Effect of shielding conditions on bead profile and melting behaviour in laser powder bed fusion additive manufacturing

Abstract

A series of experiments were performed using a 500W continuous wave fibre laser on a single powder bed layer using different processing variables. The aim was to investigate the effect of different shielding conditions on melting behaviour and bead profile in laser powder bed fusion (PBF). Through high-speed imaging, it was found that under an argon atmosphere a strong plasma plume is generated from the meltpool. Laser beam-plasma plume interactions caused strong instabilities during melting, including laser wandering, track instability and continuous fluctuations between melting regimes (conduction and keyhole). Hence, it was not possible to control the profile of the melted tracks under this condition. By using a helium atmosphere, a smaller plasma was obtained, reducing the disruptions caused by laser-plasma interactions. This led to a stable melting regime that allowed control of the melt bead profile.This condition was used to study the effect of laser-material fundamental interaction parameters on the bead geometry in powder bed melting. It was found that during melting of single tracks, the dominant regime of melting is conduction for the range of parameters tested.Penetration and melt width were found to increase with increasing energy density. For longer interaction times, melt widths were found to be up to ten times the size of the beam diameter used. Fluid flow modelling showed that this is due to strong melt flow as consequence of surface tension gradients generated by very high temperature gradients.

Absorption of Nitrogen during Pulsed Wave L-PBF of 17-4 PH Steel

“…With increasing energy density, the higher availability of nitrogen in the cover gas leads to more absorption than what is seen at lower energy densities. This corresponds with the trend of larger melt pools and increased plasma generation as laser power or energy density increases [ 23 , 34 ], both of which result in more gas absorption into the liquid metal.…”

“…Evaluation of nitrogen absorption from Yb: Fiber lasers used in welding shows that with high power lasers the plasma plume above the melt pool is sufficient to dissociate the diatomic nitrogen into monatomic nitrogen greatly increasing absorption potential [ 22 ]. At the lower laser powers of L-PBF plasma plume generation has also been observed [ 23 ] and the absorption of nitrogen cover gas has been measured [ 12 ]. While in-situ nitrogen absorption has been documented, in comparison more nitrogen has been shown to be absorbed in laser welding where higher laser powers are used [ 21 ].…”

Laser powder bed fusion for metal additive manufacturing: perspectives on recent developments

“…Similar observations were made by Caballero et al when pure helium was used instead of argon. It is observed that the strong fluctuations in melting is absence in helium atmosphere, and better control of the melt profiles was achieved compared to argon (Caballero et al 2020). However, for AlSi10Mg, there is no significant difference observed when nitrogen was used instead of argon (Ch et al 2019).…”

Process parameter optimization of metal additive manufacturing: a review and outlook

“…The shielding gas velocities can disrupt the powder bed and affect the porosity [39,40] . In addition, the shielding gas can directly influence the vapor plume characteristics [39,41,42] , where the impingement angle of the vapor plume has also been observed to disrupt the powder bed [43,44] . The vapor plume shape determined by its relative direction to the shielding gas flow can also possibly affect the oxidation and reduction characteristics [45] .…”