Research On Microstructure Evolution And Properties Of IN718/WC Coating Fabricated By Hybrid Ultrasonic-Electromagnetic Field Assisted Laser Cladding

To solve the problem of cracking,metallurgical defects,uneven distribution of microstructure in metal-ceramic composite coating fabricated by laser cladding,this paper proposed ultrasonic-electromagnetic field assisted laser cladding.Based on theory analytical and experimental,the effect of ultrasonic-electromagnetic field on cracking,pores and solidified characteristic of IN718/WC composite coating was reported.On the basis,the coating microstructure evolution and properties enhancement was studied.The main contents of this paper are as follows:（1）The synergy mechanism between ultrasonic and electromagnetic in the molten pool was investigated.Effect of nonlinear effects induced by ultrasonic/electromagnetic on melt flows and undercooling was analyzed,the theory model of cavitation effect intensified by Lorentz force was built.Based on the abnormal molten characteristic indued by nonlinear under ultrasonic and electromagnetic field,the effect of compound fields on melt undercooling and nucleation rate was researched.（2）The metal-ceramic composite coatings were fabricated by compound fields assisted laser cladding.The mutual influence mechanism of energy field parameters in ultrasonic electromagnetic field assisted laser cladding was explored.The orthogonal experimental method was used to optimize the multi parameter/multi variable in compound field assisted laser cladding,the influence of energy field parameters with different intensities on forming coefficient and defects was optimized,the optimal parameter combination as follows:ultrasonic amplitude 20μm,ultrasonic frequency 24k Hz,current intensity 300 A,magnetic intensity 0.2 m T.（3）The pore and cracks evolution and coating formation characteristic under different external field was revealed.The surface morphology,dilution and interface bonding feature were analyzed by super depth field microscope,SEM and laser confocal microscope.The influence of energy field on coating formation characteristic was revealed,the pore escape and cracks reduction mechanism in coating formed by laser cladding was investigated,the effect of energy field on coating morphology/performance and wear properties enhancement was clarified.The results showed that un-melt powder and the solidification of flashed melt was depressed by energy field,the coating surface roughness reduced from Ra 8.23μm to Ra 4.07μm.Hybrid energy field increases the escape velocity of pores in the molten pool,and the porosity in the coating decreased from 0.951%to 0.148%.Moreover,both the cracks distributed at the bottom of coating and cracks propagation at the overlapped zone were well restricted,due to energy field produced stirring effect to reduce the thermal gradient in the molten pool.（4）The microstructure evolution under ultrasonic and electromagnetic was explored.The microstructure refinement,phase,segregation,WC particle distribution,grain morphology and grain size were characterized by SEM,EBSD and XRD testing method.The influence of energy field on microstructure evolution and coating wear properties was revealed.Results demonstrated that the phase composition unchanged,the heat damage of WC reduced apparently,the elements segregation decreased,and the microstructure of coating refined significantly,the average grain size from 3～15μm decreased to 1～6μm under hybrid energy field.The concentration of WC was depressed,the reinforced particles distributed uniformly in the coating due to upward Lorentz force acted on the WC particles.（5）The influence of energy field on coating microhardness curve distribution and wear behavior was studied.The influence of energy field on microhardness of single track and multi tracks along the depth direction was characterized,the macro and micro morphology was measured by laser confocal microscope and SEM,the coating wear behavior and performance enhancement was clarified.Under hybrid energy field,results showed that the coating microhardness increased from 576.1 HV_0.2to 641.4HV_0.2,the microhardness curve distributed uniformly along the depth of coating with hybrid energy field.The friction coefficient reduced from 0.95 to 0.24,the wear rate reduced from 3.83±1.28×10^-6 mm³/（N·m）to 0.6±0.18×10^-6 mm³/（N·m）and the coating wear properties enhanced significantly.Results show that nonlinear effects induced by ultrasonic and electromagnetic fields promote the uniformity of temperature field distribution and undercooling in the molten pool,the formation of pores and cracks were depressed,the promote grains refinement,microstructure/WC particles distribution evenly,thus,wear resistance performance was increased.