What is the difference between 2xxx and 7xxx series aluminum alloys?

2xxx series (Al-Cu): Primary alloying element is copper (2-6%), Examples: 2024, 2014, 2219, Higher damage tolerance and fatigue resistance, Good toughness, Moderate strength (60-70 ksi UTS), Used for fuselage skins, lower wing surfaces, Good weldability (2219). 7xxx series (Al-Zn): Primary alloying elements are zinc (5-8%) and magnesium, Examples: 7075, 7050, 7055, 7150, Higher strength (70-85 ksi UTS), Lower toughness than 2xxx, More susceptible to stress corrosion, Used for upper wing skins, stringers, spars. Selection: 2xxx preferred where damage tolerance is critical (fuselage), 7xxx where strength is primary requirement (wing upper surface in compression). Heat treatment tempers (-T3, -T6, -T7) significantly affect properties.