WordNet records no synonym and no antonym for heat of fusion. It is the only member of its synset, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
heat absorbed by a unit mass of a solid at its melting point in order to convert the solid into a liquid at the same temperature
“the heat of fusion is equal to the heat of solidification”
Synonyms, antonyms and the broader and narrower relations for heat of fusion come from Princeton University’s WordNet. Full sources and licences.