Observations of particle-melt pool impact events in directed energy deposition

In the rapidly growing field of Additive Manufacturing (AM), the Laser Directed Energy Deposition (L-DED) process is the focus of intense technical attention due to its potential to generate high quality components with location specific composition and microstructural control. Despite the variety of experimental and modelling efforts devoted to the subject, no studies directly observe the interactions between individual powder particles and the liquid pool of metal at a high enough temporal frequency to characterize these discrete contact events. The frequency and nature of these powder-pool impingements govern overall process behavior, and are a poorly quantified fundamental building block of L-DED. In this work, we report novel results in which the melt pool is imaged at up to 200,000 frames per second, with pixel resolution of up to 3.6 μm. Video images reveal that particles often impact and float on the surface of the melt pool for several hundreds of microseconds before melting into it. Further incoming particles were observed to rebound from the melt pool by these floating particles. Through modelling this process analytically, particle self-shielding is shown to impose unavoidable upper limits on overall powder capture efficiency for the L-DED process.