In-situ Synchrotron imaging of keyhole mode multi-layer laser powder bed fusion additive manufacturing

Abstract

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

WITHDRAWN: On the in situ elimination of pores during laser powder bed fusion of a titanium alloy

“…Surface roughness might cause stress concentration and trigger crack formation [13], which had a negative effect on fatigue performance. Porosity has a great impact on the dynamic properties such as the fatigue life of the LPBF components [14,15]. Pores can be divided into two categories according the input laser energy [16][17][18]: (1) lack of fusion voids with irregular shape and large-size caused by the incomplete melting of the powder particles [12,[19][20][21], and the poor bonding between adjacent tracks or layers [22,23]; (2) near spherical and small-size pores caused by excessive laser energy input, gas retention or improper closure of key-hole [6,24].…”

Progress on In Situ and Operando X-ray Imaging of Solidification Processes

“…The heating/cooling rates of the LPBF process are approximately 10 5 –10 6 K s −1 which is three orders of magnitude faster than that of traditional casting, and the thermal gradient established inside the molten pool can reach between 10 3 and 10 4 K mm −1 . To tackle such demanding scientific challenges, research teams across the globe developed different types of additive manufacturing simulators [ 13 , 26 , 97 ] combined with high-speed X-ray imaging and diffraction facilities at Advanced Photon Source (APS) [ 151 ], Diamond Light Source [ 26 , 58 , 152 ], European Synchrotron Radiation Facility [ 153 ], Stanford Light Source [ 13 , 154 , 155 ], and Swiss Light Source [ 42 ], to probe and elucidate the molten pool dynamics during LPBF.…”

“…In addition to the melt pool geometry studies, several recent studies focused on the fundamental origin of the keyhole dynamics and the evolution of keyhole porosity during LPBF in a single layer build [ 158 , 159 ] and multi-layer build conditions [ 153 , 160 ]. Cang et al [ 159 ] suggested that the keyhole porosity is increasingly sensitive to scan speed.…”

“…Hojjatzadeh et al [25] captured other pore evolution mechanisms, including (1) pore transfer from powder feedst the molten pool, pore trapped by (2) fluctuations of the melt surface or at the depre zone (Figure 11e,f), or (3) pore formation from a crack (Figure 11g). [58]); (c) formation of open-pore due to pore bursting during multilayer laser remelting (after [153]); (d) formation of pores due to laser turning and acceleration/deceleration scan mirrors (after [155]); (e) pore transfer from powder feedstock to the molten pool (after [25]); (f) pore formation due to surface fluctuations (after [25]); and (g) pore growth from existing cracks (after [25]). Permitted for reuse from respective publishers.…”

Enhanced near-infrared absorption for laser powder bed fusion using reduced graphene oxide

“…In this work, we employ two different carbon additives: nanocarbon (nano-C) and reduced graphene oxides (rGO) to enhance the laser absorbance of the fused silica powder bed. We process these powder mixtures using a custom-built In Situ and Operando Process Replicator (ISOPR) machine [43] while performing synchrotron X-ray imaging to investigate the role played by these carbon additives and elucidate key mechanisms involved during LPBF of glass. Here, we successfully demonstrate a one-step process of fused silica using LPBF with a NIR beam, achieving high density AM glass parts prior to further heat treatment.…”