Laser powder bed fusion of Ni-Mn-Ga magnetic shape memory alloy

Abstract

Additive manufacturing (AM) has gone through major developments in the past decade, enabling the rapid manufacture of complex geometries from traditional engineering materials. This study aims to facilitate the development and additive manufacturing of a new generation of fast and simple digital components with integrated magnetic shape memory (MSM) alloy sections that can be actuated by an external magnetic field. Here, we employ a systematic design of experiments (DoE) approach for investigating laser powder bed fusion (L-PBF) of a Ni-Mn-Ga based MSM alloy. The effects of the applied process parameters on the chemical composition and relative density are determined, and detailed investigations are conducted on the microstructural properties of the as-deposited material obtained using optimized parameters. The results show that although the L-PBF of Ni-Mn-Ga is characterized by an ever-present loss of Mn, deposition of Ni-Mn-Ga with a high relative density of 98.3% and a minimal loss of Mn at ∼1.1 at.% is feasible. The material produced in this manner was compositionally near homogenous and, in as-deposited condition, consisted of a mixture of 14 M and non-modulated (NM) martensites. However, combined measurements by the low-field ac magnetic susceptibility method (LFMS) and DSC revealed that the phase transformation of the as-deposited material from martensite to austenite, and vice versa, was broad and occurred in a paramagnetic state. Inspection by SEM revealed a layered microstructure with a stripe-like surface relief that originated from the presence of martensitic twins within the sample. Additionally, AFM and MFM measurements showed that in as-deposited Ni-Mn-Ga, there exists a weak MFM contrast that can be attributed to the twinned martensite having magnetic anisotropy. Overall, L-PBF shows high potential for the production of functional Ni-Mn-Ga based MSM alloys.