Laser Cladding MoFeCrTiWAlNb_x High-melting Point High-entropy Alloy Coatings

Different from the traditional alloys,high-entropy alloys are composed of various elements.High-melting-point elements are added to the alloy system.High-melting-point high-entropy alloys are prepared on the surface of tool steels by laser cladding technology,and it is expected to produce simple structures with high hardness and high resistance.In this paper,MoFeCrTiWAlNb_x（x=1,1.5,2,2.5,3）high-melting point high-entropy alloys were prepared on the surface of W₆Mo₅Cr₄V₂AlA tool steel using Mo,Fe,Cr,Ti,W,Al and Nb metal powders as raw materials.The influence of laser processing parameters,Nb element content and annealing temperature on the microstructure and properties of MoFeCrTiWAlNb_x coating was explored by metallographic microscope,X-ray diffractometer,microhardness tester,friction and wear tester,etc.The alloy powders were finer and more uniform after ball milling for 8 h,and the structure was irregularly granular.When the laser scanning speed is constant,if the laser power is small,the laser effect is not obvious,resulting in a lighter cladding layer and defects such as pilling and holes are likely to occur.If the laser power is too high,the smoothness of the coating surface will gradually deteriorate and the symmetry of the surface will deteriorate.When the laser power is 2.4,2.6 kW,the surface morphology of the coating is relatively smooth.The microstructure of MoFeCrTiWAlNb high entropy alloy coating is mainly composed of cytoplasmic dendrites and granular structure.With the increase of laser power,the cytoplasmic dendrites increase,but the granular structure decreases slightly.As the laser power increases,the microhardness of the coating gradually increases,but when the laser power is 2.8 kW,the microhardness decreases.When the laser power is fixed and the laser scanning speed is changed,the scanning speed is low and the cracks appear in the cladding layer.As the scanning speed increases,the laser input heat gradually decreases and the quality of the cladding layer deteriorates.The scan speed of 4 mm/s is ideal.When the scanning speed is low,the coating structure is coarse and the intergranular granular structure is less.As the scanning speed increases,the cytoplasmic dendrites and granular structure become finer and more uniform.When the scanning speed is lower,the microhardness of the coating is lower,and the hardness increases as the scanning speed increases.However,when the scanning speed was increased to 6 mm/s,the hardness decreased.The optimum process parameter in this experiment is P=2.6 kW,v=4 mm/s.Under this process parameter,the coating TEM is mainly composed of a white base phase and a black particle phase,and also has a long strip shape organization.By adjusting the amount of Nb element,the microstructure of the high-melting-point high-entropy alloy of MoFeCrTiWAlNb_x is mainly composed of white petal-like dendrite and black-gray cell crystals.With the increase of Nb content,the white petal-like dendrite gradually becomes fine,and gradually disappears,black-gray cell crystals gradually grow into a polygonal shape.The coating consists of a pre-eutectic MC phase,a eutectic structure（BCC phase and（MC）_e phase）and a small amount of Laves phase.With the increase of Nb content,the amount of pre-eutectic carbides in the high-melting high-entropy alloy coating of MoFeCrTiWAlNb_x decreases and its size increases,and it gradually evolves into an irregular granular shape.The volume fraction of the eutectic structure gradually increases,and the morphology of the microstructure changes from irregular small particles and rod-like dendrites to large particles and dendritic networks.The BCC phase gradually increases and the（MC）_e gradually decreases.With the increase of Nb content,the hardness of the coating gradually decreased,and the wear resistance gradually increased.The coating mainly suffered from composite wear with wear and abrasive wear.The MoFeCrTiWAlNb₃ coating also experienced fatigue wear.After annealing at different temperatures,the main phase of the MoFeCrTiWAlNb high-melting-point high-entropy alloy coating is BCC phase,a small amount of MC and Laves phase.The coating is mainly composed of irregular granular structures and rod-like dendrites.As the annealing temperature increases,the high-melting elements accumulate in the petaloid structure and the dendrite grows up.After annealing treatment,the hardness of the coating first rose and then decreased,showing good resistance to high temperature softening at 950°C.The coating wear mechanism is mainly based on adhesive wear and abrasive wear.When the annealing temperature is 950°C,the coating wear is the lowest,the friction coefficient is the lowest,and the wear resistance is the best.