Microstructure Evolution And High-temperature Softening Resistance Of High-entropy Alloy Coating With High-melting Point Fabricated By Laser Cladding

In order to prepare the coating possessing high hardness,high wear resistance and excellent high-temperature softening resistance and to improve the service life of the cutter under the condition of high-speed dry cutting,the high-entropy alloys?HEAs?coatings possessing high-melting point and excellent high-temperature softening resistance were fabricated on high-speed steel by laser cladding.The microstructure and properties of coatings were studied by using X-ray diffractometer,scanning electron microscopy,transmission electron microscopy,microhardness tester and abrasion tester,and so on.Firstly,the MoFeCrTiW HEAs system with high-melting point was established.Secondly,the microstructure and properties of MoFeCrTiWSi_xAl_y?x=0,1 and y=0,1?HEAs coatings were researched.On this basis,the influence of Nb on the microstructure and properties of MoFeCrTiWAlNb_x?x=1,3,5,7;x is mole ratio?HEAs coatings and the evolution rule of microstructure and properties under high temperature were investigated.Finally,the mechanism of high-temperature softening resistance was explored.The main conclusions can be drawn and listed as follows:?1?The research results show that,the phases of MoFeCrTiW HEAs coating are composed of body-centered cubic?BCC?solide solution phase and a little Laves phase;this is consistent with the theoretical calculation results.The fine Laves phase particles distribute uniformly on the cellular crystal matrix.The average hardness of coating is 479 HV_0.2,which is about 3 times higher than that of Q235 steel.?2?The microstructure and properties of MoFeCrTiWSi_xAl_y HEAs coatings were researched.The results show that,adding equiatomic Si can promote the formation of intermetallic compounds and the hardness and wear resistance of the coatings are improved.The average hardness of MoFeCrTiWSi HEAs coating is 713HV_0.2,but the hardness distribution is not uniform.Adding equiatomic Al can improve the formation of BCC phase and inhibit intermetallic compounds forming and the microstructure of coating can be fined,but the hardness and wear resistance are all decreased,the hardness of MoFeCrTiWAl coating is only 345 HV_0.2.Aftre adding Si and Al,the oxidation resistance of coating at 800?is improved,and the effect of Al is higher than that of Si.?3?The microstructure and properties of MoFeCrTiWAlNb_x?x=1,3,5,7? high-melting point HEAs coatings?Nb_x coating for short?were researched.The results show that,when x is 1,the coating is hypoeutectic structure,the phases of coating are BCC solid solution,?Nb,Ti?C and Fe₃W₃C;when x is more than 3,the phases of coatings are BCC solid solution,?Nb,Ti?C and Laves phase.Nb3 coating is eutectic structure,Nb5 and Nb7 coatings are hypereutectic structure.With the addition of Nb increasing,the average hardness of coatings will be significantly raised from 345 HV_0.2 without Nb to 885 HV_0.2 and the wear resistance is also gradually improved.However,the longitudinal cracks perpendicular to the coating surface can be observed easily as x?5.?4?The research results show that,after being annealed at 750⁹50?for 4hours,the hardness of MoFeCrTiWAlNb HEAs coating?Nb1 coating for short?is declined.Especially when the annealing temperature is more than 850?,the microstructures of coating is coarsened obviously and the hardness is reduced significantly to 459 HV_0.2 at 850?and to 397 HV_0.2 at 950?.As being annealed at950?for 4¹0 hours,with the annealing time prolonging,the hardnesses of coatings are all raised.After being annealed for 8 hours,the hardness of Nb1 coating markedly be raised to 613 HV_0.2 and then declined to 569 HV_0.2 when annealing time is prolonged to 10 h.The wear resistance of Nb1 coating annealed at 950?for 8 hours is comparable to that of the cladded coating.?5?After being annealed at 850?for 4 hours,the lamellar eutectic Laves phase of MoFeCrTiWAlNb₃ HEAs coating?Nb3 coating for short?is changed to short rob and the number of Laves phase is reduced.The average hardness of coating is declined to 512 HV_0.2 and 432 HV_0.2.2 after being annealed at 850?and 950?,respectively.After being annealed at 950?for 8 hours,the hardness of Nb3 coating is markedly raised to 639 HV_0.2 and then declined to 615 HV_0.2 when the annealing time is prolonged to 10 h.The Nb3 coating annealed at 950?for 10 hours can obtain the highest wear resistance.When the annealing time is 4 h,6 h,8 h and 10 h,there are fine secondary phase particles of?phase and Laves phase precipitated from the matrix.The average size of particles is 0.408?m,0.554?m,0.659?m,0.477?m and the volume percentage of them is 3.99%,7.36%,12.37%,8.63%,respectively.?6?The research results show that,under the condition of annealing at 950?for4¹0 hours,with the annealing time prolonging,the low-melting point eutectic Laves phase is dissolved into the matrix of MoFeCrTiWAlNb3 HEAs coating,which leads to the lattice expansion and the matrix being strengthened.There are many fine secondary phase particles of?phase and Laves phase precipitated from the matrix when the coating is annealed at 950?for 8¹0 h,which leads to the dispersion strengthening be improved and the hardness and wear resistance of Nb3 coating are all increased.Therefore,the mechanism of high-temperature softening resistance of Nb3 coating is the comprehensive impact of dispersion strengthening and solid solution strengthening.