Surface Integrity And Corrosion Behavior Of Milled Al-Li Alloy 2A97

The weight reduction of transportation,such as vehicles and aerocrafts,is an important measure for the social sustainable development.Currently,the realization of the weight reduction of structures mainly depends on reducing the density of structural materials.The incorporation of active element lithium in A1 alloys not only reduces their density,but also strengthens them by the formation of A13Li phase that coheres with aluminum solid solution.Therefore,Al-Li alloys become the promising materials in the transportation industry and aerospace field owing to their lower density,higher elasticity modulus and specific strength together with specific stiffness than conventional aluminum alloys.However,the addition of element lithium makes the corrosion resistance of Al-Li alloys an obstacle to replace Al alloys.The corrosion resistances of machined surfaces of Al-Li alloy 2A97 is the research object in this study.The machined surface integrity of Al-Li alloy 2A97,the corrosion resistance and mechanism of machined surfaces of alloy 2A97,the comparison on the corrosion resistances of alloy 2A97,pure A1 and conventional aluminum alloys are investigated by milling tests and electrochemical measurements.3.5 wt.%sodium chloride solution is selected as the electrolyte to simulate the atmospheric corrosive environment.The research results can be applied in the industrial production of structural components that are made from Al-Li alloys,and promote the transformation of the materials from Al alloys to Al-Li alloys.Firstly,the machined surface integrity of Al-Li alloy 2A97 is studied.The surface roughness and residual stresses on the face-milled surfaces are measured;the microhardness and microstructures of the face-milled surface layers are studied.It is found that the extent of work hardening of face-milled surfaces ranges from 109%to 119%,and the depth of the hardened layer is about 20 μm;the residual stresses with small values are generated on face-milled surfaces.Secondly,the effects of surface integrity factors on the corrosion resistance of Al-Li alloy 2A97 are studied.The corrosion parameters,namely the corrosion potential,the corrosion current density and the polarization resistance,of the face-milled and ground-polished surfaces are compared and analyzed after potentiodynamic polarization tests.It is found that increasing the machined surface roughness of alloy 2A97 worsens its corrosion resistance,while the work hardening of machined surface enhances its resistance to corrosion.In addition,the residual stresses on the machined surface affect marginally the corrosion behavior of alloy 2A97.Then,based on the investigation on the corrosion extent of alloy 2A97 milled surfaces affecting their electrochemical behaviors,the concept of the instantaneous corrosion potential during alloy electrochemical corrosion is put forward.Then the convex function model of the instantaneous corrosion potential for alloy 2A97 at the early corrosion stage is established.According to the variation trend of current density,the anodic branches of the polarization curves of alloy 2A97 machined surfaces are divided into three phases,namely preferential dissolution,enhancing the protection,and breaking down of the protective film.With the above model and the assumption that the protective effectiveness provided by the film on alloy 2A97 machined surface is proportional to its equivalent thickness,the three phases are respectively elaborated.There are two characteristics within phase II,namely its small potential window or even no the phase and the current density is up to 10-5 A·cm-2 within the phase.Therefore,the anodic protection is not suitable to protect Al-Li alloy 2A97 from corrosion in chloride-containing media.Finally,based on the concept of the instantaneous corrosion potential,the instantaneous equivalent resistance during alloy corrosion is proposed as an indicator for comparing the corrosion behaviors of alloys 2A97,2024,6061,7050 and pure Al.It is found that the indicator can be applied in characterizing the variation of the corrosion behaviors of the above five materials during their corrosion processes.The superiority in the corrosion resistance of alloy 2A97 over the other four materials at the early corrosion stage is observed.At the steady corrosion processes,the corrosion resistance of alloy 2A97 remains almost twice as high as those of alloys 2024,6061 and pure Al.