Experimental study on the influence of wall thickness and scanning strategy in laser powder bed fusion of Ni-Mn-Ga magnetic shape memory alloys

This dissertation examines the optimization of laser powder bed fusion (PBF-LB) for the fabrication of Ni–Mn–Ga magnetic shape memory (MSM) alloys with controlled crystallographic orientation and improved magneto-functional properties. The primary objective is to facilitate additive manufacturing of single- and oligocrystalline MSM structures appropriate for high-performance microactuators. The principal contributions of this research are listed as follows: A modular build-volume reduction kit was developed to support cost-effective experimentation with small quantities of novel alloy powders. The system features a detachable substrate-preheating module and significantly reduces material use, which is critical for compositionally tuned and expensive Ni–Mn–Ga powders. Using this setup, epitaxial solidification was achieved in thin-walled Ni–Mn–Ga structures, producing coarse oligocrystalline 10M martensite with minimal crystallographic misorientation. A systematic evaluation of PBF-LB process parameters, including scanning strategy, volumetric energy density (VED), and scan vector rotation, was conducted to assess their effects on densification, microstructure, phase transformation, and magnetic performance. Results show that low VED conditions cause a Mn loss of about 1 at. %, producing compositions closer to the feedstock powder and improving magnetic response with a 10M martensite structure. Bidirectional scanning achieved higher densification and fewer defects than unidirectional paths. Additionally, X-ray diffraction and magnetic measurements show a strong correlation among process parameters, crystal structures, and resulting magneto-mechanical behavior. Intermartensitic transformations were observed for the first time in PBF-LB-manufactured Ni–Mn–Ga alloys, indicating the formation of metastable microstructures and greater magnetic anisotropy. In summary, these findings advance understanding of magneto-structural coupling in additively manufactured Ni–Mn–Ga alloys and guide the design of energy-efficient, highfrequency MSM actuators for next-generation adaptive devices.