I'm making crucibles. Don't ask. I'll let you know if it works. Thanks

I am not sure a "flux" would lower the melting point of anything. The purpose of a flux it to coat the material and prevent oxidization of the metal in the welding process. I think what you are looking for is some material to alloy the aluminia so to speak, and I am not sure that "alloy" is the correct term here but it adequately describes the process. The melting temperature of an alloy is always lower than the parent metal. i.e. brass melts at a lower temperature than copper, etc. But if that is the result you are after you will first have to melt the alumina to add an alloy material to it.