Selective Laser Melting of NiTi Shape Memory Alloy: Processability, Microstructure, and Superelasticity
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SPECIAL ISSUE: A TRIBUTE TO PROF. DR. GUNTHER EGGELER, INVITED PAPER
Selective Laser Melting of NiTi Shape Memory Alloy: Processability, Microstructure, and Superelasticity Carlo Alberto Biffi1 • Jacopo Fiocchi1 • Fabrizio Valenza2 • Paola Bassani1 Ausonio Tuissi1
•
Ó ASM International 2020
Abstract 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/mm3. 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 °C. This suggests that the microstructure & Ausonio Tuissi [email protected] Carlo Alberto Biffi [email protected] 1
National Research Council of Italy - Institute of Condensed Matter Chemistry and Technologies for Energy, Unit of Lecco, CNR ICMATE, Via Previati 1/E, 23900 Lecco, Italy
2
National Research Council of Italy - Institute of Condensed Matter Chemistry and Technologies for Energy, Unit of Genova, CNR ICMATE, Via de Marini, 6, 16149 Genova, Italy
of as-built samples is highly efficient to promote superelasticity after annealing. Keywords NiTi Shape memory alloy Selective laser melting Microstructure EBSD Martensitic transformation Superelasticity
Introduction Shape Memory Alloys (SMAs) are functional materials, which are well known for their unique properties, namely, shape memory effect (SME) and pseudoelasticity (PE) [1]. Due to its stable and optimal properties, intermetallic NiTi compound is the most widespread SMA. These peculiar characteristics are due to a diffusionless solid-to-solid phase transformation, namely, martensitic transformation (MT), which can be found in Ti-rich NiTi and Ni-rich NiTi alloys above or below room temperature, respectively. The temperature range, at which the MT occurs, dictates the use of these materials as actuators or for biomedical devices showing SME or PE, respectively. However, the use of Nirich NiTi alloy, showing SE at room temperature, was also demonstrated to be suitable for developing high-perfor
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