Herr Schmitt,
I've enclosed a scan of a page that answers your question. It describes the annealing and hardening processes used by German aircraft manufacturers for the heat treatment of Duralumin (95% Aluminium, 4% Copper, 1% Magnesium & Manganese.)
Source:
"Wir wiederholen: Metallflugzeugbau - 1000 Fragen und Antworten aus Werkstoffkunde, Arbeitskunde, Flugzeugkunde und Fachrechnen" by Dr Alfred Spraul.
Published in 1942 by Verlag für Fachliteratur, Wien (122 pages)
Hope that helps,
Leon Venter
p.s. For a detailed analysis of the composition of the metals used in Luftwaffe aircraft, I can also recommend:
"A Metallurgical Study of German and Italian Aircraft Engine and Airframe Parts" by Charles Otto.
Published in 1943 by Kennedy Press (140 pages)
It covers the Jumo 211, DB 601, BMW 132 & 801, Bramo-Fafnir 323 and Fiat A80 engines, as well as various airframe components from the Bf 110, Ju 88, He 111 and Bf 109.