Properties of Ni-Mn-Ga magnetic shape memory alloy manufactured by laser powder bed fusion

Abstract

In the category of smart materials, the magnetic shape memory alloy (MSMA) Ni-Mn-Ga has gained traction due to its giant strains of 6-10% when exposed to a magnetic field. This property makes the material a potential candidate for small-sized motion-producing devices in which conventional mechanisms fail to perform. Additive manufacturing (AM) has demonstrated novel ways of engineering with its ability to produce high-complexity shapes. Laser power bed fusion (L-PBF) is an AM method that bases on layer-by-layer melting of powder feedstock using a focused laser beam. Recently, this process has gained interest as a promising approach to manufacture Ni-Mn-Ga-based MSMAs. The overall results from this process have been positive, although the built material requires considerable amount of post-processing, for example, by heat treatments. This thesis aimed to study the properties of the samples fabricated from Ni-Mn-Ga alloy by L-PBF. A set of Ni-Mn-Ga samples with slightly different compositions were built using L-PBF and subsequently heat treated. Several characterization methods were utilized to study the micro-structure and the MSM-related properties of the produced samples in both, as-built and heat-treated, conditions. The results confirmed what was already stated in the scientific literature; the built samples exhibited homogeneous chemical compositions, modulated martensitic crystal structures, and fully reversible martensitic transformations typical for the Ni-Mn-Ga alloys fabricated by L-PBF. Internal stresses, compositional variation, local variations of lattice parameters, and disordering were suggested as the reasons of low-quality properties in as-built samples. Heat treatment homogenized and stabilized the crystal structure as well as reduced internal stresses. Heat-treated samples possessed high-quality magnetic properties and diffraction spectrums; the grains appeared larger with long-range order. EDS experiment did not reveal composition gradients in the material. It was concluded that L-PBF was a reliable and repeatable manufacturing method for Ni-Mn-Ga MSMAs.