Printing of NiTinol parts with characteristics respecting the general microstructural, compositional and mechanical requirements of bone replacement implants

Abstract

NiTinol is considered to be an attractive candidate alloy for biomedical applications. The aim of this research was to obtain porous NiTinol parts that respected the general microstructural (having 30–80 vol% interconnected pores and pore sizes in the range of 100–600 μm), compositional (an impurity content of less than 500 ppm) and mechanical (an elastic modulus close to that of cortical bone (10–20 GPa) or cancellous bone (<3 GPa), higher strength than 100 MPa at 2% strain and adequate fracture elongation of at least 8%) prerequisites of implants used in bone replacement applications. This was done by printing parts using a laser powder bed fusion technique and by employing a slightly Ni-rich pre-alloyed NiTinol powder having ∼55.9 wt% Ni in its composition. In the next setup, different post-printing heat treatments were employed to adjust the microstructural, compositional and mechanical characteristics of the printed parts step-by-step with the mentioned ones required for the bone replacement implants. The effects of different printing parameters and conducted heat treatment procedures on the microstructure, composition, phase transformation characteristics and mechanical properties of dense and porous printed parts were also systematically discussed and results were correlated together. The developed porous parts (with ∼33 vol% interconnected porosity and ∼600 μm pore width size) were printed using the processing parameters: laser power = 120 W, scanning velocity = 1100 mms−1, hatch spacing = 0.08 mm and layer thickness = 0.03 mm. Regardless of being heat treated or not, the parts generally respected the mentioned targeted properties.