In situ radiographic and ex situ tomographic analysis of pore interactions during multilayer builds in laser powder bed fusion

Abstract

Porosity and high surface roughness can be detrimental to the mechanical performance of laser powder bed fusion (LPBF) additive manufactured components, potentially resulting in reduced component life. However, the link between powder layer thickness on pore formation and surface undulations in the LPBF parts remains unclear. In this paper, the influence of processing parameters on Ti-6Al-4V additive manufactured thin-wall components are investigated for multilayer builds, using a custom-built process replicator and in situ highspeed synchrotron X-ray imaging. In addition to the formation of initial keyhole pores, the results reveal three pore phenomena in multilayer builds resulting from keyhole melting: (i) healing of the previous layers pores via liquid filling during remelting; (ii) insufficient laser penetration depth to remelt and heal pores; and (iii) pores formed by keyholing which merge with existing pores, increasing the pore size. The results also show that the variation of powder layer thickness influences which pore formation mechanisms take place in multilayer builds.High-resolution X-ray computed tomography images reveal that clusters of pores form at the ends of tracks and when variations in the layer thickness and melt flow cause irregular 2 remelting and track height undulations. Extreme variations in height were found to lead to lack of fusion pores in the trough regions. It is hypothesised that the end of track pores were augmented by soluble gas which is partitioned into the melt pool and swept to track ends, supersaturating during end of track solidification and diffusing into pores increasing their size.

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

“…They also suggested that the keyhole porosity regime varies slightly, which is also supported by a high fidelity simulation [ 158 ]. In contrast, Sinclair et al [ 160 ] revealed that the powder layer thickness alters the track height in LPBF, leading to inconsistent laser melting in subsequent build layers. They also revealed that keyhole pores can be removed via laser remelting.…”

“…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.…”

“…By studying the fundamental mechanisms of pore formation, several mitigation strategies have been proposed to reduce or eliminate porosity in LPBF components: (1) using laser re-melting techniques to reduce pore size distribution and number density either by promoting gas release from the keyhole or by inducing liquid metal flow to partially or completely filling pre-existing pores [ 152 , 160 ]; (2) reduce the input normalised enthalpy to reduce metal vaporisation during laser turning [ 154 ]; (3) use the Marangoni-driven force to drive porosity towards the laser-interaction domain (with a wide melt pool); and (4) reduce the melt viscosity (10 −3 Pa S) to avoid pore trapping [ 26 ] by maintaining the melt temperature just below the boiling point.…”

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

“…Hydrogen is expected to be present in both the virgin substrate and powder particles. During LPBF, the melt at the advancing solidification front can then become supersaturated with hydrogen, driving hydrogen diffusion from the melt into the bubble 28,30 and it is several orders faster than the diffusion of other atoms 53 .…”

Radiographic Visibility Limit of Pores in Metal Powder for Additive Manufacturing

“…For the laser powder bed fusion (LPBF) process, most defects are formed during the dynamic melting process, when the laser interacts with the solid and the melt pool [2,3]. The AM community distinguishes between lack of fusion pores, which are flat and irregular pores (voids) between non-fused powder layers, keyhole pores, which are irregular pores (voids) created by the laser and a high energy input, and gas pores, which are mostly spherical pores formed by adding gas to the melt pool [2,[4][5][6][7][8]. A detailed review of pores and voids created during the AM build process was recently published by Sola et al [2].…”