4D printing of Magnetic Shape Memory Alloys

Magnetic shape memory alloys (MSMA) are ferromagnetic materials exhibiting a plastic reversible transformation when subjected to a magnetic field. This transformation occurs within few milliseconds, making them good candidates for ultra-fast actuators. Previous studies have reported an increase of the shape memory effect in bamboo-like Ni-Mn-Ga structures. Thus, 3D Printing, using Laser Powder Bed Fusion (l-PBF), is a potential manufacturing approach to fabricate near-net-shape textured MSMAs structures. This study investigates the influence of l-PBF process parameters (laser power, scan speed, hatch spacing and scanning strategy) on the relative density and the microstructure of bulk and lattice specimens made from a gas atomised Ni-Mn-Ga powder doped with excess Mn. The as-built bulk samples showed a high relative density, up to 98% with a homogenous 14M structure and a ferromagnetic behaviour. The residual porosity in the bulk material is mainly due to gas voids, lack of fusion and cracking. Fabrication of lattice structures at a low laser power (70W) and scan speed (450 mm/s) resulted in a significant decrease in cracking. The effect of process parameters on the strut’s geometry was also investigated, in addition the influence of the lattice geometries on the magnetic properties. Microstructural analysis revealed a layered microstructure with a stripe-like surface relief that originated from the presence of martensitic twins within the sample. Further work will focus on developing a new design to enhance the magnetic properties.France-UK Dstl PhD Schem

In-situ alloying laser powder bed fusion of Ni-Mn-Ga magnetic shape memory alloy using liquid Ga

Ni-Mn-Ga-based magnetic shape memory alloys can exhibit large magnetic field induced strains (MFIS). Recently, additive manufacturing techniques, especially laser powder bed fusion (L-PBF), have been successfully used to manufacture functional polycrystalline Ni-Mn-Ga with complex geometries, such as ‘bamboo-grained’ lattice structures. However, previous approaches of L-PBF of Ni-Mn-Ga have used pre-alloyed powders, which can limit the compositional freedom of the manufactured devices. This study explores, for the first time, the feasibility of an in-situ L-PBF alloying approach using a powder blend of elemental Ni, Mn, and Ga. Promising results were obtained despite the significant differences between the elemental Ni and Mn powders and the liquid Ga. The microstructure of the as-built sample showed distinct stripe patterns from the 14 M structure confirmed by XRD analysis. Heat-treatment significantly improved chemical homogeneity, dissolved the Ni-rich phase but couldn’t dissolve MnO hindering the shape memory effect

In-situ alloying laser powder bed fusion of Ni-Mn-Ga magnetic shape memory alloy using liquid Ga

Ni-Mn-Ga-based magnetic shape memory alloys can exhibit large magnetic field induced strains (MFIS). Recently, additive manufacturing techniques, especially laser powder bed fusion (L-PBF), have been successfully used to manufacture functional polycrystalline Ni-Mn-Ga with complex geometries, such as ‘bamboo-grained’ lattice structures. However, previous approaches of L-PBF of Ni-Mn-Ga have used pre-alloyed powders, which can limit the compositional freedom of the manufactured devices. This study explores, for the first time, the feasibility of an in-situ L-PBF alloying approach using a powder blend of elemental Ni, Mn, and Ga. Promising results were obtained despite the significant differences between the elemental Ni and Mn powders and the liquid Ga. The microstructure of the as-built sample showed distinct stripe patterns from the 14 M structure confirmed by XRD analysis. Heat-treatment significantly improved chemical homogeneity, dissolved the Ni-rich phase but couldn’t dissolve MnO hindering the shape memory effect

4D Printing of Magnetic Shape Memory Alloys

Magnetic shape memory alloys (MSMA) are ferromagnetic materials exhibiting a plastic reversible transformation. Compared to thermally activated shape memory alloys, such as NiTi, the response of MSMA is much faster (less than a millisecond), making them good candidates for actuators, sensors, micro pumps and energy harvesters. To date, the Ni-Mn-Ga system is the most studied MSMA, and is the focus of this study. The shape memory effect in MSMA is driven by a phase transformation from a high ordered austenitic Heusler phase to a lower symmetry martensitic phase. The change in the shape occurs within the martensitic phase in the presence of a magnetic field. This is due to the reorientation of the twin variants. The best magnetic shape memory effect was reported in single crystalline Ni-Mn-Ga exhibiting up to 10% strain. However, in the polycrystalline form, grain boundaries create obstacles for twin boundary motion and thus the shape memory effect is reduced. Nevertheless, recent studies show a high magnetic-field induced strain, up to 8.7%, in polycrystalline Ni-Mn-Ga foams. Increasing porosity and grain size decreases the grain boundary constraint. MSMAs foams can be made using ceramic space holders, by binder-jetting or by ink-printing. However, these techniques create random distribution and/or size porosity. Further investigations are required to control porosity and grain morphology to enhance the shape memory effect. In addition, magnetic properties are orientation-dependant. Previous studies have reported the possibility to control the grain orientation via laser-powder bed fusion (l-PBF) additive manufacturing technique by tuning the printing parameters and the scanning strategy. Thus, l-PBF appears to be a potential approach to create near-net shape oligocrystalline and foam-structure Ni-Mn-Ga.DST