Effect of microstructure of 7A04 aluminum alloy forging on its anodization process
HAN Lei;PENG Jiangtao;WANG Jian'gang;GUO Rui;WANG Shuaidong;[Objective] 7A04 aluminum alloy forgings are commonly used in the primary load-bearing structures in the aerospace industry, with sulfuric acid anodizing being a standard surface treatment process. Studying the influence of 7A04 aluminum alloy forging's intrinsic microstructure on anodizing is significant for guiding production. [Method] 7A04 joint forgings underwent alkaline cleaning and anodizing after machining. The substrate and anodic oxide film were examined by macroscopic and microscopic observation, energy-dispersive spectroscopy, metallographic analysis, and microhardness testing. [Result] Due to different local forging conditions, the two ends of the joint exhibited different colors after anodizing: one end showed the normal yellow hue, while the other end displayed abnormal dark spots. During the free forging process, the areas with low deformation and high forging rate resulted in insufficient grain refinement, adversely affecting the mechanical properties. This also led to different surface flatness at the two ends of the joint after alkaline cleaning prior to anodizing, consequently producing anodic oxide films with different flatness. Differences in light reflection and scattering due to this surface morphology resulted in the observed color variation. However, the anodic oxide film at the dark spot area remained compact and continuous, albeit slightly thinner, and still provided a certain degree of protection. [Conclusion] For 7A04 aluminum alloy during free forging, it is essential to ensure that the deformation amount, forging rate, and cold forging amount meet the requirements to achieve sufficient and uniform grain refinement. This is crucial for guaranteeing that both the quality of the anodizing process and the mechanical properties satisfy the demands of engineering applications.
Effect of alkaline silanization pretreatment of aluminum alloy on adhesion of ceramic coating onto its surface: experimental study and mechanism analysis
LU Jiahua;LIN Yingjie;[Objective] The effect of alkaline silanization as a pretreatment on the adhesion of ceramic nonstick coating on the surface of aluminum alloy cookware and the related mechanism were explored. [Method] Using 3003 aluminum alloy as the substrate, silanization was conducted in a solution containing silica sol and methyltrimethoxysilane. The adhesion of the subsequently sprayed ceramic nonstick coating was measured, and its chemical structure and surface morphology were examined by Fourier-transform infrared spectroscopy(FT-IR), scanning electron microscopy(SEM), and energy-dispersive spectroscopy(EDS). [Result] A thin pretreatment film was formed on the surface of 3003 aluminum alloy by alkaline silanization, which significantly improved the adhesion between the substrate and the ceramic nonstick coating. [Conclusion] A large number of Al─O─Si bonds are formed at the interface between the substrate and the pretreatment film, and numerous Al─O─Si and Si─O─Si bonds are generated at the interface between the pretreatment film and the ceramic coating. The adhesion of the ceramic nonstick coating on the 3003 aluminum substrate is enhanced by the bonding effect of these chemical bonds.
Aging behavior of protective coatings on 2A97 aluminum–lithium alloy in a harsh marine environment
JIA Jinghuan;LI Zhao;ZHAN Zhongwei;LUO Chen;ZHAO Mingliang;[Objective] The durability of organic protective coatings on 2A97 aluminum–lithium alloy in harsh marine atmosphere of South China Sea was studied to provide data support for coating selection and application in aviation equipment. [Method] The aging behavior of Al–Li alloy specimens treated via sulfuric acid anodizing followed by epoxy primer coating after 0.5 to 2.0 years of exposure in South China Sea marine atmosphere was studied through outdoor exposure test, combined with macro/microscopic morphology observation, glossiness and color difference measurement, and electrochemical impedance spectroscopy(EIS). [Result] With the extending of exposure time, the number and area of micro-pits on coating surface continuously increased, the glossiness loss and color difference of coating significantly rose, and the low-frequency impedance modulus decreased. [Conclusion] The coating on 2A97 Al–Li alloy surface treated by sulfuric acid anodizing and epoxy primer coating gradually aged within 2 years of exposure in the harsh marine atmosphere of South China Sea, primarily due to the combined effects of hydrolytic degradation of hydrophilic groups and photodegradation in the coating.
Preparation and properties of ZIF-90/PA composite coating on magnesium alloy
LIU Wei;WANG Shaochong;JI Qian;[Objective] To improve the corrosion resistance and antibacterial properties of AZ31B magnesium alloy through surface modification technology. [Method] A zinc-ion-containing zeolitic imidazolate framework coating(ZIF-90) was prepared on the surface of silanized AZ31B magnesium alloy via a solvothermal method, followed by the chemical conversion with phytic acid(PA) to form a ZIF-90/PA composite coating. The surface morphology and microstructure of the coatings were analyzed by X-ray diffraction(XRD), scanning electron microscopy(SEM), and Fourier-transform infrared spectroscopy(FTIR). The corrosion resistance of the coated magnesium alloy in simulated body fluid(SBF) was evaluated via electrochemical measurements and hydrogen evolution experiments. The antibacterial performance was examined using Staphylococcus aureus. [Result] The ZIF-90/PA composite coating was successfully synthesized on the surface of AZ31B magnesium alloy. Compared with the PA-coated magnesium alloy, the ZIF-90/PA-coated magnesium alloy exhibited a significantly increased capacitive arc radius in SBF, a reduction in corrosion current density by one order of magnitude, a positive potential shift by 0.217 V, a notable decrease in hydrogen evolution volume, and an improvement in antibacterial rate from 25.3% to 98.2%. [Conclusion] Compared with the single PA coating, the ZIF-90/PA composite coating exhibits superior corrosion resistance and antibacterial properties, due to its thicker structure, more compact surface conversion film, strong adhesion strength, and the antibacterial effect of zinc ions.
Microstructure and properties of MAO/Mg–Al LDH composite coating on AZ91D magnesium alloy
XU Liping;HU Shengbo;MAO Jie;[Objective] This study aims to enhance the corrosion resistance of AZ91D magnesium alloy by preparing a micro-arc oxidation/layered double hydroxide(MAO/Mg–Al LDH) composite coating on its surface, and to investigate its feasibility and protective performance. [Method] The AZ91D magnesium alloy was firstly subjected to micro-arc oxidation(MAO), followed by the in-situ growth of an Mg–Al LDH coating on the MAO film via hydrothermal method. The phase composition, chemical structure, and microstructure of the composite coating were characterized by X-ray diffraction(XRD), Fourier-transform infrared spectroscopy(FT-IR), scanning electron microscopy(SEM), and energydispersive spectroscopy(EDS). The corrosion resistance of MAO/Mg–Al LDH composite coating was evaluated through potentiodynamic polarization measurement and electrochemical impedance spectroscopy(EIS). [Result] The MAO/Mg–Al LDH composite coating exhibited a uniform and compact nanosheet-like structure, which effectively sealed the micropores and microcracks in the MAO coating. Compared to the single MAO coating, the composite coating showed a positive shift of 100 mV in corrosion potential, a nearly one-order-of-magnitude reduction in corrosion current density, and a 23.5-fold increase in total impedance, demonstrating significantly superior corrosion resistance. [Conclusion] The MAO/Mg–Al LDH composite coating prepared by this process significantly enhances the corrosion protection of the substrate, providing an effective technical approach for surface protection of magnesium alloys.
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