Fused Filament Fabrication of NiTi Components and Hybridization with Laser Powder Bed Fusion for Filigree Structures

The present study introduces an approach of powder metallurgical shaping of a pseudo-elastic nickel-titanium (NiTi 44 alloy) combining two different Additive Manufacturing (AM) processes, namely Fused Filament Fabrication (FFF) as well as Laser Powder Bed Fusion (LPBF) by manufacturing filigree structures on top of sintered FFF parts. The starting point for both processes is that a commercial gas atomized NiTi powder has been fractionated into two classes. Using the fine fraction with particle sizes < 15 µm, robust thermoplastic filaments based on a non-commercial binder system have been produced and processed to different auxetic and non-auxetic geometries employing a commercial standard printer. FTIR-analysis for thermal decomposition products was used to develop a debinding regime. After sintering, the phase transformation austenite/martensite was characterized by DSC in as sintered and annealed state. Precipitates resulting from residual impurities were detected by micrographs and XRD. They have led to an increased transformation temperature. Adjusting oxygen and carbon content in the alloy remains a challenging issue for powder metallurgical processed NiTi alloys. Filigree lattice structures were built onto the surface of sintered FFF parts by LPBF using the coarser powder fraction (15-45µm). A good material bond has formed resulting in the first known NiTi hybrid which opens up new production and design options for future applications.