80% lower material, 6061 vs. 7075 ??

[DarkSoul]

I have worked in the machining trade for a long time, and am very familiar with materials since I am also now a mechanical designer, primarily with metals ranging from, of course, aluminum, several alloy steels, carbon fiber, and a few exotic materials from time to time, and I am curious why it seems almost all AR15 lowers seem to be made from 7075.

Of course I fully understand that this is a superior alloy over 6061, but if companies are now making polymer lowers, why wouldn't you use 6061 since the price of said material is substantially lower, like, by half?

We have been looking into manufacturing 80% billet lowers, and it just seems that 6061 would be a good material to work with since it is still very strong, and considerably less cost than 7075, but considerably stronger than polymer? Wouldn't 6061 be a great middle of the road material in terms of cost and strength? I wouldn't suggest putting a 50bmg or .416 upper on it, but for the majority of shooters out there, I would think it will be more than up to the task.

I think as we start machining, Ill make a 6061 to test out, and see if I can get a lot of range time on it to see if there are any issues, but I would like to hear peoples thoughts on this, just to be sure there is nothing overtly obvious that I am not thinking of.

[VaderSpade]

If logic was all that entered into the equation 6061 would be MORE than adequate.

But we've got to have the one that goes to 11, cause that’s 1 more!

[Zartan]

7075 cuts better

[DarkSoul]

LOL.... well you got me there

[eaglemike]

Given the price per lb delta and the strength difference, then add the better cutting of the 7075, to me there isn't a question.

A Lot of people like to say 6061 is good enough, or say "well, so-and-so makes theirs out of 6061." Mil-spec is 7075. Wonder why? People like the insist on mil-spec buttstock tubes that are forged - they are stronger, and will take the blows to clear a big-time jam better. Now you are transfereing to the load to either 6061 or the stronger 7075 in the fairly fragile area where the buttstock/recoil tube mounts.

Given the same cross section 7075 is FAR stronger than 6061. (both normally avaiable and common tempers.

On a side note, whenever someone says something "is made out of T6 billet" you know they don't know what they are talking about.

[couch]

So true about the term "billet".

[DarkSoul]

I agree on the term "billet", I always get a chuckle out of people that refer to aluminum products as "made from billet"......... so, billet what? billet mud, poop, steel, brass?

I hope you guys realize that my use of the word was to differentiate between starting with a forging, vs. a "billet" of , well, that's sort of the question, what actual material, a billet of..........

We have the model almost pinned down to where we want it, and will actually cut the first few from 6061 since its a little easier to machine, and easier on the cutters, (I don't know why some of you have said 7075 is easier to machine, unless I am missing the sarcasm), I think I will actually run a 6061 unit for a while to see if I can find any shortcomings.

[hypnoman]

I think it will be interesting to see. I think there always be a market for a product within the necessary strength range while costing 1/2 of the price of something over-strength. If a polymer lower shows us that plastic is strong than 6061 at 1/2 the price of 7075 definite has its place.

[ptoguy2002]

IIRC, the problem with 6061 was when you forged it, it created some sort of grain structure or inclusion that made it corode badly. That plus SE Asia and you got 7075. ....IIRC, the change didn't have to do with strength.

[NorCalDustin]

It would also be neat to be able to buy polymer 80%'s...

[hypnoman]

I agree

[couch]

6061 cuts like chewed gum. It sticks to tools, forms chips like crap and you can't plow thru it as fast as 7075 because of this. 7075 isn't as sticky or stringy. So you can drill it faster, mill it faster and tends to leave better surface finishes due to the same reasons.

[VaderSpade]

7075 Engraves better/cleaner also.

[DarkSoul]

IIRC, the problem with 6061 was when you forged it, it created some sort of grain structure or inclusion that made it corode badly. That plus SE Asia and you got 7075. ....IIRC, the change didn't have to do with strength.

That's very interesting, and makes some sense, I imagine that if they were forged in 6061 domestically, and held to tighter quality control, maybe it would fly.

6061 cuts like chewed gum. It sticks to tools, forms chips like crap and you can't plow thru it as fast as 7075 because of this. 7075 isn't as sticky or stringy. So you can drill it faster, mill it faster and tends to leave better surface finishes due to the same reasons.

Man, what are you using for tools and coolant/lubricant?, I have seen no issues of any of what you are describing on our machining centers. I even asked the boss what he prefers, and he preferred 6061 as well, although he said 7075 isn't hard to machine, but still likes 6061 better.

[couch]

Blaser Blasocut BC20. I know I can cut thru 7075 a lot faster than 6061 even with flood coolant and high helix coated end mills. The harder 7075 doesn't string or glob as easy as 6061 so chip evacuation is much easier both from the flutes in the tools and pockets on the parts.

For me, the cycle tine reduction and higher performance over 6061 far outweighs the cost difference.