Ah well, I knew I screwed up the moment I had poured the aluminium.
I am making a tool for my elderly father. He still likes to play golf, so I made a tool to stick the ball with tee and all into the ground, then automatically release the ball. The top of the handle is going to be an aluminium golf ball.
But it needed a hole to mount it on the carbon fibre rod of 21 mm diameter.
I had a piece of smooth steel, right diameter, so I used that as a core. And yes, now I cannot remove the steel.
Heated it, cooled it, drilled holes in it, nothing. After I had done all that, I realised that maybe I should have just tapped a M6 hole in it and mount it, embedded steel and all. Too late.
So I think I will have to cast another ball tomorrow and bore it out.
Another lesson learned.