The effect of post process treatments on biomechanical properties of Ti6Al4V metamaterials and spinal cage implants manufactured by selective laser melting

Abstract

Selective laser melting (SLM) is an additive manufacturing technique, which is currently on the rise of being used for manufacturing bone implants. Spinal cage, dental and hip implants can for example be manufactured using SLM. Ti6Al4V lattice structures, categorised as metamaterials, can be printed by SLM with mechanical properties close to bone tissue. Due to the lattice structure the stiffness of the Ti6Al4V is decreased, by which stress shielding can be reduced. The lattice structures enhance bone ingrowth which in turn improves the implant’s integration into bone tissue. In light of this potential, this research is focused on biomechanical properties of additively manufactured Ti6Al4V metamaterials.

The current research is aimed at improving the fatigue resistance and wettability of diamond lattice structured Ti6Al4V by applying different microstructural designs and surface engineering through hot isostatic pressing (HIP), sand blasting (SB) and chemical etching (CE). Furthermore a comparison is made between the two SLM processes in terms of continuous and pulsed laser scanning. In order to verify the developed herein post treatment procedures, the tests were also upscaled to actual spinal cage implants. Furthermore, surface modifications affect its wettability which can be linked to cell adhesion and ultimately healing time of the implant. Hence Sessile drop tests were performed to assess the wettability and compare the effect of the various surface modifications.

For both SLM methods it was found that HIP reduces porosity of Ti6Al4V metamaterials, which reduces crack initiation sites and it also serves as a heat treatment increasing the b-phase fraction and thus increasing ductility
and fatigue resistance. SB and CE were found to reduce surface indiscrepancies, which decrease the effect of stress concentration and fatigue initiation sites. Finally SB induces compressive residual surface stresses which means the surface is work hardened, increasing the overall mechanical properties.

For continuous SLM samples an increase in yield strength from 89 MPa up to 115 MPa was found by applying HIP treatment. It should be noted, however, that static mechanical properties were not affected by SB and CE treatments. Fatigue resistance, both low cycle (LCF) and high cycle fatigue (HCF), was significantly improved by a combination of HIP, SB and CE. The observed trend was similar for both pulsed and continuous SLM samples. It is worth noting that SLM samples manufactured with pulsing laser were found in general to be inferior to the
continuous laser SLM, both in terms of static and dynamic properties. The difference is likely attributed to the nature of the laser scanning process, where for pulsing laser method each bead interconnection serves as stress concentration, while for continues laser it is rather the strut interconnections that act as weakest points. Furthermore, for the continuous SLM a preferred grain growth direction was observed which indicates anisotropy. This was not observed for pulsed SLM samples.

For the wettability results it was observed that SB decreases and CE increases the contact angle. A decrease in contact angle means the surface has become more hydrophilic, hence the in this study developed SB modification could be considered as more favourable for osseointergration.

The upscaled spinal cage implants post treatment procedure showed a decrease in yield strength and an increase in fatigue resistance for the HIP+SB+CE as compared to as-processed implants. The rather limited post treatment
improvement on implants was linked to the post process treatment method, which should be modified to account for the complex geometry of these structures.