Effects of minor Sc on microstructure and mechanical properties of Al-Zn-Mg-Zr alloy welded joint

Two kinds of Al-6.0Zn-2.0Mg-0.12Zr and Al-6.0Zn-2.0Mg-0.2Sc-0.12Zr alloy plates were prepared by ingot-metallurgy. The alloy plates with 3 min thickness were welded by argon shield welding method, and the mechanical properties and microstructures of the two welded joints filled with Al-Mg-Sc welding wire were studied comparatively. The results show that firstly, minor Sc can raise the mechanical properties of the Al-Zn-Mg-Zr base alloy greatly. The reason for the increment is the fine grain strengthening, precipitation strengthening and the substructure strengthening caused by Al-3(Sc, Zr). Secondly, eta' phase (MgZn2) and grain size in the heat-affected zone of the alloy without Sc become coarse obviously, the eta' phase (MgZn2) in the heat-affected zone of the alloy with Sc becomes coarse also, but the grain size has no visible change. Al-3(Sc, Zr) particles are rather stable and can inhibit the movement of dislocation/and sub-grain boundaries, overaging softening is not serious. Thirdly, adding minor Sc can raise the strength of welded joint remarkably, the tensile strength of alloy with Se increases from 395 MPa to 447 MPa and the welding coefficient increases from 0.7 to 0.8 as well. The reason for the high strength of welded joint with Se addition is the fine grain strengthening, precipitation strengthening and the increasing of resistance to thermal cycling softening caused by Al-3(Sc, Zr).