Best aluminium alloy for the masts

Discussion in 'Materials' started by deucalion, Jan 8, 2009.

  1. deucalion
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    deucalion New Member

    I'm working for an extrusion company and I have lots of inquiry from our parthners for masts. I checked on the net and as I see, there are 4 or 5 kind of alloys to produce these masts.

    Masts for the optimists, lasers or windsurf booms, it is generally used 6060/63-T5 or T6. This is a light alloy with low mechanical properties.

    For the biggers 6061-T6 and 6082-T6 alloys can be used. I looked up for these alloys mechanical properties.

    6061-T6 / 6082-T6
    Ultimate Tensile strength 260-310 MPa / Min. 270 MPa
    Yield strength Min. 240 MPa / Min. 200 MPa
    Elongation %0,05 9-13 / 12
    Hardness (Brinell) 95-97 / 80

    I also see rarely 7075-T6/T9 alloys which has high strength but they are not so prevalent.

    Are these informations right? What's the best aluminium alloy for the masts? Which one is the most common and why? What's the priviliged mechanical property and what are the minimum values? Is there any international standard of these mechanical properties for the mast production? Can anybody help me?
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Depends if the mast is to be welded or unwelded when used.
    6061 is popular in the US because the aircraft industry requires shed loads of the stuff, so it is always in stock. It does however have 3 times the copper content of 6082, which is the reason why most Class societies no longer allow 6061 to be used. 6061 has poor corrosion properties.
     
  3. Guest62110524

    Guest62110524 Previous Member

    well that is the most misleading post you could post,
    after 30 yrears building alloy yachts I can tell you 6061 t6 stands up in all situations , especially masts
    I used it on trailers too, its propertys are amazing,and I use it with Lloyds, ABS, approval

    I used it(along with a zillion others) in all framing
    where did you get your facts, !!!!!! it is the number one alloy of preference
    It is against my principals now to give neg feed , but boy you tempt me
     
  4. deucalion
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    deucalion New Member

    As I see their mechanical properties are so close to each other, the main difference is on their chemical composition. 6061 has greater yield strength and hardness, but the tensile strength of 6082 is more than 6061. Which of these properties are most important?
    What about stiffness? Which property directly effect on stiffness?
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    Whoosh
    6082 has 0.1% copper, 6061 generally has 0.3% copper. (Can be as high as 0.4%).
    Both DNV and LR no longer accept 6061 owing to its poor corrosion properties compare to 6082. It has been well documented. DNV have not outright banded it, just prefer you don't use it. LR have removed it from its approval list. Yet in both cases if there is an application that is identical to one which is proposed and has demonstrated little or no corrosion effects, then class will allow a dispensation. To my knowledge LR have no allowed 6061 for around 5~7 years. I wouldn't recommend it either, 6082 is far superior for the aforesaid reasons.

    Copper over time weakens the protective oxide layer. Hence 7000 series ally which have added copper and those in the 2000 series are best avoided.

    Clearly LR USA has different ideas to the rest of the world.
     
  6. deucalion
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    deucalion New Member

    How can I provide documents of LR or DNV rules about these issues (aluminium masts) ? Any website?
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    Sorry, don't understand the question?
     
  8. robherc
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    robherc Designer/Hobbyist

    Ad Hoc, he's asking for a link to research the LR and DNV rules on aluminum masts for himself. ... and so he can have the documentation if anyone asks.
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    If that is all he is after, just looking at DNV rules: Pt.2Ch.2 sec.9 A1300 table A3, will give the mechanical properties, or LR Pt.2. Ch.8 Sec1. table 8.1.4 also.

    As for masts. It is like anything in design. You establish the loads, select a suitable geometry/structure design, then see what stresses are involved then select the material to suit the computed stress and predicted fatigue. Without establishing the loads and its in-service conditions and method of construction, bit hard to advise....there is not a "This is a Mast, please select one" book. All masts are unique to each vessel.
     
  10. alex folen
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    alex folen Flynpig

    Hello, forgive my ignorance. But in airplane experimental design we think of the lightest material with the greatest strength. I have looked at newer designed sailboats (for speed) and the lighter craft the faster in same design. Is this correct? I would think of aircraft material and its strength and go from there, with all the calculations thereafter. May cut out a lot of guess work as to the type of material needed? Perhaps I’m still ignorant of sailboats, heck I’m new this new found love!
     
  11. Jimbo1490
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    Jimbo1490 Senior Member

    If you think 6061 has poor corrosion resistance, you would do well to stay clear of 7075. That stuff is a corrosion nightmare! I have seen bare spots on aircraft where minor surface corrosion was dressed in preparation for a paint job, then left bare for a month, and these spots 'blew out'; went to intergranular exfoliated corrosion. In one month! The reasons this does not normally happen in service on aircraft are that 1) aircraft aluminum is 'Alclad' (plated with pure aluminum) and 2) aircraft skins are painted and 3) aircraft are virtually always subject to a rigorous inspection program.

    Of course when corrosion spots are dressed out, the Alclad coating gets removed, thus the rapid progression of new corrosion. I would consider this alloy wholly unsuited to the marine environment due to a propensity for intergranular exfoliation type of corrosion, which is always structurally significant.

    Jimbo
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    Jimbo1490
    Exactly 7075 has some 1.2~2.0% copper, which is way over than of 0.1% of 6082. Even 6061 seems better in comparrison at roughly 0.4%. And yet you see some boat builders, like the one above, think it isn't a problem to be concerned with!

    Copper, the enemy of aluminum...
     
  13. Jimbo1490
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    Jimbo1490 Senior Member

    Copper cannot be considered the enemy of aluminum in that without copper, there is no such thing as hardened aluminum. Without 'hardenable' aluminum, you are left with only the weaker, 'non-structural' aluminum alloys. Free crystals of copper intertwine into the crytalline lattice of aluminum molecules, thus preventing adjacent 'sheets' of aluminum lattice from sliding past each other. The aluminum lattice usually forms large common interface planes between 'sheets'. These interface planes become weak points in the crystalline structure since they slide past each other easily. During rapid cooling after heating, the copper crystals become embedded in adjacent sheets. In order for the 'sheets' to slide thereafter, they must either shear the copper crystals or tear the aluminum lattice within the same sheet, where it is the strongest. This makes the alloy MUCH stiffer and a bit stronger as well.

    Jimbo
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    Ahh...you misunderstand me. I'm not referring to it metallurgical composition. I am well aware of the role of copper in this aspect of making the alloys. I am referring to the finished product, ie when used on a boat.

    Bimetallic corrosion of aluminium by copper in solution, ie sea water, is a killer.

    This is why LR no longer allow 6061, too much copper..

    Aluminium has a nominal electrical potential of -750mV, whereas copper has -360mV. For bimetallic corrosion to take place a there must be, in general, a potential greater than 100mV. Clearly copper is way way over this...and as such is a killer when in solution, to copper.

    That's why copper (pipping, paint etc etc) when in direct contact or in solution of any form, must be avoided at all costs, with aluminium, when used on a boat.
     

  15. sailor2
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    sailor2 Senior Member

    That is simply a wrong statement. Copper is just one out of many metals to make aluminium hardenable.

    Mg alone will make alu work hardenable, but not allow hardening by heat treatment. It is the best alloying for corrosion resistance properties for marine use. When welded, heat will soften it again in areas that heat up.
    Using Si + Mg alloying makes alu both work hardenable and heat treatable. This is so even with absolute zero copper content. And still has very good corrosion resistance, althoug slightly less so than using only Mg. Thats the most recomendable grade for marine use when heat treatable grade is needed. 6082 grade has max copper content as stated earlier, but no minimum content. Special orders with less copper is possible from some manufacturers with same structural mechanical properties but even beter corrosion resistance.

    Pure Zn alloying also allows hardening, but is prone to corrosion far too much for marine use without very carefull sealing and regular inspection. It is still possible for masts with good coating by Al-Oxid, but not for anything under waterline.
    Include both Zn and Copper and result is the strongest Alu there is, but a nightmare regarding corrosion. Still used on aircrafts with regular inspection.
     
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