On the powder chilling effect in laser based directed energy deposition

Understanding the thermal behavior during laser based directed energy deposition (LDED) is crucial for the grain structure control for superior and bespoke mechanical performance. Transient and localized chilling effect induced by the melting behavior of injected powder particles during the LDED process, plays a similar role of the cold mold surface in casting on solidification but has received little attention in the past. Here, we employ low energy density to partially retain the fine-grained powder particles during the deposition process, serving as tracers to study the influence of powder particle melting heat absorption on solidification. High-speed camera and infrared camera are used to real-time record the dynamic and thermal interactions between the powder particles and melt pool. Results show that powder particles gradually melt and absorb heat, leading to chilling effect on the melt pool at a millimeter scale. The temperature at the interaction position determines whether powder particles can penetrate the melt pool, thereby affecting the melting mode. Compared to floating powder, powder entering the melt pool can cause larger temperature drops. The collective powder chilling effect induced by multi-particle powder flow results in significant fluctuations in melt pool shape, maximum temperature, average temperature. Consequently, the powder chilling effect increases the average solidification rate at the tail of the melt pool to nearly three times, reduces the temperature gradient at the solid-liquid interface by 45 %, promoting the columnar-to-equiaxed transition during solidification. This study could be valuable in the additive manufacturing of single crystal and fine-grained components.