Alloy 7050 stress corrosion resistance
Alloy 7050 stress corrosion resistance
(OP)
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.
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.





RE: Alloy 7050 stress corrosion resistance
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).
RE: Alloy 7050 stress corrosion resistance
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
RE: Alloy 7050 stress corrosion resistance
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?
RE: Alloy 7050 stress corrosion resistance
MIL-HDBK-5J Table 3.1.2.3.1(a) p.3-16 to 3-17 (pp. 318-319 in the .pdf version)
SuperStress
RE: Alloy 7050 stress corrosion resistance
RE: Alloy 7050 stress corrosion resistance
RE: Alloy 7050 stress corrosion resistance
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