Gas metal arc (GMA) directed energy deposition (DED), also known as wire arc additive manufacturing (WAAM), operates on the same principle as gas metal arc welding (GMAW). Both processes use the electric arc to melt and deposit filler metal onto a substrate. However, the geometry of the part being printed by DED can pose boundary conditions to arc and molten pool physics far different from those in GMAW. For example, the arc plasma and shielding gas flow behaviors for DED single bead per layer “walls” that are thin and / or tall can be quite different than those for bead-on-plate welds by GMAW. Hence, it is important to understand how the DED part geometry affects the arc and molten pool physics to ensure deposit quality. In this study, a computational magnetohydrodynamic (MHD) model was developed to simulate the complex interactions between the arc, the shielding gas, and the deposited material during DED of a tall wall.
To describe the complex physics in arc and molten pool flows, the MHD model was developed to numerically solve a range of partial differential equations. First, Maxwell’s equations were solved to obtain electric potential and magnetic field vector. These quantities in turn were used to compute the Lorentz force, a primary driving force for arc plasma and molten metal flows. The temperature and velocity fields were computed by solving the governing equations of mass, momentum and energy conservation. Moreover, two mass transport equations were solved, one for oxygen and the other for nitrogen, to obtain the chemical composition field.
Material properties for shielding gas, Ar-CO2 mixture, were taken from the literature. The material of substrate and filler wire is AISI 316 stainless steel. The model was applied to study how nozzle and substrate geometries, shielding gas flow rate, and welding parameters affect the temperature and flow fields.
Results and Conclusions:
The model was used to predict nitrogen and oxygen concentrations around the molten pool for a range of welding parameter and shielding gas flow conditions. In particular, the concentrations were found to increase when the cup to work distance was too large, or the shielding gas flow rate was too low. Shorter cup to work distances provided better local shielding at low shielding gas flow rates. The calculated results were found to correlate well to the available experimental data of weld metal composition.
Wire arc additive manufacturing; directed energy deposition; additive manufacturing; shielding gas modeling; magnetohydrodynamic