Research On Crack Control And Its Hot Corrosion Resistance Of Modified Al-Si Coating By Laser Melt Injection Fine CeO₂

The improvement of the oxidation and corrosion resistance of the metallic bond coating is a hot subject and a persistent pursuit goal in the research field of the thermal barrier coating.As one of the typical representatives of the metallic bond coating,Al-Si coating is a kind of metallic bond coating with great development potential for service at extreme temperature,which not only easily obtains a protective Al₂O₃ scale with strong stability at extremely high temperatures,but also has an inner layer diffusion barrier structure that can inhibit coating degradation.The“fine particles laser shock melt injection technique”is a new laser surface modification technology recently proposed by our research group.If this technique is used to effectively inject rare earth particles such as CeO₂ with an average particle size of 1?m into the Al-Si coating,it is expected to retain both the original high aluminum content and the inner diffusion barrier structure of the coating and introduce various oxidation and corrosion resistance mechanisms such as“blockage effect”and“refining grain”of fine particles as well as the“active effect”of rare earth elements.As a result,the diverse anti-oxidation corrosion modification of the Al-Si coating is achieved.However,the brittleness the Al-Si coating is relatively high due to the coating has higher Al content.At the same time,crack control is a common key issue of the laser surface modification technology.Based on the analyses of relevant researches state,this paper focuses on“Research on Crack Control and Its Hot Corrosion Resistance of Modified Al-Si coating by Laser Melt Injection Fine CeO₂”to study,which not only provide a reference for the crack control of the laser injection fine particles modification technique,but also give an idea for improving the oxidation and corrosion resistance of the metallic bond coating serviced at extreme temperature.The main works accomplished and results obtained are as follows:?1?Research on optimization of process parameters for laser melt injection fine CeO₂modification Al-Si coating was performed.The effect law of laser process parameters,especially scanning speed,on the quality?crack,penetration,morphology?of the modified Al-Si coating was analyzed.The optimal process parameters under the experimental conditions of this paper was obtained,and the microstructure of the modified Al-Si coating was investigated.The results show that the scanning speed is the main factor influencing the quality of the modified Al-Si coating,and the defocus amount is secondary,while the laser power has the least influence.The optimal laser process parameters under the experimental conditions are:laser power of 425W,1250mm/min of scanning speed and-2mm of defocusing amount.Fine CeO₂ particles are injected into until the bottom of the molten pool,so that dendrites at the solidification front of the molten pool are reduced,and the internal grains of the modified zone are refined.?2?Research on the crack distribution and mechanism of laser melt injection fine CeO₂modification Al-Si coating was performed.The typical cracks and their distribution rules occurred in the process of laser melt injection was collected.collating based on the analysis of microstructure and composition,and combining relevant theoretical analysis The cause of cracks was initially discussed.The results show that typical cracks mainly include oblique non-penetrating cracks,longitudinal non-through cracks,herringbone cracks,transverse cracks,and longitudinal penetrating cracks,which mainly originate from the coating surface or the solidification front of the molten pool and expand toward the longitudinal and transverse directions with the intergranular or transgranular form.The main reason for the crack is due to the larger thermal stress which caused by the high temperature and rapid cooling process of the molten pool during the laser melt injection.?3?Research on control of cracks in modified Al-Si coating by substrate preheating treatment was performed.The crack evolution of modified Al-Si coating under the substrate preheating treatment was studied.The mechanism of crack regulation was analyzed and the optimal preheating treatment process parameter was obtained.The microstructure of the modified Al-Si coating after substrate preheating treatment were investigated.The results show that the preheating treatment has obvious improvement effect on the crack problem,and the morphology of the coating without micro-cracks and complete morphology and suitable depth of penetration can be obtained through reasonable matching of process parameters.At the same time,dendrites in the solidification front of the molten pool are further reduced and internal grains are further refined.?4?Research on control of cracks in modified Al-Si coating by ultrasonic vibration was performed.The crack evolution of the modified Al-Si coating under the ultrasonic vibration was studied.The mechanism of the crack regulation was analyzed and the optimal ultrasonic vibration process parameter was obtained.The microstructure of the modified Al-Si coating after substrate preheating treatment were investigated.The results show that the effect of ultrasonic vibration has a certain effect on the crack problem,but the process parameters which can prepare the modified Al-Si coating with no micro-cracks and complete morphology has not been obtained.At the same time,a more fine cell-like solidification structure is present in the modified zone and dendritic crystals are greatly reduced at the solidification front of the molten pool.?5?The hot corrosion behavior of the modified Al-Si coating and the original Al-Si coating at 1050?C Cl-containing Na-slats was studied.The hot corrosion morphology,element distribution,and surface phases were investigated,and the hot corrosion mechanisms of the two coatings were discussed in detail.The results show that the molten salt has a strong dissolution capacity at 1050?C,which makes it difficult to form a protective oxide scale on the surface of the original Al-Si coating in the initial stage of hot corrosion.Aa a result,the coating surface forms the protective Al₂O₃ scale until 60 hours of hot corrosion while the duration of the steady state oxidation is extremely short.On the contrary,the protective Al₂O₃ scale has formed on the surface of the modified Al-Si coating after 30 hours of hot corrosion,and the duration of the steady state oxidation has been maintained for more than 50 hours,which shows good hot corrosion resistance.