Best aluminium alloy for the masts

[Jimbo1490]

It's not a 'wrong' statement; your example of work hardening is invalid as metallurgically, this type of hardening is inferior and degrades strength properties. None of these alloys you mention is considered a 'high-strength' hardenable alloy. Only the alloys containing copper can get you this mix of properties. Only copper hardens aluminum by imbedding between adjacent 'plates' of aluminum's crystalline lattice, preventing those plate from sliding, the mechanism of pure aluminum's malleability. This is why only alloys with copper can be 'precipitation hardened', the type of hardening of most interest in a structural metal. The other alloying agents harden aluminum a different (inferior) way.

I stand by the statement.

Jimbo

[hunny06]

Its really good to have all of you intelligent persons here.These are very nice posts or information matter.perhaps it ll be useful for anyone else including me.



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Jimbo1490

I think there is cross lines going on here with terminology.

Work "hardening", ie using mechanical means to increase the strength of an alloy from say O-temper to say H116-temper, is one definition. This is the terminology used by many, but is correctly termed strain-hardening in the metullurgical sense. (To "work a metal", is where the common usage comes from)

The addition of copper brings about many changes to increase the mechanical properties and also changes the alloy designation from non-heat treatable to heat treatable. These alloys can then be age-"hardened" further to increase its properties, thus confusing the layman with respect to the word "hardening", even further!

The term "hardening" is the confusing bit as it relates to many different disciplines all at the same time, with subsequent different meanings. Since hardness testing is a relatively quick and easy way to assess the strength of a material without the need to prepare tensile test samples.

As such, the term ‘hardness’ is a comparitively important for engineers, but, it is not strictly spekaing considered to be a fundamental property of matter. Even though it is refered to as such. The index of hardness is a manifestation of several related properties of the metal, which may well include a combined effect of yield point, tensile strength, ductility, work-hardening characteristics and resistance to abrasion.

It is however, most commonly used when referring to the weld quality since it is directly proportional to the UTS and as such directly related to the quality of the weld. Which is why hardness testing is done right across the weldments from parent metal to parent metal across the HAZ for class certs. Sorry, slight digression...but you get the point.

[sailor2]

Originally Posted by Jimbo1490

It's not a 'wrong' statement; your example of work hardening is invalid as metallurgically, this type of hardening is inferior and degrades strength properties. None of these alloys you mention is considered a 'high-strength' hardenable alloy. Only the alloys containing copper can get you this mix of properties. Only copper hardens aluminum by imbedding between adjacent 'plates' of aluminum's crystalline lattice, preventing those plate from sliding, the mechanism of pure aluminum's malleability. This is why only alloys with copper can be 'precipitation hardened', the type of hardening of most interest in a structural metal. The other alloying agents harden aluminum a different (inferior) way.

I stand by the statement.

Jimbo

Si + Mg type of alu with zero copper content is precipitation hardened !!!
That's a metallurgical fact. And the reason that type of aluminium alloy is hardenable by heat treatment, like T6. That's artificially age hardened as you propably know or at least should.

"None of these alloys you mention is considered a 'high-strength' hardenable alloy" I didn't claim so, but Zn alloyed types can be classified like that even with zero copper content. A high-strength classification was not a requirement in this thread anyway.

[sailor2]

http://aluminium.matter.org.uk/conte...eid=2144417047
Make sure to press all these 3 buttons that shows on that link to see all the content there !!!
* Solute Mg effects
* Stress and strain
* Corrosion and strength

Suitable info for some aluminium alloys discussed in this thread :
http://aluminium.matter.org.uk/conte...eid=2144417081
http://aluminium.matter.org.uk/conte...eid=2144417085
http://aluminium.matter.org.uk/conte...eid=2144417086
http://aluminium.matter.org.uk/aluse...asp?AlloyID=23
http://aluminium.matter.org.uk/aluse...asp?AlloyID=25
http://aluminium.matter.org.uk/aluse...asp?AlloyID=26
http://aluminium.matter.org.uk/aluse...asp?AlloyID=27
Some relevant quotes from those links :

EN AW-2024 / ISO: Al Cu4Mg1 Composition: Al 4.4Cu 1.5Mg 0.6Mn
Characteristic Properties: Heat treatable alloy. Very good machining characteristics. High strength alloy with a strength slightly higher than 2014(A) and 2017A and 2030. High fatigue strength. Suitable for welding only by resistance welding. Corrosion resistance only with cladding or other protection.

EN AW-6082 / ISO: Al Si1MgMn Composition: Al 0.9Mg 1.0Si 0.7 Mn
Characteristic Properties: Very good corrosion resistance. Very good weldability (lowered strength values in the zone of welding). Good machinability. Good cold formability in T4 temper after a stabilizing heat treatment. Heat treatable medium high strength construction. Alloy with a strength somewhat higher than 6061. Medium high fatigue strength. Not suitable for complex sections.

EN AW-7010 / ISO: Al Zn6MgCu Composition: Al 6.2Zn 2.4Mg 1.8Cu Zr
Characteristic Properties: Heat treatable very high strength alloy with a strength slightly higher than 7075. Very high fatigue strength. Stress corrosion resistant. Not suitable for high temperature applications or fusion welding. Corrosion protection is recommended also in atmosphere.

EN AW-7020 / ISO: Al Zn4,5Mg1 Composition: Al 4.5Zn 1.2Mg Mn Cr Zr
Characteristic Properties: Heat treatable high strength weldable alloy. High fatigue strength. High strength in welded structures. Suitable precautions (aging after welding) must be taken against stress corrosion cracking and exfoliation.

Note that 7020 doesn't have to contain any copper, yet classified as high strength alloy unlike 6061 or 6082, as I said earlier. don't miss the statement on 7010 that even with atmostheric conditions instead of marine ones, corrosion is an issue !

[Jimbo1490]

Note that 7020 is also prone to exfoliation, a severe and always structurally significant form of corrosion wherein the metal basically disintegrates.

Jimbo

Very informative and interesting statements above.
A laymans question:
Will they answer the question of the topic Best aluminium alloy for the masts eventually?
Regards
Richard

[sailor2]

Originally Posted by Jimbo1490

Note that 7020 is also prone to exfoliation, a severe and always structurally significant form of corrosion wherein the metal basically disintegrates.
Jimbo

Which is why I didn't recomment it for mast production. Just provided the evidence contrary to your statement that "copper is mandatory to acheave high strength aluminium alloy". Just as I did on the other statement of yours that "This is why only alloys with copper can be 'precipitation hardened"

I just don't like incorrect statements no matter who writes them, that's all there is to it. Don't take it personally, I treat others just the same way when I find claims to be incorrect instead of just being different opinions.

[sailor2]

Originally Posted by apex1

Very informative and interesting statements above.
A laymans question:
Will they answer the question of the topic Best aluminium alloy for the masts eventually?
Regards
Richard

Depends on the mast. On most cases stiffness of the stayed mast is the critical issue for dimensioning, which means a lot of alternatives are strong enough and a mast designer can from theoretical point of view consentrate on what is most corrosion resistant or in practise, what is most cost effective and easily available.
Most used alloys are 6082, 6061 and even 6063 or 6060 in small boats.

Any alloy from 7000-series is a rare exception, but those have been made.
I would guess AC boats used those before carbon took over, as longetivity was no issue for them.

Haven't heard anybody having any 2000-series alloy masts, which doesn't prove there isn't any.

There are no facts about what's best for average joe. Only opinions and mine is go with most used 6000-series alloys, easy to maintain as riggers have a lot of practical experience of those.

[Ad Hoc]

Sailor2
You state that 7020 doesn't "have to" contain copper, that in itself is misleading. Since all commercially available 7020 contains 0.2% copper.

JImbo1490.
Not sure how a heat-treatable high strength alloy as 7020 is prone to exfoliation corrosion? Since 7020 is high strength, there is no requirement to roll it (strain hardening) from its final heat treated temper, ie mechanically. The heat treatment gives greater properties. To my knowledge, I've never seen 7020 offered as rolled plate, strain hardened at that. In fact is it extremely rare to find 7000 series as rolled and strain hardened plate. Since strain hardening is generally used with alloys containing Mg/Mn to improve its mechanical properties. This is not necessary with 7000 series as it is already high strength.

Exfoliation is caused by exposed edges (of the many layers in the alloy micro structure) when an alloy has too much strain hardening, the layers just peel away. It can also be caused by cold roll process when strain hardening the alloy to a different temper, such as H321, instead of a hot rolled process, when strain hardening the alloy.

[Jimbo1490]

Exfoliation is caused (fundamentally) by different alloying agents that don't play well together. It can be kicked off by atmospheric exposure or any other cause for corrosion, but it sometimes happens without any apparent external cause, just the 'fundamental' one. I have removed new surplus Lockheed wing planks from factory crates, still bathed in cosmoline, only to find that they had disastrous exfoliated corrosion. The alloy was 7078. As far as I'm aware, all the 2000 and 7000 series alloys contain copper, which is exactly why they can be precipitation hardened. So in the end you have to decide if the loss of strength (by alloying without copper) is worth the extra corrosion resistance. But copper is not "the enemy of aluminum" even in this context. It's just another engineering trade-off.

Jimbo

[Ad Hoc]

Exfoliation is simply the delamination of the thin layers of the aluminum, from strain hardening the alloy, with white corrosion products between the layers from the environment, ie sea water and other elements in solution.

You're probably referring to the result of exfoliation corrosion owing to poor maintenance, which in reality is bimetallic corrosion. Such as often found next to fasteners where an electrically insulating sealant or a sacrificial cadmium plating has broken down, permitting a galvanic action between the dissimilar metals. This is common on poorly maintained aircraft structures.

[Jimbo1490]

The exfoliation I'm referring to happens without any dissimilar metals in contact with the structure at all (other than those dissimilar alloying agents), only other aluminum structure and aluminum fasteners. This need not be caused by strain hardening. Heat treated aluminum is always fully annealed as part of the heat treating process, so that would not be a factor.You don't see very much of this type of corrosion on aluminum in the marine industry because most boat builders know better than to use these alloys (2xxx and 7xxx) in a marine environment, but spontaneous exfoliation is quite common in aircraft made from these alloys. But as they are as strong as mild steel, they are quite attractive to aircraft designers.

Jimbo

[sailor2]

Originally Posted by Ad Hoc

Sailor2
You state that 7020 doesn't "have to" contain copper, that in itself is misleading. Since all commercially available 7020 contains 0.2% copper.

From the standard the 0.2% is the max allowed to fit under 7020 designation. The standard does not specify minimum copper content. Which means the properties of 7020 can be accheaved without any copper content.
There is allowable range for Zn content as well as Mg content as these are necessary for properties to be what the standard requires.

What commercially available 7020 contains depends what is ordered if you order enough quantities. Quite many section manufacturers on this planet, are you sure you know them all that well ?

EN AW-7020
Zn 4.0 - 5.0
Mg 1.0 - 1.4
Mn 0.05 -0.50
Fe <= 0.40
Cr 0.10 -0.35
Si <= 0.35
Zr+Ti 0.08 -0.25
Zr 0.08 -0.20
Cu <= 0.20
Total Other <= 0.15
Other Elem <= 0.05
Al Remainder

All compositions in wt%.

Jimbo wrote :

As far as I'm aware, all the 2000 and 7000 series alloys contain copper, which is exactly why they can be precipitation hardened.

so how do you explain the following then ? Are those folks at European Aluminium Association just lying or what ?

As with some of the 2xxx series, alloys in the 7xxx series also have additions of magnesium to maximise their age-hardening potential where the precipitating phases are typically of the type MgZn2. Such alloys give medium strength, but are relatively easily welded. Aluminium-zinc-magnesium alloys have a greater response to heat treatment than the binary aluminium-zinc alloys resulting in higher possible strengths. The additions of zinc and magnesium however decrease the corrosion resistance.

Possible precipitates in aluminium alloys include at least : Mg2Si, Al2Cu, MgZn2, LiAl3.
Any of these allow precipitation hardening by heat treatment.
Jimbo, the site I gave in the link for is very informative and intended to be educational. Also in metallurgy. If you really think copper based Al2Cu is only possible precipitate you should educate yourself a little more. http://aluminium.matter.org.uk/conte...geid=989848382

[Ad Hoc]

Jimbo
That's interesting. You're quite right 2000 and 7000 series are rarely used in the marine industry, for good reasons too. But i cant get my head around exfoliation corrosion occurring if the alloy has no strain hardening. Since where are all the layers of atoms in the microstructure, to delaminate? Perhaps you're referring to intergranular corrosion, which i can well understand in your situation is probably the mechanism. Since, if an alloy is not strain hardened, i find it hard to understand how exfoliation corrosion can take place.

Sailor
Again, interesting. I have limited knowledge of 7020, but each time i have referred to it, or been asked to, in the past, I've always been quoted 0.2%, but never as a max, always as an absolute value.