Hi Panja,
I found 6Al4V quite tough. I engraved it with my Lindsay Nitro G20, so with enough power it can be engraved, but it was a bit of a struggle. My Lindsay Classic would only engrave 6Al4V to half the depth of the Nitro in one pass. If you can get CP titanium or Grade 2 titanium, it is much easier to engrave.
Tom
Ti grade5 is the best one for engraving, it is more compressed and only a bit harder than CP or grade 2
arnaud