the most popular metal used to make flutes  

It's something I never looked into, I couldn't tell you.

that would be HEAVY METAL. haha.

I don't know the metal but I'm guessing it's stronger than alumium metal or someting like that.

Seems to me they're made of some metallic league similar to those of the coins, with nickel and such, most of them receive a nickel or silver bath for more durability.
But you can also find flutes made of wood/bamboo, silver, gold and even crystal.
Hope this helps a little ;)

Hello,

It depends on the quality of the flute. Student flutes are made from nickel silver. Then as you get slightly more advanced flutes, they are silver plated and then solid silver. Solid silver flutes are the most popular because they are very good quality for a relatively reasonable price. Flute are also made from gold and platinum, but obviously these are reserved for serious professionals. Many professionals play on solid silver flutes, but they are handmade, and the craftsmanship and quality of the silver are superior to what a semi-handmade flute would be.