Metal finishing Q&As since 1989

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Shot peening aluminum for fatigue resistance

I'm trying to improve the fatigue resistance of aluminum for an ultrasonic application. I've been told that anodizing and other chemical treatments will reduce the fatigue resistance. The other options would be to shot peen the surface.

Do you know of any other process that would improve fatigue? Do you have any suggestions on a starting point for shot peening?

Material is 7075 T6 aluminum.

Thanks in advance,

Michael D. Cronin
a medical supplies company

It may be of interest to you, that anodizing does reduce fatigue resistance, however shot peening prior to hard anodizing raises fatigue resistance significantly. A reference for fatigue resistance comparisons, which shows the above, is Metallic Fatigue [affil link on Amazon] Pergamon Press 1961, p 61 by Harris, W.J. As for where to start with shot peening values, I would suggest a process house who performs shot peen on a regular basis for a living. There are many good shot peen houses who will know instantly what you need.

Ward Barcafer, CEF
aerospace - Wichita, Kansas
1999

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Shot peening will improve the life of your part. but where to start depends on the use of the part. Shot peening does affect the surface finish and can greatly affect the finish on soft metals such as aluminum. For the highest fatigue resistance, you want as much depth penetration as possible, which means large shot and high velocity. This will have the maximum affect on surface finish, which may or may not create problems.

Another problem with shot peening is that it may contaminate the surface of your part with iron or glass. Since your company name suggests a medical application, I suggest you evaluate that aspect carefully.

Another process which will increase fatigue resistance is cryogenic processing. This process involves lowering the temperature to the cryogenic range and holding for a long period of time. Then the temperature is brought back to room temperature. Be careful with who does this process, as most heat treaters will just dump your part in a bath of liquid nitrogen and extract it when the stuff has evaporated.

Cryogenic processing has the advantage that it will not contaminate your parts, and it affects the entire volume of the metal, not just the surface.

F. J. Diekman
- Streamwood, Illinois
1999