Aluminum for Anchors

[DAVIDSTECKER]

I am working on a working on producing a ships anchor in aluminum instead of the normal steel (ASTM A27 Gr70-40)
I have found an anchor design with the holding power that is required and if cast in aluminum will save the weight needed by my customer.
So, ideally it would be a simple matter of finding aluminum with an equal or greater than Yield.
How ever another part of the problem is that these are ocean going vessels (salt water).
Most aluminum ships have hulls made of 5086 aluminum which in certain conditions has a yield of about 37,000 psi, very close to what I need. I have not found a cast equivalent as of yet. This is why I am writing.
My basic understanding of what makes alloys like 5086 and 6061 corrosion resistant is Magnesium. The higher the better.
We use 6061-T6 to manufacture fabricated equipment for use “above deck”, out of direct contact with the sea water. We also cast other “above deck” items in “Almag 35”, also known as 535.
So I have been comparing the Magnesium contents of these materials as well as there yields trying to find a material that we meet both the yield that I need and the corrosion resistance that this project requires.
One other note is that my customer mentioned is that an allowable aluminum casting material is ASTM A26 Gr356.
So after looking at all of these materials and talking to some foundries, I came across a cast aluminum known as 771. This is available in a T6 and a T71 heat treat.
I have compared the magnesium levels in it (Mg: 0.8 – 1)and found that it is comparable to 6061-T6, (Mg: 0.8 -1.2). The Magnesium content of the approved 356 is 0.2 – 0.45. The Magnesium content of the 5086 is 3.5 – 4.5 and the 535 is 6.2 – 7.5.
So back to my understanding that magnesium is what I am looking for to obtain the corrosion resistance that I want. Yet 771 doe's not come up as a material used in salt water applications. What am I missing? Why is this not a good use of this material?
Is there some thing else I could use?
The end product will weight in around 1,000 lbs. First run would be for 4 anchors and there may be 32 anchors for the entire contract. Is there someone that can create a custom alloy for runs of this size?

Best regards, David

 

[btrueblood]

The zinc content in 7-series aluminum alloys makes them not suitable for submerged uses.

 

[TVP]

Do not use alloy 771. 535 may be ok, but the strength will not be equivalent to steel or a wrought alloy like 5086. The design may need to be changed to reduce stress levels to an acceptable level. ASTM B 26 minimum requirements for alloy 545-F (as-cast) are yield = 18 ksi, tensile = 35 ksi, and elongation = 9%. Typical values are 20 ksi, 40 ksi, and 13%.

 

[DAVIDSTECKER]

btrueblood,
That makes sense, I use sacrificial zinc anodes under my boat to prevent electrolyses damage.
Yes the zinc content is very high compared to the others.

TVP,
Unfortunately if I use the general rule of scaling I need to select another design based on twice the loading. For the size that I would need, the aluminum version comes out to be about the same weight of the steel anchor I’m trying to replace. Just my luck.

Thank you both for your thoughts and suggestions, David

 

[swall]

Perhaps you should look at using a forging. Can probably get higher strength with a much better selection of corrosion resistant alloys than with a casting.

 

[EdStainless]

I presume that some portions see higher stress than others. How about a compound design using different strength materials for different sections? I know, this is a pain, but it might be the only way to cut enough weight.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[DAVIDSTECKER]

May be I need to approach this another way.
The anchor I am trying to replace is fabricated from basically A-36 plate, Alloy 5086-H34 does have just a little higher yield strength.(37,000 PSI)
What I see as a problem with welding the 5086 is that the weld joint strength is greatly reduced compared to the base metal. And of course all the stresses go through the welds.
If I had the means to heat treat the major sub-components, could I re-gain that strength?
If so I could stay with the fabricated design.
Can someone enlighten me on heat treating aluminum. I probably know just enough to be dangerous.
Thanks for all you thoughts, David

 

[sbi]

319.1 qunched heat treat??? i beleave you can get to 40,000

SBI
Central Ne.,USA

 

[MikeHalloran]

The only way to maintain the aluminum's full strength is with mechanical joints, which have their own problems in seawater.

How about a nice hollow welded steel anchor?





Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[mcguire]

David
I don't want to sound like a flack for stainless steel, but it's the material I know well. Why don't you switch from carbon steel to something like 2003, which has double the strength of carbon steel, and will be functionally impervious to sea water. The more common 2205 would also work. These duplex stainless steels are really the optimal material where strength/weight and corrosion resistance are major criteria.

 

[SJones]

I don't profess to being very nautical but: how will the anchors be attached to the vessel? Steel chain? Wire rope? There could be a detrimental galvanic action. It is an interesting design question as I always thought that the principal anchoring force was proportional to the amount of chain/wire in contact with the sea floor. I guess I just haven't seen a design code for anchors. If the vessels have impressed current CP, there could be an issue with hydrogen embrittlement of the duplex stainless steels if large stresses arise at stress raisers in the design.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[AlanD]

A third person showing showing caution.

Anchors designed for sand or mud bottoms require mass to allow the fluke of the anchor to dig into the bottom and a length of chain to make the pull on the anchor horizontal across the bottom, so if the anchor begins to drag it will hopefully catch again. Taking the weight out of the anchor make prevent the fluke of the anchor digging in. Taking the weight out of the chain will mean the anchor line will be straighter and more vertical near the anchor, so more likely to pull out (to overcome this you need to let more anchor line out).

An aluminum anchors designed for a rocky bottom may be ok, for that application.

If weight is such an issue, you may be able to use a smaller, lighter anchor. Give the aluminum anchor a go but I suspect that an aluminum anchor may not work because of the low weight to surface area ratio.

 

[mcguire]

There may be a little wheel reinvention going on. Take a look at westmarine.com to see examples of many types of anchors: plow, claw, grapnel, fluke, spade, mushroom, auger, and in various materials; stainless, aluminum, steel, galvanized steel, etc. Does the world need another kind of anchor?

 

[ornerynorsk]

Maraging steel? I don't know if it is suitable for seawater, but this stuff is incredibly strong. Between 8 and 10 times stronger than AL.