Investigation On The Mechanical Performance Of 3D-Printed Cellular Titanium Alloy Subjected To Static And Dynamic Loadings

As a new type of lightweight structural function material,cellular titanium alloys combine the characteristics of titanium alloys and porous metallic materials.They can be widely applied in the defense industry and civil field due to their excellent properties such as high specific stiffness/strength,high melting temperature and impact resistance.As a kind of high temperature resistant protective material,cellular titanium alloys will inevitably suffer from thermal and impact loading in the service process,thus it is important to investigate their high temperature mechanical performance and dynamic failure mechanism.Limited by the traditional fabrication processes,common porous titanium alloys are with large unit cells and simple internal structures.The rapid development of 3D printing methods satisfies the design demand of complex three-dimensional structure,which provides a more convenient way for the fabrication and multi-scale optimization of porous titanium alloys.Therefore,the 3D-printeded periodic cellular Ti-6Al-4V alloy is used as the research object in this thesis.By the combination of experimental research,theoretical analysis and numerical simulations,the compressive stress-strain relation and deformation mechanism of the material with different temperatures,loading velocities and density gradients are discussed.The results can provide some theoretical basis and experimental data references for the optimal design and engineering application of additively-manufactured porous titanium alloys.The main contents of this thesis are as follows:1)Based on the different micro-deformation mechanisms and mechanical behavior of cellular materials,with aiming at the material properties including lightweight,energy absorption and isotropic,the rhombic dodecahedron structure is adopted as the unit cell.According to the stress and deformation analysis of the representative beams in the unit cell subjected to static loading by classical beam theory,the mapping relationship between the geometric parameters of cell structure and the macroscopic elastic modulus as well as the plastic yield strength of the porous material is established.The effect of aspect ratio on the modulus,yield strength and buckling of the porous material is discussed,meanwhile,the biaxial yield surface is plotted.Afterwards,finite element analysis is conducted to verify the model predicted results,which match well with each other.2)Periodic porous Ti-6Al-4V alloy specimens with rhombic dodecahedron cells are fabricated via electron beam melting(EBM)method.By using the WDW-300 electronic universal testing machine,the quasi-static compressive properties of cellular titanium alloy with the temperature ranging from 25 ? to 600 ? are investigated.The effect of temperature on the stress-strain curve,deformation mode and energy absorption capacity of the material is revealed.Meanwhile,the superiority of EBM fabricated periodic cellular Ti-6Al-4V in the load bearing and energy absorption capacities at high temperature is validated by comparing with other common porous metallic materials.Based on the strain energy density theory,the yield criterion of periodic cellular Ti-6Al-4V at different temperatures is established.Combining with numerical simulation,the influence of temperature on the yield surface of rhombic dodecahedron cellular Ti-6Al-4V alloy is investigated.3)Dynamic compressive behavior of EBM fabricated cellular Ti-6Al-4V with rhombic dodecahedron unit cells is investigated at ambient temperature by using Split Hopkinson pressure bar(SHPB)apparatus,with the strain rate ranging from 700/s to 1300/s.The dynamic failure mechanism of cellular titanium alloy is determined by adopting high-speed photography.The effect of impact speed on the deformation mode of periodic cellular Ti-6Al-4V is analyzed by the one-dimensional shock wave model.Meanwhile,the critical velocities corresponding to different deformation modes are calculated.According to the X-ray tomography based 3D reconstruction method,a three-dimensional mesoscopic finite element model which takes the micro-defects on the struts into consideration is built.Afterwards,the dynamic response of the 3D-printed cellular titanium alloy is numerical simulated,which matches well with the shock wave model predicted results.4)By controlling the cell size along the loading direction,two density distribution modes including step-wise gradient and continuous gradient are designed.The corresponding graded periodic cellular Ti-6Al-4V specimens are manufactured by selective laser melting(SLM)method.The quasi-static,low speed impact and high speed impact experiments on the graded cellular material are conducted by electronic universal testing machine,SHPB and direct Hopkinson pressure bar(DHPB)system respectively.By adopting the digital image correlation(DIC)method,the effects of gradient and loading velocity on the deformation evolution of graded cellular Ti-6Al-4V are revealed.Afterwards,the defending abilities of the two graded materials under high speed collision are compared and analyzed,which demonstrates that the continuous graded cellular Ti-6Al-4V with negative gradient can provide better protection for the object behind.