Re: [[PSUBS-MAILIST] Aluminum subs]

["Karl Fuller" <fullerk@voyager.co.nz>]

Hi Danny and all,

Just for the record and no doubt some of you know this. The weld zone in an
aluminum weld is generally considered 60% of the strength of the surrounding
metal, so this has to be calculated when designing the sub. Steel can be
close to 100% in this same respect. Particular attention to corrosion
prevention if the sub is to be immersed for long periods must be taken.
Aluminum (like zinc) will 'sacrifice' itself to metals higher on the valance
scale. When doing wiring, make sure you don't leave any bits of copper in
the bilge where water will be sitting, it will eat out the bilge in no time
!
High degree of skills required to weld it and a big power supply for the
heat requirements, Danny has mentioned. I have seen beautiful looking welds
done by an 'aircraft level welder', literally fall apart, the weld just
sitting on the surface....You should have a way to test the weld, e.g..
Xray.
To get a similar strength to steel you need more of it, so the weight
savings are not as great as one might think and for the reason of weaker
welds you might have to go up in strength again. Higher grades of aluminum
have temper that can be destroyed by heat, just as with steel. Some of you
may have heard of cases where dive bottles have been heated to bake on paint
and then some unsuspecting filler, gets killed, it's happened.....
Riveting as Craig mentioned, is an interesting way to solve some of these
problems.
For the rest, personally, I like aluminum, easy to work, no need to paint
it.

Regards,
Karl.
----- Original Message -----
From: Danny Campbell <danny.campbell@usa.net>
To: <personal_submersibles@psubs.org>
Sent: Saturday, December 02, 2000 2:11 AM
Subject: Re: [[PSUBS-MAILIST] Aluminum subs]


> 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
>