Microstructure and mechanical properties of ultra-high strength Al-Zn-Mg-Cu-Sc aluminum alloy fabricated by wire + arc additive manufacturing

Ultra-high strength 7xxx series aluminum alloys are widely used in aerospace applications due to their exceptional high specific strength. Wire + Arc Additive Manufacturing (WAAM) is a combination of arc and wire feeding additive manufacturing technology including either the gas tungsten arc (GTA) or the gas metal arc (GMA) process. And this technology has been applied in the aerospace manufacturing industry to reduce the time of product development and “buy-to-fly” ratios. However, the existing Al-Zn-Mg-Cu aluminum alloy filler wires are not suitable for WAAM process due to their high susceptibility to hot cracks. In this study, a novel Al-Zn-Mg-Cu-Sc aluminum alloy wire named 7A55-Sc with ultra-high strength and excellent hot-crack resistance has been successfully prepared by optimizing the composition of Al-Zn-Mg-Cu alloying elements and adding appropriate amounts of inoculants Sc and Zr elements during the smelting process. And then, thin-wall components without any hot cracks were successfully fabricated by using cold metal transfer (CMT) process. Results indicated that the microstructure of the as-deposited 7A55-Sc alloy consists of predominantly fine equiaxed grains and the average size of the grains is between 5.0 and 6.0 μm. The eutectics composed of α (Al), η phase and S phase were distributed along the grain boundaries. The Al3Sc and Al3 (Zr, Sc) particles precipitated within grains act as heterogeneous nucleation nuclei, promoting the formation of the equiaxed grains. After T6 heat treatment, most of the second phases distributed within grains and grain boundaries were dissolved into the α(Al) matrix. The distribution of alloy elements became more homogeneous. The average ultimate tensile strength, yield strength and elongation of T6 heat-treated 7A55-Sc alloy in the horizontal direction were 554 MPa, 488 MPa and 1.7% respectively, whereas 523 MPa, 469 MPa and 1.4% in the vertical direction. The lower ductility was considered due to the existence of a large number of pores in the 7A55-Sc alloy fabricated by the CMT process