Do you think some christians use salvation and the word relationship interchangebly? why?

they are more or less the same. To me, salvation suggests a deeper relationship with God, while having a relationship with God can mean at any different level, like when a Christian who is just starting the religion. i think it's just the nature of language. People like to express themselves in many different forms, that's why even for a simple word like "beautiful", we have at least 20+ different synonyms for it.