Microstructure and texture control of Ni-Mn-Ga magnetic shape memory alloys manufactured by laser powder bed fusion

• Published in: Additive manufacturing Vol. 86; p. 104225
• Main Authors: Wójcik, Anna, Chulist, Robert, Szewczyk, Arkadiusz, Morończyk, Bartosz, Żrodowski, Łukasz, Wróblewski, Rafał, Kowalczyk, Maciej, Kolano-Burian, Aleksandra, Zackiewicz, Przemysław, Schell, Norbert, Maziarz, Wojciech
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 25-04-2024
• Subjects: LPBF; MFIS; Ni-Mn-Ga Heusler alloys; TEM; Texture; LPBF; Ni-Mn-Ga Heusler alloys; Texture; TEM; MFIS
• ISSN: 2214-8604; 2214-7810
• DOI: 10.1016/j.addma.2024.104225

The Laser Powder Bed Fusion (LPBF) process was used to manufacture Ni-Mn-Ga polycrystalline samples. The initial powders with a fine particle size of about 20 µm were firstly prepared by a ball milling from melt-spun ribbons. The microstructure and texture evolution of Ni-Mn-Ga alloy manufactured by LPBF were investigated employing SEM, TEM, and synchrotron radiation diffraction. Using two different scanning strategies, optimization of laser parameters and post-processing heat treatment, a homogeneous microstructure with a strong crystallographic texture was obtained. The localized heating/cooling conditions allow obtaining a layered structure with preferred fiber orientations along the growth direction. The crystal structure and crystallographic texture drastically change when the laser oscillation mode is chosen. The strong crystalline anisotropy and layered-type microstructure have a significant impact on twining flow behavior giving rise to an anisotropic mechanical response. The variant reorientation during mechanical training is realized by the so-called orthogonal shearing. It is also shown that the resulting crystal structure and characteristic transformation temperatures are strongly dependent on chemical composition related to Mn losses and internal stresses along with chemical order creating metastable phases due to the extremely high cooling rate upon the 3D printing. Therefore, all the above parameters are thoroughly monitored during the three-stage fabrication process.
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•NiMnGa alloys with microctructural and chemical homogeneity were fabricated by LPBF.•Scanning strategy (normal and oscillating) has a strong impact on microstructure.•Heat treatment allowed removing internal stresses and stabilize the structure.•Columnar, homogenous microstructure and fiber texture result in high MFIS of 0.5%.