Porous structure and compressive failure mechanism of additively manufactured cubic-lattice tantalum scaffolds

Abstract

Additively manufactured tantalum scaffolds have attracted increased attention for their high potential for bone tissue reconstruction and other industrial applications. Porous structure designs, mechanical property evaluation, and material failure investigation are needed for this new structural and functional material. In this study, cubic lattice tantalum scaffolds with porosities of 65%, 75%, and 85% were designed and fabricated by selective laser melting, and the porous architecture and microstructure were characterized. The compressive mechanical behavior and material failure mechanism were explored by experimental compression tests and finite element analysis (FEA). The designed scaffold porous structure parameters included the following: side length of unit cell 880 μm, 1020 μm, and 1220 μm; pore size 380 μm, 520 μm, and 720 μm; and strut diameter 250 μm. The pore parameters of fabricated scaffolds matched well with those of the designed scaffolds. Struts were dense without macro/microscopic cracks and other microstructure defects resulting from laser melting. Porosity significantly affected the compressive mechanical properties. As the porosity decreased from 85% to 65%, the compressive strength and elastic modulus increased from 17.6±0.5 MPa and 0.7±0.2 GPa to 40.5±0.8 MPa and 1.5±0.6 GPa, respectively. Compression testing results indicated that additively manufactured cubic lattice tantalum scaffolds show ductile deformation behavior and excellent mechanical reliability. When the strain was as high as 50%, the scaffolds with different porosities exhibited plastic failure with progressive layer-by-layer collapse behavior determined by the coupling of buckling and bending deformation, demonstrating their high ductility. Collapse initially occurred in the upper and lower lattice layers of the scaffolds. Large deformation resulted in strut fracturing with micro crack propagation. Most of the fracturing sites were at the vertical struts and conjunctions, not the horizontal struts. FEA revealed that the stress was primarily concentrated on the vertical struts, and the struts in the upper and lower layers withstood greater stress than the struts in the middle layers. The stress in the vertical struts was distributed along the inclination angle of 45° with respect to the loading direction. Stress was not concentrated at the horizontal struts. The results of the FEA were consistent with the experimental results of the compression tests and showed that the load-bearing capability of cubic lattice tantalum scaffolds can be enhanced by increasing the diameter of the vertical struts.