Molecular Dynamics Simulation of NiTi Shape Memory Alloys Produced by Laser Powder Bed Fusion: Laser Parameters on Phase Transformation Behavior.

ABSTRACT

In this study, the deposition, powder spreading, and laser fusion processes during the laser powder bed fusion (L-PBF) process were studied using molecular dynamics (MD) simulation. The effect of Ni content on the characteristic phase transformation temperatures was also investigated. Shape memory effect and superelasticity of NiTi alloys with Ni content ranged from 48.0% to 51.0% were analyzed. By employing MEAM potentials, the effects of the laser power, spot diameter, and scanning speed on the molten pool size and element evaporation were studied. Simulation results showed that a larger spot diameter renders a higher Ni content in the molten pool, also a larger molten pool. A faster scanning speed leads to a higher Ni content in the molten pool, and a smaller molten pool. The element is difficult to evaporate using small laser power and a large spot diameter. The element in the molten pool expresses a great evaporation effect when the Es is larger than 0.4 eV/ų. According to Ni content within the molten pool during laser fusion, characteristic phase transition temperatures in single crystalline NiTi alloys with variant Ni content were investigated by employing a 2NN-MEAM potential. Characteristic phase transition temperature changes as the Ni content increases from 48.0% to 51.0%. Austenite boundaries and Ni content in the boundary were found to be the keys for controlling the characteristic phase transformation temperature.