Trace Scandium And Zirconium On The Microstructure And Properties Of Al-zn-mg Alloy

This article studied the microstructure and tensile mechanical properties of two kinds of Al-6.0Zn-2.0Mg-0.12Zr and Al-6.0Zn-2.0Mg-0.2Sc-0.12Zr alloy at different treatments, analyzed their high temperature microstructure and tensile mechanical properties, examined microstucture and mechanical properties of their welded joint by testing mechanical properties, optical microscopy(OM), X-ray difrraction(XRD) and transmission electron microscopy (TEM). The results show that:(1) The addition of minor Sc and Zr to Al-Zn-Mg alloy produced a considerable improvement in strength.Tensile strength and yield strength of the alloy with minor Sc addition are raised by 40MPa and 54MPa at solution treatment respectively and by 25MPa and 35MPa at aged treatment respectively. Grain strengthening, substructure strengthening and dispersion strengthening are responsible for increase of strength.(2) The primary Al₃Sc and Al₃(Sc, Zr) can cause fine gain strengthening, while secondary Al₃Sc and Al₃(Sc, Zr) can strongly pin up sub-grain boundaries and dislocations, stabilize substructures of the alloys, so it can strongly restrain recrystallization.(3) The strength of material decreases with an increase in the test temperatures and ductility increases with it as well.The Softening mechanism is the strengthening phase transformation on high temperature.(4) Al₃Sc and Al₃(Sc, Zr) partical exist in fine dispersoid form both ambient and elevated temperature, causing the dispersion strengthening and substructure strengthening.(5) The tensile strength of alloy with Sc and Zr raises from 83MPa to136MPa on300°C.(6) The welding wire prepared by adding minor Sc and Zr has a good compatibility with Al-Zn-Mg-Zr and Al-Zn-Mg-Sc-Zr base alloy. Using MIG welding technology, the mechanical properties of the Al-Zn-Mg-Zr welding joint can reach:σ_b=409MPa,σ_0.2=248MPa,δ₅=6.8%, the welding coefficient is 0.77. The mechanical properties of the Al-Zn-Mg-Sc-Zr welding joint can reach:σ_b=447MPa,σ_0.2=289MPa,δ₅=7.6% and the welding coefficient is 0.8.(7) The precipitation strengthening particles of heat-affected zone in the welded joint of Al-Zn-Mg alloy will coarsen, due to the entering of heat, which results in the appearing of softened zone. Al₃(Sc, Zr)are rather stable and can inhibit the movement of dislocation and sub-grain boundarys, overaging softening is not serious.