K465 高温合金表面 AlSiY 涂层组织演变及化学去除过程研究

For the remanufacturing of aluminite coatings on the surfaces of aero-engine turbine blades with high quality, the non-uniform chemical stripping of the aluminite coatings should be considered. In order to solve these key problems in removing the aluminite coatings, the chemical removal mechanism should be clarified. In the present work, the microstructure evolution and non-uniform chemical stripping of AlSiY coatings on the surface of K465 superalloy were investigated. This work is the first attempt to design chemical removal reagents according to the electronic work function theory. Optical microscopy (OM), scanning electron microscopy/energy-dispersive X-ray analyzer (SEM/EDS), X-ray diffraction measurement (XRD) and other characterizations were used to analyze the microstructural evolution of AlSiY coatings. The effects of chemical removal agent components on the non-uniform chemical stripping of coatings were calculated according to the first principles. The results showed that the AlSiY coating on the surface of K465 superalloy was divided into inner and outer layers, mainly β-NiAl phases. The results of first principles calculations also showed the lowest formation energy of the β-NiAl phase as compared with other NiAl intermetallic compounds. Based on the first principles calculations, it was found that the Ni₅Al₃ phase was the diffusion barrier of Ti elements, resulting in the absence of Ti elements on the surface. By observing the cross-sectional morphology of the corrode coating, it was found that pitting corrosion was the main characteristic of the corrosion of AlSiY coatings, which indicated the non-uniform corrosion occurred on the stripped surfaces. In order to clarified this non-uniform corrosion phenomenon, the chemical stripping mechanism of AlSiY coatings was investigated by the calculation of the electronic work function. The calculated results showed that the difference in electronic work function of corroded surfaces with different crystal orientation played a key role on the chemical removal inhomogeneity. The greater the difference was, the larger the inhomogeneity was. The calculation of electronic work function showed that the chemical removal agent with a single component increased the inhomogeneity of chemical stripping. While after mixing the two chemical removal reagent components, the corrosion of the AlSiY coating became more uniform. When the single nitric acid component was applied, the large difference in electronic work function among (110), (100) and (211) surfaces was about 0.7 eV. A small amount of hydrofluoric acid could greatly decrease this value to 0.3 eV. Keeping on adding chromium trioxide into the removal reagent, the difference in electronic work function decreased to 0.2 eV, which indicated that the chromium trioxide was the key component to make the corrosion of AlSiY coating more uniform. The mixed removal reagent of nitric acid + hydrofluoric acid + chromic trioxide was beneficial to the uniform removal of the outer layer of the AlSiY coating. All experimental results in the present work have proved the correctness of the calculated results. The present study just provides a research mode for the rapid design of chemical removal reagents. By applying the method mentioned in the present work, the complex chemical removal reagent can be designed by considering the chemical components of AlSiY coatings, which will greatly shorten the research period.