Study On Evolution Of Molten Pool Dynamic Characteristics And Prediction Of Key Product Qualities In Laser Melting Deposition

As one of metal additive manufacturing technologies,laser melting deposition(LMD)can be used for free forming,remanufacturing repair and coating cladding,etc.It has been widely used in aerospace,energy,and chemical industries related to important areas of national defense and economy.However,due to the complex metallurgical behavior and process environment in LMD,the reliability and stability of product qualities need to be improved,which limits the application and popularization of this technology in high-value equipment.On-line monitoring and in-situ control technology is one of the important ways to ensure LMD quality,which is of great significance for improving its reliability and stability.However,currently,the on-line monitoring of LMD still has some problems such as incomplete process signatures,unclear quality evolution rules,and difficulty in monitoring and predicting key qualities,which limit the high product qualities of LMD.Therefore,in response to the problems mentioned above,the thesis studies the formation and evolution mechanism of dynamic characteristics of the molten pool related to the metallurgical dynamics of melting,flowing,and solidifying of the molten pool;clarifies the whole evolution process among process parameters,process signatures,and key qualities based on the dynamic characteristics of the molten pool;presents on-line monitoring and prediction method of key qualities;provides theoretical and technical support for the high-performance process of LMD.The main works are as follows:(1)A multi-source information fusion monitoring platform for LMD is built.The interaction mechanism between laser,powder,and substrate is theoretically analyzed.A finite element model of thermal-mechanical coupling based on the life-death element method is established.First,the thesis builds a monitoring platform that can obtain the LSM process signals,such as coaxial visible light,paraxial visible light,infrared light temperature,and thermal deformation.Second,the energy and mass transfer behavior between the laser head and substrate is studied,indicating that process signatures are necessary for building the quality model.Third,based on the metallurgical kinetic theory,a preliminary kinetic model of the melting,flowing,and solidifying process of molten pool is acquired.At last,the thermalmechanical coupling finite element analysis model of LMD is established.The above content provides experimental conditions and theoretical model support for the thesis.(2)The visual characteristics of laser surface melting(LSM)with the interaction between the laser and substrate are studied,which can be the research basis of visual characteristics of LMD molten pool.First,using the monitoring platform mentioned above,combined with element analysis,the formation mechanism of visual characteristics of the boundary,transition,and convection region are studied,and the characteristics result from the high-temperature solid and mushy metal,the thin film covering the molten pool surface,and the strong convective motion of liquid metal.Second,process signature characterization methods including oscillation frequency and amplitude are proposed,and the convection characteristics are more sensitive to the process parameter change.The larger convection region area and lower oscillation amplitude lead to better qualities.Finally,the mapping law between molten pool characteristics and qualities is explained.The oscillation frequency can be used to predict the different changing trends of penetration depth;the variation of convection region area can be used to predict the location with lower penetration depth;the convection region area has a better ability than the molten pool total area in real-time prediction of penetration depth.(3)The formation,evolution mechanism,and metallurgical dynamics of the visual characteristics of the LMD molten pool with an interaction of laser,substrate,and powder are studied.First,based on the molten pool characteristics of LSM,the formation mechanism of the visual characteristics of LMD is revealed.The thesis considers that the characteristics of the head,middle,and tail regions of molten pool can indicate the melting,flowing,and solidifying state.And the evolution behavior of slag is explained.Second,a metallurgical kinetic model suitable for LMD is built.Based on the competitive relationship between the melting speed vmeit and the scanning speed v,the surface binding force Fsur' and the internal impact force Fimp,and the law of heat dissipation,the thesis presents and verifies the theoretical criteria of the metallurgical dynamics and visual performance of the molten pool.At last,the change law of visual characteristics of molten pool under different process parameters was analyzed.The molten pool size,the transition texture,the middle grayscale,and the tail geometric characteristics,etc.will reach a relatively stable state as the processing state becomes stable.(4)The variation law of the temperature characteristics of the molten pool of LMD and the correlation with visual characteristics are studied.Using the monitoring platform,the influence of process parameters and scanning strategies on temperature characteristics of the molten pool is analyzed,and the temperature gradient relates to the heat accumulation state.The correlation between the temperature and visual characteristics of the molten pool is explored.In a single-layer single-track deposition,the molten pool grayscale is related to the temperature,and the length of the molten pool is related to the temperature gradient.In a singlelayer multi-track overlap,uneven heat conduction causes asymmetrical geometry of the liquidsolid boundary.In a multi-layer single-track,due to the competitive relationship between the thin wall height and the interlayer temperature,the visual characteristics of the molten pool go into a cycle with length change.The two parts above provide important process signatures and basic theoretical support for the on-line prediction.(5)The evolution law of key product qualities of LMD based on the dynamic characteristics of the molten pool is studied,and the monitoring or prediction methods of the shape accuracy of the deposition layer,the thermal deformation of the substrate,and the microstructure and properties of the thin-wall structure are proposed.Using the monitoring platform,theoretical model,and process signatures,focusing on the shape accuracy of the deposition layer,the response behavior of the dynamic characteristics of the molten pool when the defects or errors occur is revealed.The evolution mechanism of the defects or errors under layer-by-layer processing are analyzed by the hydrodynamics,and the methods for monitoring or predicting the shape accuracy based on the dynamic characteristics are proposed.For the thermal deformation of the substrate,according to the heat transfer and thermoelastic plasticity theories,the prediction method of thermal deformation based on the heated area is proposed.For the microstructure and mechanical property,using the temperature characteristics of the molten pool,a CET map suitable for 316L thin-wall structure is given,and a quantitative mapping relationship of molten pool characteristics,microstructure,and mechanical properties is constructed.A quantitative prediction method for the dendrite arm spacing and yield strength based on the dynamic characteristics of the molten pool was initially formed.