milled aluminum, which would most likely have the best 1)
>strength 2) heat transfer properties.

Crystals do not stick together too well, and separate along their
faces (when anyone tries to tell you that "there are no straight
lines in nature", bring up the subject of crystal faces). Hence the
easy breaking of cast aluminum. Just-molded cast parts cool
relatively slowly, allowing some of the atoms to organize themselves
into crystal lattices... just as with cooling molten rock. The edges
of these crystals are weaknesses in the structure. Or rather, the
crystal structure  IS the weakness.

Many crystals contain cleavage planes (utilized by diamond
"cutters"), along which those crystals will fracture under pressure
or shock. i don't know whether aluminum crystals have cleavage
planes; if they do, there is another level of weakness.

Billet aluminum is processed differently, and lacks the crystals...
or is perhaps microcrystalline? Whatever, it should be stronger...
and perhaps more malleable.

>Of course, if we are talking alloys here, we are not talking about Aluminum.
>
>With Steel, we are not talking about an element, we are talking about an
>alloy.
>
>I am not familar enough to know how much of a market pure rolled aluminum
>has, I would suspect most 'Aluminum' sheets are some type of alloy.
>
>And then there is 'tin' foil.

The aluminum used in manufacture is not pure, but contains other
metals eg magnesium. Therefore it is an alloy, or rather, there are a
host of different Al alloys.

Tin goil, tin cans... just the usual abuse of language.

>Even my old airplane, a Cessna 195 with a 300 HP radial engine had
>aluminum rods.

A Cessna with a RADIAL engine? All those I know of have Lycoming (or
Continental) boxers.

--
Andrew Grebneff
Dunedin
New Zealand
Fossil preparator
<andrew.grebneff@stonebow.otago.ac.nz>
Seashell, Macintosh, VW/Toyota van nut