Residual Stresses in Nitinol Cantilever Beams Manufactured by L-PBF

Residual stresses are those which exist within a material in the absence of any external forces, and are a common problem associated with parts that are printed via the additive manufacturing method; Laser-Powder Bed Fusion (L-PBF). Residual stresses arise during this process due to the complex thermal conditions that occur during the laser scanning of the metallic powder. The rapid heating, and subsequent cooling of the printing material results in its respective expansion and contraction, causing residual stresses to develop within the printed part. This results in detrimental effects such as cracking, layer delamination, as well as part distortion. In this study, the effects of residual stresses are examined in Nickel Titanium (NiTi) cantilever beams that have been manufactured by L-PBF. The processing parameters laser power, scanning speed and hatch spacing are varied in a Box-Behnken design of experiment, in which the effects of these parameters on the residual stresses within the samples are analysed by measuring the respective deflection of each cantilever beam, following the separation of the beam from the build substrate. X-ray diffraction examines how the presence of martensitic and austenitic NiTi, as well as any secondary metallic phases resulting from the L-PBF process, influence the deflection of the printed cantilever beams. Additionally, in-situ IR data of the melt-pool thermal signatures during laser scanning is also analysed to examine how the thermal history of each set of processing parameters affects the residual stress and respective deflection of each sample.