Metal Additive Manufacturing Conference - MAMC 2022

Recently NiTi based shape memory alloys (SMA) have been widely explored for the fabrication of complex parts through additive manufacturing (AM). Among the AM techniques, laser powder bed fusion (LPBF) is on the top and gaining momentum in reason of recent and remarkable advances. Using pre-alloyed NiTi powder as a feedstock, it was possible to correlate the processing parameters to the formation of defects such as porosity and/or lack of fusion, map transformation temperatures – of uttermost importance for the functional shape memory and superelastic effects –, and print functionally graded structures. Nonetheless, when alloying NiTi, Ti is quite sensitive to impurity pick-up (especially oxygen), which compromises the functional properties. Moreover, Ni and Ti have a different vapour pressure meaning that during the melting the former element is prone to evaporate. The aforementioned drawbacks change the chemical composition of the alloy, leading to undesirable effects that must be mitigated during the atomization process. Once this atomization becomes critical, these facts increase remarkably the powder price thus hindering research on the topic. One alternative relies on the so-called in-situ alloying, where elemental Ni and Ti are pre-mixed and alloyed during the LPBF process. Two main advantages of this method are i) compositional flexibility and ii) lower costs. In the last decade investigations on the in-situ alloying of SMA were conducted, demonstrating the feasibility of this method for printing functional and dense samples, mostly printed in a similar substrate with prior heating. The present work explored a different scenario, printing Ni-rich NiTi in a dissimilar (Ti) and non-heated substrate obtaining dense parts. For attaining adequate results, a literature-based printing parameters range was determined. Defect-less printed parts were submitted to thermophysical tests aiming to determine their transformation temperatures. Subsequently, mechanical assessment demonstrated a superelastic behaviour, attaining outstanding strain recovery after cyclic tests.