Atmosphere-assisted Strengthening Of The Commercially Pure Titanium Produced By Laser-based 3D Printing:A Research On The Processing And Machaniusms

Commercially pure titanium?CP-Ti?is one of the earliest and most widely used Ti-based materials.CP-Ti is well-known for its excellent corrosion resistance and biocompatibility,but the extensive application of CP-Ti is hindered by the lack of strength and hardness.In this thesis,we designed a novel and efficient manufacturing approach by introducing the Ar-N₂ reactive atmosphere into laser-based 3D printing?selective laser melting,SLM?processing.CP-Ti reacted with the atmosphere simultaneously during the SLM processing and yielded almost full-dense?relative density?99.5%?components.CP-Ti strengthened by the reactive atmosphere exhibited outstanding comprehensive mechanical properties.Its hardness and strength have been enhanced by 40-50%or even higher compared to those of CP-Ti printed in the inert?pure Ar?atmosphere.CP-Ti's ductility remains unharmed up to a N content of⁰.43 wt.%.The optimal specimen showed yield/ultimate tensile strength of 807/1037 MPa,with a strain-to-fracture of 19.15%.The strength of this strengthened CP-Ti is close to that of wrought Ti-6Al-4V??/?dual phase?alloy,while the ductility is more than twice as Ti-6Al-4V's.Electron backscatter diffraction,atom probe tomography,transmission electron microscopy,and laser-heated in-situ synchrotron X-ray diffraction analysis were employed to reveal the mechanism underlying the in-situ reaction between CP-Ti and the reactive atmosphere.The microstructure and its evolution were systematically studied and the features were described in detail:Ti reacted with N₂ during melting,forming a Ti-N solid solution as well as compound particles such as TiN and TiN_0.3;however,the subsequent thermal cycles of SLM processing induced the decomposition of TiN_x compounds and finally transformed the material into a homogeneous solid solution except the top surface.Nitrogen generally dissolved in the??-Ti matrix and occupied the octahedral vacancy sites as interstitial solute atoms.N solute exerted solid solution strengthening and grain refinement on the as-printed CP-Ti,while the former was the major contributing factor of the total strengthening effect.A constitutive model was developed to describe the tensile deformation behavior,such as the yield point and stress-strain curve,of the CP-Ti printed in the reactive atmosphere.The model provides a rational description of the flow behavior of the as-printed CP-Ti over various solute concentrations and grain sizes.This study has achieved the integration of 3D printing and in-situ strengthening of CP-Ti,yielded a high-performance CP-Ti which is free of detrimental alloying elements?such as Al,V,Cr,etc.?.The strengthening mechanisms have been illuminated along with the control methods of microstructure and mechanical properties.This thesis has demonstrated a promising methodology for the production of nonhazardous biomedical Ti and other high-performance metallic materials.Additionally,it has also extended the fundamental understanding of the SLM process under the reactive atmosphere.