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, 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.

Dual-channel inline coherent imaging.

We present a dual-channel inline coherent imaging system for laser machining monitoring using a single spectrometer. Sensitivity enhancement due to the added signal of the two input channels is demonstrated with a maximum sensitivity of 99 dB at a 73 kHz acquisition rate. We also treat, theoretically and experimentally, dual-channel detection in the case of signal saturation. A method to mitigate saturation artifacts while maintaining high signal-to-noise ratio is shown.