Alexi sings in a high-pitched death metal growl. I don't know why nobody realizes this but if youv'e tried to imitate his voice and succeeded you'd probably know. His style is simialar to black metal but there's a big difference: black metal is the -wrong- way to growl, it wrecks your voice in a few years usually... Alexi is smart and does it the right way, the harder but safer way. If Alexi is careful he should be able to sing for a decade or two.
The death metal influences are few and far between on the latest cd, it's hardcore thrash all the way, plus the usual bodometal. It is rather melodic tho, for having so few actual melodies (compared to FtR at least).
edit: Crash course in growling: you can make a growl with your mouth/diaprhagm and train until you have range (high and low) or you can cheat and force the sound out with your stomach muscles. In Flames singer does it the wrong way (stomach), Kalmah's singer does it the wrong way too, even worse than IF's. Norther's singer does it the right way but also pushes really hard with his stomach anyways so who knows how long he'll last, Skyfire's singer is the same way (pushes with his stomach and growls with his mouth)... Alexi doesn't push with his stomach very often, he uses his mouth and throat almost entirely... hopefully you all understand now, trust me on this, I've had a fair bit of experience and advice from professionals.