Which of the following aluminum alloys cannot be strengthened by precipitation-hardening heat treatment (that is, is non-heat-treatable)?

Answer: B. 5000 series (Al-Mg)

The 5000 series (Al-Mg) are non-heat-treatable alloys that get their strength from solid-solution strengthening by Mg and from work hardening by cold working. They have excellent corrosion resistance and weldability and are used for ships and tanks. The 2000 series (Al-Cu), typified by duralumin, are heat-treatable and strengthened by Al-Cu precipitates. The 6000 series (Al-Mg-Si) are also heat-treatable, using Mg-Si precipitates, and are widely used for extrusions. In addition, the 7000 series (Al-Zn-Mg) are the strongest heat-treatable alloys, while the 1000 series (pure aluminum) and 3000 series (Al-Mn) are non-heat-treatable. Learn to tell from the alloy system whether the strengthening mechanism is precipitation or work hardening.