If you put two blocks of an element together, why don't they bond?
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.