Research On Laser Cladding High Entropy Alloy Coating On TC4 Surface

Although titanium alloy plays a pivotal role in many fields especially aerospace and biomedical,its poor wear resistance also limits its development to a certain extent.Therefore,it is siginificant to improve the surface properties for expanding the scope of application of titanium alloy and promoting its development.The high-entropy alloy（HEAs）,as a new star in the material area,has received extensive attention due to its excellent performance.Therefore,the preparation of high-entropy alloy coating on Ti-6Al-4V（TC4）surface to improve its wear resistance is not only of guiding significance for the application of titanium alloy,but also has certain reference significance for the research of high-entropy alloy.In this paper,the CoCrFeNi₂V_0.5Ti_x series high-entropy alloy is designed,and the optimal laser cladding parameters are explored.Then,the microstructure and mechanical properties of the coating were characterized by material testing techniques,such as X-ray diffraction,scanning electron microscope,laser confocal microscope,hardness tester,and the wear resistance of the coating was characterized by two different friction pairs,Si3N4and GCr15.All of the CoCrFeNi₂V_0.5Ti_x high entropy alloy coatings consisted of BCC solid solution phase,（Co,Ni）Ti₂ phase andα-Ti phase.With the increase of Ti content,the volume fraction of the（Co,Ni）₂Ti phase gradually increased.The microhardness of the HEA coating is much higher than that of the TC4 matrix,2.6-3.0 times that of the substrate,which is the result of the combined effect of the three strengthening mechanisms of fine grain strengthening,solid solution strengthening and second phase strengthening.When 0<x<1.0,the microhardness of the coatings fluctuated around 960 HV with the increase of x,while when x increased to 1.25,the coating hardness decreased significantly to about 830 HV.Two different friction pairs,Si₃N₄ and GCr15,were used to characterize the wear resistance of the CoCrFeNi₂V_0.5Ti_x coatings.The results showed that the wear resistance of the HEA coatings has been greatly improved compared with the TC4 substrate.When using Si₃N₄,the wear rate decreased first and then increased with the increase of Ti content.Among them,CoCrFeNi₂V_0.5Ti_0.75 had the lowest wear rate,indicating that it exhibited the best wear resistance with 4.426×10^-5 mm³/（N·m）.The wear mechanism of CoCrFeNi₂V_0.5Ti_x high-entropy alloy coatings was mainly adhesive wear and oxidative wear.Ti0.5 and Ti1.25 coatings were selected as representatives,and GCr15 was used as the friction pair to characterize the wear resistance of them and the TC4 substrate.When using GCr15,Ti0.5 shows the best wear resistance,and its wear rate is 2.144×10^-5mm³/（N·m）.In the processing of dry friction,a protective film was formed by GCr15abrasive debris on the surface of the coatings to improve the wear resistance of the coatings.With the increase of Mo content,the phase structure of CoCrFeNiMo_x high-entropy alloy coatings changed from three-phase structure of BCC phase,（Co,Ti）₂ phase andα-Ti phase（x<0.4）to four-phase structure of BCC phase,（Co,Ti）₂ phase,α-Ti phase andσphase（0.5<x<1.0）.The coating exhibits a typical dendrite morphology,and with the increase of x,the BCC phase andσphase increase significantly and become a coarse massive structure.This indicated that Mo is an element that promoted the formation ofσphase andthe solid solubility of Mo element in the BCC phase is between 0.4 and 0.5 in the CrCoFeNi₂Mo_x high entropy alloys.The hardness of CrCoFeNi₂Mo_x high-entropy alloy coating is about2.45².65 times of TC4 substrate.With the increasing of Mo content,the hardness of the coating increased first and then decreased.Among them,Mo0.5 exhibited the highest hardness,about 850 HV.