I have had varied results using metal blades in a circular saw, and as you say
18v, I assume your saw is a cordless rechargable one?
I think you'd have trouble getting that to cut through a pipe that size in one go. I usually cut aluminium tube with a drop saw, which is like a circular saw, but fixed, like this:
It would be scary to try with a hand held saw, apart from the grabbing, it would be hard to keep a straight line with it. I fear it could end up looking like someone had chewed it off, they would end up different lengths, and poor mini-jester would always roll into one corner of the bed
(Trust me, I have fucked many things up by trying to do it on the dodgy...)
I'd suggest borrowing or hiring a cut-off saw, which is like a big grinder with a cut off wheel:
This will do a much quicker, safer, neater job, plus it would be way cheaper than buying a blade you will probably only use once.