Synchrotron Imaging of Keyhole Mode Multi-layer Laser 1 Powder Bed Fusion Additive Manufacturing

10 The keyhole mode in laser powder bed fusion (LPBF) additive manufacturing can be associated with 11 excessive porosity and spatter, however, the underlying physics in multilayer build conditions remain unclear. 12 Here, we used ultra-fast synchrotron X-ray imaging to reveal this phenomena. We revealed melt pool 13 dynamics, keyhole porosity and spatter formation mechanisms and their impact in all layers of the build. We 14 observed that the transient melt pool dynamics associated with the keyhole include: (I) keyhole initiation, (II) 15 keyhole development, and (III) melt pool recovery. Porosity and spatter were associated with stages (II) and 16 (III). We also discovered that droplet spatter can form due to the collapse of the keyhole recoil zone, causing 17 molten particle agglomeration and ejection during stage (III). Our results clarify the transient dynamics behind 18 the keyhole mode in a multi-layer LBPF process and can be used to guide the reduction in porosity and 19 spatter in additive manufacturing. (150 words) 20