Microstructure And High Temperature Oxidation Resistance Of Laser Cladding AlMoNbV-based High-entropy Alloy Coating

Due to its low density,high specific strength and biocompatibility,titanium alloys have applications in many fields such as aerospace,automotive,biomedical,etc.,but when the working temperature of titanium alloy exceeds 650°C,the surface of titanium alloy will be heavily oxidized,and the creep resistance and high-temperature oxidation resistance of titanium alloys will drop sharply,resulting in parts failure and loss of use value.Preparation of a high-temperature anti-oxidation coating on the surface of titanium alloy is the key to solve this problem.In view of the shortcomings of high temperature oxidation resistance of TC4titanium alloy,combined with the theoretical criteria of high-entropy alloy formation,AlMoNbV,AlMoNbTiV and AlCrMoNbV high entropy alloys were developed and designed.By means of laser cladding,AlMoNbV,AlMoNbTiV and AlCrMoNbV high entropy alloy coatings were prepared on the substrate surface of TC4.The optimal cladding process parameters were determined by observing the macroscopic morphology of the coating surface.Then the phase composition and microstructure of the coating and the microhardness was tested.Finally,samples of the three coating types and the TC4 matrix were put into the tube furnace for high-temperature oxidation experiment.The oxidation resistance mechanism of the coating was further analyzed by analyzing the structure and composition of the surface and cross section of the oxide layer.The results of laser cladding experiments show that the optimum process parameters for the preparation of high-entropy alloy coatings on TC4 matrix are:laser power P=3.7 kW,scanning speed V=600 mm?min^-1.AlMoNbV?AlMoNbTiV and AlCrMoNbV,high-entropy alloy coatings,have formed a good combination with the substrate,and the coating is mainly composed of the cladding area,bonding area and heat affected area.AlMoNbV coating is mainly composed of BCC and FCC phases.AlMoNbTiV coating phase structure of cladding layer is mainly composed of BCC phase and a small amount of TiAl intermetallic compounds,To some extent,the addition of Ti inhibited the formation of FCC phase and promoted the formation of BCC phase.Moreover,the addition of Ti element makes the coating structure more compact,but Ti element is more active and easy to react with Al element,forming dendritic intermetallic compounds.The structure of AlCrMoNbV coating phase is simple and only consists of BCC phase.The addition of Cr inhibits the formation of FCC phase,and makes coating structure fine and compact.The microhardness diagrams of AlMoNbV,AlMoNbTiV and AlCrMoNbV high-entropy alloy coatings are presented in a step shape,corresponding to the cladd ing area,heat affected area and substrate of the coating cross section,Compared with the substrate,the hardness of AlMoNbV?AlMoNbTiV and AlCrMoNbV high-entropy alloy coatings are 2.06?2.12and 2.27 times of the substrate,respectively.The addition of C r and Ti elements increases the hardness of the coating.Circulating oxidation experiment showed that The oxidation resistance of AlMoNbV coating at high temperature was almost not improved,AlMoNbTiV and AlCrMoNbV coating of high temperature oxidation resistance increased 9.4 times,4.89 times,respectively.However,Cracks appeared in the oxide layer of AlCrMoNbV coating.With the increase of time,the oxidation corrosion of the oxide layer would be accelerated.AlMoNbTiV has the best high temperature oxidation resistance,mainly due to the formation of two oxide layers on the surface of the coating,which has a double protective effect.The outermost component is TiO ₂,the secondary outer layer is Al₂O₃,Which has a certain thickness and a dense structure,and can effectively block the diffusion of oxygen atoms,Moreover,the oxidation layer of the sub-outer layer Al plays the role of a transition layer,which can not only connect the oxidation layer of the outermost layer to generate a very thin Ti,but also adhere to the coating surface tightly.So the layer and the layer are well bonded,and it is not easy to fall off.Under the double protection of the two oxide layers,the oxidation rate of the coating is greatl y reduced,and the high temperature oxidation resistance of the coating is enhanced.