Influence Of Process Parameters On Microstructure And Mechanical Propertise Of CoCrFeNiTi-(WC)_x High Entropy Alloy Composite Coating Prepared By Laser Cladding

Due to its new forming method,unique phase structure and excellent properties,high entropy alloy has rapidly become one of the hot spots in material research and development at home and abroad.It was found that the hardness,corrosion resistance and wear resistance of the high entropy alloy coating could be further enhanced by adding hard particles into the high entropy alloy coating.Therefore,the paper has made WC ceramic particles as the hard phase of the high entropy alloy coating and prepared the composite coating with strong corrosion resistance and high wear resistance on the high-chromium cast iron surface based on the coaxial powder feeding laser cladding technology.In the early stage of the experiments,CoCrFeNiTi high entropy alloy coatings were prepared based on the all-factor test.The process parameters were optimized by studying the forming quality and microstructure properties of the coating.Then,CoCrFeNiTi-（WC）_x composite coatings were prepared under the optimal process parameters,and the effect of WC content on the microstructure and properties of the composite coating was emphatically investigated.The main research contents and conclusions are as follows:（1）Effect of process parameters on microstructure and properties of CoCrFeNiTi high entropy alloy coatings.The results of SEM,XRD and EDS showed that the CoCrFeNiTi coating is mainly composed of FCC phase,（Cr,Co）Ti solid solution phase and FeCr phase,in which（Cr,Co）Ti had the high content of Ti and presents obvious dendrite.It is found that the laser power mainly affects the forming quality of the coating,that is,the higher the laser power,the less the pores,cracks and unmelted particles on the surface of the coating,and the higher the density.The scanning rate determines the cladding thickness of the coating,that is,the faster the scanning rate is,the smaller the thickness of coating.Based on the experimental analysis of the microhardness of the coating,the hardness value from the substrate to the surface of the coating presents an overall upward trend.This is due to the rapid solidification of the coating made grain refine.Therefore,the closer to the surface of the coating,the greater the cooling rate,and the higher the coating hardness.When the laser power is 1800 W and the scanning rate is 6 mm/s,the thickness of coating is larger.There is no crack,no large size unmelted particles,only a few pores inside the bonding interface.And the grain structure is closely arranged.The hardness of the coating is higher and 3.3 times that of the substrate.Therefore,it is determined that the laser parameters were the best process parameters.（2）Effect of different WC content on the microstructure of CoCrFeNiTi-（WC）_x（x=0,10,20,30,40 wt.%）composite coatings.Based on characterization analysis,WC is mainly distributed in spherical or near-spherical dispersion in the composite coating.The gray-white structure layer around the WC is the result of the interface reaction between WC and metal elements,which is conducive to the stable bonding of WC and high entropy alloy.The new W₂C phase in the composite coating is the metastable phase after the high temperature decomposition of WC phase,and the（Cr,Fe,Ni,Ti）C reinforcing phase is the hard carbide phase formed by the in-situ spontaneous generation of C and metal elements,which can significantly improve the hardness of the coating.When the WC mass fraction is 20%,the hardness of coating is the highest（67.5 HRC）.In addition,when the WC content is 40 wt.%,large pores and unmelted particles appear in the coating,indicating that excessive WC can reduce the coating density.（3）Effect of different WC content on corrosion resistance and wear resistance of CoCrFeNiTi-（WC）_x composite coatings.The anodic polarization curve and impedance spectrum of the composite coatings in Na Cl corrosion solution showed that the corrosion resistance of the coatings increases first and then decreases with the increase of WC content.When the content of WC is 20 wt.%,the corrosion current density of the composite coating is the lowest,and the corrosion resistance is the best.Based on the friction and wear tests,the wear morphology,size parameters,friction coefficient and wear loss of the coating were tested,and the wear mechanism was analyzed.The results show that the main wear of the composite coating is abrasive wear.When the WC content is 20 wt.%,the friction coefficient of the composite coating is the lowest.The wear loss of the coating is the least,which is 1/3of the wear amount of the coating without WC coating,indicating that the addition of WC could significantly improve the wear resistance of the coating.