Thermal-fluid behavior, microstructure and mechanical properties in liquid bridge transfer mode during directed energy deposition-arc additive manufacturing - Insights using NiTi as a model alloy

During Directed Energy Deposition-Arc (DED-Arc), the droplet transfer, molten dynamics and temperature characteristics greatly impact the morphology and microstructure of the as-built component. In the study, a commonly used computational fluid dynamics (CFD) model, which ignores the droplet growth (CFDc model), and an improved CFD model considering the droplet growth (CFDi model) were coupled to balance the calculation efficiency and accuracy. A NiTi shape memory alloy is used as a model alloy. The CFDi model can inherit the thermophysical data of the molten pool in a quasi-steady state calculated by CFDc model and continue the subsequent calculations. With the coupled CFDc-CFDi model, the metal transfer phenomena in the liquid bridge transfer (LBT) and free droplet transfer (FDT) modes were well compared. The numerical results are in excellent agreement with the experimental data. It is shown that the metal transfer in LBT mode is more stable than in the FDT mode, resulting in a more stable molten pool and better forming quality. Besides, the LBT mode is also superior to FDT mode due to the narrower phase transformation range and better mechanical properties of the as-built samples. The present findings lay foundations for optimizing the DED-arc process for any engineering metallic alloy.