Study On Grain Growth Of Composite Coatings In Ultrasound Aided Laser Melt Injection

Ultrasound aided laser melt injection is an ideal method of preparation for highfunctional composite coatings, and it has broad application prospects in aerospace,transportation and other fields. The microstructure of coatings plays a decisive role on theservice performance. So researching on the laser melt injection coatings’ microstructure hasgreat significance.In this paper, the grain growth of composite coatings which obtained by laser meltinjection was simulated based on cellular automata (CA) method, and researched the effect ofreinforcement particles, non-uniform temperature field and the ultrasonic vibration on graingrowth of composite coatings. Based on the theory of grain growth, which combined with thegrain boundary energy and the curvature-driven mechanism, the CA models of simulation forgrain growth of single-phase and two-phase materials were established, which analyzed theinfluence of SiC reinforcement particles on the grain growth of the316L stainless steelcomposite coatings and the AZ91D magnesium alloy substrate, researched the evolutionmechanism of composite coatings’ microstructure in ultrasound aided laser melt injection, andit has been experimental verification. Get the conclusions as follow:(1) The grain growth index of316L can be obtained as0.34, with the content of SiCparticles increases, the average grain size reduces, and with the SiC particles’ size decreases,the average grain size decreases. In laser melt injection, with the depth of molten poolincreases, the temperature of molten pool increases exponentially, the average grain size ofsimulation microstructure increases. The pinning effect of reinforcement particles on the grainboundary increase with the content increase and the size of particles decrease undernon-uniform temperature field. However, while the SiC particle sizes less than10μm, thegrain refinement mechanism by grain boundary pinning becomes heterogeneous nucleation.(2) With the content of SiC increases, the rate of grain growth of AZ91D magnesiumalloy decreases, the grain size gets smaller and the microstructure is more uniform. With thesize of SiC decreases, the pinning effect on substrate’s microstructure increases, the size ofsubstrate’s grain decreases, and the microstructure is more uniform, the Mg17Al12phase ismore favorable dissolves in α-Mg matrix.(3) With ultrasound amplitude increases, the nucleation rate of coatings increases, andthe size of grain decreases, the microstructure is uniform. When the ultrasonic amplitude is10microns, the undercooling caused by ultrasonic vibration can be obtained32.23K, thenucleation rate is279times as the one which has not applied ultrasound treatment, the grainrefinement evidently. In the experiment of ultrasound aided laser melt injection SiC-316L, the larger content of SiC, the smaller grain size of coating is and the structure is more uniform.With the size of SiC decreases, the pinning effect on coatings increases. The microstructuresof316L stainless steel with ultrasonic treatment have less columnar dendrites and finer grains,the simulation results are consistent with the experimental results.