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PRINCIPAL PUBLICATION AND AUTHORS

In-situ synchrotron imaging of keyhole mode multi-layer laser powder bed fusion additive manufacturing, Y. Chen (a,b), S.J. Clark (a,b), C.L.A. Leung (a,b), L. Sinclair (a,b), S. Marussi (a,b), M.P. Olbinado (c), E. Boller (c), A. Rack (c), I. Todd (d), P.D. Lee (a,b), Appl. Mater. Today 20, 100650 (2020); https:/doi.org/10.1016/j.apmt.2020.100650 (a) UCL Mechanical Engineering, University College London (UK) (b) Research Complex at Harwell, Rutherford Appleton Laboratory, Didcot (UK) (c) ESRF (d) Department of Materials Science and Engineering, University of Sheffield (UK)

and elucidate the phenomena that control the formation of imperfection during laser printing. As the experiments involved complex laser, powder, and X-ray safety aspects, support from ID19 s beamline scientists and ESRF s technical teams contributed significantly.

The melt pool dynamics during keyhole mode operation was investigated, including its relationship with porosity and spatter formation mechanisms in Ti-6Al-4-V (Figure 111). The melt pool was observed to change in a cyclic manner with a transient separation of the portion of the melt pool in front of the laser beam. How this cyclical process is related to the formation of microstructural features was elucidated, in particular for keyhole porosity and spatter, which are potentially detrimental to the mechanical performance of the final 3D printed components.

The evolution of a multi-layer melt track during LPBF was examined and it was found that the keyhole undergoes a transient cyclic phenomenon where a vapour depression zone is created, changing the keyhole pressure, and altering its shape by Marangoni convection. This mechanism can be summarised in three stages: keyhole initiation the formation of a vapour depression zone due to the laser-induced metal vaporisations; keyhole development

wherein the powder is being entrained into the laser-matter interaction zone and then promotes the formation of droplet spatter; and the molten pool recovery a change in pressure causing the keyhole to re-open.

From this study, it was also found that the keyhole porosity tends to form in the second and third stages described in the cyclic event where a pore is formed when the fast- moving metal vapour pushes the molten liquid around it. Besides this, the cyclic oscillation of the keyhole also promotes spatter ejected from the laser-matter interaction zone. High-speed X-ray images revealed how the droplet is formed by the agglomeration of powder particles at the second stage of the cyclic event.

The benefits of this work include a clarification of the physical understanding behind the keyhole-mode LPBF. These new insights can be coupled with modelling to improve the quality of LPBF-built components. The mechanisms observed and the understanding gained from this study can be transferrable to other beam- based processing techniques, such as laser/electron beam welding and other forms of powder bed fusion additive manufacturing where keyhole mode porosity and excessive spatter need to be avoided.

Fig. 111: Selected time series of radiographs during laser 3D printing of Ti-6Al-4V. a) Keyhole porosity forms during the first layer (L1). b) The interaction between the laser beam and prior melt features, where L1 to L4 stands for the first to fourth layers, respectively.