Abstract
The Al-Zn-Mg-Cu alloys are widely used in aerospace and automotive industries due to their excellent combination of high specific strength, great fracture toughness, and improved fatigue properties. However, Al-Zn-Mg-Cu alloys has the disadvantages of low thermal conductivity, corrosion resistance and weldability. In this case, the addition of trace alloying elements, such as Sc, is an effective means. The addition of Sc has been found to increase the strength and toughness, improve corrosion resistance and weldability of aluminum alloys. In this study, effects of Sc addition on microstructure, thermal conductivity and mechanical properties of the as-cast and as-extruded Al-2Zn-1Cu-0.3Mg-xSc (x = 0, 0.25, 0.5wt%) alloys are investigated. The average grain size of the as-cast Al-2Zn-1Cu-0.3Mg alloy is 2334㎛, however it is reduced to 914 and 529 ㎛ by addition of Sc element of 0.25wt% and 0.5wt%, respectively. Thermal conductivity of the as-cast Al-2Zn-1Cu-0.3Mg-xSc alloy is reduced to 204, 187 and 183W/MK by the additions of Sc element of 0, 0.25, 0.5wt%, respectively. On the other hand, the thermal conductivity of the as-extruded Al-2Zn-1Cu-0.3Mg-xSc alloy is about 200W/Mk regardless of the contents of Sc. This is because of the formation of coherent Al3Sc phase which decreases Sc content to cause an extremely high electrical resistivity. The maximum values of the yield and tensile strength are 126MPa and 215MPa for the as-extruded Al-2Zn-1Cu-0.3Mg-0.25Sc alloy, respectively. The increase in strength is probably because of existence of nano-scale Al3Sc precipitates and dense Al2Cu phases.