Fabrication and Characterization of Anisotropic NiTi Specimens for Load-Bearing Implants

NiTi is a promising biomaterial for load-bearing implants because of properties such as the shape memory effect, pseudoelasticity, and - in comparison to other metals - low stiffness. Still, in conventional NiTi and other metal implants, the oriented architecture of bony tissues is generally not represented. Because bone scaffolds require a specific architecture to promote cell in-growth, we have applied selective laser melting (SLM) to build a variety of specimens. The SLM process allows the straightforward fabrication of complex architectures including well-defined pore networks and patient-specific implant designs. In this study, we demonstrate the power of SLM to tailor the structural properties of NiTi parts in anisotropic manner. NiTi specimens were fabricated by SLM from pre-alloyed powder with an initial Ni-content of 55.96 wt.-%. For the building process, laser power and scanning velocity were varied from 56 -100 W and 0.11 - 0.19 mm/s, respectively. Metallographic and electron backscatter diffraction investigations revealed the anisotropic microstructure with elongated grains in the direction of heat transfer. Furthermore, we have found that the parameters of the SLM process determine shape, size, and arrangement of grains. With increasing laser power, for example, the grain width raised from (33 ± 7) μm to (90 ± 15) μm, whereas the grain length in building direction increased from (60 ± 20) μm to (600 ± 200) μm. The processing parameters were also adjusted during the fabrication process of single parts, which allows a locally tailored microstructure. As the microstructure determines the mechanical properties of the implant, the SLM process is utilized to generate anisotropic NiTi-implants.