Laser powder bed fusion for the manufacture of Ni-Mn-Ga magnetic shape memory alloy actuators

Abstract

The ability of the magnetic shape memory (MSM) alloy Ni-Mn-Ga to exhibit large magnetic-field-induced strain (MFIS) of 6-12% makes it a promising actuation material for small devices in which traditional mechanisms and piezoelectric materials are impractical. As the grain boundaries in fine-grained polycrystalline material significantly hinder twin boundary motion, large MFIS is almost exclusively obtained in oriented single crystals. However, a moderate MFIS of ~1-4% can be obtained in bulk polycrystalline Ni-Mn-Ga after a sufficient reduction of the grain boundary constraints and the introduction of a strong crystallographic texture. The drawbacks of conventionally manufactured single crystals and polycrystalline Ni-Mn-Ga, e.g. low geometric freedom and high production costs, currently limit the development of novel functional MSM devices. Therefore, additive manufacturing (AM) is attracting increasing attention as a promising method for manufacturing polycrystalline Ni-Mn-Ga, especially as it allows realization of complex geometries or device structures.

Here, a laser powder bed fusion (L-PBF) AM process and a subsequent heat-treatment process were developed for the manufacture of coarse-grained polycrystalline Ni-Mn-Ga samples. It is shown that the chemical composition and resulting MSM-related properties of the L-PBF-built Ni-Mn-Ga can be precisely changed in-situ by adjusting the applied L-PBF process parameters to control the selective evaporation of Mn. A repeatable and fully reversible MFIS of 5.8% is demonstrated for a single crystalline grain of an L-PBFbuilt Ni-Mn-Ga exhibiting a five-layered modulated martensitic structure at ambient temperature. The obtained MFIS is two orders of magnitude larger than the 0.01% MFIS previously reported for additively manufactured Ni-Mn-Ga and is similar to that of conventional single crystals exhibiting the same crystal structure.

The results indicate that L-PBF can be used to manufacture functional polycrystalline Ni-Mn-Ga, facilitating a new generation of fast and simple digital components with integrated MSM alloy sections that can be actuated by an external magnetic field. Practically, the reported results will permit the exploration of polycrystalline-MSM-based devices with a geometric freedom that has thus far been impossible with conventional manufacturing methods.