Microstructure And Properties Of Cr Reinforced Ti-Ni Matrix Composite Coatings On TC21 By Laser Cladding

Titanium and titanium alloys have low density,high tensile strength（up to 1400 MPa）,good thermal stability and good weldability,and are widely used in aerospace,marine,medical equipment,sports and automobiles.But,titanium alloys has the disadvantages of low hardness,high friction coefficient and poor wear resistance.In order to solve this problems,the surface modification technology is mostly used,laser cladding technology is the way in surface modification technology.It has the advantages of small heat input,fast cooling speed and low consumables.Etc.Laser cladding of powder materials on the surface of titanium alloy,using the special properties of the powder to improve the surface properties of titanium alloy,Ti-Ni intermetallic compound has strong toughness,wear resistance and high damping,which perfectly compensates for the shortcomings of titanium alloy.In experiment,Ti and Ni powders were laser-clad on the surface of titanium alloy,and different reinforcing phases were added to mixed powders.Reasonable process parameters were selected,and single pass cladding experiments were carried out to prepare Ti-Ni based on titanium alloy surface.The cladding layer was tested for its structure and properties.The composition of the cladding layer was measured by X-ray diffractometer,optical microscope,scanning electron microscope and electron probe.The Vickers hardness tester and the friction and wear measuring instrument were used to evaluate the hardness and wear resistance of the cladding layer.Cr element is added to the laser-clad Ti-Ni group,and the cladding layer forms TiNi,Ti₂Ni,β-Ti intermetallic compounds.When the Cr content is 8%,the hardness is up to 959.64HV,which is 3 times of the substrate,and the wear resistance is 2.8 times of the substrate.The Cr element is solid-dissolved in the Ni element,and the molten pool is rapidly cooled after laser cladding,and the excess Cr remains in Ni,causing lattice distortion,solid solution strengthening,improving the hardness of the cladding layer,but the internal stress is not uniform,dislocation accumulation,affecting the wear resistance of the cladding layer.The Cr₃C₂ ceramic reinforcing phase was added into the laser cladding Ti-Ni matrix to investigate the effect of Cr₃C₂ content on the microstructure and properties of the cladding layer.The cladding layer is composed of Ti₂Ni,TiNi,β-Ti and TiC.The hardness and wear resistance are obviously improved.When the content of Cr₃C₂ is 4%,the highest hardness value is 733 HV,which is 2.3 times of the matrix.The wear resistance is 1.35 h·mm^-3,which is 3.35 times of the matrix.The in-situ TiC hard particles are dispersed in the cladding layer,and the lattice distortion in the cladding layer is lowered to improve the hardness and wear resistance of the cladding layer.Comparing the addition of Cr elemental and Cr₃C₂ ceramic reinforcing phase in Ti-Ni matrix,it was found that in-situ formation of hard particles can significantly reduce the lattice distortion caused by Cr element.It is known from the third chapter that the inclusion of 8%Cr has the best effect.On the basis of it,different content of B element is added to in-situ TiB₂,and the hardness and wear resistance of the cladding layer are explored.The cladding layer is composed of TiNi,Ti₂Ni,Ti,CrB₂ and TiB₂ hard particles.When the content of the cladding layer B is 6%,the hardness is the highest,which is 696.11 HV,and the wear resistance is1.563 h·mm^-3,which is about 4 times that of the substrate.Adding B element on the basis of Cr elemental material improves the wear resistance of the cladding layer and reduces the hardness of the cladding layer.