Abstract

Biomaterials for implant applications require a well-balanced combination of mechanical, biological, and corrosion properties to allow their intended use within the human body. Titanium alloys fulfil these requirements excellently and are therefore often used for medical applications. One of these alloys is Ti-13Nb-13Zr – a so-called β-rich (α+β)-titanium alloy, standardized as wrought material for use in implant applications, and widely studied in terms of conventional manufacturing. However, during casting or forging, the feasible complexity of implant designs is limited. Additive manufacturing, for example the powder bed fusion – laser beam / metal (PBF-LB/M) process, increases the achievable level of complexity and allows direct fabrication of parts on the basis of alloy powder. Thus, sophisticated and, possibly, patient-specific implant designs are realizable, which could improve medical care. Ti-13Nb-13Zr is a good choice for the fabrication of such (dental) implants, since it can form the soft and ductile α''-martensite (metastable phase with orthorhombic crystal structure, potentially residual-stress tolerant and, thus, enhancing PBF-LB/M) during quenching from the β-phase field, which can be decomposed to a lamellar (α+β)-microstructure during subsequent annealing.

The comparably low stiffness of the α''-phase compared to the α'-martensite and the stable α-phase is favorable for implant applications as it limits the so-called stress shielding effect, which can result in bone resorption and implant loss during long-term use. The controlled decomposition of the α''-martensite during an annealing treatment can, thus, lead to a good combination of strength and stiffness, taking the ingrowth phase of the implant (higher stiffness needed) and long-term use (lower stiffness in order to prevent stress shielding) into consideration. This annealing treatment can be performed during hot isostatic pressing, which is usually performed after additive manufacturing in order to reduce residual porosity and, thus, improve mechanical properties of the final part.

However, open questions remain regarding PBF-LB/M-printing of Ti-13Nb-13Zr. Existing literature often describes the formation of the hexagonal α'-martensite during powder bed fusion, which, in contrast to the α''-martensite, is less ductile and exhibits a higher stiffness. Since the type of martensite formed is of high importance regarding production by powder bed fusion, mechanical properties and, thus, applicability of printed implants, a deeper understanding of microstructure formation during PBF-LB/M-printing and post-processing is necessary.

In order to study microstructure formation, multi-track laser melting experiments were conducted on conventionally produced Ti-13Nb-13Zr. These experiments, performed on a conventional PBF-LB/M machine, simulate the 3D-printing process by melting a surface layer of the alloy analogous to the processing of a powder layer. These experiments allow an easy and fast approach to study the influence of different process parameters on microstructure, phase composition, and melt pool formation. Different conditions were studied by varying the laser power, scanning speed, hatch distance, and position of the laser focus. Microstructure and phase analyses were performed by means of optical and scanning electron microscopy and X-ray diffraction combined with hardness tests. The results obtained are now being transferred to PBF-LB/M of Ti-13Nb-13Zr.