On the in-flight powder vaporization under laser irradiation in directed energy deposition

The pursuit of high-efficiency Laser Directed Energy Deposition (L-DED) has significantly increased laser power and powder feed rates, shifting the dominant heat and mass transfer mechanism from laser–substrate to laser–powder interaction. Under high-energy laser irradiation, in-flight powder vaporization—a phenomenon previously considered negligible—has become increasingly prominent. However, its underlying physical mechanisms and subsequent impact on material properties remain poorly understood. This study simplifies the complex collective behavior of multi-particle powder streams by investigating a controlled number of Ti6Al4V particles during free-fall through a high-energy laser beam. High-speed imaging was employed to directly observe the vaporization/condensation halos resulting from the in-flight vaporization and subsequent condensation of powder particles. Through grayscale and position observations of the temperature rise process of individual powder particles, combined with a theoretical model considering both laser melting and vaporization, a quantitative study of the temperature and velocity changes of the powder particles under laser irradiation was conducted. It is proposed that the non-uniform laser-induced vaporization of the melted powder is the source of powder particle velocity variation. Post-mortem characterization indicates that the vaporization is primarily driven by the preferential vaporization of aluminum, with a composition loss exceeding 2.8 wt%.As Al is a critical α-stabilizing and solid-solution strengthening element, its depletion may weaken the composition-related strengthening contribution and affect the subsequent microstructural stability. Theoretical strengthening analysis suggests that Al depletion may lead to an estimated weakening tendency of approximately 25 MPa under the same microstructural condition, which should be regarded as a theoretical indication rather than a directly measured strength decrease. Consequently, the in-flight vaporization during laser–powder particle interaction must be circumvented through meticulous laser thermal modulation, particularly for alloy systems comprising volatile elements. This work provides both a robust empirical foundation and a theoretical framework for scientists and engineers to advance the next generation of high-efficiency additive manufacturing.