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

Abstract

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.

Residual Stress in Additive Manufacturing : Control using orientation and scan strategies

“…When the layer thickness is increased, LOF defects occurrence is observed more which also narrows down the optimal processing window. In contrast, keyhole defects are formed when a very high laser power in combination with a lower speed is used, causing excessive thermal energy being focused on a tiny volume [54][55][56][57].…”