Epitaxial Re-solidification of Magnetic Shape-Memory Alloy Single Crystals under a Laser Heat Source and in Direct Laser Deposition

Magnetic shape-memory alloys (MSMAs) deform upon exposure to a magnetic field, with magnetically-driven strains of 6 – 12% to date and demonstrated application to actuators. Samples that have undergone such strains have been produced by single crystal growth, replication casting of porous foams, or extraction of a grain from a polycrystal. Additive manufacturing (AM) of MSMAs could allow fuller use of geometry in part design, and would avoid the segregation and potentially the high cost associated with single crystal growth. While much research in AM of MSMAs has pursued porous polycrystals, liquid-phase AM may produce epitaxial growth during solidification, as reported for directed energy deposition of Ni superalloys, whereby the solidifying material adopts the crystal orientation of preexisting solid and extends a single crystal. Thus, AM might be used to produce dense, single-crystalline MSMA parts with capacity for full blocking force. To explore solidification parameters without the influence of impinging powder, Ni2MnGa austenite single crystals were melted with a moving laser spot under several combinations of laser power and velocity. While tracks created with lower laser travel velocity were almost entirely epitaxial (single-crystalline), tracks created with high power (300 W, 350 W) and high velocity (10 mm/s) included grains at their tops, demonstrating a breakdown of epitaxy. Conversely, all but one track created with 200 – 250 W and 2.5 – 10 mm/s presented no significant grains on EBSD orientation maps. Far-field high-energy diffraction microscopy (HEDM) identified single-digit numbers of grains in samples from three of these tracks. Near-field HEDM experiments with three other samples revealed that mosaic spread of epitaxial material was slightly higher than that of surrounding non-re-solidified material. Epitaxial solidification had cellular or cellular-dendritic morphology, and in one track, transitioned to dendritic morphology while maintaining epitaxy. Furthermore, a track misoriented from a substrate’s [100] axis by ~40° (near [110]) was found to be near-completely epitaxial, demonstrating tolerance for misalignment and perhaps suggesting importance of nuclei density. Deposition tracks with austenite powder included grains at their tops. Although only one section per sample was mapped by EBSD, no grains extended below the former plane of the substrate surface in these sections