Robotics and AI are strongly related. Yet they differ clearly.
There is a sub-field/branch of AI (no disrespect here!) called "embodied AI" that focuses on AI endowed with a body---basically humanoid robots but also self-driving cars and more abstract models. It seems the technical aspect of this branch is at home in Robotics SE.
But there are also AIs without an explicit body. Examples today are all the "AIs" behind Facebook's face detection machinery or Google's voice/image analysis, etc. Or Apple's Siri, which is personified, but not embodied. This is arguably most of the work we hear about these days.
Interestingly, some people claim that you cannot have "true AI" or "strong AI" without some form of body, but that's another debate.
This quite famous blog post also tries to demystify AI in general, including the belief that robots and AI are them same. An exert:
So let’s clear things up. First, stop thinking of robots. A robot is a container for AI, sometimes mimicking the human form, sometimes not—but the AI itself is the computer inside the robot. AI is the brain, and the robot is its body—if it even has a body. For example, the software and data behind Siri is AI, the woman’s voice we hear is a personification of that AI, and there’s no robot involved at all.