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

TLDR

Keyhole porosity in laser powder‑bed fusion is a major concern that can affect component fatigue life, yet the mechanisms of keyhole fluctuation, collapse, and bubble growth and shrinkage remain unclear. The study aims to reveal keyhole fluctuation and bubble evolution mechanisms to inform control system development for minimizing porosity. Synchrotron X‑ray imaging was used to observe and quantify keyhole and bubble dynamics during laser powder‑bed fusion. Keyhole porosity can initiate in both unstable and transition regimes, with fast radial fluctuations (2.5–10 kHz); collapse tends to occur partway up the rear wall, and post‑collapse bubbles rapidly grow then shrink due to metal‑vapour condensation, while hydrogen diffusion slows shrinkage and stabilizes bubble size.

Abstract

Abstract Keyhole porosity is a key concern in laser powder-bed fusion (LPBF), potentially impacting component fatigue life. However, some keyhole porosity formation mechanisms, e.g., keyhole fluctuation, collapse and bubble growth and shrinkage, remain unclear. Using synchrotron X-ray imaging we reveal keyhole and bubble behaviour, quantifying their formation dynamics. The findings support the hypotheses that: (i) keyhole porosity can initiate not only in unstable, but also in the transition keyhole regimes created by high laser power-velocity conditions, causing fast radial keyhole fluctuations (2.5–10 kHz); (ii) transition regime collapse tends to occur part way up the rear-wall; and (iii) immediately after keyhole collapse, bubbles undergo rapid growth due to pressure equilibration, then shrink due to metal-vapour condensation. Concurrent with condensation, hydrogen diffusion into the bubble slows the shrinkage and stabilises the bubble size. The keyhole fluctuation and bubble evolution mechanisms revealed here may guide the development of control systems for minimising porosity.