Any real difference between forged and milled lowers?  If so, would it really matter?

Who makes milled lowers?

Are you refering to billet vs forged?
All are milled to some extent.

Every manufacturers receivers are "milled" to some extent on larger machine tools, usually a vertical milling machine.

"Forged" refers to the process of forming a heated block of aluminum by hammering it into it's general final shape between 2 forging dies.  Raw forgings are then "milled" to the final shape and all holes drilled.  This is the most prevalent method by which receivers are made today.  They are the strongest and generally cheapest to make in high volume production.

Another process involves casting the upper or lower by pouring liquid molten aluminum into casting patterns or molds.  "Milling" or machining is still required for final shaping and drilling.  This process is not often used to make AR receivers.  Cast receivers tend to be weaker than a forged receiver.

Yet another process to form upper receivers that is called "extrusion".  Semi-molten aluminum is forced through a die that creates a continuous piece of material to a uniform cross-sectional shape.  It is then cut to length and final machined to final configuration.  DPMS uses this process to make their Hi-Pro and Lo-Pro upper receivers.

The last fabrication method involves taking a big brick (billet) of aluminum and totally shaping it to the final configuration on a "milling" machine.  This type of receiver is generally referred to as a "billet" receiver.

So, there is no really advantage of billet over forged?

How do you till if a lower is cast or forged?

Cast receivers typically have raised lettering on them. Your chances of running across a cast receiver is pretty low. I'm not aware of any current production cast receivers.

Okay, well, I do have one on which I did my Delton build for my first, an Essential Arms lower.

Then why are you asking how to identify one?

Because I did not know that till you explained it, which explained the difference in my Stag lower vs. the EA lower (which has raised lettering).  I had wondered about it but did not know it was cast.

Aluminum is a crystalline material and as such, it will have a "grain".  This grain is set in the casting of the initial melt, called "blooms".  Wrought aluminum, used in both forged and milled receivers, is such bloom that has been reduced in dimension by both hot and cold forming processes.  In the milled receiver, the grain is usually only in one direction and there will be slight mechanical property differences called anisotrophy.  

In the forged receiver, high strength areas like pivot bosses and the buffer tube lug have been designed to promote optimum grain directions in the forging process.  This is done by careful design of the intermediate dies, the temperature of forging and size of the forging blank.

Cast receivers in alumnum got a bad rap because if inattention to detail.  Some were cast in dies which made the aluminum cool too fast with fine grain AND considerable built-in stresses as the materal cooled and shrunk, fighting the mould.  If done in a proper investment casting AND properly heat treated, the strength should be fine for most purposes.

Now if the receiver were cast in an alternate material, done in the precision investment method, it can be very strong.  Materials like CF8M, a cast equivalent of 316 stainless, would be incredibly strong as it is much stiffer.  And it would be heavier.  Other materials like SiAlMo bronze would have even better properties like bearing strength, something the owner of a RDIAS would appreciate.  Tool steels?  Go for it.

Most jewelry is investment cast.  Something to think about.  As are most of the FCG parts like trigger and hammer.

The easiest way of identifying a cast receiver is the lack of forge flashing.  Some forged lowers clean up all the signs of forge flash.  Cast receivers also have a rougher appearance inside the mag well.

So the question about all this that I have never seen answered anywhere yet, is how much stronger is a forged lower than a billet lower (or an upper, or whatever)?  
For many, their eyes roll back in their head, they say it doesn't matter, etc, etc.
If it doesn't matter to you, that's fine.  But I'm curious to know if anyone has enough of a metallurgical background who really does know more than the opinions and speculations.

From what I have seen here and elsewhere, the forging is stronger.   Yes, I know high quality billet lowers are made today that thicken up certain areas to account for this, and I have no doubt it is just fine.  I have a LaRue billet lower that I have no strength concerns about.   Sun Devil, POF, VLTOR etc are other examples.

I just want to know for the sake of knowing.    How much stronger is a forged lower than a typical billet lower?    Don't mean to get a flame war going, I just want to know, if anyone out there actually does have real information on it.    It's pretty much a given that for the same thickness, the forged part is stronger.   But by how much?

I'd like someone who owns both a forged lower and a nice billet lower to do destructive testing and let us know at what kind of force they went to failure.    I'll volunteer my Stealth lower for the experiment if I can find it.   Ok, I looked for it and can't find it.

There was a person who posted the actual psi rating (young's modulus - measured in Pa or ksi) for forged aluminum versus aluminum billets used in machining.  I don't remember the numbers, but the forging does have a significantly higher rating.  One way to find out is to do a search on young's modulus of T6061 aluminum as a start - then try to find its rating when forged versus not forged.  I believe it was on the order of 80ksi for forged, and 50ksi for as-is (no cold work done, no heat treating, etc).

There's really no need for destructive testing
Just find a machine shop or a lab that has access to a Rockwell hardness testing machine, and test the hardness of your billet vs your forging.  That should give you all the answers.

Forging like anything else can be good or bad. That is a quality issue.
Same w/billet. Extrusion is basically billet material drawn through tooling to produce a shape.

The main advantage that a forging has over a billet is there is less material to machine off in the manufacturing process. It is a productivity/Price thing more than a quality thing. That's it.

When you machine from billet, you have to remove alot of material to get to what would be the starting point with a forging. Therefore there is more machining and machine time required when making a lower from a billet.

There is no need to be concerned about which is better or which is stronger. I'd take either.

Now, Castings that is another thing. Avoid at all costs. Unless as someone mentioned before, you are doing investment castings, castings are to be avoided. Voids and pockets in the material can form very easily. These are often found when machining and often not. These areas are weak and can fail. These are things you really don't have to worry about with Forgings or billets.

Just FYI Custom Motor cycle wheels are made from billet material. Aircraft airframes are made from billet material. You will never see a cast part on the airframe of an airplane. Even the titanium & stainless aeroframe parts will be made from billet. Never Cast!

So if anything, I'd pick billet #1, Forging #2 and Casting "NEVER".

Actually, forgings can be inherently stronger, but many companies do not opt to forge due to the expensive equipment involved with forgings.  It is much less expensive to run a CNC machine that can make many parts than to have to shell out a lot of money for one big piece of forging die just to make one part.

Yes, forgings ARE inherently stronger of the two, when comparing articles machined from forgings and billet, which are identical when completed.

In the case of the AR receivers,  the forgins are 7075-T6, which is NEARLY TWICE AS STRONG
as the more common 6061-T6 that would be what billet milled receivers are most commonly made from.

6061-T6

T6 temper 6061 has an ultimate tensile strength of at least 42,000 psi (290 MPa) and yield strength of at least 35,000 psi (241 MPa). In thicknesses of 0.250 inch (6.35 mm) or less, it has elongation of 8% or more; in thicker sections, it has elongation of 10%

7075-T6

7075 tech sheet T6 temper 7075 has an ultimate tensile strength of 74,000 - 78,000 psi (510 - 538 MPa) and yield strength of at least 63,000 - 69,000 psi (434-476 MPa). It has elongation of 5-8%

Now consider that these are the properties of 7075 PRIOR to the forging process. Forging
increases the strength of the part by a considerable margin.


The difference is clear.  7075 forgings are > 6061.


CJ

In the engine and other parts under higher stress, would we find forged parts or billet parts?  From what I have seen, forgings are used in critical parts.

Now granted, for our purposes, billets are strong enough.   We won't be stressing our parts to the levels seen in something like an engine.    But I like knowing what the difference is.    Good info posted.  Thanks to all.

There you have it.    AND they're cheaper than billets too.

Cast receivers in aluminum suffer from ductility issues.  This is a grain structure problem that cannot be corrected with heat treatment.

Aluminum is a bitch to cast because it is low density and it forms a tough oxide film which causes bubbles to form.  Most engine blocks cast in aluminum are filled from the bottom to prevent these problems.

Most cast alloys in aluminum have inferior properties as sufficient silicon is added to promote fluidity.  This messes the grain structure.  But for things like automotive pistons, it doesn't matter.  It also adds great wear strength on the skirts, preventing most scuffing wear.