Evolution And Influencing Factors Of Process Parameters On Residual Stress,Microstructure And Mechanical Properties Of Pure Titanium Specimens Manufactured By Laser Additive Manufacturing

Titanium and its alloy have low elasticity,high specific strength,and excellent biocompatibility and corrosion resistance,which have been widely used as implanting materials.Normally,the pure Ti parts prepared by traditional casting method exhibit low hardness and strength.Moreover,by the traditional casting method,the parts with complex shapes are difficult to form,and the customized production of components can not be realized.In contrast,laser additive manufacturing technology has many advantages,such as digitalization,low cost,short period,accelerating innovation and so on,which is suitable for preparation of metal parts with the fully dense,the complex structure and the high performance.However,the titanium and its alloys parts prepared by laser additive manufacturing technology may have many defects,such as cracks,holes,high residual stress and poor plasticity.In this thesis,pure Ti parts were prepared by using coaxial powder feeding laser additive manufacturing technology,and the influence of the process parameters and the subsequent annealing treatment on their residual stress,microstructure and tensile mechanical properties were investigated.The main contents and results are as follows:1.The distributions of the residual stress on the scanning surface of the pure titanium samples prepared by laser additive manufacture under the different process parameters were studied.The results show that the process parameters have the different effects on the residual stress distributions in the combination zone,the middle zone and the top zone.The process parameters of laser additive manufacturing have a signifcant effects on the temperature gradient,thermal shrinkage of molten pool,impact force of laser and deformation limitation of the substrate material at the different zones in the cladding layer,which may be reponsible for the difference of distributions of the residual stress at different zones.2.The microstructures of the as-deposited samples prepared by laser additive manufacturing method with the different parameters were observed,and the effects of the parameters on the microstructure were investigated.It is found that the microstructure contains the deformed equiaxed grains at the bottom region of the sample.In the middle region,the sample exhibits a needle-like,flaky and strip-like microstructure.While,in the top region,the sample exhibits needle-like microstruture.Too high or too low scanning speed and the powder delivery rate may lead to the formation of the cracks or holes with the different sizes in the as-deposited sample..3.The effects of the process parameters on the tensile mechanical properties of the as-deposited samples were studied.It is found that the as-deposited sample prepared at the conditions of the laser power of 900 W,the scanning speed of 480mm/min,and the powder delivery rate of 4.02 g/min exhibits a superior comprehensive mechanical properties,which yield strength,fracture strength and the elongation are 604.31 MPa and 682.47 MPa,21.79%,respectively.The sample with a needle microstructure exhibit a high strength,but poor plasticity in comparsion with that with the sheet and strip structure.4.The effects of the annealing temperature on the mechanical properties of the asdeposited alloy were investigated.The yield strength and fracture strength of the sample annealed at 550°C for 1 h are 585.15 MPa and 672.47 MPa,respectively,much higher than that of the high purity titanium prepared by traditional casting method.The anneating treatment can promote the plasticity and reduce the residual stress of the asdeposited samples.