Research On The Formation Mechanism And Oxidation Property Of Laser Melt Injection Fine Particles In Aluminum Silicon Coating

Faced with the increasing working temperature and complex service environment,it is always a hot topic and pursuit goal in the field of thermal barrier coatings?TBCs?to improve the the oxidation and corrosion resistance of metal bonding coatings.In this paper,based on the analysis of related research from domestic and foreign,if taking the aluminum silicon coating as the object and maintaining the in-situ diffusion barrier structure of the inner layer,it is proposed that the rare earth particles were introduced into the surface by laser shock melt injection technology and giving full play to the high energy and fast cooling characteristics of laser processing technology and the synergistic effect and active element effect of fine rare earth particles,so as to realize the multiplex oxidation corrosion resistance modification idea.In the study,the formation mechanism,high temperature oxidation performance of modified Al-Si coating was studied systematically in the aspects of the theoretical basis of laser melt injection fine particles,the coupling between laser melt injection process parameters and preset layer,and the microstructure and hardness of modified Al-Si coating.The major work done and the results obtained are as follows:?1?The injection process of fine-particles,and the matching relationship between the melting point,boiling point of the matrix,micro-particles,and the binder was analyzed.At the same time,a theoretical model of the energy required for the injection of fine particles is established according to the law of conservation of energy;In addition,according to the relationship between the melting point of the fine particles and the particle diameter,it is determined that the fine particles can be applied to the laser shock melt injection,and then the theoretical basis can be established for the laser melt injection fine particles.The results show that the injection of micro-particles by laser impact melt injection requires that the melting point of the binder is not greater than the melting point of the matrix,and it is not greater than the melting point of the fine particles,and the boiling point of the binder is not greater than the melting point of the fine particles.When the particle size reaches 80 nm,it is close to the normal melting point of CeO₂.Combined with the particle size range of commercially available CeO₂ particles,the selected fine particles of the following study have a particle size of 1?m.?2?In view of laser shock melt injection fine particles technology is a new type of laser surface modification technology,based on exploratory experiments and theoretical analysis of the second chapter,the laser process parameters window under the experimental conditions and the orthogonal optimization and single-factor optimization experiments were carried which was determined by the optimal process parameters.Then the interaction between the laser process parameters and the state of the composite preset layers?rubber powder ratio,preset layer thickness?was studied.The results show that under the experimental conditions,the molten pool penetration depth D_m,the fine particle injection depth D_i,and the molten pool defect grade D_l are evaluated.The laser power is the main factor affecting the modified Al-Si coating.Second is the scanning speed,followed is defocus amount.The optimal laser process parameters are:laser power 450 W,scanning speed 1100 mm/min,and defocus amount-1mm.The change of ratio of the rubber powder in the composite pre-layer is essentially the change of the energy E_g distribution ratio for the gasification explosive absorption of the binder,and the change of thickness of the composite preset layer is essentially changing the temperature rise E_q and the energy distribution relationship of the molten pool E_s.?3?The process of phase composition,microstructure,element change of Al-Si coating before and after modification was analyzed by using SEM,EDS,XRD,OM,hardness testerand.And then the effect of power,ratio of powder to powder in the composite preset layer,and thickness of the composite preset layer on the microstructure and hardness of the modified layer was systematically studied.The results show that the modified Al-Si coating are mainly contained?-NiAl,Ni₂Al₃,and Cr₃Si,and Ni₃Si and CeSiO₄ are also present.The outer layer is composed of rare earth and fine-grained solidification structures,and the diffusion barrier in the inner layer is significantly enhanced.With the increase of laser power,the microstructure of the modified layer gradually changes from a large area of non-morphological features to a fine,solidified structure and then to a coarse structure.With the reduction of the ratio of the powder to the compound layer,the microstructure of the modified layer is fine.The dendrites gradually transform to equiaxed crystals.With the increase of the thickness of the composite preset layer,the microstructure of the dendrites gradually changes from equiaxed grains to small grains of dendrites.?4?The thermal oxidation products on the surface of the coating,the microscopic morphology of the coating surface,and microscopic morphology of the cross-section of the the thermal oxidation behaviors of Al-Si coatings at 1050°C for 150 h before and after laser-melting modification were analyzed by using SEM,EDS,XRD,OM,and the preliminary study on the anti-stripping capability of oxide films on the coating surface was conducted.Compared with Al-Si coating,the transformation of aluminum oxide film occurs on the surface of the modified Al-Si coating,but the addition of yttrium oxide causes the aluminum oxide film to rapidly complete the phase transition into the steady-state oxidation phase.The oxide film on the coating surface is mainly composed of a uniform and stable?-Al₂O₃ oxide film,and the oxide film on the surface of the modified Al-Si coating has obvious“pinning phenomenon”and improves the adhesion of the oxide film.After the thermal oxidation of the modified Al-Si coating for 100 h,the surface oxide film was mainly exfoliated.Although the exfoliation area had reached 62.7%,the exfoliation area on the coating surface did not reveal the matrix,but redistributed densely.The thin needle flake alumina film shows the self-repair ability of the modified Al-Si coating with strong oxide film.