Part of the strengthening of aluminium alloys involves "age hardening" in
which copper platelets are laid down in the structure.After some years these
revert to copper aluminium compounds significantly reducing the alloys strength.If
you want the scientific try this:- <A HREF="http://www2.umist.ac.uk/material/general/gallery/alcu1.html">
http://www2.umist.ac.uk/
Steven Robson
Ulverston