ALUMINUM ROLLER BURNISH SERVICE TEMPERATURES 3

[VMSETHAN]

Our company is producing a 6061 aluminum extrusion product which has a number of holes that require roller burnishing.
The process itself has been successful, but I am looking for input on service temp limits of the part after the burnishing has been completed. I ask the question because we have seen a spec from an OEM which limits temperature on a similar item to 250F. Does anyony have information on how or why this specific temp was chosen?
Details are: Hole size -.4375
Expansion - .0004-.0008
Surface finish - 16 Ra maximum
Any thoughts are appreciated.

 

[swall]

What is the temper of the 6061? What is the purpose of the roller burnishing?

 

[VMSETHAN]

It is T-6. Our underststanding on the roller burnish is that it was implemented long asgo in the original product to increase fatigue life of the holes during operation.

 

[tbuelna]

VMSETHAN,

Creep may be an issue with 6061 above 250F for sustained loading conditions. Check MIL-HDBK-5 for more information.

Regards,
Terry

 

[VMSETHAN]

Good idea, I will look at that for info.
Just to clarify, the final use of this part will NOT see a continuous 250F. During final assy it experiences the temps for a breif time(60 min) during an adhesive cure. Some concern has been raised wheteher this degree/time frame would do harm to the roller burnish or decrease its effectiveness.

Thanks again.

 

[btrueblood]

Look at the website for Fatigue Technology, they manufacture tooling to put compressive stresses into fastener holes for fatigue resistance. Same idea as your roller burnishing, but they helped to develop the technology some 20+ years ago, and you may find more info on the subject there.

That compressive residual stress induced by the roller burnishing, and which provides a degree of protection against fatigue crack growth, will decay (stress relaxation) with exposure to high temperatures, due to the same mechanisms that allow the allow the material to creep as tbuelna pointed out.

Yes, in a perfect world, the part would ideally be roller burnished after all paint/adhesive cure is complete...but that may not be possible.

Has the part exhibited fatigue cracking in field service?

 

[TVP]

The issue is relief of residual stresses after exposure to elevated temperatures. For Al, anything above ~ 250 F (120 C) can cause stress relief for parts that have been processed by shot peening, roller burnishing (deep rolling), or hole expansion. One reference to elevated temperature performance of roller burnished Al can be found using the following link (1st hit):

http://scholar.google.com/scholar?a...as_yhi=&as_sdt=1.&as_sdtp=on&as_sdts=23&hl=en

 

[VMSETHAN]

We have talked to Faigue Tech in the past about their cold work and bushings. I will be asking them about this also.

The the early version of this part was not roller burnished, but the OEM started to incoporates it later in production. It is a life limited part, so we assume the process was added to enhance as such. We have an STC/PMA to re-create the part and wanted to be making the best version.

You are correct in that we cannot roller burnsh afer final bonding due to tooing and part configuration. I am hoping to get some "tribal knowledge" on the 250F being a hard limit or not.

 

[VMSETHAN]

Thanks TVP, we were responding at the same time. I will look that up.
You say "about 250"?
If we got some confirmation on a 250-260 range there would be no worries.

 

[TVP]

I checked two SAE aerospace standards on shot peening, SAE AMS 2430M and SAE AMS-S-13165, both of which have a limit of 200 F as the maximum exposure temperature for aluminum parts. Metal Improvement, a company that performs a lot of shot peening for the industry, shows an approximate temperature of 250 F for Al parts. I suggest that you work with a coating that can be properly cured at 200 F maximum temperature, in order to follow industry best practices. If your customer allows 250 F, then I would definitely not exceed that, unless you want to perform some x-ray diffraction on actually finished parts in order to prove that 250-260 F is an acceptable range.

 

[TVP]

I found the following PhD thesis (overseen by Prof. Scholtes from Uni Kassel, a frequent contributor to research on this subject) that shows some data on stress relaxation vs. exposure temperature:

http://www.upress.uni-kassel.de/online/frei/978-3-89958-260-4.volltext.frei.pdf

 

[VMSETHAN]

Our Director of Engineering is from Germany, so I have handed it off to her for evaluation. Thanks for the info!!

 

[btrueblood]

VMS,

The whole paper is not in German, see page 147 and on for the section of most interest for you (all in English).

Also see equations #2 and #3 (page 12, again in English) for the theoretical equations for the decay of residual stress vs. time and temperature. You'll find that the problem is defining an "acceptable" level of decay of the residual stress, or an acceptable level of diminished fatigue life.

 

[btrueblood]

Oh, and a star to TVP for the link.