Study On Selective Laser Melting Forming Of Porous Bone Scaffolds And Process Optimization

The advent of Selective Laser Melting(SLM)technology,which has made significant medical and engineering advances in recent years,has opened up a wider scope for the development of artificial porous bone scaffolds for medical applications.In the implantation of artificial porous bone scaffolds,the mechanical properties of the scaffolds are a prerequisite for the long-term functional application of the scaffolds,while the differences in the mechanical properties of the scaffolds are caused by the structure of the scaffold itself and the parameters of the forming process.In this study,the design and optimization of the structure of porous bone scaffolds were investigated by means of finite element simulations combined with experimental methods,and the mechanism of the influence of the processing parameters of SLM technology on various aspects of the properties of porous bone scaffolds made of 316L stainless steel and post-processing stress relief were also investigated.In this study,six different porous bone structures(Cubic,Body-Bentred Cubic(BCC),Face-Centred Cubic(FCC),and three optimized structures based on them)and 18 different SLM machining parameters were designed to investigate more comprehensively the effects of structure and machining parameters on the organizational structure and performance of porous bone scaffolds.The experiments mainly used static mechanical finite element numerical simulations and experiments,hydrodynamic simulations,SLM forming numerical simulations and experiments,corrosion resistance experiments,and heat treatment stress relief simulations and experiments to summarize the mechanism of the influence of structural and process parameters on the mechanical properties of porous bone scaffolds,as well as to discuss and summarize the residual stress,dendrite spacing,grain orientation and transient melt pool stress fields in bone scaffolds caused by different processing parameters.The results of the study showed that the mechanical properties of the conventional structure were significantly improved after optimization,and it was also found that the magnitude of residual stress and the distribution of defects in the scaffolds under different processing parameters had a strong influence on the organization and performance of the scaffolds.The conventional/optimized structures formed at an energy density of 41.7 J/mm~3 have significantly lower residual stress and less surface spherification and melt pool surface cracking.In addition,as the processing energy density rises,when the energy density is≥100.0 J/mm~3 not only does the residual stress in the scaffolds rise sharply,but there is also a large amount of spheroidisation on the surface of the scaffolds and the residual deformation is more serious.The study of the microstructure of the scaffolds shows that the tissue arrangement of the scaffolds,the use of too low or too high energy densities for forming the scaffolds results in coarse grains,disorganized and unevenly shaped tissue in the melt pool,which leads to a reduction in the corrosion resistance of the scaffolds and a poor performance in terms of mechanical properties.For the heat treatment of porous bone scaffolds to relieve stress,after heat treatment at 800℃/1050℃,the residual stress inside the scaffolds can be removed significantly,and the maximum stress relief effect can be close to about 90%.It is found that as the heat treatment temperature rises,it leads to a gradual and fundamental change in the morphology and arrangement of the grains inside the melt pool.The grains inside the melt pool gradually increase in size,and the original equiaxed grains gradually evolve into large column/block-shaped grains with the increase of heat treatment temperature,the boundary of the fish scale pattern melting pool disappears,the original fine cracks were eliminated during the grain growth process and the grain was arranged more closely.The mechanical properties of the porous bone scaffolds were changed to varying degrees at this time,with the hardness of the scaffolds decreasing as the heat treatment temperature increased,and the corrosion resistance of all scaffolds decreased.Finally,it was found that the use of finite element simulations plays an important role in the SLM forming of bone scaffolds.Finite element calculations can play a role in predicting and optimizing the construction process,post-build residual stresses,and heat treatment for stress relief,thus providing a theoretical and technical basis for the study of SLM.