Study Of Reinforcement Particle Distribution Under An Electric-magnetic Composite Field In Laser Coaxial Melt Injection

Laser melt injection(LMI)is one of the most popular technologies to achieve ultra-high surface performance of materials by injecting reinforcement particles directly into the metal matrix.However,this technique still has some problems such as difficult post-machining,cracking and bottleneck of particle content,which restrict the further application of this technique in industry.Therefore,it is important to control the gradient of reinforcement particles in laser melt injection layer according to needs.The traditional LMI process usually uses a lateral nozzle to supply the powder,which results in narrow process window.It is time-consuming to adjust the complex process parameters,and the effect is limited.As a result,it is necessary to develop a new method for adjusting the distribution of reinforcement particles in LMI layer without changing the original process parameters.In this research,both a steady magnetic field and a steady electric field are coupled in the LMI process,which produces the induced Lorenz force and directional Lorenz force simultaneously.The Lorenz force,as a sort of volume force,can influence the velocity of fluid flow and the equivalent buoyancy of the reinforcement particles to control the particles' position precisely.A theoretical model is built to calculate the temperature changes during the delivering process of the particles.Then the LMI process window using the coaxial nozzle is determined based on this model and experiment results.Another 2D multi-physics simulation model is established which is coupled with the equations of the electric-magnetic composite field,heat transfer,fluid dynamics,the movement of the particles and the phase transition.The mechanisms of the heat and mass transfer in the molten pool are discussed with this model.Using the experimental results and the quantitative calculation results of the WC content in different regions of the LMI layer,the relationship between the parameters of the LMI and the gradient of the reinforcement particles is established.Finally,the graded evolution of the microstructure,the composition,the microhardness and the wear resistance of the injection layer,which are essentially induced by the gradient distribution of the WC particles,are discussed.The main research results obtained are detailed below:(1)The diameter of WC particle is the key factor for the success of laser coaxial melt injection.Using the simulation and experimental results,the detailed process window of laser coaxial melt injection is obtained when the mean diameter of WC particles is 100 ?m.(2)Both the induced Lorenz force and directional Lorenz force are formed in the LMI zone by the effect of the electric-magnetic composite field.The former is a sort of drag force,which is always opposite to the direction of the fluid flow.The latter is a sort of volume force,similar to gravity force,which changes the equivalent buoyancy acting on the particles.However,the single steady magnetic field can only provide the induced Lorenz force.(3)The distribution gradient of reinforcement particles can be reversed by changing the direction of the directional Loren force.With the effect of the electric-magnetic composite field,the WC content in the key region of LMI layer can be up to about 50%.(4)With the effect of the single steady magnetic field,the damped fluid flow and the increased equivalent viscosity in the bottom of the LMI layer are the main reasons for the concentration of the reinforcement particles in the upper region of the LMI layer.However,with the effect of the electric-magnetic composite field,the direction of the resultant electromagnetic force governs the distribution tendency of the reinforcement particles.(5)The graded distribution of the reinforcement particles is the essential cause for the graded changes of the composition,microstructure,microhardness and wear resistance of the LMI layer.When the WC particles are concentrated in the near surface,the maximum microhardness is up to HV900.The main wear mechanism of LMI layer is abrasive wear,meanwhile adhesive wear and fatigue wear are also observed in some micro zones.With the assistance of the electric-magnetic composite field,the distribution of reinforcement particles can be tailored easily with respect to different situations without changing the original LMI process parameters.This approach also breaks through the bottleneck of the tradition laser melt injection for controlling the particles' distribution and expands the application of LMI technique.