Thanks! Really interesting read. Any idea where people got that 6061 is even used in the SCAR though? If seen it posted around but nothing from FN I've seen. Also, save for corrosion (which anodizing handles well) and weldability it still seems that they went with the 6061 to save on costs primarily. That is perfectly fine if they don't think there needs to be extra strength but are there other advantages I am missing (eg better fatigue, etc) that would make them shy away from something else - say the 7004 which seems to have all the positives of the 6061 and then some?
They used 6061 because it's extruded. Which in this application is every bit as durable as it would be if it were milled to the same dimensions from billet 7075. The only real difference is milling would take HOURS and a lot of expensive carbide tooling where as extruding takes a minute or two as its spit out of a hot die already at finish dimension (for the most part) with minimal machining needed to finish. The strength gained from starting with a billet of 7075 would be negligible at best but the time/money lost would be extreme.
As as far as corrosion resistance goes, 6061 will still hold up to the elements for a VERY long time before you have any problem there. It's not going to rust away overnight like mild steel. When you hear or read about 7075 being more corrosion resistant, they're usually referring to industrial applications where the material is exposed to various other chemicals that will drastically speed up the corrosion process when used in the manufacturing of other goods.
Im going to go out on a limb and guess that you got the 2025 vs 6061 vs 7075 questions from reading other manufacturers specs on AR 15 type uppers and lowers and the materials they used? Most companies use this sort of thing as part of their marketing strategy to make their product appear superior to the rest in hopes to sell more product. If you compare those materials side by side for torsional stress, shear stress, etc.. The figures you get wont be as drastically different as if you were to compare the same materials with different manufacturing processes such as extrusion vs cast vs billet vs forged.
To sum it up, using extrusion was the logical choice for a tubular upper where as if the lower was also aluminum, it would most likely be machined from either a forging or a billet. Nothing is really going to be gained from using billet 7075 vs extruded 6061 in this particular application. At least not enough to justify the means.
