How to greet the audience at the beginning of a PhD defense talk/presentation?
I would like to second both xLeitix and Koldito's comments and convert them into an answer:


In most cases, there is no requirement, and you can just say, "Good [morning/afternoon], my name is [name], and welcome to my thesis defense."
A very few institutions have a much more formal set of requirements.  For example, when I was an examiner for a defense at TU Delft, I had to learn a few words of Dutch in order to ask my questions with the required formality.  Also, I had to come a day early to get fitted for a special archaic form of suit. Don't worry about this, though: if this is the case for your institution, then somebody will make sure that you are instructed in what to do.