| As we all know,titanium and titanium alloys are widely used in the aerospace field because of their high specific strength,good corrosion resistance,and low density.The laser solid forming technology can achieve rapid and near-net forming of complex structural parts without molds,which makes the rapid development of laser solid forming titanium alloy in recent years.However,due to the unique rapid solidification and the complex thermal cycles of laser solid forming,the structure of laser solid forming samples often exhibits the columnar morphology of epitaxial growth,and the columnar grains grows preferentially in the direction of thermal diffusion,and there is obvious texture phenomenon in the<100>direction.Due to the strong anisotropy of the texture,the strength and plasticity of the specimen by laser solid forming do not match.To solve this problem,based on the excellent comprehensive mechanical properties of equiaxed grains,how to transform columnar grains with strong anisotropy into equiaxed grains has become the key.As for the existing alloy system,Ti-15Mo alloy not only can be used as structural material but also play an important role in the field of medical alloy due to its high strength and low elastic modulus,making Ti-15Mo alloy favored by more and more researchers.Mo element is one of the important elements in the composition design and structure optimization of titanium alloy.Since the Mo element and the Ti element are isomorphic elements,and their lattice structure and lattice constant are the same,the high melting point Mo element can act as a nucleation particle in the titanium alloy.Based on the nucleation mechanism,it is finally expected to be realized the equiaxation of structure by laser solid forming.Based on this,this study mainly focuses on the existing Ti-15Mo alloy,the Ti-15Mo alloy powder prepared by the element mixing method is used as the raw material.Under the same forming process parameters,the forming characteristics,microstructure formation and mechanical properties of the Ti-15Mo alloy are studied based on the laser solid forming technology.The original?grain morphology,intragranular substructure and tensile fracture of the Ti-15Mo alloy were characterized by optical microscopy(OM),scanning electron microscopy(SEM)and electron backscatter diffraction(EBSD),and the microstructure formation,tensile and compressive fracture behaviors were studied.Main research contents and corresponding results are as follows:(1)The deposition characteristics of Ti-15Mo alloy by laser solid forming under different mixing methods of Ti and Mo element powder were studied.Under the same feeding conditions,when bond-coated Ti(150?m)+Mo(3?m)and direct mixed Ti(150?m)+Mo(150?m)as raw materials,the interaction with the solid-liquid interface of the molten pool and the movement speed of the powder are almost the same,so that the deposition efficiency of the Ti-15Mo alloy by laser solid forming in the two mixing method are the same.However,when direct mixed Ti(150?m)+Mo(3?m)as raw material,the small Mo powder particles with 3?m is easy to form sticky powder on the surface of the sample during the deposition process,thus affecting the absorption rate of laser energy of the material,so the deposition efficiency of laser solid forming Ti-15Mo alloy is decreases.(2)Based on the study of the deposition characteristics of laser solid forming Ti-15Mo alloy under different element addition methods,the formation mechanism of the original?grains were discussed.The directly mixed elemental powder Ti(150?m)+Mo(3?m)as raw material,the LSFed Ti-15Mo sample is composed of epitaxial?columnar grains from the bottom to the upper region,and only a small amount of equiaxed grains exist at the top,and there are obvious cellular crystal in the original?grains.However,when direct mixed Ti(150?m)+Mo(150?m)and bond-coated Ti(150?m)+Mo(3?m)as raw materials,due to the nucleation caused by the presence of high melting point un-melted Mo powder,the microstructure of Ti-15Mo alloy are composed of near-equiaxed and equiaxed grains,and there are still cellular crystals in the grain.(3)The formation of metastable?phase in laser solid forming Ti-15Mo alloy under complex thermal cycling history conditions was studied.Based on the actual size of the formed sample and the thermophysical parameters of the material,the thermal cycle curve of the forming process was calculated by finite element simulation.The results show that under the complex thermal history,the thermal cycle accumulation is still beneficial to the formation of metastable?phase.(4)Comparing the deposition efficiency and microstructure of the LSFed Ti-15Mo alloy under different element addition methods,it can be found that the bond-coated Ti(150?m)+Mo(3?m)as the raw material can achieve Ti-15Mo alloy structure optimization.The room temperature tensile test confirmed that the elongation(?)of the LSFed Ti-15Mo alloy reaches the forging standard,and its tensile strength and yield strength are significantly higher than the forging standard,the tensile strength(?b)is 887.7MPa,and the yield strength(?0.2)can reach 731.5MPa.Meanwhile,research shows that the tensile process of{332}<113>twin mechanism formation,is the main reason for the show excellent tensile properties at room temperature. |
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