aluminium alloy for thin pressure vessel
aluminium alloy for thin pressure vessel
(OP)
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
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





RE: aluminium alloy for thin pressure vessel
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,
RE: aluminium alloy for thin pressure vessel
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.
RE: aluminium alloy for thin pressure vessel
Are you sure you're not limited by codes and regulations?
RE: aluminium alloy for thin pressure vessel
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?
RE: aluminium alloy for thin pressure vessel
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.
RE: aluminium alloy for thin pressure vessel
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P.E. Metallurgy, Plymouth Tube
RE: aluminium alloy for thin pressure vessel
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.
RE: aluminium alloy for thin pressure vessel
RE: aluminium alloy for thin pressure vessel
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.
RE: aluminium alloy for thin pressure vessel
- 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).
RE: aluminium alloy for thin pressure vessel
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?
RE: aluminium alloy for thin pressure vessel
RE: aluminium alloy for thin pressure vessel
forming and regardless of product form and material,
shall be 1/16 in. (1.5 mm) exclusive of any corrosion allowance.
Regards
r6155
RE: aluminium alloy for thin pressure vessel
RE: aluminium alloy for thin pressure vessel
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