Alloy 7050 stress corrosion resistance 7

[Ziller]

Hy everybody,

I am designing a highly stressed component using 7050 alloy.
This component have holes throu which titanium pins will transfer the load into the piece itself. On top of this the part will work in salt environment. Do you have any advise for protecting the component? Is T73 the best heat treatment for minimizing the effects of stress corrosion cracking?

Best Regards.

 

[rb1957]

anodize is typical corrosion protection process.

if you cold worked the hole, that would put compression stresses on the edge, and prevent stress corrosion cracking (by preventing tension stresses on the surface).

 

[WKTaylor]

Ziller..

Refer to MIL-HDBK-5J, or MMPDS, para 3.1.2.3.1 "Resistance to Stress-Corrosion Cracking..." [aluminum alloys]; and the assocciated tables.

Generally "ST" is the killer. Note that typically T73xx, and -T74xx tempers have the best SCC resistance... while most -T76xx tempers have the poorest [but much better than -T6xxx under all circumstances]. "Form" [plate, extrusion, forging, etc] plays a significant role for certain alloys. Proprietary tempers such as -T77xxx have limited data.

Be cautious... because SCC is only one problem of many in a highly corrosive environment: dissimilar materials, exfoliation, filiform, fretting/wear, poor drainage, etc... could also be significant factors. Also, surface treatments [inorganic/organic], are very critical for long-term serviceability.

Regards, Wil Taylor

 

[Kwan]

If I were designing such a part I would reduce the operating stress level below the SCC threshold. This would minimize the SCC issues.

Wil is correct about the other forms of corrosion. The typical coating (used by Boeing) would be to shot peen, then anodize. Use two coates of epoxy primer, wet install the fasteners. Also use fay surface seal and fillet seal at installation. Then apply a coat of epoxy primer after installation and a corrosion inhibiting coating.

I can not locate the SCC threshold. Maybe someone can provide a reference for this?

 

[SuperStress]

They're done as a percentage of minimum yield strength.

MIL-HDBK-5J Table 3.1.2.3.1(a) p.3-16 to 3-17 (pp. 318-319 in the .pdf version)

SuperStress

 

[TVP]

As Wil Taylor mentioned, section 3.1.2.3 of MMPDS references SCC resistance for Al alloys. 7050-T73/T74 is recommended to have total stresses (service + heat treatment + straightening, etc.) less than 50% of the tensile yield strength. Depending on the product form the TYS can be as low as 50 ksi for the ST direction, which would mean fairly low total stresses. Kwan's suggestion of shot peening + anodizing + 2 coats of epoxy to seal the anodic layer and then wet installation of the fasteners is a very good one.

 

[RPstress]

I personally would hesitate to use 7000 series in these circumstances. Depending on inspectability and the consequences of failure, it might pay to keep the ST-direction stresses to <25% of Fty. (I'm assuming here that "salt environment" means something similar to a sea-water or coastal situation: a seaplane or a carrier-based application.)

 

[WKTaylor]

Guys...

My problem is that a large percentage of the acft I currently work-on is an H2 Embrittlement, SCC & Exfoliation experiment in-progress [Mid-50s design].

For our USAF customer, I follow guidelines in MIL-HDBK-1568 and MIL-HDBK-1587 to insure SCC-resistant design in repair parts, mod-kits, new replacement parts, etc. Finding a suitable replacement material for 7075-T6 Die forgings, 7178-T6, 7179-T6, etc has been a challange!

NOTE: per MIL-HDBK-1587 minimum ST SCC threshold rating [sustained NaCl exposure, no failure] for aluminum alloys MUST be 25-KSI [not 25% of FTy]. Plate Examples: 7050-T7451 is very good ~+40KSI; 7050-T7651 barely makes the cut at ~25-KSI; and 7075-T6 is way too low at ~6-KSI.

Regards, Wil Taylor