6061: further thoughts and new information

M

misanthropist

Scribe
EE Expired
Rating - 100%
43   0   0
Location
You can't stop here. This is bat country.
For those of you who want the conclusion without a long, dry, read, it’s in bold at the bottom. For those who want the whole package, here you go:


The small number of you who read my posts regularly will recall that the use of 6061 for the NEA receivers was a bit of a concern for me. The overwhelming majority of AR uppers are 7075, whether billet or forged. This led me to suspect that 7075 would be, across the board, a better choice.

Some of you will also be aware that, generally speaking, I have been very supportive of NEA.

I have had a very hard time reconciling this and consequently have been doing a lot of research on this subject.

I would like to share with you all a few links that I have been pointed to over the last few days in the course of this research:

http://en.wikipedia.org/wiki/Young's_modulus

http://en.wikipedia.org/wiki/Shear_modulus

http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MA6061t6

http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MA7075T6

http://www.thegunzone.com/556dw-5.html

http://www.wbdg.org/ccb/FEDMIL/a8625.pdf

http://d-chn.biz/2010/11/24/electrolytic-coloring-of-hard-anodized-aluminum-alloys/

https://www.corrdefense.org/Technical%20Papers/COMPARISON%20OF%20ACCELERATED%20CORROSION%20TESTS%20TO%20CORROSION%20PERFORMANCE%20IN%20NATURAL%20ATMOSPHERIC%20ENVIRONMENTS.pdf

The first link is to the wikipedia page for Young's modulus. This is the measurement of the rigidity of a material. This is important from the perspective of a user; it effectively determines how "floppy" the gun is going to be during firing. It is the only data point I felt that I really did not understand when I began looking in to this. It came up in a discussion at another site and I found the wikipedia page to be useful. It is also perhaps worth looking at the page for Poisson's ratio which is a related function which explains the degree to which materials expand in one direction while being compressed in another.

Next I have included the Wikipedia page for shear modulus. You can think of Young's Modulus as the linear rigidity - like the resistance of pulling an elastic band. The Shear Modulus is the resistance to, of course, shearing forces - that is, force applied perpendicular to the object.

It is worth noting that in addition to the per-unit materials strengths, there is also the simple factor of amount of material. That is to say, even if one material is 5% stronger by weight than another, this problem is moot if the weaker material is used, but is made 10% thicker. So you may pay a bit of a penalty in weight, but not necessarily ultimate strength. To put this simply: I’d rather be standing on a platform made of two inch thick wood than one made of 0.001 inch thick steel. Materials strength matters, but it’s not the ONLY thing that matters.

The next two links are materials data sheets for 6061-T6 and 7075-T6.

The page after that is a reference from the Chronology of the Development of 5.56x45. The important part is this:

Colt switches from 6061 T6 aluminum forgings to 7075 T6 aluminum forgings upon suggestion by Gene Stoner. The earlier forging were found to be prone to intergranular exfoliation in the humid climate of Vietnam. Thin areas of the receiver, such as the area around the front pivot pin hole, could completely corrode apart within as little as three months.
Click to expand...

Up next: MIL-A-8625F, the "milspec" for anodizing. This lays out the specs for what is acceptable in a hard coat. Every conceivable measurement is specified. It is not fun reading.

Then: Taber Testing (wear testing) of anodized aluminum. This is not my favourite reference as the testing related largely to the comparison of coloured anodizing, but the control tests were done on milspec hard anodized 2024, 6061 T6 and 7075-T6.

Finally, the incredibly long title.pdf: this is a very interesting study sent to me by someone here, not associated with NEA and who I do not think I have seen participate in the various NEA discussions.

It is a study, presented at the 2009 Department of Defense Corrosion Conference, and conducted at the University of Hawaii. The actual goal of the study was the comparison of different methods of accelerated corrosion testing...in essence they were looking to see how well they could replicate long term exposure with different chemical processes.

But for our purposes the important thing is that two of the test metals were 6061 and 7075.

It is worth reading all of these links. But for those who do not have time, I will briefly summarize what I have learned, after sifting through many arguments, which was not particularly instructive, and reading a lot of papers, which was very instructive.

6061-T6 has a Young's Modulus that is basically equal to 7075-T6: 68.9 GPa vs 71.7. They will, therefore, be about equal in terms of rigidity during firing.

6061-T6 also has a Shear Modulus that is basically equal to 7075-T6: 26 GPa vs 26.9. I think this is a fairly realistic measure of resistance to loading such as screw threads. I base this belief on papers like this:

http://www.bastionogp.com/technicalPapers/Bolt%20Thread%20Loading.pdf

which I think I have understood sufficiently, although I am open to the possibility that this is not the case.

Furthermore, fully anodized 6061-T6 appears to have a surface thickness and wear resistance comparable to 7075-T6, as per the Taber testing link.

So on to corrosion resistance:

7075 was originally implemented because of corrosion problems in Vietnam. The corroding receivers were forged 6061. This has led many people to conclude that 6061 is less corrosion-resistant than 7075. However, in the DOD paper, 6061 billets were actually found to be MORE corrosion resistant than 7075.

I believe the issues with the corroded receivers were related to the use of FORGED 6061. I have read many sources which say that 6061 does not forge well, and in discussions on several sites with aerospace engineers, I have been advised that the forging of 6061 affects the granular structure of the aluminum and makes it prone to the type of intergranular corrosion seen in Vietnam. Billet receivers would not be subject to this problem. Additionally, it was corrosion resistance that caused the switch to 7075, not strength issues, at least to the degree that I have been able to determine.

So for those of you still reading, here is my personal conclusion:

If I were buying a forged receiver, I would want 7075. However, I am now of the opinion that billet receivers may in fact be better if made out of 6061-T6.

I would like to point out that my background is not in engineering, or materials analysis. I have tried to synthesize the data I have gathered over the past week into something I, and other laypeople, can understand. I am 100% open to the possibility that I have gotten some things wrong here and I would welcome any correction that experts can provide.

But for me, for now, I am no longer concerned about the use of 6061 in the NEA receiver. I have been warned by some people that it will be susceptible to pin hole elongation and I intend to spec mine out when I get it and measure it regularly for the duration of its service life in the hopes that I can provide useful data to people.

But this resolves my concerns.
 
Good reading and great post! I think the biggest problem with the 6061 vs. 7075 debate is that the main argument has always been "but I was told 7075 is stronger" and then the M16 being made out of 7075 is given as proof.

The simple explanation I've always been given is that 7075 is harder and a bit stronger than 6061. The real question when it comes to guns is whether it is harder and stronger enough to matter when using billet. My opinion is no and I think you're data supports that.
 
Thanks for the post. I appreciate a technical post, rather than "it sucks" comments.

For those wondering, the numbers refer to the major alloying elements added to the Aluminum. Pure Al is far too soft to be of much use, so trace amounts of other elements are added.
6000 series have Magnesium and Silicon added.
7000 series have zinc added

6061 is easily extruded (squeezed out like Play-Doh or spaghetti), so is used in window extrusions, etc. It is also very weldable, so is used in structural fabrications.

7075 is considered unweldable. For the purposes of a receiver, this is of no consequence.

The T6 that follows is Tempering, a heat treatment process.

More info here
http://en.wikipedia.org/wiki/Aluminium_alloy
 
I don't know if I could play an engineer on TV, and I didn't stay at a Holiday Inn last night, but I believe that I could successfully play a guy who stayed at a Holiday Inn last night.

As long as no detail was required. I stayed at a Red Roof Motel about a month ago.

I could not afford the Holiday Inn.
 
Well researched piece.
Thank you for taking the time to synthesis all that data.

I guess the real crux of the issue is the difference between forged and milled receivers and the metal that is best for each of these applications.
 
beavis_m305 said:
Please forgive my extreme ignorance, but what exactly causes alloy to corrode, especially in relation to firearms? Certain salts?
Click to expand...
I assume you are specifically asking about aluminum. Aluminum corrodes through oxidization, like other metals. The common idea is that aluminum doesn't corrode. This is of course false. Aluminum is actually a very active metal, meaning that its nature is to oxidize very quickly. While a weakness for most metals, this quality is actually the key to its ability to resist corrosion. When oxygen is present (in the air, soil, or water), aluminum instantly reacts to form aluminum oxide. This aluminum oxide layer is chemically bound to the surface, and it seals the core aluminum from any further reaction. This is quite different from oxidation (corrosion) in steel, where rust puffs up and flakes off, constantly exposing new metal to corrosion. Aluminum’s oxide film is tenacious, hard, and instantly self-renewing.

Although aluminum has a huge advantage when compared to other metals, it is not always completely impervious to corrosion. Its protective oxide layer can become unstable when exposed to extreme pH levels. When the environment is highly acidic or basic, breakdown of the protective layer can occur, and its automatic renewal may not be fast enough to prevent corrosion.

Annodizing is a common process used to further increase aluminum’s corrosion and abrasion resistance, as well as a method to chemically bond colorant to the surface. Anodization is achieved by artificially thickening the natural oxide layer. This film can be made many times thicker than what would otherwise be formed.

The biggest corrosion issue with aluminum is galvanic corrosion. Galvanic corrosion is when two dissimilar metals are immersed in an electrolyte solution, such as water. Aluminum is typically a reactive metal in the galvanic series. This means that in many cases, when it is in contact with other metals, aluminum will act as the anode and may begin to corrode. This is likely why the pivot pin holes, where steel meets aluminum, began to corrode on early 6061 AR15's in Vietnam, which is an extremely humid environment which lends itself to facilitating galvanic corrosion. 7075 resists this corrosion better than 6061, but is not necessarily stronger, as the OP had pointed out.
 
Excellent work. Much respect regarding your research, posts, and method you used to come to your conclusion. Unlike others on this board he actually did the hard work necessary to draw his own conclusion.
 
I am not sure if we are going to expierience very harsh enviroments for our AR's considering after all that they are range guns only for us in Canada.
And the fact that NEA has a full lifetime warranty on these makes me think they are very confident in their product.
These rifles are approx. $300 more than a Norc and are made in Canada. What are Norcs made of?, I don't even know but I think they are a great rifle too, but for $300 more for an NEA why would anybody even consider a Norc
 
You must log in or register to reply here.
Back
Top Bottom