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I know it's hard to quantify, but are rocket designers and astronauts more happy about the possibility of cold welding, or more negative about it?

I mean: two "bare metal" surfaces in vacuum weld together in the ambient temperature of space when they come in contact. In one hand, it could be invaluable in construction work, saving the whole bother with welding equipment - a good smack of a hammer will take care of bringing the elements in contact firm enough to form a weld. On the other hand, everything that isn't supposed to weld itself to anything else (like the aforementioned hammer) must be coated with, or made of non-welding substances. And if the protective layer is scratched, that still can "catch". It must add quite a bit of headaches.

So - in the absolutely subjective opinion of space equipment makers - do the headaches outweigh the benefits or vice versa?

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    $\begingroup$ This ESA study should shed some light. Contact data of different materials is included, and may be accessed online. $\endgroup$ – Jerard Puckett Apr 8 '16 at 22:15
  • $\begingroup$ To get a good cold weld, the surfaces should be aligned very well before contact. A good snack of a hammer may result in a bad alignment and only a small fraction of the surfaces areas being welded. $\endgroup$ – Uwe Jun 29 '17 at 21:11
  • $\begingroup$ "A good smack of a hammer" - is not a simple thing in zero gravity. You need a special hammer with absorption of inertia. $\endgroup$ – Peter Nazarenko Aug 30 at 13:16
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Virtually everything that goes into space is assembled on earth first. Nothing (to my knowledge) is 'assembled later' in orbit using cold welding, as there's no guarantee the weld will be of sufficient strength.

In space exploration, as it is currently undertaken, cold welding is only ever a bad thing. It usually disables some mission-critical piece of moving hardware, such as a reaction wheel.

However, I can imagine in the future, when humanity's presence in space is much more established, and complex products are manufactured in space, cold welding may indeed be a technique for the joining together of different parts. I doubt it would make as strong a weld as other pre-existing techniques, but then again, high join strength isn't always strictly necessary for something to be useful (for example, the adhesive on post-it notes).

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    $\begingroup$ +1, I've never heard of it being a good thing. $\endgroup$ – Organic Marble Feb 11 '19 at 23:30
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    $\begingroup$ @OrganicMarble perhaps there's an SF plot somewhere... $\endgroup$ – uhoh Feb 11 '19 at 23:51
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    $\begingroup$ @uhoh: Yeah, you caught me. $\endgroup$ – SF. Feb 12 '19 at 9:07
  • $\begingroup$ If two parts should be cold welded, the surfaces need some preparation first. Remove any oxide layers, dust and other contamination to get a clean, level and scratch free surface. Align the parts perfectly before welding them, but avoid any accidental premature contact. No correction of alignment is possible after the first contact. But how to get a gas tight cold weld without using ultimate fit tolerances? $\endgroup$ – Uwe Feb 12 '19 at 10:37
  • $\begingroup$ @Uwe: I'd imagine adhesive protection layer removable just before assembly, and simply enough pressure (even aforementioned hammer) to make any surface imperfections non-issue by smashing the surfaces together. Apply enough force and you will get ultimate fit tolerances out of anything. $\endgroup$ – SF. Feb 13 '19 at 8:41

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