Hard anodize to help fatigue resistance?
Hard anodize to help fatigue resistance?
(OP)
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?
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?





RE: Hard anodize to help fatigue resistance?
Regards,
Cory
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Hard anodize to help fatigue resistance?
RE: Hard anodize to help fatigue resistance?
RE: Hard anodize to help fatigue resistance?
RE: Hard anodize to help fatigue resistance?
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
RE: Hard anodize to help fatigue resistance?
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."
RE: Hard anodize to help fatigue resistance?
http://www.welding-advisers.com/
RE: Hard anodize to help fatigue resistance?
RE: Hard anodize to help fatigue resistance?
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.
RE: Hard anodize to help fatigue resistance?
RE: Hard anodize to help fatigue resistance?
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.