Effect Of Ultrasonic Rolling On Corrosion Behavior Of 6061 Aluminum Alloy Gradient-strengthened Layer

In recent years,with the development and application of rail transit technology,the lightweight and safety of vehicles have been paid more and more attention.Aluminum alloy has become an ideal material of choice because of its high specific strength and easy processing,among which 6061 aluminum alloy is one of them.Due to the long-term service,the failure of aluminum alloy occured in corrosive environment,so the application of aluminum alloy was greatly limited.In order to further prolong the service time of 6061 aluminum alloy,the effect of precipitation on the local corrosion of 6061 aluminum alloy after different aging time was firstly studied.Ultrasonic rolling technology was used to treat the surface of 6061 aluminum alloy in peak aging state and the change of its corrosion resistance was studied.And the better ultrasonic rolling processing parameters were analyzed and selected,which provided the experimental basis for further expanding the application of ultrasonic rolling technology in 6061 aluminum alloy production.The main research contents and relevant results were as follows:(1)The study on the local corrosion behavior of 6061 aluminum alloy with different aging time showed that the effect of AlFeSi on the local corrosion behavior is greater than that of Mg2 Si phase,and the spread of local corrosion was mainly affected by AlFeSi phase.The effect of AlFeSi on the local corrosion behavior of6061 aluminum alloy was related to the size of AlFeSi phase,and the smaller the size of AlFeSi phase,the smaller the effect.(2)Ultrasonic rolling technology was used to treat the surface of 6061 aluminum alloy.Four different rolling pressures of 0MPa,0.08 MPa,0.1MPa and 0.12 MPa were selected as the variables.The results showed that the surface roughness decreased first and then increased with the increase of rolling pressure.When the rolling pressure were 0.08 MPa and 0.1MPa,the surface roughness were decreased by 74.5% and70.6%,compared with the untreated samples.However,with the increase of rolling pressure,the surface quality of the sample deteriorated and the roughness increased.Ultrasonic rolling treatment refined the AlFeSi phases on the sample surface,and theirs size decreased with the increase of rolling pressure.It was found that AlFeSi phase existed in discontinuous state at grain boundary on the cross section of sample,due to plastic deformation.But theirs size change had no obvious relationship with the change of rolling pressure.After ultrasonic rolling treatment,the residual compressive stress increased,which increased first and then decreased with the increase of ultrasonic rolling pressure.When the rolling pressure was 0.1MPa,the residual compressive stress on the surface of the sample reaches the maximum,which reaches-124 MPa.At the same time,the surface hardness of 6061 aluminum alloy was improved by ultrasonic rolling treatment.When the rolling pressure was 0.1MPa,the hardness reached 134.7±2.5HV,which was increased by 15.7%.And the hardness was slightly improved with the increase of the rolling pressure.(3)The electrochemical corrosion properties of 6061 aluminum alloy samples treated with different rolling pressures were tested in 3.5%Na Cl solution at a constant temperature of 30±1?.The arc radius of the electrochemical impedance spectrum of the samples treated by ultrasonic rolling was larger than that of the samples not treated.The polarization curve also showed that the self-corrosion current density and the current density of the current of the 6061 aluminum alloy samples treated by ultrasonic rolling were lower than that of the original sample,indicating that the corrosion resistance of the 6061 aluminum alloy had been improved by ultrasonic rolling.When the rolling pressure was 0.1MPa,the dimensional dull current density was reduced by one order of magnitude compared with the original sample,and the corrosion rate was the least.As the rolling pressure increased,the surface roughness increased,and the residual compressive stress decreased,and the corrosion rate increased slightly.Intergranular corrosion tests were carried out on samples treated with different ultrasonic rolling pressures according to national standards.Based on the analysis of maximum depth of intergranular corrosion and sample surface integrity,it was found that the ultrasonic rolling treatment inhibited the expansion of intergranular corrosion of 6061 aluminum alloy.The sensitivity to intergranular corrosion was the lowest when the rolling pressure was 0.1MPa.Therefore,the sample treated with ultrasonic rolling pressure of 0.1MPa had the best corrosion resistance.In this paper,the surface of 6061 aluminum alloy was strengthened by ultrasonic rolling surface treatment technology.Considering the surface integrity,mechanical properties and corrosion resistance comprehensively,it was found that the samples treated with ultrasonic rolling pressure of 0.1 MPa had better performance.