Nickel titanium (NiTi) as one of the most utilized shape memory alloy has drawn significant interest due to its unique characteristics. However, NiTi is also considered a susceptible material to smoke during electron beam powder bed fusion (PBF-EB) process, which restricts the manufacturing possibility of the components. This work investigates processing windows for pre-heating and melting of NiTi powder to allow fabricating healthy parts. The smoke tests were carried out at different focus offsets and beam currents in relation to beam speeds. It is noted that a smaller EB spot can effectively prevents smoking while it may cause the strong powder bonding which can affect powder recycling negatively. Thus, a less focused beam (or larger EB spot) was selected to reach medium but efficient sintering. Moreover, it was observed that a negative defocused EB mitigates the smoke phenomenon compared to the positive defocused EB with a similar spot size. After that, parts having a relative density over 99% were successfully manufactured with PBF-EB. It is also found that with the same level of energy input, a set of low power with low scan speed leads to denser parts compared to a set of high power with high scan speed. This is attributed to less complexities in the melt dynamic which is related to lower density of impacting electrons. Besides, the combination of low power with low scan speed also improves geometrical accuracy of the parts attributed to the smaller spot size and smaller melt pool sizes.
QC 20230807