Elsevier

Acta Materialia

Volume 154, 1 August 2018, Pages 355-364
Acta Materialia

Full length article
Sintering regimes and resulting microstructure and properties of binder jet 3D printed Ni-Mn-Ga magnetic shape memory alloys

https://doi.org/10.1016/j.actamat.2018.05.047Get rights and content

Abstract

Binder jet 3D printing was used to produce magnetic shape memory alloy samples with densities increasing from 45% to 99% with increasing sintering temperature from 1000 °C to 1100 °C for 2 h. Within this temperature range, the sintering mechanisms, microstructural evolution, phase transformation and magnetic behavior were investigated and categorized in three different sintering regimes. X-ray diffraction showed that the pre-alloyed ball-milled Ni49.7Mn30Ga20.3 powder has the austenite phase, however, twinned 14 M modulated martensite is present over the entire sintering temperature range with an increasing formation of martensite twins with temperature. At the low temperature sintering regime (<1020 °C), solid-state sintering results in densities of ∼45%, consistent composition, phase transformation, Curie temperature and magnetization. In the medium temperature sintering regime (1020 °C–1080 °C), solid-state sintering with grain boundary diffusion leads to densities up to ∼80% and otherwise similar properties as the low temperature regime. In the high temperature sintering regime (1090 °C–1100 °C), liquid phase sintering is dominant leading to densities up to ∼99% but due to segregation at the grain boundaries, broadening of transformation temperatures and lower saturation magnetization. In conclusion, binder jet 3D printing of Ni-Mn-Ga alloys show potential to enable functional, complex-shaped elements, and intentional porosity might allow these polycrystals to exhibit the magnetic field induced strain by reducing constraints between neighboring grains.

Introduction

Ni-Mn-based ferromagnetic alloys have attracted attention due to both the magnetocaloric [1] and magnetic shape memory effects [2]. Based on the composition and microstructure of the magnetic shape memory materials, they can be driven with frequencies up to the kHz regime. In combination with the very large strain and high energy density, this allows for novel applications, which are not feasible using other adaptive materials [3,4]. Ni-Mn-Ga magnetic shape memory alloys (MSMAs) are well-known for showing a magnetic-field-induced strain (MFIS). This MFIS is pseudoplastic and reversible, meaning it is permanent if the magnetic field is removed and can be reversed via an applied perpendicular magnetic field or mechanical stress [5]. The mechanism of MFIS is based on twin boundary motion [6] that results in the reordering of crystallographic domains in an applied magnetic field which lowers magnetization energy [7,8]. The largest MFIS (up to 10%) can be achieved in single crystals when optimally oriented to the magnetic field [6,7]. Polycrystalline bulk Ni-Mn-Ga alloys are not an alternative since neighboring grain are constraining each other resulting in very limited MFIS [9,10]. However, Ni-Mn-Ga single crystals show significant chemical segregation during single crystal growth affecting local composition, crystal structure, and magnetoplastic strain [11].

Two solutions have been proposed to increase the MFIS of polycrystalline Ni-Mn-Ga including (1) increasing porosity to reduce constraints [12] and (2) introducing texture, for instance by directional solidification leading to preferred grain orientation [13]. Boonyongmaneerat et al. [14] showed that introducing pores in polycrystalline Ni-Mn-Ga reduces the internal constraints of grain boundaries to the point that the material displayed a repeatable MFIS of 0.12%. Chmielus et al. [12] showed the effects of internal and external constraints on MFIS can be reduced by modifying the architecture of the foam with a bimodal pore size distribution, and thereby increase the MFIS to 8.7%. Nilsén et al. [15] used spark plasma sintering to produce Ni-Mn-Ga foam structures and found that the twin movement was facilitated by reduced grain boundary constraints, thus, the measured MFIS of 1.24% was higher than the value reported for coarse-grained Ni-Mn-Ga foams [13]. The main mechanism of reducing internal and external constraints via porosity and foam architecture is based on the reduction of twin-twin and twin-grain boundary interactions [12].

Nevertheless, foam production by replication casting or spark plasma sintering requires the use of strong acids and is time-consuming. In our earlier studies, we investigated binder jet printing (BJP) for structural materials as a fast, low-cost additive manufacturing method which enables complex geometries [16]. Binder jetting is suitable for producing near-net shape parts with controlled porosity, which may be sufficient to overcome grain boundary constraints in polycrystalline Ni-Mn-Ga. The ability to fabricate porous and near net-shape parts with complex geometries could have broad applications in energy applications, lightweight structures, magnets, and sensors [17]. Recently, Caputo and Solomon [18] and Mostafaei et al. [19] reported binder jet printing of Ni-Mn-Ga from pre-alloyed ball-milled powder with angular shape. It was found that parts with green density of ∼40% can be binder jet printed and the subsequent sintering may increase density. In our earlier study [19], sintering of the BJP Ni-Mn-Ga part is the most challenging step as oxidation and Mn evaporation may cause compositional and microstructural variations leading to the presence of oxides and different types of martensites such as five-layered (10 M), seven-layered (14 M) and/or non-modulated (NM) martensite.

Here, we propose a reliable method to produce MSMAs with controlled relative densities from 45% to 99% via binder jet printing and subsequent sintering without oxidation and composition changes and gradients. Detailed investigations are conducted on the densification, microstructure evolution, thermal and magnetic properties of the BJP and differently sintered samples. The proposed manufacturing process resulted in the formation of structures with microstructural, magnetic and thermal properties similar to the conventionally produced bulk samples.

Section snippets

Materials and methods

Ni-Mn-Ga polycrystalline ingots were prepared by induction melting of high purity elements of Ni (99.995%, Kurt J. Lesker), Mn (99.95%, Kurt J. Lesker) and Ga (99.999%, Alfa Aesar) under an argon atmosphere. Ingots with total weight of ∼400 g were crushed and ball-milled using a Retsch planetary mill (model PM100) with a speed of 450 rpm for 3 h. Powder with particle size less than 63 μm was obtained by sieving. Fig. 1 illustrates an SEM micrograph of the angular Ni-Mn-Ga powder indicating the

Porosity and relative density measurements

Fig. 2 displays the density versus sintering temperature relation of the sintered samples characterized with OM and ImageJ image analysis as well as with Archimedes' method. The results of both methods show similar densification trends as sintering temperatures increased from 1000 °C to 1100 °C. The systematically higher density for samples sintered up to 1080 °C measured via Archimedes compared to OM densities is caused by detaching weakly bonded particles during the grinding and polishing

Sintering regimes and evolution of microstructure and properties

The sintering process is a critical step in BJ3DP leading to densification of the green part happens through a diffusion at high temperature. While the strength of the green part is due to the polymeric binder added during the printing process and cross-linking during curing, high mechanical strength can be achieved after sintering with a large controlled variation of final densities and microstructures. Since the green part's density is ∼40–50%, the driving force of the sintering is primarily

Conclusion

Binder jet 3D printing was used to additively manufacture samples made from angular Ni-Mn-Ga alloy powder and then sintered at temperatures ranging from 1000 °C to 1100 °C for 2 h. The green specimen had a density of ∼40% and depending on the applied sintering temperature, solid-state or liquid phase sintering mechanisms dominated the sintering process. It was shown that three different sintering temperature ranges, here identified as low temperature sintering regime (<1020 °C), medium

Acknowledgements

Authors would like to acknowledge Dr. Joel Gillespie (director of Materials Characterization Lab (MCL), Dietrich School for Arts and Sciences Shared Research Support Services (SRSS), University of Pittsburgh) for DSC tests, Dr. Volodymyr A. Chernenko for discussions and Jakub Toman, Katerina A. Kimes and Colleen Hilla for their assistance with alloy preparation. The authors would like to acknowledge partial funding from the National Science Foundation [NSF grant number 1727676]. ELS

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