Powder-blown laser-based directed energy deposition of (14M) Ni-Mn-Ga magnetic shape memory alloy

Ni-Mn-Ga-based magnetic shape memory (MSM) alloys are renowned for their large magnetic-field-induced strains, making them attractive for compact magnetically controlled actuators requiring high response frequencies and large reversible deformations. In this study, Ni-Mn-Ga alloy samples were deposited using powder-blown laser-based directed energy deposition (DED-LB), employing gas-atomised powder pre-alloyed with excess Mn. The samples were deposited following a systematic experimental design, varying the applied laser power and employing two melting strategies: unidirectional and bidirectional. The results demonstrate the feasibility of achieving high relative densities (>97.5 %) in multi-layered Ni-Mn-Ga samples through DED-LB. All as-deposited samples exhibited a consistent 0.7–0.9 at% Mn loss in comparison to the powder feedstock, thus showing that the Mn overdosing requirements for DED-LB differ from those in laser powder bed fusion. Both as-deposited and heat-treated samples exhibited seven-layered modulated (14 M) martensite structures at ambient temperature. In contrast, the heat-treated samples also exhibited fully reversible martensite transformation at around 66 ºC and a Curie temperature at around 94 ºC. Distinct microstructural characteristics were observed based on the melting strategy, with bidirectional melting promoting the formation of large columnar grains with a strong texture approximately along the build direction. This study highlights the potential of DED-LB processes in manufacturing relatively large (mm to cm scale) functional Ni-Mn-Ga actuators and contributes to the ongoing efforts in laser additive manufacturing of functional materials.