i was not familiar w it either when i first researched these so i wiki'd it:
http://en.wikipedia.org/wiki/7075_aluminium_alloy
"7075 aluminium alloy
From Wikipedia, the free encyclopedia
Aluminium alloy 7075 is an aluminium alloy, with zinc as the primary alloying element. It is strong, with a strength comparable to many steels, and has good fatigue strength and average machinability, but has less resistance to corrosion than many other Al alloys. Its relatively high cost limits its use to applications where cheaper alloys are not suitable.
7000 series alloys such as 7075 are often used in transport applications, including marine, automotive and aviation applications, due to their high strength-to-density ratio.[2][3] Its strength and light weight are also desirable in other fields. Lacrosse shafts, rock climbing equipment, bicycle components, and hang glider airframes are commonly made from 7075 aluminium alloy. The bicycle industry is also using 7005 and 6061 aluminium alloys. Hobby grade RC models commonly use 7075 and 6061 for chassis plates. One interesting use for 7075 is in the manufacture of M16 rifles for the American military. It is also commonly used in shafts for lacrosse sticks, and camping knife and fork sets.
Due to its strength, high density, thermal properties and its polishability 7075 is widely used in mould tool manufacture. This alloy has been further refined into other 7000 series alloys for this application namely 7050 and 7020."