Anodizing 7075 Aluminum and fatigue 3

[Dougt115]

I see this has been discussed at length and I understand that the thicker the anodize the faster the part will fatigue. Also that Type I is better for aluminum than Types II or III because the Chromic acid is less damaging to the aluminum. Also that shot peening would improve the fatigue life.

But...

Does anyone have a chart, graph or other source that shows the relationship? I am really looking for a quantitative source and not just qualitative.

I have a customer that wants the part Black.

Also is there another finish that may give me the wear resistance and not decrease the fatigue life that I should consider.

 

[tbuelna]

Dougt115-

In terms of surface wear resistance, the only type of anodize that provides any useful increase in hardness is (MIL-A-8625) type III, or hard anodize. Type I & II anodize are normally used only for corrosion protection or cosmetic purposes. The thickness of type I & II anodize coatings is usually less than 70 microinch, while type III anodize coatings can be 4 or 5 times that thick. The MIL-A-8625 spec gives guidance on anodize type/thickness for various aluminum alloys/tempers, so you can find some answers to your question there.

I've also attached a table comparing how anodize types and shot peen affect fatigue properties of some 7075-T6 test coupons. There is lots of similar technical data publicly available from places like this.

Hope that helps.
Terry

 

[TVP]

IMPROVING THE FATIGUE RESPONSE OF AEROSPACE STRUCTURAL JOINTS

http://www.lambdatechs.com/documents/258.pdf

A predictive fatigue life model for anodized 7050 aluminium alloy

http://hal-mines-albi.archives-ouvertes.fr/docs/00/79/68/20/PDF/Mabru_8494.pdf

Effect of Anodising on the Fatigue Properties of Aluminium Alloys

http://dspace.dsto.defence.gov.au/dspace/bitstream/dsto/10291/1/DSTO-TN-1180 PR.pdf

Thin-Film Sulfuric Acid Anodizing as a Replacement for Chromic Acid Anodizing

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19950025351.pdf

Fatigue and fracture assessment of toxic metal replacement coatings for aerospace applications

https://dspace.lboro.ac.uk/dspace-jspui/bitstream/2134/9636/9/TransIMFPaper745(Cree et al).pdf