Can ti and tu be used interchangeably?

No. Both Ti and Tú mean you, but they are not interchangeable. It is similar in English with Words like Me and I. In both situations you are talking about yourself, but you can't just use them at random and be grammatically correct.

Tú is used as the subject of a sentence. Ti is generally used as the subject of a prepositional phrase.

This is not a hard and fast rule, but it helps: Tú goes at the beginning of the phrase. Ti goes at the end of a phrase.

Also keep in mind that Tú and Tu are two different Words. Tú (with a tilde/accent over the U) means You. Tu (without the tilde/accent) means your.