Marine Aluminum

I am hoping someone can give me a little bit input about the different aluminum alloys that could be used in a marine environment. I am aware that the 5000 series is the generally considered best.

It was my understanding that this was a corrosion issue. It has recently been brought to my attention that perhaps it is the difference in strength etc. I have some upcoming aluminum fabrication, most rail and antenna mounts. The sizes and thicknesses make me think the 6000 series would be good enough. It certainly is cheaper and easier to source.

Thanks in advance for anyone shedding a bit of illumination on this for me.

 

Alloy Choice by Application

TollyWally,

In general 5000 series is made into sheet goods and 6000 is extruded; so 6061 sheet is a second consideration of the alloy. 6061 has a higher tensile but the lower ductility means you'd poke a hole in the 6061 hull with less net force than the 5086/5083 hull because it rigidity makes for more puncture and less 'give'. The reasons for 5086 and 5083 hull plates are mainly the combined stiffness with malleability compared to the 60 series.

For railings, if you'll bend/roll/form curves buy 6063 T-6 which will 'age' into 6061 in less than 14 months but breaks less in bending. When you buy it ask to see your supplier's proof of inventory or you may get OLD pipe, this material has aged and is much more brittle than 6063 and if you'll use stanard pipe dies and form 180's in old 6063 you'll have as much breakage as new 6061 T-6.

If your rails are long slow curves with saddled butt joints like rails on top of posts (?) then 6061 is fine but it can crack at the deck to topside joint if there are hull residual harmonics at certain RPM's

Brackets, padeyes, mount plates and so on, can be cut/drilled/tapped from 6061 bar stock very well, so you don't have to spend time finding 50 series plates in small quantities.

Radar masts, antenna mounts, railing and other extrusion based fabrications are fine in 6061 unless there is much cold forming; then the 6063 is much more forgiving to form, and it will age into 6061 anyway.

hope this helps?

cheers,
kmorin

 

Interesting read.

Basically the series determines what the aluminium is alloyed with.

1000 = 99% pure aluminium
2000 = (can't remember)
4000 = Silicon
5000 = Magnesium
6000 = Copper (I think)
8000 = Titanium

These are the ones I can remember off the top of my head, I'm sure if you are really interested I can dredge up the rest. The metal included isn't the only metal alloyed in that series, but it is the donimant one. Trace amounts (decimal of percents) of things like tin and copper can usually be found across the board

The Mg is what gives the alloy its corrosion resistance, it also burns really well once it gets to a certain temp, as discovered by the Royal Navy aboard HMS Sheffield when she was struck by an exocet missile during the Faulklands war in 1984. consequently, since this incident military vessels are not to be constructed from the 5000 series.

6000, as stated is usually used to extrude. When it is rolled into sheet form it has a higher tensile strength that 5000. The 'T' designation is the heat temper, Usually we ignore this unless we are dealing with diameters over 48mm, namely scaffold tube, as T5 will allow us to bend in a pipe bender without anealing, however T6 will not.

4000 series is used more commonly for castings, as the silicon tends to make the material flow out better with less imperfections.

The 'H' number after the alloy number designates the heat temper. These are usually found as follows
H116 - High tensile
H32 - More malable and easier to work with
H321 - Similar to H116, to close to worry about unless you have gone through the exercise of having your part specially engineered.

The 5083 - 5086 alloys came about after some bad alloy came out of Russia. Basically it just fell inside (or outside) the alloy specifications for 5083 and after some time the Magnesium 'delaminated' from the aluminium. What the exact problem was I do not know, only the result. This was around or about 2003 - 2004, and as a result the US came up with 5086, which really is 5083 but with tighter specifications for material content torrerances and the like, that is why in the US you can get 5083 - 5086 'dual certified'.

 

Alloy Availability

speedboats,

In your post about alloy content and applications you mention "This was around or about 2003 - 2004, and as a result the US came up with 5086, which really is 5083 but with tighter specifications" by that wording, you don't mean to imply that 5086 has only been available for the last few years do you?

5086 has been around a long time (40 years) but maybe you were discussing the dual certification 5083-5086?

Cheers,