Study Of The Compression And Fatigue Properties Of Ti-15Mo Gradient Porous Structures Fabricated By Selective Laser Melting

Ti-15Mo is a β-type titanium alloy with high strength,excellent ductility,superior corrosion resistance,and good biocompatibility,which has great promise for application in the field of bone implant medicine.The porous structure can effectively reduce the elastic modulus and solve the problem of "stress shielding",while the gradient porous structure can well balance the relationship between mechanical properties and porosity.In this thesis,the mechanical properties and deformation behavior of quasi-static compression and compression-compression fatigue of Ti-15Mo gradient porous structures prepared by selected area laser melting were investigated,and the main conclusions are as follows:(1)When compressed perpendicular to the density gradient,the average porosity of each horizontal layer of the gradient porous structure is the same,which results in a uniform distribution of stress in the vertical direction,similar to that of the uniform structure.Therefore,the vertical compression deformation mode of the gradient structure is similar to that of the uniform structure" which is an overall homogeneous deformation,and the compressive mechanical properties of both are similar.And when compressed parallel to the density gradient,the gradient porous structure is deformed in a layer-by-layer deformation from high porosity to low porosity,and the stress plateau fluctuates,with the high porosity region of the gradient structure bearing the major deformation,which leads to lower mechanical properties than the uniform structure.The compressive mechanical response of the Kelvin porous structure dominated by tensile deformation would be higher than that of the BCC porous structure dominated by bending deformation,and its elastic modulus,yield strength and plateau stress in compression perpendicular to(parallel to)the density gradient were 1157.9 MPa(893.6 MPa),37.576 MPa(23.05 MPa),53.73 MPa(49.02 MPa),which are comparable to the mechanical properties of human cancellous bone.(2)The Kelvin-Voight model has a better prediction of elastic modulus,yield strength,and platform stress perpendicular to the gradient porous structure compression,and its error range is 3.4%～16%,4.6%～19.9%,and 4.3%～7.8%,respectively;the prediction of elastic modulus and platform stress parallel to the gradient structure based on the mixing law is more accurate.and its error range is 8.7%～8.3%and 1.9%～10.9%.respectively.(3)For BCC porous structures,the higher the porosity,the lower the fatigue strength,and the higher the stress amplitude and maximum stress,the lower the fatigue life.For gradient porous structure,the fatigue strength of load perpendicular to the density gradient will be slightly higher than that of uniform structure,and the fatigue strength of load parallel to the density gradient will be lower than that of uniform structure.Basquin equation can be well fitted to the S-N curve.(4)In fatigue experiments,the cyclic cumulative strain rate of the load parallel to the gradient structure will be higher than that of the load perpendicular to the gradient structure and the uniform structure,which results in a lower fatigue resistance.Fatigue damage begins with damage to the strut later in the fatigue process,while the cyclic ratchet effect occurs throughout the fatigue process.Fatigue damage is attributed to the growth of cyclic ratchets and fatigue damage strain,and cyclic ratchets promote crack sprouting and expansion and play a dominant role.Crack sources are often found on the rough surfaces of the struts.internal holes and unmelted powder.(5)BCC structures fracture diagonally at 45° under cyclic compression loading,and fatigue fractures tend to occur at the nodes of the struts.When the load is parallel to the density gradient.the gradient porous structure has low fatigue resistance at the top with high porosity and high fatigue resistance at the bottom with low porosity.resulting in its diagonal shear damage only at the top.And when the load is perpendicular to the density gradient,the stress concentration in the low porosity region of the gradient porous structure is obvious,and the cracks gradually initiate and expand from the low porosity region to the high porosity region with the increase of fatigue cycles.