Because @mikep is getting chatter it would suggest to me that it is a harder alloy. If it were a softer alloy I think it wouldn’t chatter rather it would be “guminess” that would be the problem. A very rough and simple way to get an idea of the hardness of the aluminium you have is to use a utility knife to cut a small slither off one of the edges. If it slices fairly easily and cleanly it is likely to be 6061 (or 6082 here in the UK).
The other thing to consider is that you piece of aluminium may have a different temper for some reason increasing the hardness.