Ni-Concentration Dependence of Directed Energy Deposited NiTi Alloy Microstructures

Localized re-melting and re-heating in regions between overlapping passes and layers drive localized microstructure development during laser-based additive manufacturing processes. Ti-rich (Ti_52.1Ni_47.9 at.%) and Ni-rich (Ni₅₃Ti₄₇ at.%) alloy builds were fabricated using a laser-based directed energy deposition (LDED) process and sectioned in three orthogonal cross-sections to provide a 3-dimensional view of the grain and precipitate morphologies in the interpass and interlayer regions. Fine equiaxed grains, which were an order of magnitude smaller than the columnar grains in the bulk regions, were observed in the interfacial regions. While these columnar grains showed preferential orientation in the highest heat flow direction corresponding to the build height, the fine equiaxed grain structures in the interfacial regions showed no orientation preferences. In addition to these changes in grain structures, precipitation of second phases is also impacted in these regions. The re-melting and re-heating characteristic of the interfacial regions produces precipitates in the Ni-rich alloys with fine oriented morphologies different from those observed in the bulk regions.