What is the best marine alluminum alloy?

[xarax]

Is it 5383, or something else from 5 series?

[Cian Groves]

If you mean the mechanically strongest then no there are better.
Corus for example do Alustar,
http://www.corusgroup-koblenz.com/english/products/shipbuilding/index.htm

Cian

[xarax]

Thank you Cian.
Many companies tried to improve 5083. It is difficult for the non professional ,like me, to choose between the varied alloys they offer. Is there any comparative test published somewhere?
Alustar looks good, better than 5083, but what about 5383?
Alcan/Pechiney also promotes Sealium:http://www.pechiney-marine.com/Internet/tmi/marine.nsf/html/FWFGHOME?OpenDocument&HP=1&LG=0&

[Cian Groves]

Hi Xarax,

There are some mechanical properties comparisions of 5083, 5383 and 5059 in their specifications/rules under DNV. If you download the Brochure - Shipbuilding you will also see some comparsions in there.

I'm not saying Alustar is the best material however. Being superior in mechanical properties also means differences in the way it needs to be welded & also a higher price, whereas 5383 is about the same price as 5083 and has better strength at weld than 5083.

Hope it is of some help.

Cheers,
Cian

[xarax]

Thank you Cian.
It seems that the mechanical properties of the new aluminum alloys are improved indeed. Now what happens with corrosion resistance ? Staying within the 5xxx series with the already excellent marine corrosion resistance characteristics, should one worry anymore about corrosion?

[CDBarry]

What do you mean "best"?

[xarax]

The price differences of the commercially available marine aluminum alloys are not so great, compared to the overall cost of building and maintaining a small boat. So "best" is the alloy that an amateur boat builder like me would choose for his boat, if we presume that all the other problems (welding difficulties, for example) are somehow solved

[Eric W. Sponberg]

Xarax,

In the US, we have 5456 alloy which is used extensively by the US Navy, so there is usually a lot of it around. It is more expensive, but it has superior mechanical properties in the welded condition. I would like to use it whenever I can but unfortunately, it is not readily available overseas, and sailors and do-it-yourselfers are usually so price conscious that they go for the cheapest materials. This has reduced aluminum boatbuilding to the basics: 5083 for plate, and 6061-T6 for the frames and stringers. These alloys are available worldwide and they do the job adequately.

If someone is really out to optimize an aluminum hull and deck structure, then he should look to the latest improved grades such as Alustar. Remember that a good structural engineer and/or naval architect should spec out the structure to get the most out of the material.

Eric

[xarax]

Thank you Eric,
5456 has double the iron and half the copper compared to the newer 5383,
Alustar and Sealium. Does this spell any differences to longterm marine corrosion resistance or am I too naive here as well ?

[Eric W. Sponberg]

I don't know enough about the corrosion characteristics of Alustar other than the advertising that is available. A lot of people in Europe are gaining experience on it.

Certainly if the US Navy relies on it, they are probably in tune with its favorable corrosion characteristics--I just have not studied that much. Generally, all 5,000 series alloys are very good and the alloy of choice in marine structures, and better than all other series (2,000; 3,000; 4,000: 6,000; and 7,000) alloys.

Eric

[BillyDoc]

I seem to be settling on the 5456 alloy for my dream yacht but have two concerns: one, does it form well under the English wheel? and, two, when it is used for a hull how does it stand up to long-term submersion in saltwater?

Does anyone have experience working this alloy?

[BillyDoc]

Well, in attempting to answer my own question I went to the property pages of http://www.suppliersonline.com/Research/Property/step1.asp and found that the 5456 alloy may not be the best one to use after all.

Of the four major aluminum alloys we can choose from, none appears to be “perfect,” so I am putting a summary of the basic properties of interest for each of these below. I still don't know which is best, overall, although the 5456 alloy looks good for boats using developable panels with very large radii. For those of us hoping to achieve more complex curvature, however, it is not so good. Anyway, here is the summary.

5083

Machinability No specific data. However the alloy is machinable by conventional means.

Forming Forming characteristics are good for either hot or cold working.

Welding Weldability of this alloy is very good by conventional means. When filler rod is required it should be the same alloy, 5083.

Cold Working AL 5083 is readily cold worked by conventional methods. In the annealed (O temper) condition plate of 0.250 " thick can be bent on 1.5 T radius.

Other Mechanical Props Shear strength in O temper is 25 ksi.

5086

Machinability Machinability in the strain hardened tempers (H34, H36, H38) is relatively good. In the annealed (O temper) machining is difficult, but still can be accomplished. Use of lubricants is advised.

Forming Forming characteristics are best with the alloy in the O temper -- less good in the H36 or H38 tempers.

Welding This alloy is readily weldable by conventional methods. Use of electric arc welding in particular produces excellent results.

Cold Working Excellent cold working characteristics in the O temper and somewhat less good in H36 and H38 tempers.

Other Mechanical Props Shear strength for O temper is 23 ksi, for H34 it is 27 ksi. Condition O Temperature 68 Tensile Strength 38 ksi, Yield Strength 17 ksi, Elongation 22.

5454

Machinability The machinability of Al 5454 is fair to poor. It is best to do machining operations with the alloy in the H-34 temper condition. Use of oil lubricants is recommended, except that very light cuts may be done dry.

Forming This alloy does posses good hot working and cold working characteristics. It may be readily cold worked in any of the temper, or annealed, conditions.

Welding All of the commercial welding methods will work satisfactorily with AL 5454. The TIG or MIG process, with AL 5554 or 5356 filler rod, works especially well.

Cold Working Cold working is easily accomplished with this alloy by conventional methods. It may be cold worked satisfactorily in either the annealed condition or any of the H-32, H-34, H-111 tempers..

Other Mechanical Props Shear strength varies slightly from 23 ksi in the annealed condition to 26 ksi in the H-34 temper. Condition O Temperature 75 Tensile Strength 36 ksi, Yield Strength 17 ksi, Elongation 25

5456

Machinability Generally difficult to machine. Carbide tooling and oil lubricants should be used.

Forming Forming of this alloy is more difficult than for other aluminum alloys. It requires generous bend radius and should not be severely cold worked. See "Cold Working".

Welding Arc welding is best for this alloy and this makes it useful for structural applications. Gas welding may be used, but results are less favorable than for arc welding.

Cold Working The alloy may be readily cold worked. However severe cold working (over 20 % reduction) should be avoided as this may make the part subject to stress corrosion cracking in application where temperatures can reach 150 F.

Other Mechanical Props Shear strengths for various tempers are: O temper, 28 ksi. H321, 30 ksi. H311, 28 ksi


Another very important consideration when choosing an alloy is the corrosion resistance. Although all the above are advertised to have “excellent” corrosion resistance, there have been serious problems with 5083 from intergranular corrosion. Apparently these problems are “batch specific” and can be avoided by specifying ASTM B 928/B 928M, which also applies to 5086. See the attached pdf for more on this.

This is all making my head hurt. I need to do something relatively easy for a while. I think I'll gather up some of the cats around here and practice my juggling.

[Eric W. Sponberg]

When you are dealing with professional shipyards which have all the tools necessary for welding, forming, and cutting aluminum, most of these considerations fall by the wayside. The one remaining factor is the strength of the alloy in the welded condition. All of the classification society rules stipulate that you have to engineer to the welded properties of the material, not the as-milled properties. In addition, when you calculate through the ABS rules, for example, you determine thickness, moment of inertia, and section modulus based on certain metals, and then you have to apply what ABS calls a Q factor for materials with different strength properties. For stiffness properties, the modulus of elasticity is used, which for all aluminum alloys is considered to be 10,000,000 psi (70 GPa).

Q factor = 92,000/(SigmaY + SigmaU)

Where

SigmaY = yield strength of the alloy in the welded condition

SigmaU = ultimate strength of the alloy in the welded condition

Therefore, you want to use alloys that have the highest welded strengths and the lowest Q factors. Distinction is made between plates and shapes:

Unwelded Unwelded Welded Welded
Alloy SigmaY SigmaU SigmaY SigmaU Q Factor
Plates
5083-H116 31.000 44,000 21,000 39,000 1.53
5086-H111 21,000 36,000 14,000 35,000 1.88
5086-H116 28,000 40,000 14,000 35,000 1.88
5454-H34 29,000 39,000 12,000 31,000 2.14
5456-H116 33,000 46,000 19,000 41,000 1.53
6061-T6 35,000 42,000 15,000 24,000 2.36

Shapes
5083 and 5086 alloys not readily available
5456-H111 26,000 42,000 19,000 41,000 1.53
6061-T6 35,000 38,000 15,000 24,000 2.36

Therefore, on this basis alone, one can see that 5083 plating and 5456 plating and shapes give the most amount of strength for the least amount of weight.

Eric

[BillyDoc]

Thanks Eric!

Very good information there.

Bill