Effect of Sc and Zr addition on microstructure and mechanical properties of high strength 7A52 aluminum alloy welded joints fabricated by double-wire gas metal arc welding

Abstract

A new idea for improving the mechanical properties of medium-thickness 7A52 aluminum (Al) alloy welded joints was proposed. In this work, 40 mm 7A52 Al alloys were welded by double-wire gas metal arc welding (DW-GMAW) using Al-4.76Mg and Al-4.45Mg-0.70Sc-0.30Zr (wt%) alloy wires, respectively. The results showed that after adding 0.70Sc and 0.30Zr, the microstructure of weld metal (WM) transformed from dendritic grain to equiaxed grain, and the average grain size reduced from 45.04 µm to 5.55 µm (reduced by 87.7%). At the same time, the microstructure growth morphol-ogies of the molten pool changed from equiaxed grain + columnar grain + dendritic grain to equiaxed grain + fine columnar grain + equiaxed grain, while the microstructures significantly refined because micro-Al3(Sc, Zr) phases appeared in WM. The yield strength (YS), ultimate tensile strength (UTS), and elongation (El) of Al-4.76Mg welded joints were 155 MPa, 288 MPa, and 5.5%, respectively. After 0.70Sc and 0.30Zr were added, the YS and UTS increased to 226 MPa (an increase of about 45.8%) and 336 MPa (an increase of about 16.7%), respectively. The fine grain strengthening caused by the micron-Al3(Sc, Zr) phase and the precipitation strengthening caused by the nano-Al3(Sc, Zr) phase were the main reasons for the strength improvement. The findings of this study provided a theoretical basis for high efficiency and high quality for 7A52 Al alloys with medium thickness.