Hard anodize to help fatigue resistance?

[UCengno1]

MATERIAL: 7075-T6
Has anyone ever heard if a hard anodize coating can help surface fatigue crack resistance? Doesn't make sense as I believe the process imposes tensile stress at surface but I wanted to check around.

We are experiencing fatigue failure at the root of an aerodynamic bladed element in a high speed air/nat gas turbine. Confusion reigns supreme as this design worked with few issues for many years and has begun to run into problems. An anodize and seal process was removed from the processing (cost reduction)a few years back but I am struggling to see if the change is a possible link. Could it have been scrubbing the surface to an extent that it helped with fatigue crack initiation?

Surface micrographs show significant impurity levels aligned with crack propagation direction in recent failures. Is bar stock material "dirtier" than it used to be?

 

[CoryPad]

Impurities certainly are something to consider. Perhaps the anodize + seal provided corrosion resistance. Without it, the impurity inclusions could be initiating a corrosion fatigue failure.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Jabberwocky]

It has worked (we think) for us on Ti 6Al 4V ELI. We didn't dig too far into the metallography, but the theory was that surface crack initiation was delayed by the surface treatment.

 

[israelkk]

According to MIL-A-8625F COATINGS FOR ALUMINUM AND ALUMINUM ALLOYS specifically says: "6.1.3 Type III. ..... Hard coatings may reduce fatigue strength....."

 

[swall]

Hard anodizing will reduce fatigue life, so no, its removal would not have caused a fatigue problem. What kind of bar stock was used--extruded (the norm) or rolled (extra cost)? Are or were these parts shot peened at the root?

 

[Kwan]

I used to believe that the anodize process would always reduce fatigue properties. However, I said this to a very experienced stress guy and he produced test data that shows specific processes (at certain thicknesses) will improve fatigue life.

These are the processes that improved fatigue life (based on a test of 7075T6 Rod, Ftu 89.2):
15% H2SO4 .0010 thick and less
10% CrO3 .0002 thick and less

Hope this helps

 

[israelkk]

In

http://arioch.gsfc.nasa.gov/313/pdf/tip122.pdf

It suggest to change the T6 to T76 to improve the resistance to stress corrosion cracking that I suspect you have due to the removal of the coating and the sealing especially in the hot gas environment. Here is a quote from the NASA tip document "One of the strongest aluminum alloys is 7075 in the T6 heat treat condition. However, the alloy with this heat treatment has very low resistance (Table III, Ref.) to stress corrosion cracking (SCC) and may crack at applied tensile stress levels as low as 10% of its yield strength (see Reference). Such susceptibility is greatest in the short transverse direction, and somewhat less in
the long transverse direction, as related to the worked direction of the original stock."

 

[Goahead]

Could you apply glass bead peening unto the root to improve fatigue resistance? This should be done before chromic acid anodizing or chromate conversion coating. These are believed to be less harmful than hard anodizing to fatigue properties.

http://www.welding-advisers.com/

 

[swall]

Aircraft wheels, made of 2014-T6, where traditionally chromic acid anodized (in the range of .00005") because it was known that fatigue strength was reduced with the thicker (.0005")sulfuric anodize films. But, our design engineers figured out (as did our competitors) that since most fatigue failures started at a corrosion, pit, the fatigue life would be extended with the thicker film due to the enhanced corrosion resistance, and sulfuric acid anodize became the standard.

 

[UCengno1]

We currently specify 7075-T6 as material and do not specify form or even AMS. We are avid extruded bar stock users and I am sure we look for the best deal available. The environment is either high pressure air or methane at or near RT.

The original plating pre 94 was anodize with dichromate seal which was switched to anodize and seal in 94. The anodize was completely removed in 03.

We considered the SCC factors but wondered why things worked for so long without issues. That is why I am considering impurities as the preverbial straw that creates enough instability to cause all of the other negative contributors, i.e. SCC sensitive material, high cycle fatigue conditions, etc..., to become significant factors in the failure.

Will know more in a couple days as we are having a metallurgist compare an older version (5-7 years) against a current version.

 

[UCengno1]

We are speaking with a peening vendor about the issues. It often comes down to cost so we will see how appealing the price is.

 

[israelkk]

7075 is very sensitive to corrosion it should not be used in the uncoated condition. I have seen aerospace quality machined parts made of 7075-T6 heavily corroded between machining stages. We had to dry and oil them after each machining stage and before anodizing. On the other hand I have seen uncoated machined parts sitting on a desk for 20 years with no corrosion.

You can not trust your past experience with uncoated 7075 because each heat lot will behave differently. The spec allows variations on the chemical content and metallurgical characteristics of the alloy and that will greatly affect the corrosion resistance.