[Jewelry making - Article 00096] - [Orchid] Laser Welding Niobium

[Alberic]

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Help needed: laser welding niobium

Greetings:

    I have a problem: I've got a commission that's due early next week
    that requires several pieces of niobium to be laser welded together.
    No problem: I've got a buddy with a Rofin Starweld that I can use.
    Unfortunately, he's a chip-engineer, and uses it for micro work on
    computer prototypes. Thus, he knows *nothing* about using it for
    reactive metals. (Didn't even know what they were until yesterday.)
    We did some samples, and they worked reasonably well, enough that I
    pulled out the real pieces and started tacking them. Suddenly, the
    god Murphy reared his ugly head, and the welds went to hell. We'd
    been having trouble with getting brittle welds, but we increased the
    argon flow, and that seemed to help. As soon as I pulled out the real
    pieces, everything got glass brittle. The samples were done in the
    off-cuts from the real pieces, so they were exactly the same pieces
    of metal. 

    Here's the setup: The critical welds are on 12Ga Nb wire straight
    through a sheet of 20Ga Nb sheet, so as to get a 'through' joint.  I
    wasn't too trusting of the welds, so I designed it so that most of
    the stress will really go to the pin going through. They're coming in
    at an angle, so there's some unavoidable slop in the joint initially.
    The main problem is that the joints are just brittle as hell, even
    with the argon blasting on them from about 1/4" away.  In reading the
    archives (yes, of course I read the archives before posting.  Doesn't
    everybody?) I note a post that makes me think that high-velocity
    argon close up may be part of the problem, and I should slow it down
    and back it away a bit. I'll try that tomorrow. This particular Rofin
    has the ability to tailor the profile of the laser pulse. Currently,
    we're using 350-400 volts, 10Ms, and the beam diameter set to about
    .5mm, with the beam at full power for the entire 10Ms pulse. We've
    both had the thought that perhaps a high power spike initially
    followed by a 'cool down' phase might be useful. Or perhaps a slow
    ramp up to a big hit. We don't know. In one of Peter Rowe's old post
    he says something about messing about with the nature of the pulse
    being useful for reactives, but unfortunately, he doesn't say *what*
    changes in pulse profile to use. Anybody got any clues? This is a
    "cannot fail" sort of job. 

    One other issue: I'm getting intermittent coatings of black crud
    that *looks* like carbon flashing....except there's no carbon
    anywhere to be burning. What is that, and what causes it?  I'm also
    getting a little dusting of some sort of light blue vapor deposits
    coming out of some of the joints.  (Looks like the zinc that burns
    out of overheated brass.  But it isn't, of course.  So what *is*
    it?) 

    I'm thinking that I may have the power up too high as well?  The
    initial welds I'm doing with it jacked up high enough to burn about
    half way through a piece of 12Ga wire. The idea being to burn in
    half way, then fill a little with some 24 Ga wire, and finish up with
    the beam down low and wide to polish the surface. They looked
    great...and snapped like glass. Pretty clearly it's gas embrittlement
    of *some* sort, but we're out of ideas on what or how. Rather than
    burning irreplaceable parts all night, I decided to head home and
    look for answers.

    Any help will be most appreciated. 

    Brian Meek. 

    PS--> anybody else with Rofin experience: the only manuals we've got
    are in German, and mine isn't that good any more. There are two flex
    hoses for gas delivery in the weld area. The right side one is the
    argon. Looking at the manual, I suspect the left one is an air jet
    for air-cooling of parts?  It's not another argon jet is it?

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