Effect Of Zn Content On Quenching Sensitivity Of Al-xZn-1.5Mg-1.0Cu Alloy

The quenching sensitivity of Al-Zn-Mg-Cu alloys is key to their application in thick plate materials.As one of the main alloying elements,Zn plays an important role in controlling quenching sensitivity of these alloys.The continuous quenching rates of 400-4℃/s was obtained by immersion end quenching in this work,the effect of Zn content(6.00 wt.%-9.08 wt.%)on quenching sensitivity relative to mechanical properties and corrosion properties of Al-x Zn-1.5Mg-1.0Cu alloy was investigated using hardness test,room temperature tensile test,intergranular corrosion,exfoliation corrosion and electrochemical test.The mechanism was discussed based on the calculation of phase diagram,continuous-cooling-transformation and time-temperature-transformation curves,and microstructure characterization by optical microscopy,scanning electron microscopy,transmission electron microscopy and differential scanning calorimetry.The main conclusion are as follows:(1)The effect of Zn content on quenching sensitivity relative to room temperature tensile properties was revealed.In the range of 200℃/s to 70℃/s,the quenching sensitivity relative to strength is the highest for8.10%Zn alloy,intermediate for 6.00%Zn alloy,the lowest for 7.09%Zn and 9.08%Zn alloy.With the further decrease of quenching rate,the quenching sensitivity relative to strength of 9.08%Zn alloy increases rapidly;below about 32℃/s,quenching sensitivity relative to strength remains basically unchanged with the increase of Zn content from 6.00%to 7.09% and then increases rapidly for 8.10% and 9.08% Zn alloy.(2)The influence mechanism of Zn content on quenching sensitivity relative to strength was clarified.In the range of 200℃ to 70℃/s,grain boundary is the main nucleation position of quenching-induced phase.In8.10%Zn alloy,the precipitation driving force for quenching-induced phase is high,and there are a large quantity of grain boundaries acting as nucleation positions,consequently,the quenching sensitivity is the highest.The 6.00%Zn alloy has medium quenching sensitivity due to the highest number of nucleation positions of quenching-induced phase but low driving force.When the quenching rate decreases to be below 70℃/s,the quenching-induced phase in grain becomes the main factor affecting quenching sensitivity.When the quenching rate is 4℃/s,the total area fraction of quenching-induced phase of the 6.00% and 7.09%Zn alloy is similar and the lowest.When the Zn content is greater than 7.09%,the total area fraction of quenching-induced phase increases rapidly.(3)The influence of Zn content on quenching sensitivity relative to intergranular corrosion and exfoliation corrosion was investigated.At the quenching rate of 4℃/s,quenching sensitivity relative to intergranular corrosion depth decreases for the alloys with Zn content of6.00%>8.10%>7.09%≈9.08%.In the range of 200℃ to 70℃/s,the quenching sensitivity relative to exfoliation corrosion of 8.10%Zn alloy is the highest,followed by 6.00%Zn alloy,and the quenching sensitivity of7.09% and 9.08%Zn alloy is the lowest.At the quenching rate of 4℃/s,quenching sensitivity relative to exfoliation corrosion depth decreases for the alloys with Zn content of 6.00%≈7.09%>8.10%≈9.08%.(4)The influence mechanism of Zn content on quenching sensitivity relative to exfoliation corrosion depth of the alloy was clarified.With the increase of Zn content,the precipitation free zone(PFZ)near grain boundary widens,and the contents of Zn and Mg in the second phase at grain boundaries increase,leading to the increase of local corrosion depth of the alloy.As the quenching rate decreases from 400℃/s to 4℃/s,the widening rate of PFZ of 6.00%Zn alloy is smaller than that of 9.08%Zn alloy,but the increment rate of Zn element content in grain boundary phases of 6.00%Zn alloy is larger and the increment rate of Cu element content is smaller.Therefore,the electrical potential difference between grain boundary phase and the matrix is larger due to decreased quenching rate,so the quenching sensitivity relative to exfoliation corrosion depth is higher.