Alloying Design And Strengthening,corrosion Mechanisms Of 7xxx Series Aluminum Alloys For Oil Drilling

7xxx（Al-Zn-Mg-Cu）series high strength aluminum alloys have been widely used as structural materials in aerospace and other fields due to their low density and high strength.In recent decades,the application potential of the alloys used in oil drilling has been gradually valued.However,7xxx series aluminum alloys show poor corrosion resistance in the Cl^-rich environment,which leads to the risk of failure in service.There are two effective methods to improve the comprehensive properties of alloys:one is to optimize the alloy composition and introduce microalloying elements;another is to optimize the alloy production process,mainly to optimize the heat treatments.Therefore,it is of great scientific significance and industrial guiding value to further study the effects of composition and heat treatment optimizations on the microstructure and properties,and to clarify the strengthening and corrosion mechanisms of the 7xxx series aluminum alloy.In this paper,several groups of 7xxx series aluminum alloys（based on aluminum alloy 7075）were selected for investigation.The Zn/Mg ratio was adjusted,trace elements such as Ag,Zr,and Er were added respectively,and the solid solution and aging treatments were optimized.The alloys were analyzed by using scanning electron microscopy（SEM）,transmission electron microscopy（TEM）and three-dimensional atom probe（3DAP）techniques,tensile and hardness measurements,intergranular and stress corrosion tests.Researches have been focused on the following aspects:（1）Effect of Ag on aging precipitation behavior of 7xxx series aluminum alloy;（2）Effect of trace Er on the microstructure and properties of 7xxx series aluminum alloy;（3）Effect of heat treatments on microstructure and properties of 7xxx series aluminum alloy;（4）The establishment of aging kinetics models for the tested 7xxx series aluminum alloys.The main conclusions can be drawn as follows:（1）The effects of adding 0.3wt.%Ag on the microstructure,properties and precipitation behavior of aluminum alloy 7075（commonly used in the drilling industry）during aging are investigated.TEM and 3DAP results show that there are T-type andη-type clusters/precipitates in both aged alloys 7075 and 7075+Ag.The addition of Ag accelerates the aging precipitation response,makes the precipitates smaller and denser,and increases the hardness and strength of the peak aged alloy.Meanwhile,the precipitate free zones become narrower,which is beneficial to improve the plasticity and corrosion resistance of the alloy.Besides,at the early aging state,Ag atoms interact with vacancies and Mg,forming a large number of"vacancy-Ag",and"Mg-Ag"co-clusters.They act as formation sites of clusters,leading to an increase in the proportion of T-type clusters with higher Mg content in the early aging stage.Therefore,more T-type precipitates are formed in the Ag-containing alloy,and the average Mg content of clusters/precipitates in the Ag-containing alloy is higher than that in the Ag-free alloy at all aging stages.（2）Based on the research of aluminum alloy 7075,a group of alloys is designed in this study.The Zn/Mg ratio has been adjusted and the thermal stability microalloying elements such as Zr and Er are added.The effects of Er on the microstructure and properties of Al-Zn-Mg-Cu-Zr alloys after casting,homogenization,solution and aging treatments are emphatically studied.After adding Zr and Er into the alloy,fine Al₃（Er,Zr）particles are formed in the matrix after homogenization,resulting in the second phase strengthening.Also,the particles have high thermal stability and can pin grain boundaries,subgrain boundaries and dislocations during the solution treatment after extrusion,which significantly inhibits the recrystallization and reduces the proportion of large angle grain boundaries of the alloy.Therefore,compared with the alloy without Er,the hardness and yield strength of the aged 0.10wt.%Er-containing alloy increase by 14%（25.4HV）and12.7%（70MPa）,respectively,and the tensile strength of the alloy exceeds 670 MPa.Meanwhile,the intergranular corrosion depth of the Er-containing alloy is reduced by 65%and the stress corrosion sensitivity is greatly reduced compared with that of the Er-free alloy.In conclusion,the addition of Er can significantly improve the mechanical properties and corrosion resistance of the Al-Zn-Mg-Cu-Zr alloy,so that the alloy can meet the property requirements of aluminum oil drilling materials.（3）The effects of solid solution and RRA treatments on the microstructure,mechanical properties and corrosion resistance of commercial aluminum alloy 7085（based on aluminum alloy 7075,the Zn/Mg ratio is increased and Zr element is added）are systematically studied.After an optimized RRA treatment（120℃/24h（pre-aging）+160℃/1.5h（retrogression）+120℃/24h（re-aging））,a large number of plate-likeη’strengthening precipitates are dispersed in the matrix,so the hardness and strength of the alloy are improved.At the same time,after the retrogression treatment,the coarseηphases with higher Cu content are dispersed along the grain boundary,which reduces the potential difference between the GPB and matrix,and inhibits the path of corrosion cracking along the GBPs.Therefore,the intergranular corrosion and stress corrosion resistance of the alloy in Na Cl solution are greatly improved.（4）Based on the results of the aging hardness and microstructure evolution of the above alloys,the effects of Zn/Mg ratio and the addition of Ag on the aging precipitation kinetics of 7xxx series aluminum alloys were studied combining with Johnson-Mehl-Avrami（JMA）equation.The results show that the maximum hardness of 7xxx series alloy can be obtained by the single-stage aging system as 120～135℃/20～26h.For 7075,7075+Ag,Al-Zn-Mg-Cu-Zr and 7085 alloys,with the increase of Zn/Mg ratio or the addition of Ag,the diffusion activation energy decreases,and the aging precipitation kinetic increases.The relationships between aging hardness,time and temperature of different alloys are also calculated,which is in good agreement with the experimental values,and can provide theoretical references for the formulation of the aging process for7xxx series aluminum alloys.