At the right temperature and pressure, copper (also aluminium, steel, etc.) becomes plastic. Shaping or adhesion can then be accomplished without introducing much in the way of contaminants. You can also avoid many of the fractures that normally form when a liquid metal cools from a liquid state back to a solid - thus preserving (and sometimes improving even) the crystalline structure of the original metal. I wouldn't be surprised if "cold casting" is using some of the same techniques. Casting can often introduce not only impurities and unwanted oxidation, but stress-fractures too. I can’t imagine that fractures of any type would improve the thermal conductivity of a waterblock.
Industry uses this technique extensively with friction-stir welding.
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