||In recent years, additive manufacture (AM), or called as rapid prototyping or 3D printing, has become a flourishing industry suitable to fabricate bio-implants, due to the benefits such as the reduction in process steps, complexity of parts, and customization. Among of them, the application of human bone has been most widely studied. However, the bone is a porous structure, subjected to wide variations as a result of human movement and inevitable changes with age, sex, and location. Therefore, it is important to study the relationship between the porous structure and mechanical properties of various bionic implants.|
In this study, we used the selective laser melting (SLM) and electron beam melting (EBM) methods to produce porous structures of bone bio-implants. The highly biocompatible Ti-6Al-4V alloy was applied as experimental.
For the first part, using Ti-6Al-4V alloy powders, the Ti-6Al-4V porous samples were fabricated by SLM, with the help from computer-aided design (CAD) for different porosities. Compared with the CAD models and porous samples fabricated by SLM, the relevant relationships are characterized with morphology, physical properties and mechanical properties. The difference between the CAD model and porous SLM parts leads to the larger ligament widths and smaller pore sizes for SLM parts, due to the laser beam broadening and laser melting edge effects. Due to the higher porosity samples with a higher pore number density, this difference between the CAD model and porous SLM parts could be more obvious, so that the designed porosity will be greater than that of the actual porosity of porous samples. The difference can be reduced by decreasing the size of laser beam and the used powders. The structure of Ti-6Al-4V prepared by SLM was seen to possess higher hardness favorable for wear resistance and beneficial for the application of human bone implant. The mechanical properties (elastic modulus and yield strength) of porous SLM parts decrease with increasing porosity, matching well with the human bone. In terms of the matched elastic modulus, it can avoid the risk of stress shielding effect. By applying the Gibson and Ashby model, the relationship between porosity and mechanical properties of SLM porous foams can be described and predicted.
According to many previous studies, the porous samples with high porosity can effectively reduce the stress shielding problem, but mechanical strength would also be reduced. For the second part, the high porosity of porous Ti-6Al-4V samples are fabricated by EBM, and the physical and mechanical properties are characterized. The results indicate that the porosity of porous parts can be as high as near 80% by increasing the ligament and pore size. The elastic modulus of such EBM porous Ti-6Al-4V structure with high porosity is found to match well with that of the human cancellous bone. However, it can obviously be seen that the higher ligament width of porous samples will enhance the endurance to fracture. Therefore, the relationship between the ligament width and work of fracture is systematically studied. According to the relation, it is concluded that when the ligament width is smaller than 401 μm, the porous structure cannot bear any strength for fracture. However, because the AM is basically a powder metallurgy technique, the porous samples would be inherent with rough surface, prone to cause stress concentration and premature fracture. Therefore, the relationship between the ratio of the ligament width and surface roughness corresponding to the fracture energy is derived in this study. From this relation, the fracture energy of porous samples with the larger ligament width will be less affected by the surface roughness. For an average ligament width of about 650 μm, the maximum acceptable surface roughness cannot be greater than 91 μm; otherwise, the porous sample can barely withstand the energy to fracture.
The experimental results are imported into the physical formulae and the relationships between the mechanical properties corresponding to the porous structure prepared by SLM and EBM are deduced. It is expected that these relationships can be used to prepare bionic implants using those processes to estimate its mechanical properties before the implants can be applied, effectively increase the value of their practical application.