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ReviewA design for the additive manufacture of functionally graded porous structures with tailored mechanical properties for biomedical applicationsJayanthi Parthasarathy, Binil Starly, Shivakumar Raman

Journal of Manufacturing Processes 13 (2011) 160170Contents:1. Introduction1.1. Additive manufacturing of metals.1.2. Necessity for innovative design for bone replacement.1.3. Periodic cellular structures and design of internal architecture.2. Methodology2.1. FEA prediction of effective mechanical properties.2.2. Input CAD design.2.3. Additive manufacturing electron beam melting processing of Ti6Al4V.2Contents:2.4. Evaluation of structural and mechanical properties.2.5. Biomechanical evaluation of patient-specific implants.3. Results and discussion3.1. Predictability of structural properties.3.2. Estimation of strength.3.3. Performance of mandible and hip implants.4. Conclusion

31. Introduction1.1. Additive manufacturing of metalsAdditive manufacturing (AM) is defined by ASTM as a process of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies.Alternate names to AM include additive fabrication, additive layer manufacturing, layer manufacturing and freeform fabrication.

1. Introduction1.1. Additive manufacturing of metalsThese processes create components from CAD models by adding material layer by layer, and the final components are often produced in a single step. 1. Introduction1.1. Additive manufacturing of metalsAM typically uses STL data (triangulated model) of the part to be fabricated as input data. The CAD model of the part is sliced into 2D layers, and each contoured layers data is transferred onto the machine. Parts are built through a directed solidification of the metal powder using a high-energy electron beam or laser power.Structural integrity was examined using a micro CT scanner. The CT image data reconstruction software MIMICS was used to reconstruct the 3D model of the fabricated parts. By analyzing the 3D model, it was possible to determine any internal defects, measure internal strut dimensions, and find blocked pores.2. Methodology2.4. Evaluation of structural and mechanical propertiesPorosity is measured using a pycnometer.The effective stiffness and eventually the compressive strength of the part reduces with increasing porosity.2. Methodology2.4. Evaluation of structural and mechanical properties

Axial compression tests were done to evaluate the stiffness of the porous Ti6Al4V parts.Average values of the stiffness of the sample groups were taken as the stiffness value for the given porosity. Since the parts were fabricated layer by layer by melting of Ti6Al4V powder, shear testing was done to evaluate the interlayer strength.2. Methodology2.4. Evaluation of structural and mechanical propertiesTwo CAD models, a mandible and a hip implant were used for biomechanical evaluation.A patient-specific 3D model of a hemi mandible was reconstructed from CT scan data.Von Mises stresses generated in the mandible as a result of vertical masticatory forces are studied with the elastic modulus values derived from the mechanical testing to evaluate the function of the implant in a clinical scenario.2. Methodology2.5. Biomechanical evaluation of patient-specific implantsOne of the primary interests of this study was to determine the predictability of the dimension of individual structural elements and weight of the final part.The average measured length of the parts is 1% more than the intended design. The average volume of the parts is 3.29 % over the design volume.3. Results and discussion3.1. Predictability of structural propertiesUsing an optical microscope, 24 pores, 4 in each of the 6 sides of the cube, were randomly selected and measured and average values calculated. For sets 1, 2, and 3, the struts are of the same size, 800 m, while the distance between the struts is increased. 3. Results and discussion3.1. Predictability of structural properties

3. Results and discussion3.1. Predictability of structural propertiesSurface deformations were observed in the optical microscope in all the samples. Structural deformations were also observed in the reconstructed micro CT images. The surface irregularities/deformations could be attributed to the large variations in pore and strut sizes.

3. Results and discussion3.1. Predictability of structural propertiesThe porosities of the parts as measured using the helium pycnometry method for sets 14 are given in Table 4. The estimated porosity and the intended porosity were found to be within acceptable limits.3. Results and discussion3.1. Predictability of structural properties

The mechanical properties, especially the strength, depend on the strut size, apart from the overall porosity.3. Results and discussion3.1. Predictability of structural properties

Mandible implantIt is observed that the sample with maximum porosity of 70% failed at loads of 18156 Mpa against a normal mastication load of 180 MPa, which is less than 20% of the ultimate tensile strength and compressive strength, giving a safety factor of 5.The implant would weigh 80.57 g as against the weight of dense titanium being 268.57 g.3. Results and discussion3.1. Predictability of structural propertiesA design strategy has been developed for eventual fabrication of porous titanium structures with periodic cellular structures targeted to biomedical applications. Cellular structures with porosities ranging between 49.75% and 70.32% targeted to biomedical applications have been designed and fabricated.Design compensations would be required with smaller pore sizes and strut sizes.4. ConclusionThank You

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