Microstructure and Properties of Micro-nano Ceramic Coating Obtained by High Velocity Oxygen Fuel and Laser Remelting
Author:Guo Hua Feng
Supervisor:huang yin hui
Ti6Al4V titanium alloy has the advantages of low density,high ratio strength and good corrosion resistance.It is widely used in aerospace,petrochemical and civil fields.The poor wear resistance has become a research focus for scholars at home and abroad.Surface coating technology is an effective way to improve the tribological properties of titanium alloy surface.It is necessary to further study how to prepare high quality wear resistant coatings and achieve quality control.In this paper,using the titanium alloy substrate as the research object and follow particle acceleration-flight-heating-impact-flattening-cooling solidification-forming coating rules.Based on theoretical analysis,numerical simulation and experimental research,the micron structure and micro/nano structure WC-12 Co coating were successfully prepared by high velocity oxygen fuel technology.The microstructure and properties of the two coatings were analyzed.The multiple regression mathematical model of process parameters and coating quality was established,and the coating quality was optimized.Finally,laser remelting technology was used to realize the change of interface behavior between the coating and substrate.The main work and results of this paper are as follows:(1)On the basis of analyzing the characteristics of particle momentum transfer,a finite element model was established and the effect of powder particle size,flight speed and spray distance on the evolution of melting model were studied quantitatively.The theoretical model of particle melting mechanism of WC-12 Co powder was established.(2)The spatio-temporal independence of WC-12 Co powder particles was calculated,and the impact and flattening behavior of WC-12 Co particles were obtained.The rebound mechanism of WC particles was revealed.The calculation model of residual stress in coating was established which provided the basis for revealing the material and technological factors for its contribution.The interface state between coating and substrate with mechanical combination was clarified.The heterogeneity of the microstructure in coating was characterized and the size type and distribution of pore were studied quantitatively.(3)The microstructure and tribological properties of the two coatings were compared and analyzed.The influences rule of material properties and spraying process parameters on microstructure and properties of coatings were discussed.The wear mechanism of coating and substrate was revealed.Micro/nano coatings are mainly composed of submicron and nano-scale WC particles.Micron coating is mainly micron and submicron size.The porosity and friction coefficient of micronano coatings are relatively small,and the hardness and anti-deformation capability are better than micron coating.The wear mechanism of the substrate and the coating is obviously different.The Ti6Al4 V substrate shows the comprehensive wear mechanism with abrasive wear,adhesion wear as the dominant and oxidation wear as the auxiliary.Micron WC-12 Co coating is characterized by the abrasive wear mechanism of micro-cutting and peeling,while micro/nano WC-12 Co mainly adopts the abrasive wear mechanism of micro-cutting.Both of the coatings show excellent wear resistance and the micro/nano coating is more advantageous.(4)The dependence of residual stress on powder characteristics and momentum transfer characteristics is revealed.It is clear that the residual compressive stress in the coating is caused by the blast effect of the unmelted WC particles which is not related to the spraying process.(5)Based on statistical theory,a multivariate regression mathematical model of technological parameters and coating quality was established.The reliability and precision of the model were verified by the supplementary test.The prediction and optimization of coating quality are realized.The high quality WC-12 Co coating was prepared,which improved the quality controllability of coating.(6)The Marangoni effect multi-physical coupling field model was established.It is revealed that the convective heat transfer nature of laser molten pool and the driving force of the homogenization of coating components.The 3D finite element model of temperature field of laser remelting thermal spraying coating was established,and the influence of technological parameters on the evolution of molten pool temperature field was analyzed.A non-porous and metallurgical bonded laser remelting coating was successfully prepared by optimized process parameters.The technical route of high quality laser remelting WC-12 Co coating was developed.