The precipitates will coarsen, which reduces yield and tensile strength considerably. Ductility/formability will be increased. Fatigue strength will be reduced.
The reduction will depend on microstructural variables like grain size, precipitate size, presence of non-metallic inclusions, etc. Only a metallurgist from an aluminum producer would likely be able to assign a number to this, and that would depend on having all of the pertinent details.
Aluminum is very poor in fatigue anyway. If fatigue is a concern, Aluminum might not be a good choice for your application. In my training, we were tought that Aluminum is unique in the metallurgy world in that there is no stress level that will not eventually result in fatigue failure. It is my understanding that this thinking has changed slightly, that there may very well be a fatigue limit for aluminum alloys but the stress level is so low it might as well not exist for all practical purposes.
