Effect Of Sc, Zr On Microstrures And Properties Of Al-Zn-Mg-Cu Alloy

The effects of Zr and Sc on microstructures and properties of Al alloys have been studied extensively.But the expensive of Sc is high, which limits its large application. In present work, the effects of Sc and Zr on the microstructures and mechanical properties of Al-Zn-Mg-Cu were investigated.And the optimum contents of Sc and Zr were determined, also, the effects of Sc and Zr on microstructures and properties of Al-Zn-Mg-Cu alloy after solid solution and ageing treatments were also studied.The results show that when adding Sc and Zr separately, grain size decreases obviously and strength of as-cast alloy increases with the increasing in adding content. While the elongation drops sharply. When the content of Zr increases to0.25%, elongation dropps to7.1%. Adding both Sc and Zr, with increasing of Sc+Zr, the strength increases and elongation decreases gradually. When adding0.04%Sc and0.16%Zr, tensile strength reaches the maximum,775MPa, elongation is9.85%. Increasing the content of Sc+Zr, continuelly, tensile strength decreases.Adding both Sc and Zr, A large amount of fine disperse Al3(Sc,Zr) particles having petal-like shape precipitate during homogenization, These Al3(Sc,Zr) particles can pin dislocation and sub-boundaries and hind grain boundary migration and sub-grain growth, also have strengthening effects.The extruded rods of Al-Zn-Mg-Cu alloy adding Sc and Zr have smaller grain size and higher strength and hardness than those without Sc and Zr. When the alloy solid solution treated at420℃×3h+465℃×2h, the tensile strength and elongation of the alloy are777Mpa and11.84%, respectively, which increased by54%and11.84%respectively comparing with those of extruded rods.Al-Zn-Mg-Cu alloy with adding both Sc and Zr has artificial ageing strengthening effect. At the early stage of single aging, the strength, hardness, elongation and electrical conductivity rise. When onset time reached to peak in8h, the tensile strength of alloy was755MPa, which was increased by54%and elongation was13.28%, which was increased by 37%compared with no heat treatment. With the prolonging of ageing time, the elongation decreases.