Effect Of Sc And Zr On Microstructure And Properties Of Al-Mg-Zn Alloys

Al-Mg-Zn alloy has moderate strength,excellent corrosion resistance and an agehardening response through precipitating T-Mg32(Al,Zn)49 phases.The microstructure of Al-Mg-Zn alloy is regulated by microalloying to further improve the properties of the alloy,which is of great significance for the development and application of new alloys.The effects of Sc and Zr microalloying on the microstructure and properties of Al-Mg-Zn alloys with different Sc and Zr contents were studied.The relationship between microstructure and properties of Al-Mg-Zn alloy was determined.The following conclusions are reached:The effect of Sc and Zr microalloying on the as-cast microstructure of Al-Mg-Zn alloy was studied.The content of Sc and Zr had a significant effect on the as-cast structure of AlMg-Zn alloy.The average grain size of the alloy ranged from 197 to 260 μm when Sc or Zr was added alone.The as-cast microstructure of the alloy was refined and the average grain size ranged from 150 to165 μm when the total mass fraction of Sc and Zr was not less than 0.27,and the mass fraction of Sc was less than 0.1 and more then 0.07.The critical content of Sc and Zr which can produce a remarkable refining effect on Al-Mg-Zn alloy was that the total mass fraction of Sc and Zr was not less than 0.27,and the mass fraction of Sc was more than 0.13.The average grain size ranged from 30 to 44 μm.The result of first principles calculations was that the lattice misfit of Al/Al3Zr was higher than that of Al/Al3(Sc,Zr),which indicated that the Al3(Sc,Zr)phase can serve as a better heterogeneous nucleation for a-Al for its lower interface energy.The formation energy of the Al3(Sc,Zr)was lower than that of the Al3Sc phase.Al3(Sc,Zr)particles have an effect on the recrystallization process of the alloy during rolling deformation and solution heat treatment.The highest number density of Al3(Sc,Zr)particles in 0.10Sc-0.17Zr alloys had the most obvious inhibition on recrystallization.The effect of Sc and Zr microalloying on recrystallization behavior of Al-Mg-Zn alloy was studied.The effect of two-stage homogenization system on the precipitation behavior of Al3(Sc,Zr)particles was investigated.In the range of the first stage homogenization temperature from 380℃ to 440℃,the particle size gradually increased and the number density gradually decreased with the increase of temperature.Compared with the first stage homogenization,the particles size did not increase obviously after the same second stage homogenization.After different homogenizing heat treatment,Al3(Sc,Zr)particles showed a bean morphology and had a good coherent relationship with Al matrix,which means that the Al3(Sc,Zr)particles have good thermal stability.The number density of Al3(Sc,Zr)particles in 0.10Sc-0.17Zr alloys was higher than that in 0.13Sc-0.14Zr alloys.After hot rolling,the alloy grain was flattened and elongated into a fibrous shape.The smaller the original grain size was,the finer the grain was after rolling.After intermediate annealing,the 0Sc-0.14Zr alloy recovered and partially recrystallized.After solution treatment at 460℃/1 h,The 0Sc-0.14Zr alloy had obvious recrystallization and some grains grew abnormally.The recrystallization degree of 0.10Sc-0.17Zr alloy was low,and the grain size was fine.Partial recrystallization occurred in 0.13Sc-0.14Zr alloys and 0.17Sc-0.10Zr alloys.The effect of Sc and Zr microalloying on the properties of Al-Mg-Zn alloys was studied.The composite addition of Sc and Zr can effectively improve the mechanical properties of the alloy,and 0.13Sc-0.14Zr alloy and 0.17Sc-0.10Zr alloy had small grain size,and the contribution of fine grain strengthening was about 60 MPa,while that of 0Sc-0.14Zr alloy was 25 MPa.The relative strengthening contribution of T precipitation in the alloy was about 290MPa,which is the main strengthening mechanism.The intergranular corrosion performance of the alloy was related to the grain boundary precipitated phase distribution.The grain boundary precipitated phase of 0.10Sc-0.17Zr alloy was discontinuous distribution,and the intergranular corrosion performance was excellent.The maximum depth of intergranular corrosion was 75 μm,and the grain boundary precipitated phase of other alloys was continuous distribution,and the intergranular corrosion performance was poor.The laser welded joint of the alloy was ’ U ’ type.The composite addition of Sc and Zr can weld the mechanical properties of the joint structure.The tensile strength of the welded joint was about 330 MPa.