Influence Of Zn/Mg Ratio On Quench Sensitivity Of Al-Zn-Mg-Cu Alloy

Abstract:Al-Zn-Mg-Cu series alloys have been widely used in the aerospace field as structural materials,for its high strength,high fracture toughness,good stress corrosion resistance and low density characteristics.The mechanical properties of the alloys can be increased by increasing alloying elements, which makes the quench sensitivity of the alloys become more obvious.It has become popular research topic in material science and engineering in order to get the alloys which has higher mechanical properties and more excellent hardenability. Research shows that the content and proportion of main alloying elements of Zn, Mg, Cu are the important factors that affect quench sensitivity.But at present the research about Zn/Mg mainly focus on the alloys which have low Zn content and high Mg content.This paper concentrates on the alloys whose Zn contents are8.0～9.0, with1.5～2.5Mg and1.0Cu, which are studied systemically.Therefore, the influence factors and mechanism of Zn/Mg ratio (3.0,4.25,6.0) on quench sensitivity of alloys were investigated by phase diagram calculation,experiment and testing analysis.Main conclusions are listed in the following:(1)The influence of the Zn, Mg elements and Zn/Mg ratio on quench sensitivity by calculation shows that the increase of Zn and Mg content makes the incubation period of the TTT curve shorten to a certain extent,which increases the quench sensitivity.With increase of Zn/Mg ratio,incubation period of TTT curve extends, begenning time of precipitation temperature reduces and the quench sensitivity of the alloys decreases.(2)The microstructural evolution of the alloys with different ratio of Zn/Mg in heat treatment and machining process shows that in as-cast,there is no other new phases observed in addition to the Al matrix and MgZn2phase.In the homogenization process,the phase of the alloys doesn’t transform into S phase,and the phases are dissolved more fully in subsequent homogenization and solid solution.The microstructure of the three alloys after solution treatment shows that a certain degree of recrystallization occurs,the alloys with lower Zn/Mg recrystallied with higher degree than the two higher ones.(3)The influence of the solid solution on the quench sensitivity is showed and the optimal solution process is obtained. Compared with475℃/2h treatment, the treatment450℃/8h+475℃/2h refines the grain which increases the number of recrystallization grains and (sub)grain boundaries,promates η equilibrium phase precipitation during the slow queching process,and leads to the obvious increase of quench sensitivity.475℃/2h single solution treatment is the best one.(4)The influence of Zn/Mg ratio on quench sensitivity of Al-Zn-Mg-Cu alloy shows that with increase of Zn/Mg,(from3.2,4.25to6.0),the depth of hardening layer is65mm,100mm above,90mm respectively,and the quench sensitivity of the alloys decreases.The best ratio of Zn/Mg is4.25, the depth of hardening layer is more than100mm.46figures,19tables and80references.