When building your AR15 and selecting components, an inevitable question will arise on 6061 aluminum vs 7075 alloy. There is a significant cost difference between these two materials. Customers want to know if it's worth paying this material upgrade cost vs putting the funds into something else like a better trigger or stock. To make an informed choice, I'll explain some differences.
Aluminum alloys are always a blend of several metals with a set goal in mind. 6061 aluminum is primarily silicon and magnesium and it's known as a "jack of all trades" alloy for it's hybrid blend of characteristics like excellent weldability , malleability, corrosion resistance and strength at a low material cost. Like many blends though, when you maximize one aspect, you "rob Peter to pay Paul".
7075 aluminum alloy mainly uses zinc as the alloying agent of the aluminum. It also has a higher percentage of magnesium and copper than what is found in 6061 alloy. This 7075 alloy has the strength of many steels and is stronger than 6061 while maintaining the lightweight goal of aluminum. 7075's tradeoffs are lower corrosion resistance than 6061 and higher cost. 7075 os commonly used in military aircraft, boats, and auto parts.
To decide on the part for your AR15 build, I would consider where the part is being used, possible cost trade offs and what your needs are. 6061 aluminum has been used since the origination of the AR without fail and is a "tried and true" choice. Very few places in your AR will ever see a need for the superior strength that 7075 aluminum can provide. Areas that do require the highest strength such as the barrel trunnion are high grade steel. Generally speaking, forged 6061 is more than adequate for your upper and lower receivers. If your AR may be used for a "Buttstroke to the head" series, then investing in 7075 for the buffer tube may be a wise move that justifies the added expense. The ALG Defense True 7075 Buffer Tube is a nice option at an affordable price. If your budget is limited though, I would always recommend a trigger upgrade investment over the standard milspec lower parts kit trigger more than any other single cost upgrade. 6061 aluminum is more than adequate for the AR15 and the vast majority of civilian AR15 users. For hard military use, and/or the guy for whom "money is no object", then 7075 aluminum is a superior option in your AR15.