I'd say it generally depends on what you mean by "free will," however, more importantly is what you mean by "metaphysical." Metaphysics is hard to pin point given all the different branches of philosophy that are considered "metaphysical" and especially given the historical context of the word. Since the 17th century, though, "metaphysics" is usually meant to explain the nature of "being." It is meant to explain how we understand things like existence, objects and their properties, the notion of cause and effect, just to name a few. Free will is also one such metaphysical issue.
Although I would say the issue of free will and all it entails is more metaphysical than it is physical, physicality may contribute too. Thus the question "is free will metaphysical or physical?" is a false dilemma.
Free will is more metaphysical, either which way you define it, as it deals with causality, moral accountability, decision making which is related to the brain and mind, etc. The aspect of the physical that could be related to free will would concern that of decision making. Brain scans reveal that brain activity takes place up to 10 seconds before we actually make a decision, unconsciously making our decisions well before we consciously do. That brain activity I would say is purely physical and in fact relevant to the issue of free will.
Such a find implies determinism, too. It implies the truth of determinism moreover. That's the heart of the issue of free will: Is determinism compatible with any sense of free will?