Design and Investigation of a Novel Local Shielding Gas Concept for Laser Metal Deposition with Coaxial Wire Feeding

Abstract

Laser metal deposition with coaxial wire feeding is a directed energy deposition process in which a metal wire is fed to a laser-induced melt pool. Oxidation occurring during the process is a major challenge as it significantly influences the mechanical properties of the produced part. Therefore, an inert gas atmosphere is required in the high temperature process zone, whereby local shielding offers significant cost advantages over an inert gas chamber. In this work, a novel local shielding gas nozzle was developed based on basic methods of fluid mechanics. A gas flow-optimized prototype incorporating internal cooling channels was additively manufactured by laser-powder bed fusion and tested for its effectiveness via deposition experiments. Using the developed local shielding gas concept, an unwanted mixing with the atmosphere due to turbulence was avoided and an oxide-free deposition was achieved when processing a stainless steel ER316LSi wire. Furthermore, the effects of the shielding gas flow rate were investigated, where a negative correlation with the melt pool temperature as well as the weld bead width was demonstrated. Finally, a solid cuboid was successfully built up without oxide inclusions. Overheating of the nozzle due to reflected laser radiation could be avoided by the internal cooling system. The concept, which can be applied to most commercially available coaxial wire deposition heads, represents an important step for the economical application of laser metal deposition.

A Review on Wire-Laser Directed Energy Deposition: Parameter Control, Process Stability, and Future Research Paths

“…Beads produced with local shielding exhibited a brighter surface, indicative of reduced oxygen presence, as discoloration serves as an indicator of residual oxygen. Additionally, as illustrated in Figure 25, an increased flow rate was found to decrease ripples in the bead [176], which is attributed to a higher cooling rate resulting from convection in increased shielding gas flow [174,177].…”

“…Conventional LDED heads typically feature central shielding gas feed mechanisms, i.e., filling the entire fabrication chamber with inert gas. However, central shielding can be costly and has disadvantages, including limitations on the possible size of fabricated parts due to the chamber size [174,175]. This method is known to cause turbulence in the gas stream, resulting in a higher and problematic intake of oxygen [54].…”

“…A further reduction of oxidation can be obtained by implementing local shielding only in the fabrication area [101]. The latter approach, as developed and evaluated by Bernuar et al for coaxial W-LDED, offers benefits regarding gas usage, equipment complexity, and setup duration [174]. Their study examined the effects of shielding gas flow rate on bead geometry and the temperature of the melt pool.…”

“…Therefore, it is commonly used in W-LDED to maintain process stability. Various controllers such as PI (Proportional-Integral), PLC (Programmable Logic Controller), MPC (Model Predictive Control), and ILC (Iterative Learning Control) have been utilized in research to implement closed-loop control strategies in W-LDED processes [128,135,174,213,222,223].…”