Study On Microstructure And Properties Of Co-Based Coatings By Laser Cladding Coupled With Electromagnetic/Ultrasonic Compound Energy Field

Laser cladding technology has become one of the key technologies in the field of surface coating preparation due to its characteristics of rapid cooling rate,the fine microstructure,the flexible material selection and good interface bonding as well as controllable dilution rate.However,in the practical application,many problems such as micro-cracks,severe composition segregation and poor controllability of solidification structure inside the cladding layer seriously restrict the further promotion of the technology in the field of surface engineering.Aiming at the uneven distribution of microstructure and composition segregation in laser cladding layer,this paper proposes a new process for preparing coating with electromagnetic/ultrasound compound energy field.Take the cobalt-based alloy coating as research object,combination of theoretical analysis,numerical simulation and experimental research to explore the influence mechanism of energy field on convection,heat transfer and mass transfer during solidification of molten pool,and reveals the influence of energy field on microstructure evolution and composition segregation of cladding layer.On this basis,the mechanism of improvement of wear resistance and resistance to high temperature oxidation and hot corrosion of cobalt-based coatings was further studied.The main contents are as follows:(1)The influence of energy field on convection,heat and mass transfer in molten pool is systematically analyzed.Based on the theory of laser and material interaction,the Marongoni convection in the molten pool caused by both buoyancy and surface tension was studied,meanwhile the influence of convective driving force caused by temperature difference and concentration difference in molten pool on composition distribution and heat transfer mode of cladding layer was analyzed;the mechanism of the applied single electromagnetic field,single ultrasonic field and its compound energy field on the laser molten pool is discussed in depth by comprehensive consideration of the existence of molten pool and the influence of multi-parameter coupling;the metallurgical dynamics and thermal mass transfer behavior of the moving pool induced by the energy field were clarified,which provides a theoretical basis for revealing the energy field-assisted effect to improve the composition distribution,homogenize the microstructure and enhance the mechanical and high temperature performance of the cladding layer.(2)Establishment of electromagnetic/ultrasonic compound energy field experimental device and then carry out laser cladding experiment.Based on the existing IPG-2k W fiber laser processing system,combining a spatially variable dual excitation coil used to form a specific magnetic field configuration and coupled with the ultrasonic vibration device that generates longitudinal high-frequency vibration to establish an electromagnetic/ultrasonic-assisted laser cladding experimental platform.Take the aspect ratio and dilution ratio as index,the cladding layer width,height and dilution rate data obtained by orthogonal experiment were analyzed by comprehensive scoring method to obtain the optimization process parameter;then an electromagnetic/ultrasound compound energy field was introduced to carry out a series of lase cladding cobalt-based coatings experiment by single energy field-assisted and compound energy field-assisted.The effect of applied energy field strength on the molten pool and its microstructure evolution was studied.The numerical simulation method is used to study the dynamic change of the temperature field of the molten pool under different energy field strength,which provides a basis for the selection and optimization of energy field parameters.(3)Exploring the influence of energy field on the microstructure evolution and composition segregation of cladding layer.Taking single track cladding layer as the research object,the thermal process of molten pool and the formation mechanism of microstructure and morphology were studied by theoretical analysis and numerical simulation.Microstructure,phase composition,secondary dendrite arm spacing and macro/micro segregation of alloy elements Cr and C were analyzed by optical microscopy(OM),scanning electron microscopy(SEM)and XRD X-ray diffractometer;The effect of compound energy field on the undercooling required for nucleus formation and growth during solidification were explored.The solidification kinetics and grain homogenization as well as refinement mechanism of laser cladding assisted by electromagnetic/ultrasonic compound energy field were revealed.The effect of single electromagnetic field,single ultrasonic field and compound energy field on macro/micro segregation of main alloying elements were compared and analyzed.The Weibull distribution of microhardness and wear properties of cladding layer with and without energy field-assisted were discussed based on the results of the composition distribution and microstructure evolution.(4)The high temperature oxidation and hot corrosion behavior of cladding layerunder energy field-assisted were studied in depth.SEM and EDS were used to observe the oxide film morphology and element distribution after oxidation at 650?,750? and 850? for 100 hours.The oxidation kinetics model of the cladding layer was established according to the oxidation kinetic curve,and the influence of oxidation temperature and microstructure on the high temperature oxidation performance of cladding layer was analyzed.The changes of micro-morphology,phase composition and concentration gradient of anti-oxidant element Cr on the surface of coatings under the action of energy field-assisted were investigated systematically,then the mechanism of the compound energy field on the high temperature oxidation resistance of cladding layer was understood.Meanwhile the hot corrosion performance of energy field-assisted cladding layer after exposure at 750 °C for 100 hours was studied,the mechanism of thermal corrosion resistance of cladding layer under the action of energy field-assisted was clarified by combining the morphology of corrosion surface/cross section and element mapping scanning.