Effect of Unit Cell Type and Pore Size on Porosity and Mechanical Behavior of Additively Manufactured Ti6Al4V Scaffolds

Abstract

Porous metal structures have emerged as a promising solution in repairing and replacing damaged bone in biomedical applications. With the advent of additive manufacturing technology, fabrication of porous scaffold architecture of different unit cell types with desired parameters can replicate the biomechanical properties of the natural bone, thereby overcoming the issues, such as stress shielding effect, to avoid implant failure. The purpose of this research was to investigate the influence of cube and gyroid unit cell types, with pore size ranging from 300 to 600 µm, on porosity and mechanical behavior of titanium alloy (Ti6Al4V) scaffolds. Scaffold samples were modeled and analyzed using finite element analysis (FEA) following the ISO standard (ISO 13314). Selective laser melting (SLM) process was used to manufacture five samples of each type. Morphological characterization of samples was performed through micro CT Scan system and the samples were later subjected to compression testing to assess the mechanical behavior of scaffolds. Numerical and experimental analysis of samples show porosity greater than 50% for all types, which is in agreement with desired porosity range of natural bone. Mechanical properties of samples depict that values of elastic modulus and yield strength decreases with increase in porosity, with elastic modulus reduced up to 3 GPa and yield strength decreased to 7 MPa. However, while comparing with natural bone properties, only cube and gyroid structure with pore size 300 µm falls under the category of giving similar properties to that of natural bone. Analysis of porous scaffolds show promising results for application in orthopedic implants. Application of optimum scaffold structures to implants can reduce the premature failure of implants and increase the reliability of prosthetics.

Keywords: Ti6Al4V; Young’s modulus; cube; gyroid; porous; selective laser melting; stress shielding effect.

Figure 1

Unit cell architecture: (a) cube; and (b) gyroid.

Figure 2

Unit cell pore size and strut thickness for: (a) cube; and (b) gyroid.

Figure 3

(a) Cube sample with plates on top and bottom; (b) cube sample from front view; and (c) top view of cube porous scaffold without top plate.

Figure 4

(a) Gyroid sample with plates on top and bottom; (b) gyroid sample from front view; and (c) top view of gyroid porous scaffold without top plate.

Figure 5

3D Printed (i) cube and (ii) gyroid samples with pore size: (a) 0.3 mm; (b) 0.4 mm; (c) 0.5 mm; and (d) 0.6 mm.

Figure 6

CT reconstruction of samples: (a) cube; and (b) gyroid.

Figure 7

CT Image of (i) cube and (ii) gyroid sample: (a) 0.3 mm; (b) 0.4 mm; (c) 0.5 mm; and (d) 0.6 mm.

Figure 8

Porosity comparison for cube and gyroid samples in bone porosity range.

Figure 9

Deformation produced due to applied force in: (a) cubic sample; and (b) gyroid sample.

Figure 10

Numerical and experimental stress-strain curve for: (a) cube 0.3 mm; (b) cube 0.4 mm; (c) cube 0.5 mm; (d) cube 0.6 mm; (e) gyroid 0.3 mm; (f) gyroid 0.4 mm; (g) gyroid 0.5 mm; and (h) gyroid 0.6 mm.

Figure 11

Comparison between numerical and experimental Young’s modulus for cube and gyroid sample in the range of bone modulus.

Figure 12

Comparison between numerical and experimental yield strength for cube and gyroid sample in the range of bone yield strength.