On the crystallographic texture and torsional behavior of NiTi shape memory alloy processed by laser powder bed fusion: Effect of build orientation

There is a growing industrial and academic interest in additive manufacturing (AM) techniques such as laser powder bed fusion (LPBF) both for producing parts with complex shapes and for tailoring the microstructure and properties of the resulting parts. This interest when it comes to NiTi-based shape memory alloys (SMA) has a third reason due to the poor machinability and need for several steps of additional treatments to create functional parts from these alloys. Generating the printability map and achieving fully dense parts is one of the priorities in AM communities, despite the aforementioned benefits of AM that open new doors for controlling the microstructure and thereby enhancing thermomechanical properties of additively manufactured components. Motivated by crystallographic texture-dependence of NiTi thermomechanical properties, a build orientation approach is utilized for controlling the crystallographic texture of LPBF-processed NiTi specimens. The microstructural and phase analyses of parts fabricated at varying angles show the significant effect of building orientation on the crystallographic texture of the as-printed samples. The mechanism of the effect of the proposed approach for controlling texture is discussed. SMA rotary actuator is a potential application of this methodology. We assessed the thermomechanical properties of the as-printed rod specimens under simple torsion using 3D digital image correlation (DIC) to capture the shear strain field. The recovery shear strains of the samples are then correlated to the crystallographic textures. It is shown that the recovery strain of the textured NiTi samples is in good agreement with the theoretical transformation strains obtained from lattice deformation theory.