Say you have two lumps or blocks of an element, like lithium for example, say in the form of two bars.

Why, when you bring the two bars together so that they touch each other, do they not instantly bond with each other forming one larger bar or block? We can weld elements together so they 'stick' to each other, but what is the process that actually causes two like elements to bond together? Why do we need to 'weld' two bars together - why don't they just bond on their own?

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    $\begingroup$ Here on earth, the surface of a metal may react with the atmosphere to form an oxide. This oxide layer prevents two blocks of metals from bonding. Even if the metal does not have a layer of oxide, gas molecules get adsorbed to the surface of the metal, preventing metals from bonding. In general, surface area minimization is favorable for metals. Metals near the surface are in an energetically less favorable "surface states". $\endgroup$ – CoffeeIsLife Oct 20 '16 at 2:30
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    $\begingroup$ I recall a process that does exactly this, with fused quartz. It’s called “optical bonding” or something like that, since the surfaces to be mated are lapped mirror smooth. Then the pieces are brought together and become one. I read about it in conjuction with Gravity Probe B, I beleive. $\endgroup$ – JDługosz Oct 20 '16 at 7:59
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    $\begingroup$ Gauge blocks are of great relevance to this question $\endgroup$ – Nicolau Saker Neto Oct 20 '16 at 10:50
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    $\begingroup$ en.m.wikipedia.org/wiki/Cold_welding $\endgroup$ – The Garage Chemist Oct 29 '16 at 9:38

Why, when you bring the two bars together so that they touch each other, do they not instantly bond with each other forming one larger bar or block? ... Why do we need to 'weld' two bars together - why don't they just bond on their own?

The problem is generally one of two things: gases (air) or metal oxides get in the way.

You can actually bond two pieces of metal this way: it's called cold welding. But, in order to get it to work with large pieces of metal, you have to (1) get all the air out of the way, and (2) you have to clean both surfaces very thoroughly in order to remove all traces of surface oxides, and (3) you have to make sure both surfaces are perfectly matched, either precisely flat or with precisely the same curvature.

Once you've taken care of those preconditions, my expectation is that all metals should cold weld. No guarantees as to how hard it might be to actually satisfy these conditions, though.

We can weld elements together so they 'stick' to each other, but what is the process that actually causes two like elements to bond together?

In the most common welding methods, both of the surface to be welded are actually melted in the area right around the weld, which takes care of (1) and (3) above. Requirement (2) is dealt with in a variety of ways, such as by surrounding the weld area with an inert gas blanket or by using a 'flux' material that either chemically reacts with any metal oxides to turn them back into metal, and/or that "floats" on top of the weld puddle to protect it from oxidation.

ADDENDUM: After reading DarioOO's answer, I realized that I should note that cold welding does not satisfy the "forming one larger bar or block" aspect of your question. An assembly of cold-welded parts is not attached as strongly as it would be if it were machined from a single piece.

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    $\begingroup$ And until NASA learned how to prevent it this was a problem for spacecraft. While the contact points were small the force that could be applied was also small. $\endgroup$ – Loren Pechtel Oct 20 '16 at 4:24
  • $\begingroup$ I simpli do not believe two metals may bond togheter if clean enough :/. Metal is organized into crystals, what one could wrongly believe is cold wielding is simply atmospheric pressure that keep surfaces united by pressure. $\endgroup$ – CoffeDeveloper Oct 20 '16 at 8:14
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    $\begingroup$ @DarioOO well, have a citation from the ESA on how it happens in a vacuum: esmat.esa.int/Publications/Published_papers/STM-279.pdf (they talk a lot about "fretting", vibration at the contact surface) $\endgroup$ – pjc50 Oct 20 '16 at 9:19
  • $\begingroup$ They mention "Impact, Vibrations, Energy". Wielding do not happens if you do not apply energy. which is what this answer allow to understand (But that is wrong, wielding do not happen just putting metal togheter, which is what I say, and what is sad in the paper) u.u $\endgroup$ – CoffeDeveloper Oct 20 '16 at 11:24
  • $\begingroup$ @DarioOO there are other sources: three further papers are referenced by the Wikipedia article. $\endgroup$ – OrangeDog Oct 20 '16 at 13:21

You cannot simply put two bars together and expect them to weld at room temperature.

  • If two surfaces are flat enough they will adhere just because of atmospheric pressure.

Metal pieces adhering because of atmospheric pressure

In red you see air pressure force vectors (I did not draw them all), in green the resultant vectors. This is not a real weld, and if you apply enough force (or if you just make the pieces slide in opposite directions), you are still able to separate them without actually breaking them.

If you put a bar on top of another bar the those still remain two separate metal pieces, because at microscopic level their surfaces won't just stick, regardless of how good you make them flat.

All metals are made by many small crystals, those crystals are kept together mainly by mechanical forces (they just adhere because of irregular shapes, like puzzle pieces). There is a small EM force but it is mostly negligible.

However, there is a good deal of strength in the bonds between atoms inside the same metal crystal.

The reason why two metal pieces do not weld spontaneously is exactly this: the crystals do not adhere to each one even if flattened because they have no way to grip on each other. In order to weld two metal pieces you have to give extra energy:

  • Applying enough heat: the atoms rearrange their positions, forming new crystals.
  • Applying enough mechanical force: the crystals penetrate each other and start to adhere just because there is now friction between them.

In order for crystals to adhere to each other they need to have big contact surface:

  • When you repeatedly bend a metal it will break because you gradually make crystals slide between each other reducing the contact surface between crystals and smudging out the irregularitites that cause the inter-crystal friction.
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    $\begingroup$ Ah, I guess you're taking issue with the "forming one large bar or block" part of the OP's question? Yeah, that's problematic.... $\endgroup$ – hBy2Py Oct 20 '16 at 11:22
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    $\begingroup$ If you edit to clearly state that the joined pieces do not become one indistinguishable piece of metal, that may reverse the downvote trend. Thank you for pointing this out! $\endgroup$ – hBy2Py Oct 20 '16 at 11:27
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    $\begingroup$ Why do you claim that "crystals of the same metal do not [weld] together inside the same metal"? Most everyday metal objects are polycrystalline, consisting of large numbers of differently oriented crystals that nonetheless adhere to each other quite solidly. (Also, please fix your spelling. You "wield" a tool or a weapon in your hand, but you "weld" metals together. They're not even pronounced the same way.) $\endgroup$ – Ilmari Karonen Oct 20 '16 at 11:50
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    $\begingroup$ Very clean (i.e. UHV deposited) films will quite happily stick together, which is one way to make well-characterized bi-crystals. $\endgroup$ – Jon Custer Oct 20 '16 at 15:58
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    $\begingroup$ Do you have a reference for your claim that the crystals in polycrystalline materials "are kept together mainly by mec[h]anical force (they just adhere because of irregular shapes, like puzzle pieces)"? While bonding across crystal boundaries is certainly more strained and thus somewhat weaker than within crystals (pretty much by definition), the difference is typically much less than an order of magnitude. Also, as far as I know, the crystal grains in most polycrystalline materials tend to be mostly convex, and thus incapable of mechanically interlocking in the absence of chemical bonding. $\endgroup$ – Ilmari Karonen Oct 20 '16 at 17:42

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