Pressure vessel weld

[btrettel]

I want to build a small low pressure air tank. I'm considering thin-walled aluminum tubes for their low weight. I know a bit about solid mechanics so I can calculate the stresses in the tube. The maximum stress in the tube is about 2.6 ksi at 100 psi, which is a safety factor of about 13.5 for 6061 T6 aluminum.

The problem is the effect welding would have. While I am a mechanical engineering student, I'm only a sophomore and haven't learned much about materials and manufacturing processes. This aluminum is heat treated, so I'm thinking that welding some plates on the end would affect the heat treating and therefore the strength of the aluminum.

Is there anything to worry about? I figured it would be best to ask about the weld before building what could be a small bomb.

Edit: Of course I check Wikipedia afterward to find the aluminum near the weld simply goes to 6061-0 and I can use those figures for comparison. I suppose any suggestions are welcome at this point unless this information is inaccurate.

[Supe]

Buy the tank.

There are a LOT of other considerations in designing any pressure vessel, and just as many considerations for welding them (even from a legal standpoint, not just a design standpoint).

If you've ever seen the results of a PV that fails a pneumatic test in the field, you'd get a very good grasp of what it is you're dealing with.

[A_DAB_will_do]

I'll second the first recommendation. Buy the tank.

To answer some of your later questions, yes, welding will severely affect the mechanical properties(YS, UTS) of 6061-T6 aluminum. You are correct in realizing this is a heat treated grade, and it takes very little additional heating to over age or, in the case of welding, fully anneal the heat affected zone(HAZ) around each weld.

T6 has an approximate YS = 241MPa(35 KSI) . Annealed 6061 YS = 82MPa(12KSI). That's a factor of 3 reduction in YS, and I would call that a generous estimate...

Of lesser concern are the fatigue properties of aluminum. It may or may not be an issue in this application....But Aluminum alloys, and all FCC structured materials have no lower fatigue limit; unlike BCC materials like steel. If stresses are less than ~1/2 the YS in steel fatigue is generally not a concern. Fatigue is ALWAYS accumulating in Aluminum. This is not a good property for a pressure vessel.

I'm not suggesting that aluminum can't be used in this kind of application; but where it is used there are special precautions made to inspect and periodically replace the stressed aluminum before it fails.

[btrettel]

Thanks for the replies.

I'm looking at other things right now because it just doesn't seem to be a good idea (not that I was thinking it was too great of an idea to begin with). At worst I'll continue using my heavier HPA tank and regulator. I'm considering other options too, though, it doesn't seem most of them will fit my budget so I'll probably continue with the HPA tank.

If anyone could suggest a small (75 to 125 cubic inch) pressure vessel for sale or a company that makes them I'd appreciate it. I am having difficulty finding anything with a listed price.

Edit: Something like these are exactly what I want. That company didn't list a price though. I emailed them but from my dealing's with companies like this before when a price isn't listed it's too high. I'll wait and see though.

I'll start looking at lightweight steel pressure vessels too.

[wiLlweLd4FoOd]

wikipedia is only good to get the general idea on a subject. Most college professors will not accept it as a reference. It is mostly opinions and the information on it is wrong slot of the times. I have known of people to use 6in pipe for a bumper and also use it for on board air. Very bad idea.

[btrettel]

You can spare me the lecture on Wikipedia--I've heard it many more times than necessary. This is not a reference, rather, it is verification of what I assumed. As you said, Wikipedia is a good for a general idea of a subject, which is all I needed.

A little more research on my part makes me think the low pressure aluminum tanks I've linked to above shouldn't cost more than $50, which would be great. But I'm still waiting for a reply. If anyone has seen anything similar I'd appreciate some information. That link was all I could find about small low pressure tanks.

[hotrodder]

FWIW there's a suprisingly large range of aluminium pressure vessels around, the high pressure flavours tend to be manufactured from a single billet (diving, industrial gases etc), lower presure types welded (automotive applications such as air suspension tanks, even airbag systems).

While it's annoying when websites have no prices i wouldn't automatically assume that it's because they're pricey- may just be because they are a manufacturer or wholesaler and don't deal direct with the public. The link to catalinacylinders website... they're a manufacturer, doubt they're set up for dealing with the odd single sale so no point in prices. Intrestingly those vessels are manufactured from 1100 and 3003 aluminium, non heat treatable alloys with very low properties even with the strain hardening that'll occur from manufacture

I'd be suprised if a little time with google didn't turn up something suitable at a reasonable price.

small aluminium air tank Didn't have a proper read but it appears that these may all be a bit too small for your requirements- dirt cheap though

[firestick]

Yes Buy The Tank - Round The World Bodgy Homemade P/v Tanks Prevail.
Check Out A Professionaly Made Tank , Nameplate Attached Or Stamping Mark Will Have The Following Or Similar.
Manafacturer.working Pressure, Test Pressure,date And The Tester Number Or Mark, They Have To Conform To Regulations Test Certificates For Material , Fittings And Much More.
Keep An American In A Job Or Even An Aussie Buy Marked Us Or Even Aus.

[btrettel]

I called the manufacturer on the link I posted earlier. They are a manufacturer, but they didn't have a problem with selling a single tank. The tank I was looking at cost only $17.50. And a higher capacity one was only $40. This is very reasonable, especially given that either tank weighs less than a pound!

I almost feel stupid for considering making my own. Making my own would have been more expensive, less safe, and heavier. Hopefully someone looking for a similar aluminum tank will just go for manufactured options.

Thanks again to everyone who replied.