| Titanium alloys have excellent comprehensive properties and are widely used in aerospace,energy power,marine engineering,weaponry and biomedical fields.The working conditions of modern high-end equipment put forward higher requirements on material properties,titanium alloys have wider application fields in the future.For example,they play a very important role in the field of various high-performance blades,such as aero-engines,part of medium-temperature blade substrates of gas turbine compressors,and the new titanium alloy final-stage large blades in the field of steam turbine manufacturing,but the service environment puts high demands on the wear resistance of the materials.Due to the poor wear resistance of the titanium alloy substrate,it is not possible to meet the service conditions of high-end products in some cases.Effectively improving wear resistance is an important technical means for the application of key components of titanium alloys in extreme working conditions,and has important engineering application value for exerting the advantages and expanding application fields of titanium alloys.Laser gas nitriding is one of the most flexible,economical and effective surface modification methods for large-sized anisotropic parts,it has the advantages of small deformation,short preparation period,controllable microstructure,deep nitriding,high metallurgical bonding strength between substrate and nitrided layer.This paper aims at realizing the demand for deep nitriding technology of large titanium alloy blade under the conventional production conditions,and the common problems on the surface nitriding process of large complex anisotropic parts are studied.Firstly,laser gas nitriding is applied to the surface of titanium alloy plate by diode laser,and the process characteristics are mainly studied.The influence of process parameters on the cross-sectional morphology,geometrical dimensions,surface forming characteristics of nitrided layer,surface roughness,crack,microstructure,hardness,phase and wear resistance of nitrided layer was verified.The feasibility of applying the process to such complex components is verified and an optimal process interval is obtained.However,the optical axis of the diode laser is fixed,the laser gesture cannot be changed with the curvature of the three-dimensional surface,and the curvature change is greater than the adjustment sensitivity of the its focal distance.The implementation of the surface nitriding complex anisotropic components is difficult.In order to adapt to the actual production conditions,a fiber laser nitriding system for large and complex anisotropic parts was designed,and the process of laser gas nitriding large blade surface wasrealized based on robot control.Through the above content research,the following conclusions were obtained:The results of surface nitriding of titanium alloy plate by diode laser show that:(1)When the laser gas nitriding is carried out in the pure nitrogen environment,The cross-section of the nitrided layer appears hemispherical,finger-like,multi-finger and W four typical morphology with decreasing the moving speed of the substrate.Their convection lines are obvious,the convection dominates the nitrogen transport process during laser gas nitriding;the cross-section morphology of the nitrided layer in the diluted nitrogen environment is different from that in the pure nitrogen environment,as the ratio of nitrogen to argon decreases,the convection line gradually disappears,and diffusion dominates the nitrogen transport process.However,when the ratio of nitrogen to argon is as low as 5%,the flow behavior of the molten pool is abrupt,the convection effect is enhanced and dominates nitriding paocess,and the cross-section morphology of the nitrided layer is transformed into a hemispherical morphology.In general,the ideal cross-section morphology is hemispherical regardless of the nitriding atmosphere.(2)When the laser gas nitriding is carried out in a pure nitrogen atmosphere,as the laser power increases or one of the moving speed and the focal distance decreases,the penetration depth and the melting width of the nitrided layer increase,and the forming coefficient decreases.With increasing of nitrogen flow rate,the penetration depth and melting width of the nitrided layer decrease first and then tend to constant,and the forming coefficient increases.The influence of the nozzle distance on the geometrical dimensions of the nitrided layer is not obvious,but it has a great influence on the surface forming quality.In the diluted nitrogen environment,as the ratio of nitrogen to argon decreases between 100% and 18%,the melting width of the nitrided layer is basically constant,and the penetration depth first increases and then decreases,However,when the ratio is as low as 5%,the penetration depth increases.(3)In the pure nitrogen environment,as the laser power or gas flow increases and the moving speed or focal length decreases,the surface color of the nitrided layer changes from dark to bright,but when the heat input is too large,the surface of the nitrided layer becomes silvery white;When the ratio of nitrogen to argon is reduced in the diluted environment,the surface color of the nitrided layer is gradually darkened by bright golden yellow and gradually becomes silvery white.In the pure nitrogen environment,when the heat input is large,the nitrided layer has a smooth zone and a rough zone,and the surface of the nitrided layer becomes rough when the heat input is insufficient;In the diluted nitrogen environment,the nitrided layer appears the smooth zone and the rough zone,and the ripple structure appears in the smooth zone as the ratio of nitrogen to argon decreases.However,when the ratio of nitrogen to argon is aslow as 5%,the surface of the nitrided layer becomes smooth.(4)In the pure nitrogen environment,longitudinal and transverse cracks will occur on the surface of the single nitrided layer,and longitudinal cracks will occur in the cross-section of the nitrided layer,mesh interlaced crack appear inevitably in the multi-pass overlapping nitrided layer;With the ratio of nitrogen to argon decreases in the diluted nitrogen environment,the cracks in the single and multi-pass nitrided layer reduce,when the ratio is as low as 5%,since the hardness of the nitrided layer decreases and microstructure of the nitrided layer is composed of martensite,cracks in the single and the overlapping nitrided layer can be completely eliminated.(5)Laser gas nitriding is implemented in pure and diluted nitrogen environment,the nitrided layer and the substrate achieve good metallurgical bonding,and the nitrided layer structure is composed of dendrites and acicular martensite.As the ratio of nitrogen to argon decreases,dendrites gradually decrease and the acicular martensites gradually increase.When it is reduced to 5%,the nitrided layer is composed of acicular martensite.In the pure nitrogen environment,the hardness and hardness gradient from the surface to the inside of the nitrided layer is largest.The hardness and hardness gradient decrease as the ratio of nitrogen to argon reduces.The process parameters affect the hardness of the nitrided layer by influencing the area fraction of the dendrites,dendritic morphology,distribution,size,and phase.(6)When the laser power is 1400 W,the moving speed is in the range of 3-20mm/s,the focal distance is 280-285 mm,the gas flow rate is 25-30L/min,the nozzle distance is 3-5mm,the ratio of nitrogen to argon is5%,overlapping rate is 38%,the nitrided layer with penetration depth of more than500 um,surface hardness of more than 600 HV,a good surface formation and no cracks can be obtained,which meets the technical requirements of the blade.(7)Wear resistance of the titanium alloy by laser gas nitriding is greatly improved compared with the substrate.Comparing the process characteristics and performance of nitrided layer of diode lasers and fiber lasers,the result shows that the optimal process specifications for diode laser are also applicable to fiber laser.The blade positioning fixture is designed independently for the large titanium alloy blade,and by optimizing the process,programming the robot,planning the nitriding path,surface nitriding of large titanium alloy blade samples was achieved based on robotic control using fiber laser. |
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