7075 alloy

[J Neeves]

I was looking for something else and was sidetracked, as one does, after finding a manufacturer of 7075 alloy nuts and bolts. I am slightly familiar with this quality of alloy as Anchor Right use it for the shank of their alloy Excel which is identical in design to their steel model except for the shank that is a slightly thicker width (to give it a strength comparable to Bis 80).

7075 alloy has a yield strength of 500 MPa vs 690 MPa for Bis 80 and 200 MPa for 316 stainless.

I then wondered - most of the fittings on my mast and boom are stainless and are attached commonly with stainless rivets or stainless bolts tapped into the alloy of the mast and all of these need Duralac, or something similar, to retard galvanic corrosion. Using Duralac is both messy and expensive. My genoa track is alloy but has little plastic inserts between the bolts that hold it to the deck and the track itself. etc

So I then wondered - why are not all the fittings and bolts made from 7075 alloy.

I'm guessing 7075 is expensive but a few alloy bolts and fittings are not going to make much difference to the cost of a stg 100,000 or stg 250,000 yacht.

So what is the catch?

 

[vyv_cox]

I suspect (without looking it up) that the answer may be associated with the treatment that the alloy has received to achieve the quoted strength. For example, as you know, the Bis 80 undergoes a two stage heat treatment to give it a strength of 690 MPa. Without that treatment it would have a tensile strength comparable to mild steel or AISI 316. Aluminium sheet comes in a variety of strengths known as soft, half hard and hard, dependent upon the degree of rolling it has had in the mill. Many more complex aluminium alloys require solution treatment, a two or three stage heat treatment process, to get to their high strength.

Pop rivets need to be relatively soft to take the plastic deformation necessary to make them up. Not sure about bolts but I cannot think of any reasonably strong ones made in aluminium. Another factor might be that aluminium is not great in fatigue, which could mitigate against its use as fastenings.

 

[J Neeves]

Thanks Vyv,

I looked a bit further to find that 7075 bolts are actually common place, there was me thinking they were few and far between, but anyone with a focus on cycling and weight (of the bicycle) will be using 7075 bolts and lots of other bits and pieces made from 7075.

7075 bolts have almost 3 times the strength of similarly sized 316 stainless though I have not found anything bigger than M6 yet.

I'm thinking of testing a few, look out for the results in 6 -12 months time

Jonathan

 

[bogman]

7075 is used in the aircraft industry along with 2024. 2024 has useless corrosion resistance in salt water and I suspect that 7075 may be the same. Also the strength that you quote is for heat treated 7075 usually quoted at T6 or T3.

no harm in giving it a try but I'd watch the corrosion resistance.

 

[J Neeves]

Noted, just out of interest, what are masts made from (the actual alloy).

All the bolts I have seen, for bicycles, are T6. I assume, and one should never assume and do any work on that assumption, that 7075 plate could be sourced in a T6 treatment.

I'd be using in a non critical area to start with.

Jonathan

 

[bogman]

masts if they are welded are 5056 or 6061....as a general rule if you can weld Aluminium it is ok for marine use. 7075 and 2024 are not weldable.

 

[langstonelayabout]

And aren't most rivets used in modern alloy masts made out of Monel?

 

[vyv_cox]

And aren't most rivets used in modern alloy masts made out of Monel?

Not unless essential, I suspect. Monel is very expensive compared with aluminium.

 

[macd]

most of the fittings on my mast and boom are stainless and are attached commonly with stainless rivets or stainless bolts tapped into the alloy of the mast

That is largely true also on my mast/boom, except that most of the larger/high load fastenings employ stainless steel thread inserts. This seems to be standard on Selden spars (and is not necessarily obvious from a cursory look).

 

[J Neeves]

That is largely true also on my mast/boom, except that most of the larger/high load fastenings employ stainless steel thread inserts. This seems to be standard on Selden spars (and is not necessarily obvious from a cursory look).

Maybe I should have mentioned:

Our mast was built by a local spar maker (I think the extrusions are also local) who is the distributor for Selden who might influence how it was assembled. But all the tangs are stainless and fixed with stainless 'bolts', we have clutches and winches on the mast, fixed with stainless bolts and the aluminium castings on the mast (for the sheaves) are all secured using stainless bolts.

When the outhaul failed and we had to take the boom apart to replace it (in the middle of nowhere) the last thing we worried about was Duralac (which we used later) but if it had been a masthead fitting climbing a mast just to apply some Duralac would be a real pain - so I just thought alloy bolts would be so much more convenient.

Obviously not a very emotive issue

I'll report back once I've checked the weather performance of 7075 T6 in situ - so patience

edit Vyv wil correct me, a device that is fully threaded is called a 'screw' but one with an unthread portion is called a 'bolt' ?
- in which case replace my word bolt with screw, but I assume everyone knows what I'm talking about. close edit

 

[macd]

...so I just thought alloy bolts would be so much more convenient.

I certainly see your reasoning and will be interested to learn how you get on with 7075.
My existing preference, as you may have guessed, is for thread inserts -- additional to those already used as standard by Selden. They don't inherently prevent corrosion, of course, but confine it to the region between the insert and mother metal, where it is relatively harmless, rather than between thread and fastener, where it's a major pain. (Chief amongst these are those 2, 4 or 6 fasteners that connect the bottom of the furling drum to plates (or whatever) from the deck fitting. Most furling gear manufacturers invite you to remove and lubricate these annually, but almost no-one does. Add the fact that they're often button-head cap screws, which don't accept a lot of torque...)
'Recoil' is the simplest insert system I've encountered and...guess what...they're Australian.

 

[vyv_cox]

Fully threaded is a 'set screw' to me but there may be other definitions.

 

[howard easton]

the common useage when I did business with fasterner makers was to call anything fully threaded either a wood screw or a machine screw. But certainly a bolt is only part threaded.

I reckon the answer to mr Neeves original question is cost and convenience. If the stainless is strong elnough , it is readily available and fairly cheap. The mast maker isnt concerned about the corrosion you might suffer when the mast is 4 or 5 years outr of guarantee anyway. And you can see that approach right round your boat where stainless fasteners into aluminium is a common sight. And equally common is the way they seize because no one has given them any protection.

And its not unique to boat builders. The structure of my Lotus car is all aluminium and such things as the undertray are bolted into the aluminium with unprotected steel machine screws, most of which arent even stainless.

 

[PuffTheMagicDragon]

Fully threaded and without a head is 'set screw' as Vyv has said. If fully threaded but WITH a head I, have always known them as 'machine screws'.

 

[J Neeves]

Most of the things I have called bolts are not bolts at all!

So what is a stud, as it seems to be the same as a set screw?


Vyv, a question or 2

If one were testing for corrosion between dissimilar metals, stainless and alloy or stainless and lead (as it came up recently) or simply looking at corrosion of 7075 what is the environment one needs (for marine application). Does 'whatever' need to be simply splashed with seawater, or in seawater. And for dissimilar metals - what is the actual mechanism - does one need water, or oxygen or is it simply the 2 metals in contact (no oxygen (so sealed, somehow).

I was going to replace one of the stainless bolts on the mud palms of the Fortress as a start, they are not load bearing and there are 3 other bolts. I also have various pieces of alloy (5083 from memory 5 something in any case) I can use.

Jonathan

 

[vyv_cox]

Most of the things I have called bolts are not bolts at all!

So what is a stud, as it seems to be the same as a set screw?


Vyv, a question or 2

If one were testing for corrosion between dissimilar metals, stainless and alloy or stainless and lead (as it came up recently) or simply looking at corrosion of 7075 what is the environment one needs (for marine application). Does 'whatever' need to be simply splashed with seawater, or in seawater. And for dissimilar metals - what is the actual mechanism - does one need water, or oxygen or is it simply the 2 metals in contact (no oxygen (so sealed, somehow).

I was going to replace one of the stainless bolts on the mud palms of the Fortress as a start, they are not load bearing and there are 3 other bolts. I also have various pieces of alloy (5083 from memory 5 something in any case) I can use.

Jonathan

A stud doesn't have a head!

The standard test for most corrosion mechanisms is a salt spray cabinet. The ones I have used were a perspex box using compressed air to spray a standard salt solution very similar to seawater, at controlled temperature. The solution is sold by many laboratory chemicals suppliers. In practice anything in the cabinet, standing on racks, is saturated with the solution constantly. I guess you could make something up, in the absence of a compressed air supply, with a big stirrer throwing the solution around a container (bucket with a lid?).

The galvanic mechanism requires two metals to be in contact, immersed in solution. Electrons then pass from one to the other. No need for air, although in practice there will always be some in normal situations.

 

[J Neeves]

Thanks Vyv,

I'll conjure something up - but I'll simply use seawater, its cheap!

Jonathan

 

[vyv_cox]

I googled 'salt spray cabinet' to try to find some suitable examples for you. As with almost every piece of laboratory equipment, in 30 years they have acquired so many bells and whistles that they are now unrecognisable to me. However, I found the exact one that I have used, with two employers, at http://www.indiamart.com/amruta-electronics/powder-coating-equipments.html. Scroll down and you will find it.

The last time I used one I was assessing the corrosion resistance of bolts, coated and with different compositions. There's continuity for you!

 

[sailorbaz]

Short metallurgy lesson!
I am now retired but worked all my life in the Aluminium industry.
7075 is an Aluminium-Zinc-Magnesium-Copper alloy developed for aircraft and has high strength, good fracture toughness but poor corrosion resistance. It has about 0.3% iron as an impurity. Later versions, 7475 have less iron and slightly better corrosion resistance. On aircraft coatings are used to protect from corrosion but in my opinion these alloys have no place on a boat exposed to salt water!
Masts and spars are extrusions made from Aluminium-Magnesium-Silicon alloys which have lower strength but much better corrosion resistance especially when anodised. 7xxx alloys do not anodise successfully.
6061 is the American alloy with Chromium to stabilise the grain structure and 6351 is the British alloy using Manganese as a grain stabiliser. Both have a similar resistance to corrosion, although recent work has shown 6061 to have better resistance to stress corrosion.
7xxx and 6xxx alloys are heat treatable to develop maximum strength.
Boats with Aluminium hulls and superstructure are usually made from 5083 which is an Aluminium-Magnesium-Manganese alloy and is not heat treatable. It's strength is developed through work hardening during the rolling process.
This alloy has the best corrosion resistance.
So to sum up all alloys are a compromise - increase the strength and corrosion resistance goes down and the job of the metallurgist is to find the best compromise.
Sailorbaz

 

[noelex]

On aircraft coatings are used to protect from corrosion but in my opinion these alloys have no place on a boat exposed to salt water!

As a general rule the 5000 series aluminiums have by far the best corrosion resistance. The 6000 series aluminums are OK. They are used for some applications on a boat and are frequently protected by a coating like anodising. The 7000 series aluminums are significantly worse again.

If calculating strength it is worth noting that for this sort of aluminium the tensile strength is only slightly above the yield strength. The yield strength of 7075 depends on the temper etc but is in the range 370-480. Unfortunately, the higher strength T6 tempers have the worse corrosion resistance.

As has been noted yield strength of 7075 is better than 316, which has a yield strength of 200-300. However, the tensile strength of the two materials are similar, so take this into account, depending on the application, before you reduce sizes.

I think aluminium can be excellent in the marine environment (I have an aluminium boat so I have to say that ), but it needs to be the right grade of aluminium used in the right application. Too often aluminium gets a bad rap because non marine grade aluminium alloys are used in a salt water environment.