Towards the additive manufacturing of Ni-Mn-Ga complex devices with magnetic field induced strain

Laser powder bed fusion (L-PBF) is used to produce foam-like Ni-Mn-Ga with tailored microscale and mesoscale features. Ni50-Mn28.2-Ga21.8 (at%) powder was gas atomised and processed in an L-PBF system with a range of energy density from 26.24 and 44.90 J/mm3. We characterised microscale and mesoscale properties, such as the chemical composition, crystal structure, magnetisation measurements, density, and porosity measurements as a function of process parameters, in a systematic design of experiment. Preliminary research on macroscale properties included tensile testing and magnetic field induced strain (MFIS) measurements. Results show how controlling process parameters allows tailoring the Ni-Mn-Ga polycrystalline microstructure. Hence, obtaining twinned martensitic structures with a predominant orientation going across the visible grain boundaries. All the processed samples showed a 56 Am2/kg magnetisation level, close to Ni-Mn-Ga 10 M single crystals. Mesoscale results show a distinctive porosity pattern that is tailored by the process parameters and the laser scanning strategy. In contrast, macroscale mechanical tensile test results show a brittle fracture of Ni-Mn-Ga due to the high porosity with yield stress 2-3 times higher than shown in single crystals. In sum, we built geometrically complex demonstrators with (i) microscale twinned martensitic structures with a predominant orientation going across the visible grain boundaries and (ii) mesoscale tailored periodic porosity patterns created by modifying power, scanning speed, and scanning strategy systematically. L-PBF demonstrates great potential to produce foam-like polycrystalline Ni-Mn-Ga, reducing grain boundary constraints and thus the magnetic force needed for MFIS.