Effect of Cu content on microstructures and properties of Al-Mg-Si-Cu alloys

The microstructures and properties of Al-Mg-Si-Cu alloys were investigated by differential scanning calorimetry, Vickers hardness, electrical conductivity, tensile properties test, intergranular corrosion and transmission electron microscopy. The hardness of the alloys aged at 180°C increases to a peak value and decreases subsequently. The hardening rates of the alloys increase with the increase of Cu content, and the softening rate decreases due to overageing. The electrical conductivity of the alloys firstly decreases to a minimum value and increases subsequently. The electrical conductivity decreases with the increase of Cu content. With the Cu content increasing, the tensile strengths of the alloys in T6 and T4 increase, and the resistance to intergranular corrosion decreases, but the alloy without Cu does not present intergranular corrosion sensitivity. The kind of the precipitates in the T6 alloy varies with the Cu content increasing. β″ phase dominates in the alloys at the Cu content lower than 0.6, but β″ and Q' coexist in the alloys with Cu content of 0.9. The number density and volume fraction of the precipitates increase with the increase of Cu content and the size decreases. The precipitates are solute clusters in the T4 alloy.