Investigation On Microstructure And Performance Strengthening Mechanism Of Laser Composite Remanufacturing TC4 Titanium Alloy

TC titanium alloy has been a kind of widely used titanium alloy in aeronautic and astronautic industries,particularly for critical applications such as engine fans,compressor blades and heavy load components due to its outstanding comprehensive service performances.However,these components are especially vulnerable in such extreme service environments and the damages caused by foreign object damage,machining and manufacturing become the problem as these defects promote the crack propagation and increase the risk of accidents.In order to avoid expensive integral replacement,remanufacturing process is necessary.Laser metal deposition is considered an attractive remanufacturing process,but single laser melting deposition process can not meet the requirements of components that serve in extreme working conditions.Laser metal depositon combined with surface deformation technique is a new kind of additive manufacturing process by applying surface deformation process every few layers to ease the residual stress,refine the microstructure of titanium and improve mechanical properties.The transent temperature filed of laser metal deposition and the influence of multilayer laser shcok peening on the residual stress distribution of TC4 titanium alloy depsotion was studied through numerical simulation and experimental analysis.Base on these data,laser metal deposition with interlayer laser shock peening was applied on TC4 titianium alloy plates to repair the grooves and the reisudal stress distribution,microstructure evlution and mechanical property improvements were studied.The results revealed that laser deposting with interlayer laser shock peening could effectively refine the microstructure,reduce the tensile residual stress and strengthen the desposition.The research contents and conclusions are listed as follows:(1)The finite element models of laser deposition and laser shock peening was established by ANSYS and ANSYS / LS-DYNA.Transient temperature filed during laser depositing and residual stress distribution after laser shock peening were studied,providing theoretical basis for the laser metal deposition with interlayer laser shock peening.The transient temperature field revealed that the temperature of the spot zone increased rapidly beyond melting point to form a molting pool,and then declined.Substrate and deposition could achieve metallurgical bonding by laser metal depsosition parameters employed and the remelting depth was approximately 0.3 mm.Tesnile residual stress perpendicular to the scanning direction was induced during laser deposition.Post laser shock peening treatment could transform the tensile residual stress into compressive residual stress.The average value of the compressive residual stress and the depth of the compressive residual stress layer were 435.78 MPa and 0.98 mm,respectively.(2)The effects of Multi-layer laser shock peening on the residual stress,microstructure and microhardness of the TC4 titanium alloy deposition were studied.The average surface residual stresses values after 1,2 and 3 layers of laser shock peening were 260.7 MPa,398.3 MPa and 474.8 MPa,respectively,and the compressive residual stress layer thicknesses were 0.7 mm,0.83 mm and 0.85 mm,respectively;the average surface microhardness values were 372 HV,388.5 HV and 405.4 HV,respectively,and the influence depth were 0.7 mm,0.8 mm and 0.9 mm respectively.Results revealed that with increased peening layers,the average of residual compressive stress,thickness of the compressive residual stress layer,average of the microhardness and the influence depth increased,but the improvements delinced with increased peening layers.In addition,a large number of twins and high-density dislocations were induced by multi-layer laser shock peening,refining the grain size of the surface layer of the deposition,and the average size of ? lath reduced from 10 ?m to 6 ?m.(3)Laser metal depositon with interlayer laser shock peening was used to repair the prefabricated grooves on the TC4 titanium alloy plate.Residual stress distribution,microstructure evolution and microhardness distribution of the deposition were analyzed and the grain refinement mechanism after interlayer laser shock peening was discussed.The tensile properties of the repaired plates were compared and fracture morphology was analyzed.Introduction of interlayer shock peening reduced the tensile residual stresses of deposition,and the microhardess along the depth increased from 360 HV to 403 HV.The application of laser shock peening to each adjacent layer was effective in both disturbing the epitaxial growth of prior ? grains and refining the sizes of ?/?? phases,transforming columnar structure to equiaxed structure.Dynamic recystallizaiton could be the main reason for grian refinement after interlayer laser shock peening.The ultimate tensile strength and yielding strength was retored by 98% and 97% after the combined remanufacturing process,but the ductility reduced from 11% to 6%.In addition,the fracture positions of the tensile samples were located in the substrate,and the fracture of samples with interlayer laser shock peening revealed more and denser dimples,accompanied with delamination.