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gun toting monkeyboy
12-01-2011, 2:27 PM
Ok, I have found a billet lower that is machined out of a solid block of aluminum. The only thing is that it is 6061-T6511 instead of the regular 7075T6. How much of a problem is this? Is this something that is a non-issue, or will the 6000 series not hold up anywhere near as well as the 7000 series alloys?

-Mb

AeroEngi
12-01-2011, 2:51 PM
6000 series aluminum generally has less strength than 7000 series aluminum. 6061-T6511 is actually one of the "stronger" tempers in the 6000 series. Now with that being said, I don't think it should be a problem to use as a material for a lower receiver because the lower doesn't see much load, espcially compared to the upper. Also, the lower doesn't see much temperature fluctuations either. I honestly believe making lowers out of 7075-T6 aluminum is a marketing gimmick. You don't need the strength of 7075 for a lower.

spits55
12-01-2011, 2:51 PM
You are good to go....

http://www.ehow.com/info_8078719_difference-between-6061-7075-aluminum.html

eaglemike
12-01-2011, 3:23 PM
The common versions of 6061 compared to the common versions of 7075 = 7075 is much stronger for the same cross section.

The only reason to machine a "billet" lower out of 6061 is due to the lower cost. 7075 is anywhere from $1 to $3 per pound more in cost. Variance in cost due to amount purchased, size, where produced, and availability. Cold finished bar stock costs more than extruded. If you like to calculate the cost difference you can use .1lb/cubic inch. This will get you close enough.

7075 actually machines better/easier, and hard anodizes as easily as 6061.

dieselpower
12-01-2011, 5:54 PM
6061-T651 has a higher elasticity than 7075. Another thing to remember is just because its 7075 doesn't mean its better....

Search AR15.com and read the Industry sections for Mega Lowers. They explain why they think their blend of 6061 is better than most 7075 lowers.

eaglemike
12-01-2011, 6:15 PM
6061-T651 has a higher elasticity than 7075. Another thing to remember is just because its 7075 doesn't mean its better....

Search AR15.com and read the Industry sections for Mega Lowers. They explain why they think their blend of 6061 is better than most 7075 lowers.

Ummm, you really need to look at some charts. When you start with the same cross section, there is no contest. 7075 is waaaay stronger. Please don't refer me to AR15.com - I know more than a lot of them - and I have a real life jet engine engineer for a friend. Yes, he is certified and all that. He and I have had this exact discussion MANY times. I have several material handbooks on the shelf. Why do you think aerospace engineers prefer 7075 over 6061 for STRUCTURAL parts? (I'm also a former aerospace QA manager, heat treat inspector, hardness and conductivity test certified, etc)

This is the setup : you start with 2 parts. One is 7075, the other 6061, both same temper, ex T651. If both start off using the same strength/different wall thickness (the 7075 is thinner) after a few million cycles the 6061 is better. If you start off using the SAME wall thickness/cross-section, there isn't a contest. 7075 wins by a wide margin.

hope this helps.......

AeroEngi
12-01-2011, 6:22 PM
One thing everyone forgot to mention...if people are successfully using Plum Crazy Polymer lowers, then I think you'll have no problems whatsoever going with the 6061 aluminum lower.

dieselpower
12-02-2011, 5:25 AM
Ummm, you really need to look at some charts. When you start with the same cross section, there is no contest. 7075 is waaaay stronger. Please don't refer me to AR15.com - I know more than a lot of them - and I have a real life jet engine engineer for a friend. Yes, he is certified and all that. He and I have had this exact discussion MANY times. I have several material handbooks on the shelf. Why do you think aerospace engineers prefer 7075 over 6061 for STRUCTURAL parts? (I'm also a former aerospace QA manager, heat treat inspector, hardness and conductivity test certified, etc)

This is the setup : you start with 2 parts. One is 7075, the other 6061, both same temper, ex T651. If both start off using the same strength/different wall thickness (the 7075 is thinner) after a few million cycles the 6061 is better. If you start off using the SAME wall thickness/cross-section, there isn't a contest. 7075 wins by a wide margin.

hope this helps.......

I am too and if you think elasticity (http://en.wikipedia.org/wiki/Elasticity_(physics)) is the same as strength you fail at understanding real world vs paper. If all you care about is hardness, again you fail at real world vs paper. I guess if you are building a safe queen and never take it into the real world you have a point 7075 makes a better wall display. I am not saying 6061 makes a better firearm, but to suggest strength is the only thing to care about shows you complete lack of understanding of why things are made differently in the first place. Instead of just inspecting what others tell you, the next phase of your life how about taking some engineering classes. When you have to start factoring in 100 variables into an end design, you will START to understand what I am saying.

I refer you to a FIREARMS MANUFACTURER who has their own forum on AR15.com, yet you an aerospace inspector with a bookshelf think you know better than them...

I'm done here. Have a good day.

Nathan Krynn
12-02-2011, 5:32 AM
We used to use 6061 T651 for our 308's when we started making them a few years ago. There are some benefits and detractors for it. The only real downside for 6061 I ever saw was don't dry fire it with the upper off as the hammer might indent the front of the FCG if you do this repeatedly, I used to do this all day long so its not like a one time deal will do this. We also found our anodizers a lot of time had a hard time getting good color on the 6061 vs the 7075.

Seriously on a lower you should be fine, like others have said there are not a lot of stresses on lowers.

eaglemike
12-02-2011, 8:39 AM
I am too and if you think elasticity (http://en.wikipedia.org/wiki/Elasticity_%28physics%29) is the same as strength you fail at understanding real world vs paper. If all you care about is hardness, again you fail at real world vs paper. I guess if you are building a safe queen and never take it into the real world you have a point 7075 makes a better wall display. I am not saying 6061 makes a better firearm, but to suggest strength is the only thing to care about shows you complete lack of understanding of why things are made differently in the first place. Instead of just inspecting what others tell you, the next phase of your life how about taking some engineering classes. When you have to start factoring in 100 variables into an end design, you will START to understand what I am saying.

I refer you to a FIREARMS MANUFACTURER who has their own forum on AR15.com, yet you an aerospace inspector with a bookshelf think you know better than them...

I'm done here. Have a good day.
I'm also a machine shop owner with 34 years of experience in manufacturing. I started out sweeping a floor in the machine shop and pushing a file. Don't talk to ME about real world.I didn't insult you in any way - why are you doing so????? I have taken some engineering classes. I just don't have a degree in engineering. I have several friends that are certified engineers I ask for help when I think I need it. Every time I've been right on the money, so they tell me. How many of the mil-spec lowers are 7075? All of them I've checked. Yupper, I still own a hardness tester.

One more thing. Why are all the uppers 7075?????? :) If all the load is on the upper and 6061 is better?????

eaglemike
12-02-2011, 9:55 AM
Here's some objective data which might be helpful.

6061T651 tensile strength = 45,000
yield = 40,000
elongation (after yield, before tensile failure) 1/2" round sample 17 percent in a 2" section


7075T651 tensile strength = 83,000
yield = 73,000
elongation (after yield, before tensile failure) 1/2" round sample 11 percent in a 2" section

The above shows the 7075 will deform less, but the 6061 will have failed long before the 7075 started to yield. The 7075 can still deform elastically after the 6061 has failed. The difference between 45ksi and 73ksi.

Nathan Krynn
12-02-2011, 10:03 AM
Here's some objective data which might be helpful.

6061T651 tensile strength = 45,000
yield = 40,000
elongation (after yield, before tensile failure) 1/2" round sample 17 percent in a 2" section


7075T651 tensile strength = 83,000
yield = 73,000
elongation (after yield, before tensile failure) 1/2" round sample 11 percent in a 2" section

The above shows the 7075 will deform less, but the 6061 will have failed long before the 7075 started to yield. The 7075 can still deform elastically after the 6061 has failed. The difference between 45ksi and 73ksi.


I am not an engineer and run the office not the machine shop (but have learned a lot). So you know more about these numbers then I so I will ask you this, does an AR lower come near any of these numbers for failure?

eaglemike
12-02-2011, 10:12 AM
I am not an engineer and run the office not the machine shop (but have learned a lot). So you know more about these numbers then I so I will ask you this, does an AR lower come near any of these numbers for failure?
Nathan,
I've not run the numbers. It would take a lot of work for me to do the calc's or pay someone else to do so. I rather doubt it, as we see so few failures, especially in 5.56/.223. I have seen some of the carbon lowers fail. There were posts about those here a few years ago. In theory they should have been fine.... :) I prefer to play it safe. :)

BTW, heard nothing but good things about your work. Congrats!

AeroEngi
12-02-2011, 10:29 AM
I am not an engineer and run the office not the machine shop (but have learned a lot). So you know more about these numbers then I so I will ask you this, does an AR lower come near any of these numbers for failure?

I think if the polymer lowers can handle the loads associated with standard operation of the rifle then 6061 aluminum should have no problem at all. Being able to use polymer lowers should tell you how low the loads and stresses really are.

Nathan Krynn
12-02-2011, 10:31 AM
Nathan,
I've not run the numbers. It would take a lot of work for me to do the calc's or pay someone else to do so. I rather doubt it, as we see so few failures, especially in 5.56/.223. I have seen some of the carbon lowers fail. There were posts about those here a few years ago. In theory they should have been fine.... :) I prefer to play it safe. :)

BTW, heard nothing but good things about your work. Congrats!

Yeah the old polymer lowers (not the plum ones) I have seen fail personally. It was funny as this guy and wife came to a shoot and were ripping everyone a new one that we all over paid for our lowers and guns and then on the 1st stage one of their plastic guns failed then on the third the second. Broke at the buffer tower area if I remember.

I can tell you we went to 7075 t651 from 6061 t651 (on our 308's as the AR-15's have always been 7075 forgings) not because the difference in strength which there is one but mostly to the color never matched up with 7075 uppers and sometimes it was WAY off. There was a public demand for 7075 too but I can say I have one of our first 308's and it has thousands of rounds and no problem what so ever. The only thing I ever saw was dry firing with no upper on it but you shouldn't do that anyway and it took a lot of dry firing and the lower was still good to go.

Please know I am not trying to fan the flames, defend some company I don't know, or anything else.

TY for the compliment, we try to do our best as corny as that sounds.

eaglemike
12-02-2011, 10:35 AM
I think if the polymer lowers can handle the loads associated with standard operation of the rifle then 6061 aluminum should have no problem at all. Being able to use polymer lowers should tell you how low the loads and stresses really are.
Yupper!
Long term it will be very interesting to see how these polymer lowers hold up. Some of the chemicals people use on guns are pretty harsh to some plastics, and it's hard to control what a gun owner will do..... :)

I'm thinking there is a reason the big name guys use 7075 forgings, as opposed to 6061. It's not like either of them are new materials. :)

SoCalSig1911
12-02-2011, 10:43 AM
Bottom line is 7075 is a stronger metal and is better suited for the platform than 6061. I also own and operate an Aerospace manufacturing facility and i make parts for some of the biggest names in the business and my family has been doing so since the 70's, I'm a 2nd gen aerospace manufacturer. Any aluminum aircraft part that goes under heavy load like a wing flap hinge or similar is always 7075 rather than 6061 due to the strength properties. The cost is not much of a difference unless in bulk quantity. The better material is used the more you will pay in the end so if someone is cutting corners and using 6061 as opposed to the 7075 I'm sure it was to save a few pennies. Maybe i should start making titanium lowers and selling them to the ballers! lol.

17+1
12-02-2011, 11:55 AM
Never had a problem with my 6061 Sun Devil lower. I doubt many people have experienced complete mechanical failure with them.

I always considered the lower as a low stress part where material selection is less critical.

rdmax
12-02-2011, 1:07 PM
From what I researched when picking my next lower, 7075 is not only stronger, but a lot more dent resistant than 6061. Lowers like Sun Devil are not forged. Forging also makes the metal stronger. Bullet 6061 is made stronger by leaving more metal on the critical stress areas. If you like the looks of a billet lower, then go for it. I decided to go with a standard Mil-spec lower because why pay more for a lower that really seems less durable.

gun toting monkeyboy
12-02-2011, 2:25 PM
Which polymer lowers did you see fail? I am just curious, as I have been using a Cav Arms one on my lightweight build for several years without incident. I didn't know if there were any other polymer ones out there until the Plum Crazy came along.

-Mb

eaglemike
12-02-2011, 2:59 PM
The lowers I've seen were the carbon fiber version. Was it DPMS that was making them? IIRC someone else had designed and done the first runs, then the product and design was purchased by them.

The stress taken by the lower isn't huge, but I think it's more than some people think. It does make sense the polymer lower failed at the buffer tower. A lot of the stress is transferred right there. The overall Eugene Stoner design is pretty cool.

Sicarius
12-02-2011, 3:06 PM
Both are fine. My Sun Devil is still rocking without issues and I am not exactly kind to my firearms. I just wanted a billet lower is the only reason I got one. The rest of mine are 7k series. As for an upper, I wouldn't get anything but 7k series AL.
Kevin

dieselpower
12-02-2011, 6:14 PM
I manufacturer cookie sheets. My cookie sheets can sit on the surface of the sun for 60 seconds! My closest competitor can only last 30. Thats my claim to fame and why my pans are better for your 350 degree baking needs....:grilling:

HOPE THIS HELPS....

gun toting monkeyboy
12-02-2011, 6:31 PM
I want some of those cookie sheets!

dw1784
12-02-2011, 7:54 PM
I'm thinking there is a reason the big name guys use 7075 forgings, as opposed to 6061.

I can't believe all you "aerospace" guys missed what the OP's asking: 6061 T6 511. You can't compare a forging to an extrusion. Technically it's apples to oranges. Had the OP asked about forgings between 6 and a 7, or a billet made from a slab of 6 to 7 series, then that's a different story.

Technically it's an engineered material for different applications. Just because BBS wheels are 6 series forging doesn't mean it's inferior to the 7 series tubings found in cheaper bicycle frames(6 series cost more, but for a different reason). Totally different application.

For this low stress, low load bearing application, it's more than adequate.

P.S. Nathan Krynn's obervation about the "take of color" of coating(I'm assuming it's the quality of anodizing): have you tried T4 instead of T6? it might be the tempering, not the alloy itself. Then again, someone will ask "is your T4 as good as a T6?" it never ends...

bottomline OP: was it a good buy? bang for the buck? that's all it should matter:)

SoCalSig1911
12-02-2011, 11:12 PM
I can't believe all you "aerospace" guys missed what the OP's asking: 6061 T6 511. You can't compare a forging to an extrusion. Technically it's apples to oranges. Had the OP asked about forgings between 6 and a 7, or a billet made from a slab of 6 to 7 series, then that's a different story.

Technically it's an engineered material for different applications. Just because BBS wheels are 6 series forging doesn't mean it's inferior to the 7 series tubings found in cheaper bicycle frames(6 series cost more, but for a different reason). Totally different application.

For this low stress, low load bearing application, it's more than adequate.

P.S. Nathan Krynn's obervation about the "take of color" of coating(I'm assuming it's the quality of anodizing): have you tried T4 instead of T6? it might be the tempering, not the alloy itself. Then again, someone will ask "is your T4 as good as a T6?" it never ends...

bottomline OP: was it a good buy? bang for the buck? that's all it should matter:)

I simply ignored the T6511 and interpreted it as T651 since 99% of people out there don't know the difference unless in the field of manufacturing. I didn't assume that he was asking about extruded material for this application but was just comparing the different strength properties between 7075-T651 and 6061-T651.