Effect Of Scandium Addition And Synergistic Heat Treatment On Microstructure And Properties Of Al-Zn-Mg-Cu Alloy With Fine Grain Structure

Aluminum alloys are widely used in aerospace,high-speed trains,and automotive industries.In order to obtain high-performance fine-grain structure materials,and to broaden the engineering application range of aluminum alloy materials.In this paper,four kinds of alloy ingots are prepared by melting and casting process of Al-Zn-Mg-Cu(7xxx)alloy with low Zn content,low Zn 7xxx aluminum alloy with Sc,high Zn 7xxx aluminum alloy and high Zn 7xxx aluminum alloy with Sc.The ingot undergoes homogenization heat treatment,hot rolling,solution aging heat treatment,friction stir processing(FSP),and rolling and heat treatment after processing.Use optical microscope,transmission electron microscope,scanning electron microscope,Vickers hardness tester,universal testing machine and other characterization and testing methods to study the effect of Sc-added collaborative heat treatment process on the microstructure,mechanical properties and corrosion properties of FSP state 7xxx aluminum alloy.FSP transforms the grain structure of each alloy from fibrous to fine equiaxed.Direct aging heat treatment has little effect on the grain size of FSP low Zn and FSP high Zn samples,but it can precipitate Mg and Zn solid solution atoms dissolved in the FSP process to form??phase,and improve the yield strength and tensile strength of FSP alloys.The solution-natural aging(T4)treatment significantly coarsened the crystal grains of the low-Zn and high-Zn samples in the FSP state.The solution treatment provides high enough energy for atom migration,which in turn leads to coarsening of crystal grains.However,the solution treatment also causes the soluble precipitation phase in the FSP alloy to re-dissolve,thereby enhancing the solid solution strengthening effect of the alloy.The solid solution strengthening caused by the T4treatment and the softening effect caused by the grain coarsening of the process cancel each other out.Therefore,the T4 treatment does not significantly affect the strength of the FSP state alloy.The solution-artificial aging(T6)treatment resulted in the precipitation of?'phase with high strengthening effect,which significantly improved the strength of samples with low Zn in FSP state and high Zn in FSP state.Adding a small amount of Sc element to the 7 series alloy melt will form a micron-level primary Al₃(Sc,Zr)phase during the solidification process,which can promote the non-uniform nucleation of the melt and refine the as-cast alloy grains.In the subsequent long-term high temperature homogenization heat treatment process,nano-scale Al₃(Sc,Zr)phase is precipitated in the Sc-containing aluminum alloy.This phase can effectively pin the grain boundaries and position in the aluminum alloy rolling deformation,heat treatment and FSP process.inhibiting recrystallization and recrystallization grain coarsening,and then refine the alloy grains and optimize its mechanical properties.The structure of the Sc-containing aluminum alloy in the FSP state includes fine equiaxed crystals and high-density?phases.The nano Al₃(Sc,Zr)phase can effectively suppress the grain coarsening of the Sc-containing aluminum alloy in the FSP state during the subsequent heat treatment,so that each sample still has a fine-grained structure.The grain structure of the FSP state Sc-containing low Zn aluminum alloy after hot rolling and heat treatment changes from equiaxed to fibrous,while the grain structure after cold rolling and heat treatment is still equiaxed.After the FSP state Sc-containing aluminum alloy is directly aging or T6 treated,the strength of each sample has been greatly improved,which is mainly attributed to the precipitation strengthening effect caused by the high-density?phase.T4 treatment has little effect on the mechanical properties of the FSP state Sc-containing low-Zn aluminum alloy,but it can significantly optimize the mechanical properties of the FSP state Sc-containing high-Zn aluminum alloy.FSP can optimize the corrosion resistance of Sc 7-containing aluminum alloys,and the evolution of precipitates caused by subsequent heat treatment can significantly affect the corrosivity of Sc-containing aluminum alloys in the FSP state.Direct aging and T6 treatment promote the precipitation of Zn and Mg solute atoms in FSP samples,thereby increasing the corrosion potential of the alloy and optimizing the corrosion resistance of FSP samples.At the same time,the discontinuous distribution of grain boundary phases and narrower PFZ can further optimize the corrosion resistance of the alloy.Therefore,the Sc-containing aluminum alloy after T6 treatment has the most excellent corrosion resistance.The T4 treatment increases the supersaturation of the FSP sample,therefore,the alloy has the worst corrosion resistance.