| Due to the high strength and good mechanical properties,Al-Zn-Mg-Cu alloy has been widely used in the fields of transportation and aerospace.As a precipitation strengthened aluminum alloy,the properties of Al-Zn-Mg-Cu alloy depends on the precipitation sequence,size,density and distribution of the precipitates,and there is a strong tendency of stress corrosion cracking(SCC)when the heat treatment process is not appropriate.Traditional heat treatment methods have less effect on corrosion resistance enhancement,generally showing a 5-15 times enhancement,while the enhancement effect of superhydrophobic surface treatment is often over a thousand times or even ten thousand times,so it is important to study the superhydrophobic surface modification of Al-Zn-Mg-Cu alloy and reveal the mechanism of superhydrophobic surface modification to improve corrosion resistance.Low heat input friction stir welding(FSW)is the best method for the alloys.However,due to the complex evolution behavior of precipitates in the FSW joint of Al-Zn-Mg-Cu alloy,the research about the precipitation behavior in FSW joint of Al-Zn-Mg-Cu alloy needs to be further investigated.The present work focused on the corrosion resistance of Al-Zn-Mg-Cu alloy,analyzed the evolution behavior of Al-Zn-Mg-Cu alloy and FSW joints in the precipitates under different processes and aging heat treatment conditions,and investigated the mechanism of improving the corrosion resistance of the alloy by different surface superhydrophobic treatment,takes FSW joints as an application case,reveals the effect of superhydrophobic treatment on the SCC behavior of FSW joints.The main research contents of the present work are as follows:(1)The alloy was treated by different heat treatment,and the effect of heat treatment on the comprehensive performance and precipitates evolution of alloy was evaluated.The results show the strengthening mechanism of T6 and RRA treatment.The small and diffuse η’ phase under T6 treatment shear dislocations in the matrix.Due to the formation of largee MgZn2 phases under the RRA treatment,the dislocation movement as an Orwan bypassing mechanism in the RRA treatment matrix.RRA sample has coarse and discontinuous distribution GBPs with high Cu content,which can slow down anodic dissolution and hydrogen cracking,thus increasing the SCC resistance of Al-Zn-Mg-Cu alloy.The SCC process of the Al-Zn-Mg-Cu alloy can be divided into three stages,and the trend of charge transfer resistance(Rct)can be divided into three parts.The Rct values of RRA sample in all stages are larger than those of DA and T6 samples,and the RRA sample has a longer SCC resistance time.(2)The fabrication method applicable to joints was successively explored,then the mechanism of corrosion resistance for superhydrophobic surface was studied.A superhydrophobic surface was successfully fabricated via electrolytic polishing and 0.7-g/L MgCl2 aqueous solution etching followed by Perfluorooctyltriethoxysilane modification,featuring typical micro-nano hierarchical structures,including a nanosheets-composed petallike morphology.The contact angle of the superhydrophobic surface is as high as 167.3°.The electrochemical results show that the corrosion current density of the superhydrophobic surface decreases by more than 2 orders of magnitude compared to that of the sanded AA7055 surface,and the corrosion inhibition efficiency of the superhydrophobic surface is 99.67%.The superhydrophobic surface can maintain great stability in the concentrated NaCl solution environment,and the superhydrophobic layer on the surface will not be damaged by highconcentration chloride ions because of its strong protective effect against chloride ions.(3)Based on the study of preparing superhydrophobic layer by etching method and considering the ease of preparation process,a high corrosion-resistant and environmentally stable superhydrophobic layer was fabricated on the surface of Al-Zn-Mg-Cu alloys using surface silanization and constructing a secondary micro-nano structure with polytetrafluoroethylene and SiO2 nanoparticles on the polydimethylsiloxane surface.The sprayed nanoparticles on the low-energy surface layer can form a micro-nano secondary structure,and the surface contact angles is as high as 156.0°.The Rf value of superhydrophobic sample is much larger than the sanded Al-Zn-Mg-Cu alloys sample by 5 orders of magnitude.The corrosion current density of the superhydrophobic surface decreases by more than 2 orders of magnitude compared to the sanded Al-Zn-Mg-Cu alloys surface,and the corrosion inhibition efficiency of the superhydrophobic surface reaches to 99.38%.The superhydrophobic surface has excellent long-lasting stability,wear resistance and anti-ice properties.The freezing time of the superhydrophobic surface is 6 times greater than that on the surface of Al-Zn-MgCu alloys.(4)FSW parameters and pre-weld heat treatment conditions for Al-Zn-Mg-Cu alloys were optimized.The maximum tensile strength and elongation of FSW joints were 452.24 MPa and 5.07%,up to 74.4%tensile strength of the base material.TMAZ is the weak region of SCC throughout the FSW joint,which reduces the strength of joint,and the fracture position occurred in this region.TMAZ has higher stored energy and more residual internal stresses than other regions of the joint,while the shear stress is maximum and the crack is easier to expand in this region.The high heat input at low welding speeds resulted in the dissolution of the small η’phase in TMAZ.RRA alloy joint TMAZ has a lower number and a higher average size of MPs compared to DA alloy joint TMAZ.RRA alloy joint has good plasticity,it is better shaped and easily deformed,and the effects of stretching can be offset by the deformation during SSRT,which helping to improve the SCC resistance of RRA alloy joint.More GP zone and η’ phases dissolved in the regression and welding thermal cycling process in RRA alloy joint TMAZ,which increases the Zn,Mg content in the matrix and reduces the corrosion potential.Compared to DA alloy joint TMAZ GBPs,RRA alloy joint TMAZ GBPs were more discrete and coarser.RRA alloy joint TMAZ GBPs showed higher Cu content due to the diffusion of Cu elements in RRA alloy joint during the longer regression aging process.These grain boundary properties of RRA alloy joint can slow down anodic dissolution and hydrogen cracking,thus increasing the SCC resistance of RRA alloy joint.(5)The superhydrophobic surface treatment was carried out on FSW joints,and the SCC performance of superhydrophobic joints and FSW joints with different pre-weld heat treatment conditions were compared and analyzed by in situ electrochemistry,and the mechanism of the effect of superhydrophobic treatment and different aging heat treatments before welding on the SCC resistance of FSW joints was investigated.The results show that the SCC resistance of AlZn-Mg-Cu alloy joints can be significantly enhanced by superhydrophobic treatment due to the hindering effect of the superhydrophobic layer on the corrosive medium,and the superhydrophobic film ruptures after 29 h under SSRT conditions,and the SCC resistance time of the joints increases by 61%. |
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