Is expiation obligatory for one who swore not to break his voluntary fast, then broke it?

It appears from your question that you swore not to break your fast with your friends. If that is the case, then if you do break your fast with them, an expiation for breaking an oath becomes incumbent upon you. However, if your intention was different, then what matters is what you intended, because an oath is based on the intention of the one who swears it, as mentioned by Ibn Qudamah.