The influence of selective laser melting defects on the fatigue properties of Ti6Al4V porosity graded gyroids for bone implants

Abstract

Porosity grading of metallic lattice structures is becoming a favorable design for bone implant applications since these structures mimic the bone mechanical and biological properties. Especially when using bio-mimetic unit cell designs created by triply periodic minimal surfaces (TPMS), such as gyroids, making these types of lattice structures dominant over other CAD-based designs. In this study, the manufacturability of gyroids is tested by studying three different designs: uniform porosity with thin struts (G2), thick struts (G4), and graded porosity (G24). The main aim is to understand the influence of selective laser melting (SLM) defects on the static and fatigue properties of uniformly and porosity graded gyroids. Qualitative and quantitative analysis revealed some dimensional deviation between nominal designs and printed parts. A finite element analysis (FEA) model based on a single unit cell is developed to predict the mechanical properties of uniformly graded porosity parts. The numerical results give insight into how manufacturing irregularities can affect the mechanical properties of lattice structures. Both G24 and G4 designs showed higher apparent modulus of elasticity (Eapp) and higher compressive strength (σp), whereas they both exhibited less fatigue strength than the G2 designs. It was observed that the Eapp was significantly affected by geometrical errors in strut shape. At the same time, fatigue strength was dependent on the surface quality and the internal defect percentage.