low creep aluminum

[soptic]

We are constructing high precision parallel spring flexure structures by EDM machining of aluminum parts and we have problems with creep effects of the final structures. Could anyboby tell me which aluminum alloy should we employ for lowest creep? Where may I find such data?

 

[AJone]

It's a little out dated, but the book "dimensional Instability" by Marschall from 1977 is a good primer.

2024-T6 is often employed for low capacity sensors.

There's couple charts for microyield behavior of of this alloy I could provide if you interested.

What kind of alloys have you tried so far?

Also if you find any other good references, please let me know.

Thanks,
Mark

 

[TVP]

sensoptic,

Are you certain that creep is the problem? Springs often have problems with high stresses near the limits of the intended travel, which can result in yielding. Creep is associated with elevated temperatures, so what is the temperature range of this design? Is the flexure essentially a cantilever beam with fully reversed loading? Reiterating Mark's previous question, what alloys have you used so far?

 

[tannerad]

There was a review paper recently published in the Journal 'Materials Science and Technology' on creep in aluminum alloys. It may be of some use ...
Citation:
Creep resistant aluminium alloys and their applications
Robinson J.S.; Cudd R.L.; Evans J.T.
Materials Science and Technology, 1 February 2003, vol. 19, no. 2, pp. 143-155(13), Institute of Materials.

Regards
David.

 

[soptic]

Thank you for the suggestions. We are using AA7075 at room temperature for our flexure structures. As suggested by Mark, AA2024 could be a better choice for precision applications. Our structure (10x10x1 cm3)is a linear translation stage operating as a pendulum for an inclination sensor application (like a swing moving laterally, only 2 deg), the two hanging arms have each two blade flexures. In fact, we are not sure that the creep (about 2% in one week) observed is due to the material or due to the structure design!

 

[TVP]

1. At room temperature, alloy 7075-T6 is essentially immune to creep. Sources for creep data include the following:

MIL-HDBK-5H

http://www.grantadesign.com/MILpdfs/MIL5-H/MIL5H-3-7-4.pdf

Atlas of Creep and Stress-Rupture Curves

http://www.asminternational.org/Tem...=Ecommerce/ProductDisplay.cfm&ProductID=10554

2. It doesn't sound as if your application involves creep-- time-dependent strain based on the application of a static force. I would recommend investigating the structure design & geometry, as there may be a problem with fixity/constraint, stresses, etc.