| Abstract Scope |
Laser powder blown directed energy deposition (DED-LB) has become a major focus within metal additive manufacturing due to its 5-axis processing capability and propensity for functionally graded materials. While it is known that shield gas flow rate and powder flow rate play a critical role in deposition of material, it is not well understood how shield gas interacts with the melt pool. To better understand the role of shield gas in DED-LB, in-situ high-speed Schlieren imaging was incorporated into the System for AM Alloy Development (SAMAD) at NIST. Single tracks of IN625 were deposited at shield gas flow rates of 5 LPM, 10 LPM, and 15 LPM. Schlieren videography results reveal that the flow field is dominated by the particle-laden carrier-plus-shield gas jet directed at the deposition surface, creating an axisymmetric stagnation point flow with the melt pool at its center. Complicating this classic stagnation point flow is the presence of a counterflowing vapor jet emanating upwards from the melt pool. The schlieren results show that initial momentum of the vapor jet is significant, in some cases rising up to reach the tip of the nozzle. The vapor jet momentum appears to differ depending on the laser power, spot geometry and scan speed. More concentrated laser spots and slower scan speeds lead to a larger initial vapor jet. During deposition, the two counterflowing jets gives rise to a “vapor dome” which fluctuates in size as the nozzle moves across the deposition surface. The height and width of Schlieren activity is found to be minimal at a shield gas flow rate of 10 LPM, and approaches to a stability model are discussed. |