Study On The Microstructure And Mechanical Properties Of CrMnFeCoNi High-entropy Alloy Fabricated By Laser Additive Manufacturing Technology

CrMnFeCoNi high-entropy alloy（HEA）has become one of the most impressive structural materials due to its fascinating cryogenic mechanical properties.However,till now most researches on its microstructure and mechanical properties are carried on the bulks fabricated by arc melting and drop casting.It would be a good choice to fabricated CrMnFeCoNi HEA by laser additive manufacturing（LAM）processs,which will not only broaden its research fields but also promote its application development.In this paper,a well-formed equimolar CrMnFeCoNi HEA coating is prepared by coaxial powder feeding LAM technology on Q235 steel.The manufacturing technology,microstructure,composition and mechanical properties are studied in detail.The LAM processing parameters have great influence on the forming and defects of the deposited coating.It’s useful to improve the forming,eliminate the crack and mend the pores by optimizing processing parameters.Severe composition segregation is found in the coating fabricated using ball-milling powders and the Mn content is much lower than the theoretical value because of the high activity and low melting point.Besides,the coating fabricated by powder preset technology is suffering from the pores.So the coaxial powder feeding LAM technology is regard as the final manufacturing technology and the optimal LAM parameters（laser power:1.6¹.8 kW,scanning speed:2 mm/s,powder feeding rate:10 g/min）were determined after a series of preliminary experiments.The alloying elements are homogeneously distributed and a FCC single solution phase is formed in the coating.Slight elements differences exist between dendrite and interdendrite with Mn/Ni gathering in the interdendrite.The microstructures from bottom to the top in the coating are plane,cellular and columnar dendritic,equiaxed dendritic crystals.The columnar dendritic is taking up most of the area and the cooling rate herein is about 1000¹500℃/s.The microhardness of the coating is around 180²00 HV_0.5,which is not affected by processing parameters.The tensile properties have strong temperature dependency.The yield strength and ultimate tensile strength increase by⁶0%and⁶5%to 564 MPa and 891 MPa respectively,while ductility increases from 0.26 to 0.36 with a decrease from 298 K to 77 K.Tiny and dense dimples are observed in the ambient/cryogenic fracture,identified as the feature of the ductile fracture mode.Large initial dislocation density of the coating(¹0¹⁰ cm^-2)is introduced by the laser processing and the dislocation density increases dramatically with the increased strain.Therefore,dislocation motion is the dominant deformation mechanism while deformation twinning is a large addition at large strain levels under cryogenic condition.These two mechanisms and their interaction produce the excellent cryogenic tensile properties of the HEA coating.Brittle-ductile transition is not found in cryogenic temperature,while the impact toughness is almost a constant as 100 J·cm^-2.The wear mechanism of the coating is micro-cutting and the friction coefficient is about 0.4 under test condition of50 N load and 200 rpm rotation speed.