Re: [[PSUBS-MAILIST] Aluminum subs]

[Danny Campbell <danny.campbell@usa.net>]

Personally, I love aluminum.  We use it all the time during the construction
of components that go into aircraft simulators - typically 6061-T6 extruded
tubes and angles as well as sheet up to about a quarter inch thick.  Great
stuff.  Strong, lite, machines well, readilly available.  Of course 99% of our
construction is BOLTED together.  On that rare occassion when I'm looking at a
design which "sure would be nice if it were welded" I always step back,
scratch my head and sigh.  Aluminum is not like steel when is comes to
sticking them together unless you really, really know what you're doing.  I'm
an engineer, not a welder, but have seen what can happen if the process is
done wrong - even by purported pro's.  Everyone knows how nice it is to weld
steel.  It's a well-behaved material.  Aluminum however requires special
handling.  

First of all, aluminum has a very high thermal conductivity which means when
you weld, the rest of the material "sucks" the heat away from the weld site
therefore it is imperative, especially for thick pieces, to preheat the
aluminum.  A problem with preheating aluminum is that it's hard to tell
visually when the material is hot enough.  With steel you can actually see a
slight color change of the material.  If you don't preheat properly then
you're not going to get the weld penetration you want.

Second, aluminum welds are not forgiving of contamination.  You MUST prepare
the surface before welding.  Usually this involves a mechanical abbrasion but
there are also chemical treatment available.  Once cleaned, welding must occur
within a very short period of time because the surface of aluminum oxidizes
rather rapidly.  You do NOT want aluminum oxide in the vicinity of the weld
because it does not melt as readily as aluminum itself and will result in
solid particle contamination - weakening the resulting weld.  And on the
subject of contamination, you need to use an inert gas technique (MIG, TIG...)
because traditional arc welding allows oxygen in the air to react with the
metal during welding to form aluminum oxide and you'll end up with the same
contamination problem noted above.

Third, cool-down is critical.  Not that you would ever do this anyway but
aluminum should not be quench-cooled.  It needs to cool gradually and
uniformly or the weld will most likely crack - once again, especially with
thick welds you may have on a pressure vessel.

Fourth, post weld treatment of the surface.  True, aluminum does not rust in
the same sense that steel rusts but it does oxidize.  We typically abrade the
part and then alodine (iridite) it.  It would be difficult if not impossible
for the backyard guy to do this level of post weld treatment to something as
large as a pressure hull sense the piece being treated is submerged in a tank
of the chemical.

Fifth, hey, aluminum is not as dense as steel and when building a sub, density
is not necessarily a bad thing.  The denser the outer hull the less lead you
have to put inside.

So anyway, let me step down from my soapbox and summarize.  Aluminum CAN be
welded if you know what you're doing and have the equipment to do so properly.
 Welding thick sections of aluminum is not something an amateur should
attempt, especially on something as critical as a pressure hull.

Good luck! (think steel),

Danny

CWall@swri.edu wrote:
Have there been any personal subs built using welded aluminum for the
pressure
hull?

I mean homebuilt, of course, not a project like the Aluminaut....

Craig Wall


____________________________________________________________________
Get free email and a permanent address at http://www.netaddress.com/?N=1