Microstructure And Properties Of Laser Cladded AlCoCrFeNiTix High-entropy Alloy Coatings On TC4

Titanium alloy is widely used in aerospace industry for its high specific strength,heat resistance and corrosion resistance.Especially,based on the core design concept that the modern fighter should be light weight and high thrust-to-weight ratio,the content of titanium alloy has become one of important indicators to measure engine advanced level.However,the low hardness and poor wear resistance limit its further application as a key component.Multi-principal high-entropy alloys have become another hotspot in the field of materials because of their excellent high-temperature wear resistance,high-temperature oxidation resistance and corrosion resistance.In this paper,the AlCoCrFeNiTix high entropy alloy coatings were prepared on Ti-6Al-4V?TC4?by laser cladding technology,and the microhardness and wear resistance were further improved by adding appropriate amount of TiB₂.The results show that the quality of HEA coatings were improved with the increase of scanning speed,and the microstructure changed from coarse dendrites to typical dendritic?DR?and interdendritic?IR?structures.The microhardness values of coatings were slightly decreased,gradually.When the laser power P=1500W and the scanning speed v=20mm/s,the HEA coating was metallurgically bonded to the substrate and no obvious defect was detected.Under the optimal parameter,the AlCoCrFeNiTix HEA coating consists of FCC+BCC two-phases structure,in which DR was FCC phase rich Al-Co-Ni-Ti,and the non-eutectic structure of IR was BCC phase rich Fe and Cr,then the eutectic structure was FCC+BCC phases.With the increase of TiB₂ content,TiB₂/AlCoCrFeNiTix composite coatings appeared Laves phase,?Ni,Co?Ti₂ and?Fe,Co?Ti intermetallic phases,which contributed to the brittleness of coatings increased.The average microhardness of AlCoCrFeNiTix HEA coating was 706.3 HV,which is 2.32 times?304.3 HV?that of the TC4 titanium alloy.The microhardness of5%,10%,15%and 20%TiB₂ composite coatings were increased by 19.3%,27%,36.4%and 43.4%,respectively,compared to the coating without TiB2.At room temperature,the wear mechanism of TC4 titanium alloy was mainly abrasive wear,and the wear mechanism of AlCoCrFeNiTix coating and 5%TiB₂+AlCoCrFeNiTix composite coating were both mainly adhesive wear.The wear resistance of two coatings was 42 and 80 times higher than that of the substrate,respectively.At 600?,the wear mechanism of TC4 titanium alloy was mainly abrasive wear and oxidative wear.The wear mechanism of AlCoCrFeNiTix coating and 5%TiB₂+AlCoCrFeNiTix composite coating were both mainly adhesive wear and oxidative wear.The high temperature wear resistances of two coatings were increased by about 12 times and 6 times,respectively,compared to the TC4 alloy.The oxidation test shows that the isothermal oxidation rate of TC4 titanium alloy was 0.918 mg·cm^-2 and the thickness of the oxide film was about 8.75?m.While,the isothermal oxidative rate of AlCoCrFeNiTix coating and 5%TiB₂+AlCoCrFeNiTix composite coating were only 0.232mg·cm^-2 and 0.22mg·cm^-2,respectively.For AlCoCrFeNiTix coating,the thickness of oxide film of DR region was about 0.95?m,the IR is about 1.80?m.Besides,for composite coating,the thickness of the oxide film was about 0.91?m.The oxide films of two coatings were mainly composed of Al₂O₃,TiO₂ and?Cr,Fe?₂O₃ oxides.The dense oxide film can prevent the diffusion of oxygen into the interior of coatings,which indicates HEA coatings exhibit excellent high-temperature oxidation resistance.