what are these ants doing?

what are these ants doing?

Observation - what are these ants doing? - UK and Ireland. Description: ants clustering around what look like black aphids. all activity on elder branch. are they working with the tree against the aphids? are they eating them, killing them or perhaps the tree is excreting something both species like to eat / use? any ideas? w

ants clustering around what look like black aphids. all activity on elder branch. are they working with the tree against the aphids? are they eating them, killing them or perhaps the tree is excreting something both species like to eat / use? any ideas? whatever it is they are all very busy