Aluminum stress relief anneal

[dwillms]

We're doing some machining to a beefy 6061-T6 extrusion and are VERY concerned about residual stresses in the material afterwards(as a result of extruding and machining). Two questions:

1. Any idea if dimensional creep is likely as a result of machined in stresses over the life of the part? For this application 5 micron is too much! In general, can someone describe the mechanics of micro creep as a result of residual stress? Does such a thing occur??

2. What heat treatment would remove residual stresses but preserve the T6 temper? What exactly would this heat treat do to the micro structure of the material?

Thanks for your help!

 

[BrianR]

While we have not done precision machining on 6061-T6 we do have experience with other alloys used in the manufacture of military jet engine components.

In this instance there was an issue with distortion resulting from residual stresses from previous processing. The planning sequence normally includes a stress relieving operation prior to machining and in some cases an additional relief process part way through the machining.

This could involve rough machining to remove excess metal, leaving enough for a machining allowance to cater for distortion that will occur during the final stress relieving operation. The work is now close to the final size and the residual stress generated in the final machining will be minimised.

Sorry we cannot give you a specific answer, except to warn that there is an issue.