Laser powder bed fusion of nitinol shape memory alloy with superelastic characteristics on Ti substrate

To fabricate complex structures made of nitinol (NiTi), additive manufacturing (AM) has drawn significant interest. However, due to its high microstructural and compositional sensitivities, it is still challenging to fabricate functional NiTi devices via AM. It has been widely reported that evaporation of Ni and formation of precipitation phases during fabrication significantly diverts the expected functional properties. To this date, laser powder bed fusion (LPBF) was the choice among many AM techniques to fabricate NiTi devices but successfully only on a NiTi substrate because of its poor bonding to other substrates (i.e., steel and Ti). Expensive NiTi substrate significantly limits the economic feasibility of fabricating NiTi parts. In this work, a multi-step approach was systematically developed, which enabled printing NiTi on a Ti substrate using a very low laser energy density of 35 J/mm3 without any visible defect. This printing method avoided the Ni evaporation as well as facilitated fast cooling by avoiding the formation of undesirable precipitation phase during solidification. The phase transformation peaks in differential scanning calorimetry (DSC) data clearly indicated that the printed sample is in austenite (B2) phase at room temperature. In the cyclic compressive test, the samples showed superelasticity with recovery ratios of 73 % and 92 % in the 1st and 10th cycle, respectively, from an imposed strain of 8.04 %. Electron backscatter diffraction (EBSD) analysis proved the presence of major [001] oriented B2 grains along the building direction which was attributed to the shallower melt pool obtained by the low laser energy density scanning. It was also found that higher oxygen level in the printing chamber reduced the austenite finish (Af) temperature and negatively affected the printability. These results showed the feasibility of LPBF in printing NiTi on a substrate other than nitinol, providing a possible route to reduce the cost of NiTi fabrication via AM with improved superelastic properties.