Abstract
The present study introduces an approach to the powder metallurgical shaping of a pseudo-elastic nickel–titanium (NiTi 44 alloy) combining two different Additive Manufacturing (AM) processes, namely fused filament fabrication (FFF) and Laser Powder Bed Fusion (LPBF), by manufacturing filigree structures on top of sintered FFF parts. Both processes start with commercial gas atomized NiTi powder, which is fractionated into two classes. Using the fine fraction with particle sizes <15 µm, robust thermoplastic filaments based on a non-commercial binder system were 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 led to an increased transformation temperature. Adjusting the oxygen and carbon content in the alloy remains a challenging issue for powder metallurgical processed NiTi alloys. Filigree lattice structures were built onto the surfaces of the sintered FFF parts by LPBF using the coarser powder fraction (15–45 µm). A good material bond was formed, resulting in the first known NiTi hybrid, which introduces new production and design options for future applications.
Highlights
Recent years have witnessed a growing interest in NiTi Shape Memory Alloy (SMA)due to its combination of outstanding functional features, such as shape memory and pseudoelasticity/superelasticity [1,2]
Additive Manufacturing is a key enabling technology for programmable materials, according to the Fraunhofer Cluster of Excellence Programmable Materials (CPM), due to its unique potential to combine the processing of smart materials with superior properties with design freedom, e.g., for metamaterials based on small-scale unit cells [5]
The results showed that the filament does not cause any problems when manufacturing parts with these commercially available standard machines
Summary
Recent years have witnessed a growing interest in NiTi Shape Memory Alloy (SMA)due to its combination of outstanding functional features, such as shape memory and pseudoelasticity/superelasticity [1,2]. The major reasons for the processability can be ascribed to the high ductility, strong work hardening, high toughness and compositional sensitivity of NiTi alloys [4] To overcome these difficulties, a solution is needed, which has been found by using Additive Manufacturing (AM) processes as an excellent alternative process to fabricate complex NiTi structures with adjustable or even programmable characteristics. Due to their inherent properties, so called ‘programmable materials’ have the potential for high functional integrability with low system complexity at the same time, since their internal design enables the adoption of functions of complete systems. Additive Manufacturing is a key enabling technology for programmable materials, according to the Fraunhofer Cluster of Excellence Programmable Materials (CPM), due to its unique potential to combine the processing of smart materials with superior properties with design freedom, e.g., for metamaterials based on small-scale unit cells [5]
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