Understanding the formation of laser-induced melt pools with both wire and powder feeding in directed energy deposition

Abstract

Wire-powder fed directed energy deposition (WP-DED) has been applied to produce high-performance metal parts. Compared to DED with the feed of a single filler (either powder or wire), simultaneous WP-DED exhibits more complex interactions between fillers and laser-induced melt pools, such as additional powder collisions by wire, different filler properties, intricated heat transfer paths, etc. At present, there is little mechanistic understanding of the interactions among wire, powder particles, and laser-generated melt pools in the WP-DED. In this work, we discovered the filler-melt pool interactions in a coaxial wire-powder fed DED (CWP-DED) process by in situ experimental characterizations. In particular, we investigated the wire-influenced particle dynamics before entering the melt pool. When in-flight particles interact with the melt pool, the temporal-varied melt pool geometry as well as melt flow dynamics were characterized. The bubble formation and evolution in CWP-DED were also revealed. We found that the extruded wire would affect the trajectory of in-flight particles and reduce their velocity before the entry of melt pool due to the inelastic collision. The melt pool size decreased with powder feed rate; however, the powder feed rate had limited influences on the melt pool oscillation. Four modes of fluid flow were observed in the melt pool, including backward flow, upwards flow to the wire-melt pool interface, rebounded flow, and clockwise flow. At a high powder feed rate, more gas-induced bubbles occurred along the melt pool boundary and wire-melt pool interfaces. This study provides a fundamental understanding of filler-melt pool interactions during the CWP-DED process to advance the development of next-generation, fast-rate additive manufacturing technology.