4D printing of Ni-Mn-Ga magnetic shape memory alloy through laser powder bed fusion

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Milleret, Anastassia (2024). 4D printing of Ni-Mn-Ga magnetic shape memory alloy through laser powder bed fusion. University of Birmingham. Ph.D.

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Abstract

Ni-Mn-Ga magnetic shape memory (MSM) alloys represent a remarkable class of smart materials capable of undergoing reversible magnetic-field-induced strains (MFIS) of up to 10%. This unique property, characterised by rapid shape changes occurring within milliseconds, positions Ni-Mn-Ga alloys as promising candidates for a wide range of applications, including ultrafast actuators, sensors, micropumps, and energy harvesters. However, the substantial MFIS is predominantly observed in single crystals, primarily because grain boundaries act as barriers against twin boundary motion in polycrystalline materials. To overcome this limitation, research on reducing the impact of grain boundaries by creating porous and highly textured structures has been explored. The largest MFIS in polycrystals, up to 8.7% has been achieved in foam-like samples featuring ‘bamboo-like grains’—where grains are stacked on top of each other, spanning the diameter of the struts. These foam-like structures possess random porosity. Another promising approach is the manufacture of structures with ‘engineered porosity’, such as lattice structures. In these lattices, each strut is composed of a single large grain connected by nodes. This level of design freedom is achievable through additive manufacturing techniques, notably laser powder bed fusion (L-PBF).
This study focuses on the 4D printing (3D printing of smart materials) of Ni-Mn-Ga lattices using L-PBF. Initially, an attempt was made using an in-situ alloying powder blend, in order to gain compositional freedom, but this approach yielded inconclusive results due to segregations in the final part, which would negate the shape memory effect. Subsequently, an optimisation of process parameters was conducted using gas atomised powder for both bulk and lattice samples, with considerations made for Mn evaporation, density, and geometrical integrity. These samples exhibited a 14M crystal structure, not favourable for large MFIS. Following heat treatment, lattices featuring fully dense 200 μm diameter struts with a bamboo-like grain structure were successfully produced.
Further investigations focused on methods to enhance the crystallographic texture and control microstructure to increase the MFIS. These methods encompassed printing on a heated platform, using a double scanning strategy, and varying strut orientation according to the build direction. The study determined that the platform temperature and the double scanning strategy minimally impacted the microstructure and the magnetic behaviour, while the strut orientation had a substantial influence on the crystallographic texture and a small influence on the magnetic behaviour.
Additionally, the research explored the mechanical properties of Ni-Mn-Ga bulk and lattice samples fabricated through L-PBF, particularly in compression loading, with the intention of paving the way for future application prospects. Nanoindentation was used to determine the elastic moduli of both heat-treated and as-built lattices. The findings revealed that the samples exhibited brittle behaviour, with heat treatment increasing ductility but reducing strength. Notably, lattice samples fractured along a 45º diagonal and displayed significantly lower strength.
Lastly, this study underscores the potential of L-PBF in fabricating Ni-Mn-Ga lattices by demonstrating an MFIS of up to 6.6% in a lattice sample with a 10M crystal structure. These findings open up exciting possibilities for the application of functional Ni-Mn-Ga lattices in various fields.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):