The Influence Of Rare Earth On The Microstructures And Properties Of The Laser Cladding Of The Ti-B-N System

The low density,high specific strength and good corrosion resistance of titanium alloy make it a high quality material for making turbine blades.However,the poor wear resistance of titanium alloy has limited its application in the field of severe friction.Laser cladding the ceramic material reinforced metal matrix composite coatings on the titanium alloy surface is one of the effective methods to improve the wear resistance of the titanium alloy.In addition,due to the special physical and chemical properties of the rare earth elements,it is conducive to promoting the grain refinement,improving the uniformity of the organization and improving the wear property of the cladding layer.At present,the relationship between the microstructures and properties of the ceramic material reinforced metal matrix composite coating with the addition of rare earth is not systematically studied.So it is meaningful to study the crystallization process of the laser cladding of Ti-B-N-Re system,and further discuss the relationship of the microstructure and wear property of the cladding layer.Firstly,the influence of the laser power and laser scanning speed on the microstructures and properties of cladding layer was studied.By using suitable laser cladding process parameters,TiB-TiN ceramic reinforced composite coatings with good metallurgy combination can be in situ synthesized on the surface of Ti-3Al-2V with low dilution rate and no obvious defects.In situ synthesized TiB is needle bar-like microstructure and TiN is equiaxed dendrite microstructure.With the increase of the laser power,the macroscopic geometry of the cladding layer increases and the dilution rate decreases.The driving force of nucleation and growth of TiN is superior to TiB,so TiN is preferred to nucleating and growing in the molten pool.The size of TiN and TiB of cladding layer decreases gradually and the microhardness of the cladding layer increases.With the increase of laser scanning speed,the dilution rate decreases,TiN and TiB size of the cladding layer decreases gradually and the microhardness of the cladding layer decreases.The effect of different RE content on the microstructural evolution,microstructural distribution,microhardness and wear property of the laser cladding coating were studied.Results indicated that the addition of appropriate CeO₂ refined the structure of the reinforced phase,enhanced the microstructural uniformity and improved the microhardness and wear property of coating.Addtionally,with the addition of 2 wt.%CeO₂,the surface hardness and the wear property of coating are optimum.And the wear mechanism of the cladding coating is abrasive wear mechanism.The addition of appropriate amount of Y can refine the reinforced phase and the organization of cladding layer.However,the excess addition of Y would instead coarsening the reinforced phase and the organization of cladding layer.With the addition of 1 wt.%Y,the surface hardness and the wear property of coating are optimum.And the wear mechanism of the cladding coating is abrasive wear mechanism.Laser cladding is a non-equilibrium rapid solidification process.When Ti:BN is 4:1,the Ti-B-N-Re system crystallization process is as follows:I)The liquid phase L is formed under the radiation of high-energy laser.II)The primary phase TiN will begin to nucleate and grow.III)The liquid phase and the primary phase TiN generate?-Ti through peritectic reaction.IV)Rare earth elements and other elements generate rare earth compounds RE_xM_y,promoting the nucleation and growth of primary phase TiB.V)TiB-?-Ti will be generated through eutectic reaction.VI)?-Ti will be transformed into?-Ti.The crystallization of TiN takes precedence over that of TiB.From the top to the bottom,TiN changes from fine equiaxed microstructure to coarse lamellar dendritic microstructure.The crystallization process of TiB is dominated by the kinetic factors.The coarse bar-like primary phase TiB mainly exists at the top and bottom of the cladding layer.There is only eutectic structure composed of TiB and?-Ti existed at the middle of the cladding layer.The microstructure of reinforced phase in the top of cladding layer is fine and dispersed,with better hardness and wear property.The amount of reinforced phase has a greater influence on the wear property than the size of reinforced phase.