Identifying the keyhole stability and pore formation mechanisms in laser powder bed fusion additive manufacturing

The keyhole phenomenon is uncovered to exist in the printable range of laser powder bed fusion process. The pores due to unstable keyhole collapse can be divided into bottom-pore (B-pore) and rear-pore (R-pore) based on the formation mechanisms. Till now, how to distinguish keyhole stability and predict pore formation remains unclear. Here, a dimensionless number (Rkl) is determined to establish a relationship between the keyhole morphology and pore formation. When Rkl is below 0.8, B-pore occurs at the keyhole bottom due to the bridge formed between the front and rear keyhole walls. With the increase of Rkl and the standard deviation, pore formation mechanisms gradually change to R-pore in which a bubble pinches off at the rear wall. The cases with no pore formation corresponding to stable keyhole. These precise thresholds can be further specified with more in situ imaging data in the future. The fluid dynamics under the two pore formation mechanisms are further explored via a multiphysics simulation model. R-pore is smaller and has large probability to escape due to the smaller escaping distance and thicker liquid film. In addition, there is a tendency from B-pore to R-pore under higher ambient pressure, providing a solution for pore regulating in laser powder bed fusion. Based on the analysis results, it is expected to guide the laser-material interaction process to fabricate high performance parts by the keyhole morphology, especially under the high energy input to improve the efficiency, such as high-energy printing and welding.