Fusion
Melting = fusing
The word you're looking for is - weld.
meet
co-operates
organ
Amass-meaning to gather together.
united
freezing,solidifying
Cluster is a 7 letter word that means gather together. Other words include collect, combine and compile.
The four-letter word you are looking for is "melt." It describes the process of a solid turning into a liquid as it is heated, such as ice melting into water.
Seam.
choir
Group