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Posted: 3/6/2006 7:53:04 AM EDT
I am trying to find out what is better. I would rather register a high quality lower for my SBR project than a low quality...Thanks....Please IM me if you could or I can check back here thanks....
Link Posted: 3/6/2006 9:19:39 AM EDT
[Last Edit: 3/6/2006 9:29:37 AM EDT by glock24]
6061 Alloy

Extremely versatile, this heat-treatable alloy combines good weldability and formability, high corrosion resistance, and medium strength. Use it for chemical equipment, vehicle parts, scaffolding, and pipe fittings. Yield strength is 40 ksi. Hardness is 95 Brinell. Melting range is 1080 C to 1205 C.

7075 Alloy

One of the hardest aluminum alloys, this exceptionally strong, heat-treatable alloy has good machinability and fair corrosion resistance. An "aircraft alloy" material, it's ideal for aviation parts, keys, gears, and other high-stress parts. Yield strength is 73 ksi. Hardness is 150 Brinell. Melting range is 890 C to 1175 C

7068 Alloy

Offering extreme high strength, this heat-treatable alloy is significantly stronger than Alloy 7075 with comparable corrosion resistance. Originally developed for ordnance applications, it's now being used for aircraft and vehicle parts as well as for other applications requiring extreme high strength. Yield strength is 99 ksi. Hardness is 190 Brinell. Melting range is 890 C to 1175 C


To answer your question, 7075 is better than 6061 because it is harder and stronger. As you can see, 7068 is better yet, but nobody offers this grade in AR receivers. Interestingly, 7068 is used in Beretta's pistol frames.

BTW, 1 ksi = 1000 psi
Link Posted: 3/6/2006 10:02:56 AM EDT
Defintely go with the 7075T6 FORGED. As far as a manufacturer, Colt, Stag, CMT, Armalite are probably your best bets as far as the quality of the material and the quality of the overall workmanship. There are a few others that are really good, too....someone will most likely bring those up.
Link Posted: 3/6/2006 4:19:53 PM EDT
Link Posted: 3/6/2006 9:26:27 PM EDT
Since those or I should say most are forged where does billet come into this fray?
Link Posted: 3/7/2006 10:27:10 AM EDT
Someone out there is offering Titianium lowers. Any comments on it?
Link Posted: 3/7/2006 10:57:05 AM EDT

Originally Posted By MT-Borg:
Someone out there is offering Titianium lowers. Any comments on it?



I would say WAYYYYY overkill. Ti lowers will be heavier and since the lower is technically a "float house" more than anything your not gaining a lot by using Ti. 7075T6 is probably the most optimum thing you can use. For aluminum, it is REALLY FRIGGIN HARD and a proper anodized finish makes the exterior VERY tough. Remember, it is a surface conversion that essentially creates an aluminum oxide case over the already tough 7075T6 body....and aluminum oxide is TOUGH.

I must give a tip of the hat to 7068.....I'd love to see a full chart and comparison of specs between 7075 and 7068.

Anyhow, Ti would, in my opinion be overkill and I am not sure how a Ti upper ( we'll assume matching uppers and lowers here ) would handle the bolt carrier racing through it at a gagillion miles an hour. Ti tends to gall, though there are ordanance grade alloys that can be used in moving assemblies.
Link Posted: 3/7/2006 11:20:19 AM EDT
[Last Edit: 3/7/2006 11:23:02 AM EDT by glock24]

Originally Posted By mstennes:
Since those or I should say most are forged where does billet come into this fray?



Here's an overview link explaining aluminum manufacturing;

www.aluminum.org/template.cfm?Section=The_Industry

From what I can understand, alumium billets start their lives in the foundry, where molten aluminum is cast into all kinds of shapes, including rectangular bars.

A billet aluminum lower would be CNC machined from one of these cast bars of aluminum.

This is different than a forging. Apparently a forged lower is produced by reheating a formed billet and smashing into a rough shape with tons of pressure.
Link Posted: 3/7/2006 11:23:57 AM EDT
The book "The Black Rifle II" talked about the move from 6061 to 7075 aluminum for AR receivers. Seems the original receivers were made of 6061 (Stoner did not approve of this, he specified 7075 from the get-go). After awhile the military started getting reports back to the effect that receivers had been found to be corroding in the jungles of Vietnam. They determined that this corrosion could be caused not only by the jungle environment but also by acids produced by a mans sweaty hands. A common corrosion point was the front pivot pin area. Some cases were so bad that the rifles became unservicable!! So they made the move to 7075 aluminum.
Link Posted: 3/7/2006 11:32:37 AM EDT

Originally Posted By MT-Borg:
Someone out there is offering Titianium lowers. Any comments on it?



Titanium - Very strong yet weighs about half as much as steel, nickel, and copper alloys. Titanium has excellent corrosion resistance and superior resistance to chlorides, saltwater, and acids. It's essentially non-magnetic and ideal for applications where electromagnetic interference must be minimized.

Titanium Grade 2 (40KSI-YS)

A commerically pure annealed titanium that's easy to form, bend, and weld. It has better chemical resistance than Grade 5, although it is not as strong. Melting point is 3020 F. Yield strength is 40 ksi. Hardness is Rockwell B80

Titanium Grade 5 (6AL-4V)

Can be heat treated to significantly increase its strength. It has good weldability and a very high strength to weight ratio at cryogenic temperatures. Melting point is 3000 F. Yield strength is 120 ksi. Hardness is Rockwell C33

Link Posted: 3/7/2006 11:36:38 AM EDT
So what is better? a forged receiver, or billet receiver?
Link Posted: 3/7/2006 11:53:34 AM EDT
"forged" is like any piece of cast that is then cut and machined to spec. "billet" is machined from a solid chunk of "stock" and usually contains fewer imperfections.

billet is better, no doubt. the next logical question is: is billet worth the extra money? i can't answer that, because i don't have a comparison...

HTH.

septic tank
Link Posted: 3/7/2006 11:55:41 AM EDT

Originally Posted By beemerman:
So what is better? a forged receiver, or billet receiver?



I wish a metallurgist would step in here, but I'll go out on a limb and say a forging is better.

From what I gathered from the above website, a forging starts life as a preformed billet and then undergoes further strength enhancements through the forging process. The forces and temperatures involved with forging "hot work" the aluminum billet and increase it's yield strength over an "unworked" billet of the same grade.
Link Posted: 3/7/2006 12:03:07 PM EDT
there is a great deal of conversation about topics of this nature in terms of metallurgy on www.benchrest.com/.

maybe they can shed some light on this. no doubt forged is better. the question remains: how much better?
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