2024-T4 vs 2024-T351 per qq-a-225/6

[cougfan]

My customer has removed 2024-t351 per qq-a-225/6 as an alternative temper to 2024-t4 citing only "property discrepancies". Does anyone know what the discrepancies are?

 

[rb1957]

QQ225/6 is "rolled, or drawn, bar, rod, wire"

T351 is a plate extrusion form, T4 not (rolled plate?)

strength-wise there isn't much in it, about 5% (T351 is stronger than T4)

another day in paradise, or is paradise one day closer ?

 

[cougfan]

I have looked at the standard properties. I am not sure about stress corrosion, exfoliation corrosion, or some fatigue differences. For 30 years it was acceptable and now -t351 for -t4 is not allowed citing property discrepancies???

 

[3DDave]

What form is used for the product? Note that -T351 properties are variable with plate thickness, which may not be acceptable. For the thickest plate, Alcoa indicates elongation as low as 4% for -T351 (

http://www.alcoa.com/mill_products/north_america/en/product.asp?prod_id=595)

 

[rb1957]

is it a plate problem ? qq-a-225/6 looks like bar material, qq-a-250/4,5 would be the plate call out, no?

i doubt there's much in it, i'd be looking for their engineering buy-off ... is it an alternate material on the drwg ? is there some production paperwork (RNC?) for the change ?

another day in paradise, or is paradise one day closer ?

 

[cougfan]

The material form is round bar.

 

[rb1957]

'k, but there should some engineering coverage for the substitution ... not just some "yobo" saying "i can't be bothered to get the right material, and i've got this lying around".

another day in paradise, or is paradise one day closer ?

 

[cougfan]

Yes, for 30 years the customer had a "material substitution" document that allowed the substitution of -T351 en lieu of -T4. A couple years ago they removed the substitution with the comment "property discrepancies".

 

[rb1957]

and now they want to reinstitute it ? i understand the "logic" of what's happening, but it looks ugly.

clearly the substitution can be allowed (since the drwgs allowed it for 30 yrs).

however, someone recently decided it wasn't a good substitute, for "property discrepencies".

but now the question has to be "what were these discrepencies?" and "why were they so significant as to change the drawing?" and "why can we accept these discrepencies now?"

this is more involved than this forum permits.

another day in paradise, or is paradise one day closer ?

 

[cougfan]

Yes, I am very curious what the property discrepancies are. They have not re-instituted the substitution.

 

[rb1957]

you can see the material properties easily enough in MMPDS-01 or Mil Hdbk 5 ... should be able to find either online.

another day in paradise, or is paradise one day closer ?

 

[WKTaylor]

Cougfan...

I suspect that MMPDS-* and AMS-QQ-A-225/6A spec mechanical-allowables differences between -T4 and -T351 are what drove this revision.

However, I-too am baffled by the change none-the-less, since -T351 [mechanical] allowables are equal-to, or [generally] superior-to, -T4 allowables [especially as section thickness increases]... and all other properties are generally identical [e, E, density, strength-VS-temperature, KIscc, KIc, etc]. Unsure about fracture mechanic properties [dA-dN etc].

NOTE.
As a rule I would NOT specify any 2XXX-T3xxx or -T4xxx Temper due to very poor KIscc [SCC] and EXCO ratings. Specify 2XXX-T8xx or -T62 temper for Higher mechanical allowables and MUCH better environmental durability, where possible [and as-allowed by stress/DADTA weenies].



Regards, Wil Taylor

Trust Me! I'm an engineer!

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

Unfortunately, in science what You 'believe' is irrelevant û "Orion"

 

[rb1957]

sorry Will, but IMHO that's "odd" advice. we've used 2024T3 for fuselage skins for a long time ... thin sheet doesn't use Kic, but Kc; T8 is quite a bit weaker than T3/4 albeit with better corrosion characteristics. most of the time it'll be clad sheet (and fair enough you can say that you were talking about unclad 2024). Heavy plate and machinings ... that's a different question ... 2024 wouldn't be a material of preference ... usually 7075 or 6061 depending on the strength needed.

another day in paradise, or is paradise one day closer ?

 

[WKTaylor]

RB1957...

I agree 100% with Your statement RE sheet metal, thin extrusions, thin bars, etc...

However, MIL-STD-1568 TABLE I. Rating for resistance to SCC aluminum alloys in the short transverse grain direction (STDG) , MMPDS-08 Table 3.1.2.3.1(a). Resistance to Stress Corrosion Ratingsa for High-Strength Aluminum Alloy Products, several other references [ASMH, in-house design documents, etc], and personal/field experiences clearly indicate significant SCC* and EXCO** problems with rolled plate, rod/bar [rolled and/or drawn], extrusions and forgings. In general the -T3 and -T4 tempers should be restricted to as-wrought [rolled, drawn, extruded, etc] stock material thicknesses of less than 0.249-inches to minimize potential for SCC and EXCO.

*SCC = stress corrosion cracking.
**EXCO = exfoliation corrosion

Regards, Wil Taylor

Trust Me! I'm an engineer!

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

Unfortunately, in science what You 'believe' is irrelevant û "Orion"

 

[tbuelna]

MMPDS-01 does list slightly different min mechanical properties for 2024-T351 and 2024-T4 bar/rod rolled, drawn or cold finished IAW QQ-A-225/6 (tables 3.2.3.0). If the analysis report for this part showed an extremely small stress margin, and some meticulous engineer or analyst reviewing the report took the time to verify the material properties used still matched current standards, he may have found that the part would have negative stress margins using the alternate material with its slightly lower min mechanical properties.

 

[3DDave]

In MIL-HDBK-5J, 31 January 2003 page 3-83 is the following notation for Design Mechanical and Physical Properties of 2024 Aluminum Alloy Bar and Rod; Rolled, Drawn, or Cold-Finished is the note:

"a The T4 temper is obsolete and should not be specified for new designs." under the AMS 4120 and AMS-QQ-A-225/6 column.

Shouldn't the design originator be able to tell why the substitution was withdrawn?

 

[dgapilot]

Perhaps they are trying to specify the raw material form rather than the actual material properties. 2024-T351 would be AMS 4034 Sheet or Plate. 2024-T4 would be AMS QQ-A-255/6 drawn, rolled, or cold finished bar, rod, or wire. Cougfan indicated that the raw material for the product was round bar, so the 2024 T4 per AMS QQ-A-255/6 is correct. Not knowing the final configuration of the part, if the design required a cylindrical part, then the spec makes sense since it's not as easy to make something round out of something flat. On the other hand, if the part is basically flat, and the manufacturing engineer envisioned cutting slabs from a round bar and then milling the part, the option of using plate would be worthwhile. It all depends on the manufacturing process used and the configuration of the finished part. This shows that the process of design should not be done in a vacuum, and the designer not only has to consider what the part does, but how it will be made. Consultation with manufacturing engineering on issues like this can have significant part cost implications. Likewise, consultation with purchasing to determine the availability and cost of the various forms of raw material that could be used should also enter into the material spec decision and potential lead time to produce the part.

 

[WKTaylor]

All...

MIL-HDBK-5A [1966, first time QQ-A-225/6 appears]
TABLE3.2 .3.O(g)D. Design Mechanical and Physical Properlies of 2024 Aluminum Alloy (Bor, Rod, and Wire); Rolled, Drawn or Cold Finished; Drawn Tubing
Mechanical allowables [S-basis] for '-T4 or -T351' are identical [same collums]

However, in MIL-HDBK-5B [1973] the [S-Basis] mechanical allowables for QQ-A-225/6 -T4 and -T351 have been separated and minor differences are apparent. -T351 is generally equal-or-superior-to -T4... EXCEPT for Fbru [very odd/unexpected/minor]. This 'difference' [-T4 VS -T351] generally tracks true from 5B~thru-MMPDS-08.

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]

 

[rb1957]

for what we see there's no significant difference between the tempers. I'd ask your customer, why T351 is unacceptable ? what are these "poperty discrepencies" ? if you really want to push the matter, maybe say that that drawing change increases to cost of the part.

T4 carries the "obsolete for new design note" mentioned above. Both T4 and T351 have the caution about poor SCC rating (in the LT direction).

On the face of it, T351 is a reasonable substutite for T4.

another day in paradise, or is paradise one day closer ?

 

[tbuelna]

It would help to know what basis was used for material properties by the customer for this particular component design. It seems strange that the 2024-T4 material that MMPDS-01 lists as being obsolete for new designs is now the only acceptable material for producing this legacy component.

On the other hand, I have seen similar situations while working on legacy programs like the Space Shuttle. I recall a situation where NASA asked the company I worked for to manufacture some spare parts that had not been produced for 20 years. Some of the process specs listed on the old paper drawings were obsolete, and there were no vendors still qualified to perform the work required by the old specs. So I had to create lots of paperwork to cover the use of current process specs in place of the obsolete ones. It was a huge headache.