Additive Manufacturing Of Ti-6Al-4V Lattice Materials By Selective Electron Beam Melting:Manufacturability And Properties

Lattice materials can be defined as three-dimensional(3D)open-celled structures formed by the arrangement of a repeating typical unit cell,made up of connected struts or plates.Metal lattice materials have quickly emerged as a class of important materials by virtue of functional and engineered structure,offering low density and unique heat exchange,fluid and gas flow control,energy absorption,and damping capacities.Due to the development of 3D printing,novel intricate metal lattice materials that contain quantitatie unit cells can be quickly and controllable fabricated in a cost-effective way and patient-specific lattice implant can be realized,therefore,3D printing metal lattice materials have quickly emerged as a research focus in the biomedical area.Selective electron beam melting(SEBM)is an established metal 3D printing process and most suited to the 3D printing of reactive metals such as Ti and Ti-6A1-4V lattices‘The fundamental research of SEBM Ti-6Al-4V lattice falls far behind its widely biomedical application.This research provides a systematic study of SEBM Ti-6Al-4V lattices,including struts with little integrity,dimensional accuracy,surface roughness,internal defects,and poor ductility.The most notable results are as follows:(1)The first part of work studied the problem of struts that have little integrity.A theoretical model combines with experimental results revealed the factors affecting struts integrity,including strut diameter d,strut inclination angle θ and layer thickness h.The relationship between manufacturability coefficient,strut diameter d,strut inclination angle θ and layer thickness h was established for the first time.Besides,the critical coefficient was also obtained.Providing guideline for robust design of lattice and also the development of powder-bed-fusion based AM system.(2)A systematic study of the dimensional accuracy,surface roughness,defects and microstructure of SEBM Ti-6A1-4V lattice struts was done in the second part.A total of 165 strut samples with design diameters of 0.05-3.0 mm were manufactured at 11 different inclination angles from 0° to 90°.The discrepancies between the design and manufactured diameters and the surface roughness,3D internal defect features and microstructure of the strut samples were characterized in detail and discussed.(3)The third part of work based unit cell design on the above two parts and studied the influence of unit cell shapes on the mechanical behaviour of SEBM Ti-6Al-4V lattices.Three kinds of unit cells,including cubic,hexagonal and tetrahedron were designed at a similar porosity around 50%,and different lattices displayed disparate strength.(4)The purpose of this part is to improve the mechanical properties of SEBM Ti-6Al-4V lattice by applying post treatment(chemical etching,hot isostatic pressing,β-annealing)and elucidate the mechanism behind it.Afterβ-annealing treatment,the compressive strength and strain of SEBM Ti-6Al4V cubic lattice increased to 321.98MPa and 32.88%,respectively.Both of its strength and strain are much higher than that of human cortical bone and published data of SEBM Ti-6Al-4V lattice.In summary,the specific goals of this work have achieved therefore:(1)have addressed an effective way to avoid lattice struts that have little or no structural integrity;(2)have solved the low compressive strength and brittleness of current SEBM Ti-6Al-4V lattice;(3)have provided theoretical and practical values for further studying of SEBM Ti-6Al-4V lattice.