Evaluation of spatter particles, metal vapour jets, and depressions considering influence of laser incident angle on melt pool behaviour

Building of practical parts involves the application of metal-based laser powder bed fusion using a laser beam (PBF-LB/M) owing to its high-precision manufacturing. However, the quality of the built parts obtained via the PBF-LB/M processes varies with the building conditions, and a thorough understanding of the building mechanism has not been achieved owing to the complex and interrelated process parameters involved. The incident angle of the laser beam, which changes on the platform during the laser beam scan owing to the designed three-dimensional data, is among the principal parameters that affects the building aspects. In this study, the melt pool in the singletrack formation during the PBF-LB/M processes was visualised using a high-speed camera, and the influence of the laser incident angle on the ejection characteristics of spatter particles formed around the laser-irradiated area was investigated. Consequently, the spatter particles and metal vapour jet behaviour varied with the laser incident angle. There was a reduction in number of spatter particles owing to the origin of the incident direction being from behind the laser irradiation area. In addition, the laser incident angle also affected the melt pool morphology because of the depression in the melting. Furthermore, the burial depth of the pores varied with the laser incident angle, and is related to the depth of the depression during the melt pool formation.

Keyhole fluctuation and pore formation mechanisms during laser powder bed fusion additive manufacturing

Keyhole porosity is a key concern in laser powder-bed fusion (LPBF), potentially impacting component fatigue life. However, the dynamics of keyhole porosity formation, i.e., keyhole fluctuation, collapse and bubble growth and shrinkage, remain unclear. Using synchrotron X-ray imaging we reveal keyhole and bubble behaviours, quantifying their formation mechanisms. The findings support the hypotheses that: (i) keyhole porosity can initiate not only in unstable, but also transition keyhole regimes, created by high laser power-velocity conditions, causing fast radial keyhole fluctuations (~ 10 kHz); (ii) transition regime collapse tends to occur part way up the rear-wall; and (iii) immediately after keyhole collapse, the bubble grows as pressure equilibrates then shrinks due to metal-vapour condensation. Concurrent with condensation, hydrogen diffusion into the bubble slows the shrinkage and stabilises the bubble size. The physics revealed here can guide the development of real-time monitoring and control systems for keyhole porosity.