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Old 28-02-2018, 14:04   #1
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Mixing Metals

i see a boat with Hull Material Steel .Decks Material Aluminum .
I have been lead to believe these two metals don't go together as the steel will turn the Aluminum to powder . However this boat was built in 1988 . Do you know how this works ??

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Old 28-02-2018, 17:52   #2
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Re: Mixing Metals

Once, I was aboard a steel motor vessel of which the superstructure was aluminum. The owner told us that the join was electrically isolated from the steel. How effective it is over time, I really don't know. But at least, your example isn't the only one.

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Old 28-02-2018, 20:26   #3
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Re: Mixing Metals

Very commonly done, with little trouble. There are bi-metallic weld strips available- al on one side and st on the other. You simply weld using the right electrode to either side. They are not isolated electrically, but are made using an interesting “explosion welding/forging” process.

Joints done like this- with appropriate coatings and maintenance- can last for a very long time with few issues.
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Old 28-02-2018, 21:35   #4
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Re: Mixing Metals

If I ever built another steel boat I would have an alloy deck. You can use the transition strips or bolt it on.

I would bolt it on with a 1/8" poly gasket Sikaflexed on both sides of the gasket. I would also hang all the bulkheads from the alloy deck and use bolts through SS tabs welded to the steel hull to locate them at the bottom and sides.

If I was able to hang up the deck in a shed I would fit the bulkheads and hoist the deck out and hang it so I could complete the fit out outside the hull then drop the completed fit-out/deck back into the hull and bolt it down.
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Old 28-02-2018, 22:04   #5
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Re: Mixing Metals

Navy ships I was on in the late 60s had aluminum superstructure on steel hulls. There was a material like thick tape that separated the two. The metals overlapped at the deck level and were riveted together. There was minor corrosion, but the aluminum alloys probably weren't as good as today. They quit doing aluminum on warships when they discovered how well it burned.
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Old 01-03-2018, 02:34   #6
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Re: Mixing Metals

Quote:
Originally Posted by NSboatman View Post
Very commonly done, with little trouble. There are bi-metallic weld strips available- al on one side and st on the other. You simply weld using the right electrode to either side. They are not isolated electrically, but are made using an interesting “explosion welding/forging” process.

Joints done like this- with appropriate coatings and maintenance- can last for a very long time with few issues.
Indeed.

NobelClad’s DetaCouple™ Structural Transitions, for instance, are used for welding aluminum superstructures and bulkheads to steel hulls, framing and decks.

NobelClad - Markets

Explosion-welding makes economically useful clad materials
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Old 03-03-2018, 13:31   #7
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Re: Mixing Metals

Thanks Guys
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Old 03-03-2018, 13:47   #8
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Re: Mixing Metals

So long as they are electrically isolated there isn't much of an issue. And this is a pretty easy thing to check. Just take a multi-meter and check for continuity between the hull and the bridge in a couple of spots.
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Old 03-03-2018, 14:06   #9
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Re: Mixing Metals

Years ago I helped a guy who was making fittings for Aircraft Carrier decks. The fittings were a composite of I think aluminum, titanium and steel. He made them by explosive forming. He layered the metals, the Ti was just a thin sheet between them, set off an explosive charge and the force caused the metals to molecularly bond with each other like welding. It was pretty interesting.
maybe the Ti was the barrier that kept things from corroding... not sure. I think it was done so they could weld the fittings to the deck.
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Old 03-03-2018, 14:40   #10
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Re: Mixing Metals

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Years ago I helped a guy who was making fittings for Aircraft Carrier decks. The fittings were a composite of I think aluminum, titanium and steel. He made them by explosive forming. He layered the metals, the Ti was just a thin sheet between them, set off an explosive charge and the force caused the metals to molecularly bond with each other like welding. It was pretty interesting.
maybe the Ti was the barrier that kept things from corroding... not sure. I think it was done so they could weld the fittings to the deck.
This is pretty far outside my wheel house, but based on the tests I have read, explosion forming Al-Ti will galvanically corrode at the boundary. My guess is the US Navy simply doesn't care about the corrosion and were trying to accomplish something else, but I have no idea what that may have been. Perhaps to join aluminum/steel with titanium acting as a filler, or provide a thermal expansion joint... I really don't know.
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Old 03-03-2018, 15:02   #11
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Re: Mixing Metals

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This is pretty far outside my wheel house, but based on the tests I have read, explosion forming Al-Ti will galvanically corrode at the boundary. My guess is the US Navy simply doesn't care about the corrosion and were trying to accomplish something else, but I have no idea what that may have been. Perhaps to join aluminum/steel with titanium acting as a filler, or provide a thermal expansion joint... I really don't know.
I think the main thing was to weld the fitting to the deck which didnt match it's metal. I suppose the deck was steel? but they wanted the aluminum fitting or vice versa.
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Old 03-03-2018, 16:52   #12
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Re: Mixing Metals

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I think the main thing was to weld the fitting to the deck which didnt match it's metal. I suppose the deck was steel? but they wanted the aluminum fitting or vice versa.
I couldn't even begin to guess... I know the deck itself is made from an armored steel, and that they can't use aluminum because in the event of a jet fuel fire (~1000C) the aluminum could melt (at ~600C). But what titanium would be doing there I really don't know.


I am sure the engineers were trying to do something with it, and likely succeeded, but I just can't imagine what it would be.
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Old 07-03-2018, 15:41   #13
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Re: Mixing Metals

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Navy ships I was on in the late 60s had aluminum superstructure on steel hulls. There was a material like thick tape that separated the two. The metals overlapped at the deck level and were riveted together. There was minor corrosion, but the aluminum alloys probably weren't as good as today. They quit doing aluminum on warships when they discovered how well it burned.


The new LSC ships are all aluminum. Magnesium burns, aluminum just puddles and melts
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Old 07-03-2018, 21:09   #14
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Re: Mixing Metals

I did see a steel hull with an alloy cabin bolted to a stainless steel flange that was welded to the deck. Plenty of sealant isolated the materials and it didn't seem to have had corrosion issues. The owners said it worked out far cheaper than the explosion bonded strip.
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