Microstructure And Properties Of CoCrFeNi-based High Entropy Alloy Coatings Prepared By Laser Cladding

With promising high temperature oxidation resistance,wear resistance,and corrosion resistance,high entropy alloys can be adapted to extreme working conditions,and show great potential in application.In this work,we aim to study the laser process parameters to prepare well formed CoCrFeNi,Al_0.5CoCrFeNi,and Al_0.5CoCrFeNiMo cladding layers,so as to achieve surface modification of H13 steel molds,and improve its oxidation resistance and wear resistance,and then effectively extend the service life of the molds.For the CoCrFeNi cladding layer,the optimal parameters,laser powers of 1800 W and2000 W,scanning speed of 3 mm/s,and 50%overlap rate were used to prepare the CoCrFeNi cladding layer.During the oxidization at high temperatures from 700℃to 1000℃,the thickness of the oxide layer on the surface of CoCrFeNi coating is less than 20μm,which is much smaller than the oxide layer thickness of H13 steel（98.6～904μm）.During cyclic oxidation at 600℃,the CoCrFeNi coating forms a thinner oxide layer after undergoing the fifth cycle（3.9μm）.As for H13 steel,a loose oxide layer was formed（14.4μm）after the first cycle.This indicates that laser cladding of CoCrFeNi coatings can significantly improve oxidation resistance.When the laser power is 2000 W and 2200 W,the cladding speed v=2 mm/s,and the overlap rate is 50%,a well formed Al_0.5CoCrFeNi coating can be obtained.The laser cladding Al_0.5CoCrFeNi coatings show a single-phase FCC structure with a higher hardness than the substrate.During the conditions of steady-state high-temperature oxidation and 600℃cyclic oxidation,the Al_0.5CoCrFeNi coatings show better oxidation resistance than H13 steel.The addition of Al element is conducive to form a dense Al₂O₃ oxide layer on the surface of the coatings,which plays a protective role for the cladding layer.Moreover,in 3.5%NaCl solution,the corrosion resistance of the cladding layer prepared at the laser power of 2000 W is better than that of the cladding layer prepared at 2200 W.The addition of Al element can greatly improve the corrosion resistance of the alloy.The Al_0.5CoCrFeNiMo high entropy alloy cladding layer mainly consists of FCC phase,BCC phaseσ,μPhase.A maximum hardness of 556.8HV can be achieved.Above 700℃,after 60 hours of oxidation,the oxide layer of the Al_0.5CoCrFeNiMo cladding layer includes an external oxide layer rich in Fe,an intermediate oxide layer composed of white Ni rich phase and black Crrich phase,and a deep internal oxide layer rich in Crand Mo phases.After high-temperature cyclic oxidation,the substrate shows a thicker oxide layer than the cladding layer.The Al_0.5CoCrFeNiMo cladding layer has good wear resistance,low friction coefficient,and low wear rate.Overall,the hardness,wear resistance,and surface oxidation resistance of Al_0.5CoCrFeNiMo cladding layer are superior to those of the substrate.In summary,laser cladding CoCrFeNi based high entropy alloy coatings has achieved surface performance optimization of substrate H13.During high-temperature oxidation environment,the CoCrFeNi cladding layer performed better oxidation resistance than Al_0.5CoCrFeNi and Al_0.5CoCrFeNiMo high entropy alloy coatings.In the 3.5%Na Cl solution,the Al_0.5CoCrFeNi cladding layer has shown better corrosion resistance than CoCrFeNi and Al_0.5CoCrFeNiMo high entropy alloy coatings.Due to the emergence of BCC phase and Topological Close Packed phase,the Al_0.5CoCrFeNiMo high entropy alloy cladding layer shows high hardness and excellent wear resistance.