Research On The Design And Mechanical Properties Of Controllable Porous Structures Based On Selective Laser Melting

As a kind of special structure and function material,porous structures widely exist in nature and industrial applications,and its special geometric,physical,mechanical and biological properties have been the hotspots in related fields.In the field of medical implants,porous structures are introduced to address "stress shielding" problems and to achieve biological immobilization of the "metal-bone" interface.Due to the irregular nature of the microstructure of the bone,the traditional uniform structural design methods are difficult to meet the special requirements.In this paper,based on the Voronoi-Tessellation theory,a controllable irregular porous structure based on "probability sphere model" was proposed.The distribution of the lattice was controlled by the probability spherical diameter so as to realize the design of the irregular and gradient porous structures.The main research contents of this paper are as follows:(1)Controllable porous structure design.Voronoi-Tessellation is a method of spatial division,which is determined by the number and spatial distribution of seed points.In this paper,we proposed a method for controlling the distribution of seed points in a probability sphere.The seed points were randomly distributed in the sphere of probability and their irregularities were controlled by the probability sphere diameter.Based on this design idea,this paper designed a controllable porous structure on the parametric design platform Grasshopper,which could achieve the controllable design with specific porosity and gradient pore structure,and meet the design requirements of the porous structure in terms of morphology and geometry.(2)Analysis of basic geometrical parameters of controllable porous structure.The basic geometric parameters of the controllable porous structure are irregularity,porosity,pore size and pore diameter.In this paper,the basic geometric parameters of the characterization method were researched,using statistical methods to analyze the relationship between geometric parameters and design parameters and their distribution.The control method and distribution of porosity gradient in gradient structure were analyzed as well.The analysis results showed that the controllable porous structure had good controllability of the basic geometric parameters.By varying the reduction factor,the number of seed points allowed for a wide range of porosities from 60% to 95%.By changing the probability sphere diameter,a controlled change of irregularities from 0 to 0.51 could be achieved.(3)Selective Laser melting process for the preparation of porous structure.Using laser selective melting technology to prepare the block structure,the density as the optimal target,the orthogonalexperiment was carried out with laser power,scanning speed and scanning distance as the optimized objects.The optimized process parameters were: P = 200 W,v = 1200 mm/s,h = 0.08 mm.Tensile test results showed that the yield strength,tensile strength and elongation of rectangular samples prepared by the optimized process were greater than the ASTM F136 standard requirements.At the same time,the simplified porous structure test model with different diameters were prepared by using the optimized technology,and the defects and microstructure of the porous structure were analyzed.Horizontal bars had significantly more internal defects than vertical bars and 45° bars,and their surface quality was significantly lower than that of vertical bars and 45° inclined bars.The microstructure of the simplified porous structure prepared by the optimization process was a typical basketweave.(4)Mechanical properties of porous structure control test.Two series of porous structures,porosity and irregularity,were prepared by the optimized process.The equivalent elastic modulus and ultimate strength of porous structure were tested by quasi-static compression test.The test results showed that the equivalent elastic modulus varied from 0.14~2.37 GPa at the same irregularity level(0.5)with the porosity ranging from 63.18% to 91.86%,and the ultimate strength varied from1.94~116.61 MPa.The porosity had a power function relationship with both the equivalent elastic modulus and the ultimate strength,which was in good agreement with the Gibson-Ashby formula.Under the same porosity level(85.5%),the variation range of irregularity was in the range of 0.11~0.5,the range of equivalent elastic modulus was 0.91~1.35 GPa,and the ultimate strength ranged from27.59~33.05 MPa.With the increase of irregularity,the equivalent elastic modulus decreased linearly.The ultimate strength decreased first,then increased,and finally tended to be stable.The compression curve of gradient porous structure had three stages of elasticity,platform and densification.Different from the uniform porous structure,the stress platform increased with the increase of strain,and the bigger the gradient of porosity was,the larger the angle of elevation would be.The elastic modulus was close to that of trabecular and porous tantalum,and the strength was greater than that of porous tantalum close to the maximum of trabecular.This could meet the medical requirements.The compression curve of gradient porous structure had three stages of elasticity,platform and densification.By contrast with human trabecular bone and porous tantalum,it is expected to be a new generation of orthopedics implant experimental samples.