  Fabrication of nickel-titanium shape memory alloy through additive
manufacturing has attracted increasing interest due to its advantages of
flexible manufacturing capability, low-cost customization, and minimal defects.
The process parameters in directed energy deposition (DED) have a crucial
impact on its molten pool characteristics (geometry, microstructure, etc.),
thus influencing the final properties of shape memory effect and
pseudoelasticity. In this paper, a three-dimensional numerical model
considering heat transfer, phase change, and fluid flow has been developed to
simulate the cladding geometry, melt pool depth, and deposition rate. The
experimental and simulated results indicated that laser power plays a critical
role in determining the melt pool width and deposition rate while scan speed
and powder feed rate have less effect on cladding geometry and deposition rate.
The fluid velocity has a huge influence on the distribution of elements in the
molten pool. The temperature gradient G, solidification rate R, as well as
shape factor G/R were calculated to illustrate the underlying mechanisms of
grain structure evolution. The grain morphology distribution of cross-section
from the experimental samples agreed well with the simulation results. The
model reported in this paper is expected to shed light on the optimization of
the deposition process and grain structure prediction.
