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?
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.
You cannot simply put two bars together and expect them to weld at room temperature.
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:
In order for crystals to adhere to each other they need to have big contact surface:
Two separate pieces of metals can be stuck together in a specific environment under the phenomenon of Casimir effect. If we take two metallic uncharged plates in vacuum and place them extremely close to each other, the distance between them around 10 nanometers, they can stick to each other with an approximate pressure of 1 atm which is significant.
We need to carry out this experiment in vacuum as air contain a lot of other stuff that would interfere in between the plates. What causes this attraction is the energy of the vaccum itself that comes from quantum fluctuations. When two uncharged plates are brought this much close to each other, it creates a low vacuum energy density region in between them and a higher vacuum energy density on either side of the two plates which manifests as a physical force between them. Thereby acts unbalanced forces between the plates by the virtue of energy density difference and brings them closer.
A little background info from Wikipedia
Casimir's original goal was to compute the van der Waals force between polarizable molecules of the conductive plates.
This way two metallic blocks can be bonded not chemically but in terms of physical forces which would be strong enough to bind them.
A drop of rain and a pond are 2 separate bodies. But once the raindrop falls into the pond, both bodies become one. Now, if you try the same thing with an ice cube and a frozen lake, the result will be different. The ice cube will not become one with the frozen lake. Why is that? The only difference between the first and second scenario is the state of matter. With liquids, intermolecular forces are just strong enough to preserve the volume of the substance, but not the shape. As a result, external matter can easily mix in with liquids. Solids on the other hand, have much stronger intermolecular forces, so it becomes much more difficult to mix external matter into a solid object. So, to answer your question, the state of matter is the factor that determines whether or not 2 bodies of a substance can automatically ‘weld’ together upon contact. Solids can’t do that, but liquids and gases can.
