Novel in-situ alloying strategy of laser powder bed fusion for fabricating crack-free and composition-homogeneous NiTi alloys

In-situ alloying for adjusting the composition of alloys is a promising approach utilizing pre-mixed powders in laser powder bed fusion (LPBF). However, inhomogeneous composition has always hindered the application of this approach. In this study, the Ni_51.8Ti_48.2 at.% alloys via in-situ alloying exposed severe cracking, due to inhomogeneous composition and inappropriate heat accumulation. To overcome these difficulties, we optimized the process parameters between volumetric energy density (VED) and layer thickness (t) by adjusting the scanning speed (v) and fabricated the crack-free, homogeneous, and dense NiTi alloys, thereby proposing a novel in-situ alloying strategy (VED-t). Computational fluid dynamics (CFD) results demonstrate that the synergistic effects of VED and t significantly extend the melt pool lifespan and change the energy distribution of the melt pool, thus achieving compositional homogenization and porosity minimization. The strategy can also regulate the cracking tendency and crystallographic orientation by changing the melt pool type (connection, keyhole, and flat). The alloys with a B2 matrix possess a strong [001] orientation along the building direction and thus exhibit good superelasticity and elastocaloric cooling under compressive loading. These findings provide a novel approach for printing brittle alloys and drive the adoption of the in-situ alloying strategy of LPBF.