When pouring copper onto another copper block and I want to have a good solid welding of the poured copper onto the block, do I need to use a flux? Or, if the block is freshly cleaned and free of most oxidation, would that be enough?
There are a couple of issues here. To get a proper fusion weld both surfaces need to melt and whether or not this happens depends on the details of the heat transfer for the whole system. It is certainly possible that heat could be conducted way from the interface too quickly for the surface to melt well enough to get good fusion.
In this situation the cleaner the surface the better, if metals are clean enough and have a good enough surface finish they can fuse cols in some circumstances and it is certainly true that a welded joint can never be too clean.
The problem with using flux is that it need a certain amount of heat to activate and it works by dissolving metal oxides from the surface so you still need to prevent flux/slag from being included in the joint (which in this case may be difficult.
It certainly is possible to get decent fusion between solid copper and molten cast iron in a mould because I have done it, although here the mass of iron was significantly greater than that of the copper and it is very sensitive to the poured metal having a decent amount of super-heat.
It really depends on exactly what you want to achieve and without more context it is difficult to be more helpful.
I doubt if pouring liquid Cu onto solid , of similar mass, will produce a weld. At one time I poured various copper alloys into a simple mold. The mold was about twice the mass of the liquid alloys ; there was never a trace of bonding. However the alloys were at temperatures of 100 to 200 F below melting point of Cu.