| Laser oriented energy deposition(Laser Directed Energy Deposition,LDED)technology is focused laser with pre-programming mode as the scanning path,and metal powder is continuously guided to the molten pool through nozzles to melt and accumulate layer by layer to make parts.It has great potential in manufacturing aerospace large complex titanium alloy structures or repair.In view of the high oxidation tendency in the on-site manufacturing / repair process of large TC4 titanium alloy parts,it will cause internal embrittlement,which will affect the forming quality of the parts.The microstructure and properties of TC4 titanium alloy fabricated by laser deposition in different oxygen environment were studied.The effect of oxygen content on the oxidation degree of single-pass and multi-layer surface of TC4 titanium alloy fabricated by laser deposition was studied,and the anisotropy of oxygen content on the microstructure and properties of TC4 block fabricated by laser deposition was analyzed.The main research contents and conclusions are as follows:(1)In order to solve the problem that process parameters affect the forming quality,the oxidation behavior of TC4 titanium alloy deposited by laser directional deposition under inert gas was studied,and the effect of color difference on the oxidation degree of cladding layer was investigated.The mathematical model of the relationship between sample surface oxidation degree and process parameters was established,and more process windows were provided for titanium alloy forming.The results show that the value of oxygen concentration in the environment has the greatest influence on the degree of oxidation.Reducing the laser power and reducing the intake of oxygen concentration is beneficial to reduce the degree of oxidation.(2)In view of the directional characteristics of laser directional deposition microstructure growth,the microstructure evolution law of laser directional deposition TC4 titanium alloy in different directions was studied.The experimental results show that with the increase of oxygen content,the surface color of the deposited sample gradually appears dark green from silver to white,and the degree of oxidation is obvious;at the same time,the thickness and aspect ratio of the deposited microstructure increase,and the microstructure is fine and fine.the width of the lath decreases from 3.07 μ m to 1.36 μ m and the size of the lath decreases.After heat treatment,the secondary α and β phases with large volume fraction of α appear,and the microstructure of both deposited and annealed state shows typical net basket structure.at the same oxygen content,the α lamellar of annealed state is slightly thicker than that of deposited state.(3)Aiming at the characteristics of the strong plasticity of laser directional deposition of titanium alloys under different process atmospheres.The effect of different oxygen content on the mechanical properties of titanium alloy in different states was studied.The room temperature tension was tested,the fracture morphology was observed and the fracture failure mechanism was determined.The results show that the yield strength and tensile strength of the alloy increase gradually with the increase of oxygen content,and the strength of the material below 60 ppm reaches 904 MPa.When the oxygen content is 0.1wt%,the strength increases 115 MPa,but the elongation decreases,the value reaches 8.71%,and the section shrinkage decreases to 6.6%.The fracture failure mechanism changes from ductile fracture to quasi-cleavage and ductile fracture.The hardness of both annealed and deposited samples increases gradually,and the microhardness of deposited samples is higher than that of annealed samples,and both of them increase with the increase of oxygen content.With the increase of oxygen concentration,the wear volume decreases gradually,and the wear reduction effect of the sample with high oxygen concentration is more significant. |
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