Hi All:
This is a fairly difficult part to even think about trying to make as one piece.
For those who have not done the conversion yet, and still think mostly in Imperial measurements; the bottom surface is 4 tenths...yes, that's 0.0004" thick.
You'll never turn it or mill it or EDM burn it successfully in one go, so a reasonable alternative to consider, is to attach a foil to a ring (as you've been doing) and ignore the fillet that's drawn in the corner of the pocket if you are allowed to.
At this thickness, there is a possible benefit to preserving the physical properties of the foil that it gets when it's rolled to thickness, so that excludes low temperature processes like furnace brazing or soldering unless it's also OK to lose those physical properties when you cook it.
You also may need to consider potential galvanic corrosion effects of dissimilar metals in the joint.
Trying to laser weld very thin foils to thick pieces has not been very successful for me in the past...I have had far better luck when a fin can be provided that I can fusion weld to that is the same thickness as the foil so I can do an edge weld which allows me to drop the weld energy right down so I don't blow a hole into the foil.
It is also theoretically possible to do a keyhole weld through the foil but your fitup must be perfect at these foil thicknesses, whether the energy source is going to be an electron beam or a laser beam.
As you can imagine, you cannot easily squeeze the joint surfaces together and still get access to the joint without some kind of indexable fixture that exposes a small segment of the joint surface at a time while acting simultaneously as a pressure plate.
You also need more filler metal than the foil alone can provide, so it's technically not really keyhole welding.
Another alternative to consider is resistance welding.
With an annular electrode, you can get a continuous weld and "may" be able to achieve a hermetic seal, but the weld engineering and process control necessary to do it reliably will not be trivial.
Since the electrode acts as the pressure plate, this approach is simple in terms of component fitup; just lay the foil on the ring and zap it.
For laser, EB or plasma welding approaches, a tiny machined fin on the outside corner of the ring, a pressure plate on the fixture that holds the foil against the surface of the ring, and then a fusion edge weld to melt the corner together is how I would probably approach it initially; assuming that this kind of re-design meets the other requirements of the part.
Implied in this approach, of course, is a crevice between the foil and the under surface of the ring, and this may be unacceptable for a variety of reasons.
Also be aware there is a stress riser at the apex of the crevice inherent in this kind of joint, so if it sees pressure differentials, it will be vulnerable to failure from cracking of the weld.
This will be true for any type of welded joint.
If you must make it one piece, the best way forward (although it will be a production nightmare), is to machine the pocket in a thicker block, then wire EDM slice it so the floor is as thin as you dare to go, then fill it with something like Cerrobend, then face the front dead flat, then flip it and surface grind and/or lap the underside to final thickness, finally melting out the Cerrobend.
If you can make resistance welding work, it will be the fastest and most production friendly, but you WILL have a crevice, even if it's just a small one, and you WILL have leakers so there will be 100% leak testing inspection in your future if you care about a hermetic seal.
You may also choose to resistance weld, then over-weld with EB or laser to improve your chances of a hermetic seal.
You will lose the physical properties of the foil in the HAZ with any welding process you choose; even EB welding.
You may or may not care.
On a last note, I concur with those who advise not using brass, but choose a grade of stainless steel suitable for your application.
The better physical properties of hard drawn stainless steel foil will help you in production in a whole bunch of ways, as will the wider range of joining options.
On a very last note; have you considered exploring diffusion bonding?
A customer of mine uses it in a filter application but I know virtually nothing about it.
They are bonding very thin stainless steel foil to itself... the process involves heat, pressure and controlled atmosphere but that's the limits of my knowledge about it.
On a very VERY last note, have you looked at the foil seal on your jar of peanuts that you peel open when you sit down with them in front of the TV?
Cheers
Marcus
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