Process development and microstructural evolution of laser powder bed fusion processed novel β Ti–42 Nb and Ti–20Nb–6Ta alloys

The d-electron alloy design method, introduced by Morinaga et al. in 1988, is a useful theoretical calculation method for predicting the phases that form in transition metals and their alloys, such as titanium. It can be particularly useful in the design and selection of β-titanium alloys. These alloys offer a wide range of useful material properties that can be tailored for specific applications, depending on the alloying elements and phase composition. However, conventional methods make processing beta titanium alloys very time-consuming and costly, which makes their widespread use difficult. Additive manufacturing is a promising technology for establishing β-titanium alloys further. In this work, the novel β-titanium alloys Ti-42Nb and Ti-20Nb-6Ta were selected as promising new candidates for biomedical applications based on the d-electron alloy design method. The laser powder bed fusion method was used to process these alloys, achieving high relative densities of 99.96%. X-ray diffraction confirmed the predicted crystal structures of pure β for Ti-42Nb and pure α″ for Ti-20Nb-6Ta. In contrast, both pure titanium and the Ti-6Al-4V alloy consisted solely of α′. To gain insight into the microstructural features, metallographic analyses were conducted using optical microscopy, scanning electron microscopy, and energy dispersive X-ray spectroscopy. The extremely high cooling rates during laser powder bed fusion allowed for an almost homogeneous element distribution. Both β-titanium alloys displayed visible laser tracks and residual melt pool contours in their microstructure, while pure titanium and Ti-6Al-4V exhibited checkerboard and columnar structures. Furthermore, a transition from planar to cellular solidification mode was observed in Ti-42Nb and Ti-20Nb-6Ta alloys. This phenomenon is likely due to the influence of constitutional undercooling during solidification of the micro-melt.