Selective Laser Melting of NiTi Shape Memory Alloy: Processability, Microstructure, and Superelasticity

Nowadays, thanks to the growing interest regarding the manufacturing of 3D complex parts with integrated functionalities, the additive manufacturing of NiTi shape memory alloy is a challenging technological issue. Particularly, 3D printing of NiTi components requires a strong interaction between technological and metallurgical approaches, due to the significant correlation among the process conditions, the microstructure, and the functional performances. The goals of the present work are to define the processability of NiTi powder for realizing fully dense samples using Selective Laser Melting process and the correlation between the microstructure and the superelastic response of specimens processed in different process conditions. It was found that highest relative density values can be obtained for a laser fluence in the range 63-160 J/mm(3). The resulting microstructures exhibit variable degrees of orientations, according to the different cooling rates and melt pool size, specific for each condition. Finally, mechanical testing in compression indicated that the as-built alloy exhibits a limited superelastic behavior. A typical flag-like behavior, characterized by 6% of complete recoverable strain, was obtained through heat treatment at 500 degrees C. This suggests that the microstructure of as-built samples is highly efficient to promote superelasticity after annealing.