Effect Of Cu Content On Mechanical Properties And Localized Corrosion Resistance Of Al-9.0Zn-1.5Mg-xCu Alloy Sheets

Al-Zn-Mg-Cu alloys have been widely used in aerospace as structural material because of high strength,high toughness and good corrosion resistance.Cu is the main alloying element of Al-Zn-Mg-Cu alloys,and significantly affects the microstructure and properties of these alloys.However,the effects of Cu content on the microstructure and properties of Al-Zn-Mg-Cu alloy with high Zn/Mg ratio are complex and controversial.In this work,the effects of Cu content with 0～2.6wt.%on the mechanical properties and localized corrosion resistance of Al-9.0Zn-1.5Mg-x Cu alloys of T6-temper were investigated by means of tensile test,Kahn tear test,immersion test and electrochemical tests,and the mechanism was discussed based on microstructure examination using optical microscopy（OM）,scanning electron microscopy（SEM）,electron backscattered diffraction（EBSD）and scanning transmission electron microscopy（STEM）.The main results are as follows:（1）With the increase of Cu content,the average size of residual Fe-rich phase increases from about 3.0μm to about 5.5μm,with the area fraction increasing from 0.86%to 1.51%,and the chemical formula changes from Al₃Fe to Al₃（Cu,Fe）and then to Al₇Cu₂Fe;the area fraction of recrystallized grains increases from 18.3%to 61.2%.（2）With the increase of Cu content,the yield strength and ultimate strength of the aged alloys increase from 506.2MPa and 526.5MPa to575.9MPa and 617.9MPa,respectively;and the improved precipitation strengthening is due to the higher density and volume fraction ofη’（Mg Zn₂）strengthening precipitate in the alloys caused by higher Cu content.（3）With the increase of Cu content,the fracture toughness of the aged alloys increases first and then decreases,and unit initiation energy and unit propagation energy reach the maximum values of 116.3 and 60.6N·mm^-1respectively when Cu content is 1.0～1.3 wt.%,and decrease significantly to 73.5 and 23.4N·mm^-1 when Cu content is 2.6wt.%.The increase of the area fraction of recrystallized grain is beneficial to the fracture toughness,while the increased size and area fraction of residual phase,the higher density and volume fraction ofη’strengthening precipitate and the largerηphase particles at the grain boundaries are not beneficial to fracture toughness.（4）With the increase of Cu content,the intergranular corrosion resistance of the alloy decreases.The maximum corrosion depth increases from 10.4μm to 116.2μm and the corrosion grade increases from 2 to 4after 6h immersion.Pitting occurs in all the alloys,while intergranular corrosion only occurs in the alloy with Cu content of 2.6wt.%.The increase of Cu content,size and area fraction of residual phase has positive effect on pitting initiation and expansion.The increase of potential difference between grain boundary particles and the matrix is favorable for the occurrence of intergranular corrosion.53 figures,12 tables and 131 references.