| Al-Cu-Mg alloy is widely used in electric transportation.In China's southeast coastal areas such as Jiangsu and Zhejiang,the extremely harsh service environment of high temperature,high humidity and high salinity has caused severe corrosion of Al-Cu-Mg alloy wiring board,which seriously threatens the safe operation of substations.How to improve the corrosion resistance of Al-Cu-Mg alloy has become the focus of attention in the field of electric transportation.In Al-Cu-Mg alloy,the precipitates generated by ageing treatment can induce strong corrosion susceptibility,and its precipitation behavior at grain boundary or within grain interior directly affects the intergranular corrosion resistance and pitting resistance,so the regulation of precipitation becomes the key to improve the corrosion resistance of Al-Cu-Mg alloy in high temperature,high humidity and high salt environments.At present,the main methods for controlling precipitation in Al-Cu-Mg alloy include alloying,ageing treatment and thermomechanical treatment,etc.However,the alloying method costs a lot and consumes a lot of resources,so it is of more environmental protection and economic value to study other precipitation regulation processes besides alloying.In this dissertation,the effects of precipitation behavior under three processes of ageing treatment,thermomechanical treatment and electropulsing treatment on the corrosion resistance of Al-Cu-Mg are studied.The conclusions are summarized as follows:For the ageing treatment,the results showed that the precipitates within grain interiors grow up gradually with the increasing of ageing time at 190?.When the ageing time reaches 6h,the size of precipitates within grain interiors is larger than 10nm,and GPB-? zone is generated.GPB-? zone can induce severe lattice distortion,so the pitting corrosion susceptibility is sharply increased.When the ageing time is less than 6h,the pitting corrosion susceptibility of the alloy is small.However,continuous corrosion channels are easy to form at grain boundaries due to the continuous distribution of precipitates,which decreases the intergranular corrosion resistance.When treating the as-quenched Al-Cu-Mg alloy with 190?/20h peak ageing,280?/20min retrogression treatment and 190?/20min re-ageing,the distribution of precipitates at grain boundaries is transformed from continuous to discontinuous,improving the intergranular corrosion resistance.On the other hand,the precipitates within grain interiors undergo growth,dissolution and precipitation again.Finally,the size of precipitates within grain interiors is reached below 10nm,so the severe lattice distortion is unable to form and the pitting corrosion susceptibility is not significantly increased.Therefore,the ageing treatment can simultaneously improve the intergranular corrosion resistance and pitting resistance,but the ageing procedure is complicated,so it is necessary to further study the regulation of precipitation in short process.For the thermomechanical treatment,the results showed that when 30%cold rolling deformation is applied after solution treatment and then natural ageing,the grain boundaries are unable to be decorated by precipitates,which reduce the potential difference between grain boundaries and the Al matrix around the grain boundaries and decrease the driving force of the corrosion reaction,decreasing the susceptibility to intergranular corrosion.Moreover,the cold rolling deformation does not promote the precipitation of large-sized(>10nm)precipitates within grain interiors,and the corrosion susceptibility induced by S phase precipitates is larger than GPB zone,T phase and the entangled structure of T phase and dislocation,so the pitting corrosion susceptibility is decreased.By thermomechanical treatment,the intergranular corrosion resistance and pitting resistance of Al-Cu-Mg alloy are improved,furthermore,the treating process is shorter than ageing treatment.However,this process requires cold rolling immediately after solution treatment and is difficult to carry out in industry.The regulation process of precipitation of shorter process and easy to carry out is required to further study.For electropulsing treatment,the results showed that when the parameters of 0.5V and 0.1kA is applied to 190?/1h under-aged Al-Cu-Mg alloy for 5min,the current density is larger at the interspace of the adjacent precipitates at grain boundaries,so the diffusion flux of atoms increases.Furthermore,the dissolution process of the precipitates is a process in which the electric free energy is reduced.Therefore,the precipitates at grain boundaries are partly dissolved and the distribution of precipitates is transformed from continuous to discontinuous,which decrease the susceptibility to intergranular corrosion;the size of precipitates(lnm)within grain interiors is not enlarged,so the pitting corrosion susceptibility is low.After that,when treating 190?/20min re-ageing treatment,the discontinuous precipitates at grain boundaries continue to grow up due to the diffusion of the solutes,and the distribution of the precipitates is maintained as discontinuous.The continuous corrosion channel can not be formed,so the susceptibility of intergranular corrosion is not increased;the precipitates with a size of less than 10nm are precipitated within grain interiors.Because the lattice distortion degree does not change significantly,the pitting corrosion susceptibility is not increased significantly.By electropulsing treatment,the resistance to intergranular corrosion and pitting corrosion is improved when ensuring the maximum hardness.This process is shorter and easy to carry out in industry,so avoids the complexity under ageing treatment and overcomes the difficult application under thermomechanical treatment.This study throws a new light on improving the corrosion resistance of Al-Cu-Mg alloy by electropulsing treatment. |
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