Preparation of Laser-Assisted Micro-Arc Oxidation Coating on TC4 Alloy Surface and Thermal Corrosion Failure Study
Ti6Al4V(TC4)titanium alloy is favored by aerospace,marine and defense industries because of its light weight,high specific strength,corrosion resistance,etc.TC4 accounts for 70％of the world's total production of titanium alloys,is currently the most widely used titanium alloy,commonly used in the manufacture of aero-engines within the pressurizer disk and rotor blades,fans,guides,spacers,intake magazines and other need to withstand a lot of stress on the rotating and heat parts.Sulfur oxides generated by aviation fuel combustion easily react with salt spray evaporated from seawater to form eutectic mixed salts adhering to the surface of tita-nium alloys,which accelerates corrosion of the material at high temperatures and significantly reduces the mechanical properties of the material.With the gradual increase in the working temperature of aircraft engines,the demand for titanium alloy for naval gas turbines has expanded,and there are higher requirements for resistance to high-temperature oxidation and corrosion,and the surface strength-ening technology can improve the heat and corrosion resistance of titanium alloys while retaining the mechanical properties to the great-est extent possible.Micro-arc oxidation(MAO)technology can prepare a ceramic coating with high bonding strength on the surface of titanium alloys,but there are a large number of micropores and cracks on the surface,and the densification is poor,making it difficult to provide long-term protection.In order to optimize the coating structure,a ceramic protective coating was prepared on the surface of TC4 alloy by laser-assisted micro-arc oxidation(LMAO),and the effects of simultaneous laser irradiation on the structure and thermal corrosion behavior of the coating under NaAlO2 salt electrolyte system were investigated,discussing the thermal corrosion behavior.Scanning electron microscope(SEM)was used to analyze the surface and cross-section micro-morphology and elemental distribution of the coating before and after thermal corrosion,X-ray diffractometer(XRD)was used to detect the physical composition of the coating,and analytical balance was used to test the mass gain of the samples after each thermal cycle,to obtain the kinetic curves of thermal corrosion,and to characterize the thermal corrosion performance of the coatings.The result showed that the surface flatness of LMAO coating was improved,the porosity and pore size of the surface were reduced,and the densification and homogeneity were improved.Al content on the surface of MAO coating was 16.79％,while Al content in LMAO coating was 19.86％,and the laser energy increased Al content in the coating.XRD patterns showed that the coatings were mainly composed of Al2TiO5,rutile TiO2,anatase TiO2 and a small amount of α-Al2O3 high-temperature stabilized phases,and the energy provided by the laser was beneficial to the formation of high-temperature phases.After 20 h of hot corrosion,a large number of corrosion pits and deep grooves were formed on the surface of TC4 substrate,and the oxides were in the form of clusters with a loose and porous structure.In the oxide layer,Ti+O atoms accounted for more than 90％,and the corrosion products were mainly composed of TiO2.Due to the relatively small proportion of Al elements in the substrate,a dense and continuous Al2O3 film could not be formed,and the structural difference between TiO2 and Al2O3 produced large thermal growth stress,which was an important reason for transverse cracks in the oxide layer.The corrosion products preferential-ly grew around the micropores and pits of the coating and spread to the micropores,and the rough surface and large pore size of MAO coating provided more attachment sites and infiltration channels for the corrosion salts,and the molten salts were more likely to gather.The thermal corrosion kinetic curve of TC4 substrate showed a linear increasing trend,and the maximum oxidized weight gain amount-ed to 2.88 mg·cm-2,which formed a 33.3 μm thick oxide layer without protection.The oxidized weight gain of MAO and LMAO coat-ings were small,and the curve was roughly parabolic with a gentle trend.The maximum oxidation gains of MAO coating decreased to 1.13 mg·cm-2,with an oxide layer of 1.05 μm.In contrast,the maximum oxidation gains of LMAO coating further decreased to 0.91 mg·cm-2,with the thinnest oxide layer of 0.33 μm.The dense layer of MAO coating had a hindering effect on O diffusion,leading to the bias of O elements near the interface of the substrate.S content in the oxide layer was small,and Cl2 as easy to form volatile chlo-rides that left holes between the grains of the oxide layer,which was an important reason for the cracking of the oxide layer.LMAO coating hole defected less,high Al element content in the coating made the corrosion products denser,reduce cracks and voids forma-tion,corrosion area was effectively controlled,enhancing the coating on O and corrosive media blocking effect,improving the protec-tion of titanium alloy substrate.
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