A stronger alternative to 7075-t6? 3

[SorryOciffer]

Looking to make some parts (nuts, about 1.5" diameter with a 1.25" bore) that will be torqued to about 30 ftlbs several times. Is 7075 adequate or is a stronger alloy available?
Thanks.

 

[WKTaylor]

Thublena...

I was privileged to take a 5-day corrosion course from the premier Army corrosion engineer, LLoyd O Gilbert, back in the 1986 time frame.

Eventually I discovered the corrosion course notes [binder full] were essentially transformed into MIL-HDBK-735 MATERIAL DETERIORATION PREVENTION AND CONTROL GUIDE FOR ARMY MATERIEL, PART ONE, METALS.

The presenter had a specific example of a M-16 forged/machined receiver, 7075-T6 black anodized, that had developed massive EXCO due to perspiration from a sweaty hand during actual SEA combat operations. He had an actual M-16 receiver as part of the course show-n-tell... absolutely amazing damage matching where finger of troops normally gripped the receiver... and was similar to what we were seeing on older Acft parts [-T6 temper] in a sea-coast environment.

During his discussion I am, certain he stated that the soultion to this problem was heat treating the die forged receiver to -T73 [EXCO & SCC resistant], instead of -T6 [EXCO & SCC prone]... then black anodizing and application of a baked-on SFL. For some reason, I don't remember shot-peenining... which I now know to be an excellent way to retatrd SCC & EXCO initiation... I think because of the complexity the interior cavities of the M-16 machined reciever.

Here is the extract from MIL-HDBK-735 RE M-16 corrosion.

4-2.4.2 Exfoliation
After a [M-16] rifle had been in service for 3 yr in a hot and
humid climate (Southeast Asia), exfoliation corrosion
was detected on parts of the lower receiver (Ref. 33). The
part affected had been protected with an anodic coating,
but the affected areas were those frequently in contact
with the hands of the soldiers and thus were exposed to
the chloride ion in perspiration. The original specification
required a minimum yield strength for the aluminum
forging of 448 MPa (65 ksi). The manufacturers supplied .
alIoy 7075 in the T6 temper, which did have adequate
strength.
An investigation was conducted on alternative tempers
of alloy 7075 that might have adequate strength and
greater exfoliation resistance than the T6 temper. Newer
a310ys with adequate strength and greater resistance to
exfoliation were also investigated.
Some conclusions of the investigation are
1. Alloy 7075 die forgings can be thermally treated to
produce an exfoliation-resistant material with a minimum
yield strength of 448 MPa (65 ksi). a
2. In the T6 temper, 7075 forgings exhibiting a
randomized grain structure, as found in those produced
from rolled bar stock, are more resistant to exfoliation
corrosion.
3. Other 7XXX series alloys, such as X7050 and
premium grade 7175, can be thermally treated to produce
a variety of exfoliation-resistant forgings with much
higher strength than 7075.
4. Temperature and length of aging are very
important factors to control to insure that the optimum
combination of strength and exfoliation corrosion
resistance is obtained.

NOTE.
During an investigation of an OV-10A that developed an unusual/sudden problem of the main landing gear hanging up during retraction and extension. We discovered that a hot-air leak from a poorly installed "S-exhaust duct" was directed against a large area of the machined frame containing the MLG trunnion. This leak severely damage the epoxy primer [classic sign of over-heating]. We identified a technical study where 7075-T73 was exposed to hot conditions exceeding aging-bake temperatures [like exhaust leak]. The report concluded that the material would distort [grow] substantially due to over-heating effects... which accounted for the sudden binding of the MLG due to Trunnion bearing misalignment cause by permanent material distortion.

In truth I believe that a new generation 2xxx series alloy would be a much better choice. Even old standbys, such as 2219-T852 or a 2124-T852, would work better. These 2xxx-T8 alloys/tempers have good EXCO/SCC resistance, good strength, good fracture toughness and outstanding long-term heat exposure resistance.

NOTE, RE M4.
I understand that the SEALs discovered that the M4 was prone to failure during extended/intense combat operations, due to over-heating of the receiver, bolt and/or gas-operating parts... and possible gun-gas-residue accumulation. This phenomena was discovered during unexpectedly long combat engagements, when a few hundred shots were fired through these weapons in almost non-stop action. This went against the grain of SEAL training where brief/intense engagements were the expected NORM... not prolonged engagements which they have actually experienced. Needless to say this has caught the SEALS attention in a very negative way.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true.
o For those who believe, no proof is required; for those who cannot believe, no proof is possible.
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can break them like an artist. [Picasso]

 

[tbuelna]

wktaylor-

As far as I know, the M4s supplied to the military still use a 7075-T6 die forging for the upper and lower receiver. The exterior of the upper receiver is shot peened after machining, then anodized, and then the bore surfaces that contact the bolt carrier are coated with DFL. One interesting thing is just how loose the RT fits are between some sliding components, such as the bolt carrier OD and upper receiver bore. Likely to permit operation at conditions of extreme temperatures and contamination, and allow for any CTE mismatch.

If you look at some of the M16/M4s that have been in service for long periods, you can see many areas of the upper/lower receiver (such as corners) where the anodize has been worn thru. You might take a look at MIL-DTL-71186 which is the model spec for the M4A1. The number of qual cycles the spec lists for the upper/lower receivers is quite low, and nowhere near what would pose a fatigue issue.

 

[WKTaylor]

Tbuelena...

Generally speaking 7075-T6 DF parts have a pretty good fatigue endurance [and fracture toughness]. What hurts us in aerospace is EXCO/SCC failures.

I am genuinely surprised that 7075-T6 DF [or possibly forged block] is still the norm for the M-16 family of small arms after all these years of substandard performance in aerospace... especially considering it's poor resistance to EXCO, SCC, abrasion and temperature exposure.

However, 'it-is-what-it-is'. Apparently the shot peening does the trick on the exterior... and the SFL & lubricants protect the interior.

Curious
Are the production upper/lower receivers MF die forgings or impacts? Are the basic DFs/Impacts hollowed-out... or have deep cavities... for interior spaces... or are they simply forged shapes?

NOTE.
South East Asia [IE: Vietnam, Cambodia, Laos] and the Middle East [Iraq, Afghanistan, Syria, Iran, etc] have substantially different climates: hence much different climate induced damage potential.


Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true.
o For those who believe, no proof is required; for those who cannot believe, no proof is possible.
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can break them like an artist. [Picasso]

 

[ornerynorsk]

Will, I can probably help you on that last post. Every blank I've seen is a die forging. They essentially are supplied by the foundry to the machine-house as shown in the attachment. The uppers are very similar, with the interior needing bulk removal and finishing ops.

The most rudimentary application of care and maintenance will altogether prevent those types of corrosion in the field. There are undoubtedly extreme cases which have come to light, but honestly, in my opinion, corrosion of the basic receiver halves is the least of the worries on the Stoner platform.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[WKTaylor]

Ornerynorsk, Tbuelna...

I wonder how these newer receivers will last in the tropical/corrosive SEA climate. Apparently M4A1s have been sold in-mass to the Philippines to replace M-16s, so the real-world tropical experience will be 'telling'. Line-troops can be far-less fastidious in cleaning, oiling, and general maintenance relative to SF troops... although a healthy weapon can be the difference between living and dying.

NOTE... for what it's worth. the Forging parting plane [FPP] has been a nagging source of SCC/fatigue failure in aerospace parts... especially when machined-thru. Amazing that the FPP of the receivers hasn't caused problems. Exterior peening retards exterior crack-initiation... so exterior initiation is a low probability. However there appear to be other vulnerable points where the FPP has to be machine thru that are not peened.

OH yeah... and how many fairies can dance on the head of a pin??? [sorry, I get a bit anal after being bitten by so many SCC/EXCO failures over the last 37 years].

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true.
o For those who believe, no proof is required; for those who cannot believe, no proof is possible.
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can break them like an artist. [Picasso]

 

[WKTaylor]

OH yeah.. and I'm glad to see the classic M-16 'suit-case [receiver] handle'* has been eliminated in-favor of the butt-strap hanger [off-the shoulder]. What a pain ... not to mention it prevented mounting any optics/attachments to the upper receiver.

*A VN vet once stated that there should be a 'suit-case-arms' command for M-16 drill... simply because VN GIs got in the habit of carrying the -16 like a brief case, when potential for 'contact' was low.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true.
o For those who believe, no proof is required; for those who cannot believe, no proof is possible.
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can break them like an artist. [Picasso]

 

[ornerynorsk]

In the early 1990's, I spent a bunch of time in southern Mindanao, back when the Philippines was manufacturing their own M16's under Colt contract. I could be wrong, but the yellow-green tint of the service weapons I observed at that time sure looked like a chromate conversion rather than anodizing. I'm not brushed up on the processes and metallurgy enough to know which would be better in a tropical climate, but I don't recall ever seeing any with severe corrosion problems. Would be interesting to see what the current contract specs call out.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[ornerynorsk]

Interesting info about the fatigue failure and stress corrosion cracking at the die line. I wonder if any going concern has ever experimented with lithium doped alloys?

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[WKTaylor]

Ornerynorsk...

The forging parting plane [die line] is where excess material is extruded-out between dies; hence beneficial surface-aligned grain-flow is re-directed toward the edge. When sectioned and etched, the adverse grain-flow toward the edge is evident.

ASME Forging Design Handbook 1972 has an excellent discussion of this phenomena.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true.
o For those who believe, no proof is required; for those who cannot believe, no proof is possible.
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can break them like an artist. [Picasso]

 

[ornerynorsk]

Thanks for that Will. Very familiar with the process, we are presently using around 60,000 pieces a year of aluminum forgings at my present employer, but these are all very low criticality parts and we don't have the capability in-house to perform that level of inspection. I'll try to get my hands on the handbook.

With a typical M4 lower receiver, such as the photo I posted yesterday, there is a significant amount of the parting line machined away, with die-face surfaces remaining largely net, as-forged. The buffer tube protrusion and handgrip tang being notable exceptions.

Have you seen any lithium aluminum alloys being used in forgings in the AC industry? With its strain hardening characteristics, I have no idea how it responds to severe forming operations. The reason I mentioned that yesterday is that Smith & Wesson, and possibly other firearms OEM's, have been adopting Al-Li, albeit on a very limited basis, in certain models and components to address fatigue cracking issues. It additionally offers a very modest weight reduction, which would be of more importance in aero.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[WKTaylor]

Aluminum-lithium alloys seem like a fairly 'chancy' material selection for ordnance and small arms... for a ~10--15% weight reduction in aluminum parts only. Lithium presents some unique design/manufacturing/recycling issues/challenges.

In aerospace we do a lot for airframe weight reduction... but payback is worth the risk.
----
Interesting article on this subject of AR-15, M-16-thru-M-16A4 & M-4.

http://world.guns.ru/assault/usa/m16-m16a1-m16a2-m16a3-e.html

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true.
o For those who believe, no proof is required; for those who cannot believe, no proof is possible.
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can break them like an artist. [Picasso]

 

[tbuelna]

Curious. Are the production upper/lower receivers MF die forgings or impacts? Are the basic DFs/Impacts hollowed-out... or have deep cavities... for interior spaces... or are they simply forged shapes?

The M4 upper/lower receivers are made from matched die forgings. I have a new Colt M4 (SAO) which is manufactured using the same materials/processing as their military M4, and the upper receiver has a "keyhole" forging mark which indicates it was produced by Cerro Fabricated Products. You can actually purchase a raw 7075-T6 flat top upper receiver forging for $23, which is incredibly cheap. FN has recently replaced Colt as the primary supplier of M4s to the US military. I'm sure they still use a matched die forging for the upper/lower receiver, but it's possible they opted to use a different alloy and/or heat treatment.

I found some old public source prints of the upper receiver forging/machining and made CAD solid models from them. Then a friend and I purchased a couple raw forgings and finish machined them. They came out very nice. Below are some pictures from the project.

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1457052774/tips/FLAT_TOP_UPPER_PROJECT_iialtq.pdf[/url]