Characterization of structural and magnetic phase transformations in Ni-Mn-Ga magnetic shape memory alloys built via laser powder bed fusion

This research analyses PBF-LB-built samples after high-temperature heat treatment, comparing them to conventionally manufactured (cast) materials. The PBF-LB-built samples were produced using variable scanning speeds of 400, 600, and 800 mm/s, while keeping the other parameters constant, resulting in variable Mn losses during melting in PBF-LB. The ambient temperature X-Ray diffraction patterns revealed a non-modulated martensite structure for the sample with the highest volumetric energy density (VED = 100 J/mm 3 ), a seven-layered orthorhombic martensite structure for the sample with medium VED (67 J/mm 3 ), and a five-layered modulated tetragonal martensite structure for the sample with the lowest VED (50 J/mm 3 ). Additionally, it was found that selective Mn loss during the PBF-LB process led to enhanced antiferromagnetic interactions, reducing the martensitic transformation (MT) temperature to near-ambient levels. This resulted in larger magnetization jumps at MT and narrower MT hysteresis. The results also showed that, similarly to the cast reference sample, the PBF-LB sample with the lowest VED exhibited an intermartensitic transformation due to internal stresses resulting from the metastable state and phase coexistence.