Printing regime for single metal microdroplet deposition in laser-induced forward transfer

Abstract

Laser-induced forward transfer (LIFT) is proposed as an additive manufacturing technique for high-resolution metallic microstructures. However, the printing regime of single microdroplet deposition is lacking, making it difficult to determine the processing parameters for single microdroplet printing. In this study, both LIFT experiments and thermal simulations are carried out to investigate the printing behaviors of a single microdroplet under different laser fluences and spot sizes. From the LIFT experiments, three printing behaviors are observed, including the non-release deposition, the single microdroplet deposition, and the splash deposition. The printing regime for single metal microdroplet deposition is obtained via varying laser pulse energy and spot size, where three printing behaviors are bounded by three thresholds. Lower threshold of the laser fluence for single microdroplet deposition is required to be constant regardless of the spot size, owing to the induced kinetic energy just overcoming the surface energy, which satisfies the ejection condition of Weber number greater than 1. Upper threshold of the laser fluence for single microdroplet deposition is observed to linearly increase with spot size, indicating that the rupture mode of the molten film determined by the nominal capillary number is closely related to the laser spot size. By analyzing the physical basis of microdroplet ejection at the threshold, Weber number and nominal capillary number are proposed as the criteria for the low and high thresholds of single microdroplet deposition.