Study On Microstructure And Corrosion Resistance Of Al-Cu-Li-Mg-Ag-Mn Alloy

Al-Li alloys not only have low density,high ratio strength and low fatigue crack expansion rate,but also have good low-temperature mechanical property and processing performance.Due to their light weight and high strength,Al-Li alloys have become one of the most potential materials in aeronautics and aerospace field.Alloying and heat-treatment could improve the microstructure and mechanical property of Al-Li alloys.At the same time,due to the complexity of some service environments,the Al-Li alloy sometimes needs to serve in a wet or salty environment for a long time.The precipitates of Al-Li alloy not only has an significant impact on the mechanical property of Al-Li alloys,but also affects the corrosion resistance of Al-Li alloys.Therefore,in this thesis,the research has been conducted on precipitates evolution of the novel Al-Cu-Li-Mg-Ag-Mn alloys during homogenization treatment,the microstructure and corrosion resistance of the aged alloys.The main conclusions of this thesis are as follows:（1）There were Al₂Cu Li phase,Al₃Li phase and Al₇Cu₄Li phase in the homogenized Al-5Cu-1Li-0.6Mg-0.5Ag-0.5Mn alloys.Al₇Cu₄Li phase were dissolved with increase of homogenization temperature and holding time.Al₂Cu phase and Al₂Cu Li phase coarsened during the homogenization process.Moreover,it could be found that there was segregation of Ce and La elements at the Cu-rich precipitates,and the segregation could not be removed by homogenization treatment.（2）The homogenized Al-5Cu-1Li-0.6Mg-0.5Ag-0.5Mn alloy was subjected to a two-stage aging treatment.Peak-age Al-5Cu-1Li-0.6Mg-0.5Ag-0.5Mn alloy,which had gone through aging treatment at 120°C for 4 hours and 180°C for 6 hours,was mainly composed ofα-Al,Al₂Cu Li,Al₂Cu,Al₃Li and Al₂₀Cu₂Mn₃ precipitates,and needle-like Al₂Cu Li,Al₂Cu precipitates and dot-like Al₃Li precipitates were tiny and scattered.（3）Tafel polarization of the peak-age Al-5Cu-1Li-0.6Mg-0.5Ag-0.5Mn alloys revealed the corrosion potential and corrosion current density to be-651m V and 5.861μA/cm²,respectively.The over-age alloy had more positive corrosion potential of-618 m V but presented higher corrosion current of16.368μA/cm².It was found that the peak-age alloy has better corrosion resistance by comparing the corrosion surfaces of peak-age alloy and over-age alloy.（4）The homogenized Al-4.5Cu-1Li-0.4Mg-0.4Ag-0.4Mn-0.5Zn alloys were mainly composed of Al（α）,Al₂Cu Li,Al₃Li and Al₇Cu₄Li precipitates.All precipitates coarsened with increase of homogenization temperature and holding time.Moreover,it was found that there was segregation of Zn elements at Cur-rich precipitates.（5）Peak-age Al-4.5Cu-1Li-0.4Mg-0.4Ag-0.4Mn-0.5Zn alloys was mainly composed by Al₂Cu Li precipitates and Al₂₀Cu₂Mn₃ precipitates.Comparing with the homogenized alloy,the needle-like Al₂Cu Li precipitates were scattered and smaller.It was found that there were segregation of Zn and Mn at Cu-rich precipitates at Cu-rich precipitates.Al₂₀Cu₂Mn₃ precipitates was small and dispersed,with an average size of approximately 100 nm.（6）For the aged Al-4.5Cu-1Li-0.4Mg-0.4Ag-0.4Mn-0.5Zn alloy,Tafel polarization testing of the peak-age alloy in a 3.5%Na Cl solution revealed the corrosion potential and corrosion current density to be-611 m V and 2.005μA/cm²,respectively.The over-age alloy had more negative corrosion potential of-653 m V.It was found that the peak-age alloy has better corrosion resistance by comparing the corrosion surfaces of peak-age alloy and over-age alloy.And alloying with Zn element could improve the corrosion resistance of the aged alloys.