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I Cold welding

  1. May 30, 2017 #1
    I've read that when two pieces of the same kind of metal touch in space, they will fuse because there is no way for the electrons to "know" that they are in different pieces of metal.

    What would happen if two different metals were to touch in space? Would electrons still flow freely between the two pieces? If so, I'm guessing one piece might attract electrons more strongly than the other, and the two pieces would be held together by electrostatic attraction (without fusing).

    Also, has anyone ever measured how long it takes for cold welding to happen? Would be close to instantaneous or would you have to press the two pieces together for a long time?
     
  2. jcsd
  3. May 30, 2017 #2
    A reference to where you read that might be nice, but it doesn't sound reasonable to me. The only cold welding I've done is wire wrapping, that is wrapping a copper wire around a metal post. Because the post is squarish the corners exert considerable pressure on the wire fusing the outer layer of the wire to the post. I think most cold welding relies on pressure to form the bond.

    Cheers
     
  4. May 30, 2017 #3
    Thanks. I can't remember exactly where I heard about it first. It was during a video about some NASA mission where the hatch got stuck for several minutes due to the parts cold welding. I found a Wikipedia article about it, but it doesn't go into much detail.
     
  5. May 30, 2017 #4
    Oh OK, that sounds a bit like your tongue getting stuck on a metal rail on a freezing cold day. It's a bit different than what I understood by cold welding.

    Cheers
     
  6. May 30, 2017 #5
    I would think the process is essentially instantaneous and requires little to no pressure. After all, the successful weld is energetically much more favorable than the separate pieces, so there's nothing to overcome first.

    Regarding two different metals, those don't have the benefit of what makes a cold weld so strong. The cold weld is so strong because there truly is no difference between two pieces of cold-welded metal, and just a contiguous metal block. So, separating two cold-welded pieces is like tearing apart a block of metal.
     
  7. May 30, 2017 #6
    In principle, yes, pure metals can cold weld; however, you need to have very clean and smooth surfaces. Most metals have oxide coatings that inhibit such welding, and most surfaces do not experience intimate contact when pressed together because extremely small unevenness causes only relatively few contact points. Thus, only a very small amount of the total contact area is actually touching, resulting in weak welds.
     
  8. May 30, 2017 #7
    Yeah, I think accidental cold-welding really only happens in space where you have no oxidization, and perfectly machined surfaces.
     
  9. May 31, 2017 #8
    Cheers. Thanks
     
  10. May 31, 2017 #9

    Vanadium 50

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    If you don't want oxidation, you need to have done the machining in space.
     
  11. May 31, 2017 #10

    Nidum

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    Dynamic cold welding is a known problem with sliding surfaces . The weld is not usually permanent but more like a continuous weld/break/weld/break cycle which causes high drag and tears metal from the surfaces and/or deposits metal on the surfaces .

    Not the best article but explains the basics : Galling
     
  12. May 31, 2017 #11

    Nidum

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    Not cold welding as such but there is the related subject of friction welding where initially cold surfaces are welded together with heat generated by mechanical power input .

    This can be an unwanted destructive effect and is one of the main causes of bearing seizure .

    Paradoxically though it can be a very useful effect and it is used deliberately to weld together parts for both cheap consumer products and expensive high performance components for the automotive and aerospace industries .

    Friction welding
     
  13. Jun 1, 2017 #12

    JBA

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    Cohesion between two identical highly oxidation resistant high hardness alloys can occur in our atmosphere if the surfaces are very highly polished to the point that no air or gas can exist between the two surfaces when they are in contact, even at low surface contact pressures. Some years ago, we experienced this phenomena on gas tight relief valves on the polished contact surfaces of the seat and disc. The result was not the same as nor have the strength of of a "cold weld" but it was sufficient to cause (after a period of contact) an overpressure valve opening set point error in excess of the allowable limit. It took some time for us to identify the issue, but in the end, changing the alloy of one of the seat components completely solved the problem.

    A similar phenomenon is experienced while utilizing highly polished gaging blocks once used in precision measuring assemblies; that, when they are carefully cleaned and stacked, are subjected to what is (or was once) known as a "wringing" process (a process of sliding and rotating the two surfaces together to the point that the two surfaces are tightly in contact). After the reference measuring is completed it can be extremely difficult to separate the blocks for return to storage. (As to how much of this is due to metal cohesion vs. atmospheric pressure on the blocks opposite outer faces is an unknown to me)

    I also think there is a similar issue with metals freshly cleaned and polished using the ionized water process.
     
  14. Jun 1, 2017 #13

    OCR

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    :check:

    Gauge blocks
    ...
     
  15. Jun 1, 2017 #14
    At one point, we had a lot of left over Silicon Nitride shapes in the lab. They were used in a project development of ceramic armour, with acrynom WHISTLE. Anyway, the surfaces of these blocks were micro-machined to very good flatness and smoothness, and in fact, we used them to sharpen razor blades with no lubricant. As you would guess, you could put two blocks together with matching machined surfaces and they would stick, presumably because pressing them together would expell most all the air between them, thus forming a vacuum. (With absolutely no air in the joint other than Silicon Nitride surfaces, there would theoretically be a perfect vacuum.) These blocks would not cold weld, as far as I could tell, so I conclude that not all materials will cold weld. Someone mentioned friction weld, which can be a useful process for plastics and metals. We discovered this process on our own in the machine shop, spinning shapes on the Bridgeport. But friction welding is indeed hot welding. Galling is a kind of enhanced cold welding, with sometimes hot welding, depending how fast the parts move in relative motion. Molybdenum is alloyed with steel in order to resist galling.
     
  16. Jun 4, 2017 #15

    JBA

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    With regard to the usage of the term Gage. Gage is a variant of the word Gauge. Ref: Webster's Dictionary, definition(s) for the word Gauge: " 2. usu gage ; an instrument for or a means of measuring or testing as: a : an instrument for measuring a dimension or for testing mechanical accuracy b: and instrument with a graduated scale or dial for measuring or indicating quantity" usu = usually

    (Note: This copied in verbatim, including the usage of bold type for the word and letter designators.)

    Clearly, in current times many use the general spelling for this word; but, some of us that have been around for a bit longer are also familiar and seen the variant used, particularly as to the instruments used for quality control; and, in older technical reference documents for dial type instruments as well.
     
  17. Jun 4, 2017 #16

    OCR

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    Last edited: Jun 4, 2017
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