aluminium alloy for thin pressure vessel

[scander]

Hello,

I am working on an aluminium pressure vessel design application with one very thin area of (ideally) 0,5-1mm thickness. With such thicknesses even 0.5-1bar of differential pressure is too much for traditional 5000 or 6000 alloys so naturally I turned towards 7000 and in particular 7075. I have never used it before in a design (not done any pressure vessels in the past either) but its mechanical properties are impressively superior to 5/6000 series ( >200% higher yield strength) and could be just what we need in order to make this vessel. Cost is nowhere near as bad as I expected either: Our local workshop quoted 10-15% more for the exact same vessel if made from 7075, compared to a typical 5xxx alloy.

So I was wondering where is the catch? Weldability? We do need to weld certain areas and ensure as strong joints as with 5000 alloys but the workshop din't complain on a first discussion we had with them.

Other than the higher cost, is anyone aware of other drawbacks of the 7000 series? Right now I am wondering how come not everybody uses it as the default material for thin pressure vessels, so am I missing something here?

Thanks,
George

 

[marty007]

I am not familiar with aluminum for pressure vessels, but some questions I would be asking:

What is my code of construction? (ASME VIII-1?)
In what specifications does this material appear? (SB-209?)
Is the 7075 grade of this material allowed in this code of construction?

Good luck,

 

[scander]

Hi Marty,

This is not about a commercial/mass-production item, more like a one-off testing prototype so the choice is not limited in terms of codes and regulations (there still are some internal codes but they don't prohibit these alloys). Basically our only requirement for this application is to minimize the thickness and so my question is limited to mechanical / machining / welding properties and possibly unique characteristics of the 7075, as my first read on its properties does not reveal any particularities.

 

[marty007]

We build a lot of equipment for testing labs, and they all still must be registered pressure vessels in compliance with our local boiler authority...

Are you sure you're not limited by codes and regulations?

 

[metengr]

Regardless if this is one off or your making a million of these vessels, safety is paramount.

First, is this a pressure vessel that falls under the scope of a construction code like, ASME B&PV Code, Section VIII, Div 1? Better review and determine this because should an accident happen you will be held responsible for not using a recognized international standard for pressure vessel design and construction.
Second, what is your design basis for construction? NDT?

 

[scander]

I appreciate the concerns for construction codes but I can assure you that it is not the case with the vessel in discussion. Given the region, I assume it falls under European Pressure Equipment Directive, certainly not ASME. We do have an internal vacuum handbook also and to my knowledge there should be no problem with 7075, but in any case, the final design will undergo review and approval by a separate vacuum department at a later stage. The current task is to suggest geometry, size and possibly materials as a basis for the detail design that may or may not be carried out in-house as this is a side equipment of what we are actually working on. Due to 7075's properties we could avoid much complexity and keep the cost down while also meeting the thickness target. The vessel itself is not the main design, it is merely enclosing the main assembly with slightly pressurized air or some other non-toxic gas, operating at steady temp, no flammable/hazardous materials involved.

So before straying away into a norms and codes debate, I'd like to re-iterate that the question here is of a more technical nature, ie. what are people's experiences with 7075 in pressure vessels (if any), assuming that it is in accordance with each application's codes and regulations of course.

 

[EdStainless]

Since 7075 wasn't designed to be welded I would strongly suggest that you look at 6061. If this is going to be used in vacuum then you need to look at this again. Buckling under vacuum is only governed by the materials modulus, and Al has a very low value.

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P.E. Metallurgy, Plymouth Tube

 

[XL83NL]

Buckling under vacuum is only governed by the materials modulus, and Al has a very low value.

Perhaps that the catch?

Anyway, if PED is applicable, there's a clause in the PED that allows design for experimental research (article 14, sub 7), meaning no CE-mark (i.e. no conformity assessment procedure). Saves some effort, but still requires a safe design. Furthermore, some contries have excluded this sub 7 of the PED in their national regulations (Netherlands for example), meaning scientific research design is not allowed in those countries per their law. Since you havent stated the design pressure nor the country it will be placed in, it's very difficult to help you further on that aspect.

 

[israelkk]

There are scuba dive tanks made of aluminum alloy.

 

[scander]

So, the material has to be Al, this is a requirement. As Ed wrote, default choice would normally be 6061 and 1 bar of design pressure wouldn't be a problem unless we didn't want a specific area of the vessel to be as thin as possible, even tin-foil thin (as long as it's relatively leak-tight). This is probably not your typical hard-duty, high pressure vessel (the whole thing would resemble a small box of 0.5m x 0.5m x 0.5m in the end) and the only challenge is how thin we can make a certain rectangular area for safe 1 bar operation (no hazardous or corrosive materials/conditions) over a few hundreds load cycles in terms of fatigue. In that sense 7075 seems far superior to 6061.

Ed how hard is to weld 7075? The workshop I contacted didn't seem to have a problem although it could be it was just a quick "yes we can do it" kind of reply, since no drawings are yet made.

 

[XL83NL]

You just introduced 2 words which makes this even more non-nstandard, and which can introduce some requirements that can bump up the required wall thickness a lot.
- rectangular area ( a small box of 0.5m x 0.5m x 0.5). You need a speical design for that, like ASME VIII-1 app. 13.
- fatigue design due to cycling. What type of cycles are you referring to, pressure or temperature, or both, or ...?

Lean design (little excess wall thickness) can however be a good thing for improving fatigue resistance (like smooth transitions, no sharp geometries due to excess weld metal,etc).

 

[scander]

Your lean design suggestion has already been part of the design so far and wherever possible, edges and transitions have been smoothed as per common engineering design practice. However, due to requirements the shape can't be a proper cylindrical, round-end vessel and the simulated stresses are too much for 6061 unless we make it thicker, which is what we're trying to avoid. I will have a closer look at the part you are quoting but it's true that it is largely a non-standard design (scientific research facility)

As I said operating temp will be steady but we expect a few hundred cycles of pressure loads during the vessel's operation. Is there a reason to believe 7075 is considerably different to 6061 in that aspect other than the higher mechanical properties?

 

[metengr]

The reason 7075 aluminum is not regarded as weldable is the significant loss of strength with stress corrosion cracking, liquation grain boundary cracks , and crater cracks. 7075 is one of the strongest aluminum alloys and is typically not welded for structural purposes. It is not used for PV applications.

 

[r6155]

Per ASME VIII Div.1 the minimum thickness permitted for shells and heads, after
forming and regardless of product form and material,
shall be 1/16 in. (1.5 mm) exclusive of any corrosion allowance.

Regards
r6155

 

[Compositepro]

A google search of 7075 Al for pressure vessels reveals some good information.

 

[EdStainless]

Scuba tanks are not thin, and they are not 7xxx alloys (or 6xxx for that matter).
Vacuum tight welds in 7075 are difficult. You would want to use it in the annealed condition, so the strength advantage is not that great.
Scander, have you checked this design for deflection and not just stress?


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P.E. Metallurgy, Plymouth Tube