Preparation Of CoCrFeMnNiB_x High-entropy Alloy Coating By Laser Cladding And Study Of Its Microstructure And Properties

High-entropy alloy（HEA）,as a new type of alloy,has excellent mechanical properties,corrosion resistance,high temperature oxidation resistance,etc.At present,high-entropy alloy has become one of the most promising materials,but their widespread use is subject to many factors.The most important limitation is the optimization of the preparation method and the alloy composition.Currently,H13 steel is used as a hot forging die for automobile connecting rods,its service life is greatly reduced due to thermal wear,high temperature oxidation and thermal fatigue on the surface due to harsh service conditions.How to improve the wear resistance,corrosion resistance and high temperature oxidation resistance of H13 steel has become a key factor in improving its service life.Therefore,according to the design concept of high-entropy alloy,this paper uses laser cladding technology to prepare CoCrFeMnNi high-entropy alloy coating on the surface of H13 steel,and coatings with adding different content of B element（x=0,0.2,0.4,0.6,0.8 at%）.The microstructure,phase structure,mechanical properties,wear resistance and high temperature oxidation resistance of CoCrFeMnNiB_x high-entropy alloy were studied,and the following research results were obtained.The CoCrFeMnNiB_x high-entropy alloy coating has a typical dendritic structure and interdendritic structure.When x=0,the dendritic structure is a multi-principal FCC solid solution,and the interdendritic structure is a Mn-rich eutectic structure.After adding the B element,the dendritic structure still uses the multi-principal FCC solid solution as the matrix phase,and the strip-shaped Cr-rich M₂B phase（M is mainly Cr,Fe）precipitated from the interdendritic structure.And with the continuous increase of B elements,the M₂B phase structure changes from thin strip-like（Cr,Fe）₂B to thick strip-like Cr₂B phase structure.The Nanoindentation method was used to analyze the mechanical properties of the matrix phase and M₂B phase in the CoCrFeMnNi B_0.8 high-entropy alloy coating structure.It was found that the average Young’s modulus and hardness of the M₂B phase in the CoCrFeMnNi B0.8 high-entropy alloy coating structure reached 329.71 GPa and14.69 GPa,which were 1.5 times and 3.12 times that of the matrix phase,respectively.The hardness and wear resistance of Co Ce Fe Mn Ni B_x high-entropy alloy coatings increase with the increase of B content.When B is not added,the hardness of the high-entropy alloy coating is only 192.38 HV0.2,and the relative wear resistance is 122.93m/mm3;when x=0.8,the alloy hardness and relative wear resistance are respectively when B is not added 3.85 times and 2.19 times.The wear mechanism is mainly changed from oxidative wear and adhesive wear to adhesive wear.The content of B element has a certain effect on the high temperature oxidation resistance of CoCrFeMnNiB_x（x=0,0.2,0.4,0.6,0.8）high-entropy alloy coating.The high-temperature oxidation resistance of CoCrFeMnNiB_x high-entropy alloy coating is between complete oxidation resistance and non-oxidation resistance.The structure of the oxide layer is mainly composed of the outer oxide layer,the inner oxide layer and the diffusion layer.The outer oxide layer is relatively loose,which adversely affects the oxidation resistance of the alloy.The inner oxide layer is relatively dense to prevent further oxidation.The diffusion layer is mainly composed of Fe and Cr oxides in contact with the alloy matrix.The parts where the alloy matrix is in contact are mainly M₂B-containing micro-protrusions,which pin the oxide layer and resist Oxidation plays a positive role.