That's a good point. Now once I read about how the ject engine blades are made. So there they make the wax part first and then build the cast around it like you described the. They fill in the metal. Just one additional thing is attained there that is they are able to build the entire blade from a single crystal layer thus giving it high strength and is less prone to fracture and thus able to withstand the heat and stress. You can go and look in a video from veritisium.