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Or in other words: how do cracks inside metal survive given the phenomenon of cold welding? If metal is overstretched, one or more fine cracks result along imperfections in the metal lattice. Why don't they disappear again if the stress force is removed or even changes the direction, f.e. if a force is applied that enables the 2 adjacent surfaces to touch again or even to be pressed together as in case with oscillating forces. From length standard calibration gauge blocks it is known that they can be cold welded together if 2 of them are put one above the other. That way both polished surfaces can be linked with each other's lattice. It's not a matter of atmospheric pressure as with bathroom hooks. Normal contacts between 2 metal parts as in screw threads, flanges and many other technical connections probably do always have enough separating layers of oxides, grease, dust etc. even if pressed together with high forces to avoid cold welding.

A crack is not surrounded by even surfaces but those fresh surfaces should perfectly match if no tiny bits did break away, and if the crack is completely inside the metal, no sulphuric or oxide layers can settle, what is the case with every metal surface that gets in contact with air after very short time.

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  • $\begingroup$ Just found a similar question: physics.stackexchange.com/questions/87107/… $\endgroup$
    – xeeka
    Commented Nov 14, 2017 at 6:13
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    $\begingroup$ Similar, but still unanswered. Maybe some of the cracks are crystalline defects, and without huge perturbations those probably can't be "welded". And by huge I basically mean "making the crystal again" or giving it quite a bit of energy. Even when playing with beads of similar sizes in a 2d surface stuff like that happens. $\endgroup$
    – Vendetta
    Commented Nov 21, 2017 at 16:16
  • $\begingroup$ For instance, a very handy demonstration of that can be found in youtube.com/watch?v=O3RsDIWB7s0 . $\endgroup$
    – Vendetta
    Commented Nov 21, 2017 at 16:23

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Indium repairs itself at room temperature but it is very soft with a melting temperature of 157C. The answer is therefore that at room temperature self diffusion in most metals is very much suppressed: the cracks are frozen.

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  • $\begingroup$ But cold welding is a serious problem not only with indium in space vehicles and stations. And the problem with the gauge blocks appear at room temperature - are they made of indium? $\endgroup$
    – xeeka
    Commented Dec 5 at 18:44
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It seems to me that the impurities that have coalesced at the triple points between the crystals forming the metal would be happy to migrate back along the crystal surfaces, preventing cold welding. We can also assume that local forces exceeded what is needed to permanently deform local areas, preventing the surface from ever mating again. (Reversing the force will not cause local deformations to collapse.)

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There are many who have attempted to repair cracked blocks. The main difficult is that you cannot simply weld an engine block since it is made from cast iron, not steel.

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    $\begingroup$ The question was not about welding a cast iron part at all, this answer isn't really relevant. Also, you can still weld cast iron. $\endgroup$
    – JMac
    Commented Apr 26, 2019 at 11:35

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