Effect Of Zn Content On Microstructure And Properties Of Al-Mg-Zn-Mn-Cu Alloys

Al-4.6Mg-xZn-0.23Mn-0.15Cu(x=0.1,0.4,0.7,1.0wt.%)experimental alloys with different Zn content were prepared by smelting-casting method.The microstructure of the alloys was observed by metallographic microscope(OM),x-ray diffraction(XRD),scanning electron microscopy(SEM),electron back-scattering diffraction(EBSD).The properties of the alloy were analyzed by vickers hardness,room temperature tensile test,exfoliation corrosion test,intercrystalline corrosion test and electrochemical test.The following results have been achieved:(1)The microstructure analysis of Al-Mg-xZn-Mn-Cu as cast and homogenized alloys shows that the alloys are mainly α-Al phase and a small amount of al6(Fe,Mn)and Al3Mg2 phase.With the increase of Zn content,the amount of intermetallics increases gradually,and the average grain size of the alloy does not change much.(2)The mechanical properties of as-cast and homogenized Al-Mg-xZn-Mn-Cu alloys show that the as-cast vickers hardness increases from 67.2 HV to 84 HV,and increased by 17.2 HV.with the increase of Zn content from 0.1 wt.%to 0.7wt.%.The homogenized vickers hardness increased from 60.1 HV to 74.6 HV,and increased by 14.5 HV.When the Zn content is 1.0wt.%The vickers hardness of the alloy was not significantly improved when the alloy was used.After homogenizing annealing,the unbalanced second phase in the tissue is eliminated greatly,the amount of intermetallics is reduced,and the vickers hardness of homogenized state is lower than that of as-cast alloy.(3)Microstructure analysis of Al-Mg-xZn-Mn-Cu cold-rolled alloy shows that the metallographic structure is all fibrous with elongation along the rolling direction,and some broken second phase particles can be observed.There are Al6(Fe,Mn)phases and dispersed particle phases discontinuously distributed along the rolling direction in the backscatter morphology of the alloy,and the grain size is smaller than that of the cast and homogenized phases.When the Zn content is 1.0wt.%,Mg,Zn and Cu elements are obviously enriched,which decreases the solid solution strengthening effect of Mg and Zn elements and decreases the mechanical properties of the alloy.(4)The mechanical properties of Al-Mg-xZn-Mn-Cu cold-rolled alloy show that the Zn content is added from 0.1 wt.%to 0.7wt.%.The vickers hardness increased from 128 HV to 142 HV,and increased by 14 HV.The tensile strength and yield strength increased from 388.41 MPa and 336.21 MPa to 407.63 MPa and 361.84 MPa,respectively,with an increase of 19.22 MPa and 25.63 MPa,When the Zn content is 1.0wt.%,the vickers hardness and tensile strength of the alloy decrease when the alloy is used.fracture morphology observation shows that the fracture mode of the alloy is ductile fracture.(5)The results of corrosion performance of cold-rolled Al-Mg-xZn-Mn-Cu alloy show that with the increase of Zn content,the self-corrosion potential of the alloy moves forward gradually,the corrosion current density decreases gradually,and the maximum intercrystalline corrosion depth decreases,indicating that the corrosion resistance of the alloy increases gradually.Among them,the content of Zn is 0.7wt.%,the self-corrosive potential is the largest and the corrosion tendency is the weakest.The corrosion current density is the lowest and the corrosion rate is the slowest.The intercrystalline etching depth is the shallowest,and the alloy with 0.7wt.%Zn content has the best corrosion resistance.It can be concluded that the Al-Mg-Zn-Mn-Cu coldrolled alloy with 0.7wt.%Zn content has the best comprehensive properties.(6)The ideal annealing process for Al-4.6Mg-0.7Zn-0.23Mn-0.15Cu cold-rolled alloy is to keep it warm at 275℃ for 1 hour.At this time,the alloy has good comprehensive mechanical properties,with a tensile strength of 362.87 MPa,a yield strength of 312.22 MPa and an elongation of 10.14%,and a good combination of strength and elongation.The fracture morphology shows a typical ductile fracture with a large and deep dimple shape.(7)The EBSD results of Al-4.6Mg-0.7Zn-0.23Mn-0.15Cu alloy at different annealing temperatures show that the cold-rolled alloy mainly consists of deformed grain and small-angle grain boundary.With the increase of annealing temperature,the proportion of small-angle grain boundary and deformed grain decreases gradually,the proportion of large-angle grain boundary and recrystallized grain increases gradually.After complete recrystallization,the deformed grain disappears,and the alloy mainly consists of recrystallized grain.