That looks good mate. I was going for alu mainly because of weight, the hammer head is basically going to be a solid block of alu with a pyramid shape at the top (wish I was decent with CAD so I could put pics up) and im planning for the base of the head the be level with the floor of the arena. I thought since im going to need alu for the size of the head (hardox would be way too heavy) I might as well use alu for the arm as well.

I will have a look around from some prices of hardox though, maybe if it works out cheaper than the alu would be I might go for it.