Effect of Processing Gas on melt pool Dynamics and Microstructure of 316L SS in L-PBF Process

Abstract

Laser Powder Bed Fusion (L-BPF) is an emerging Additive Manufacturing field that allows rapid prototyping of parts with any geometrical complexity directly from a CAD file. This technology established itself as one of the most attractive and developing fields in research and industry. However, several factors such as the lack of understanding the laser-powder interactions and unexpected defect formation hold this technology from being adopted in major industries. Many extensive efforts have been made by the community to find the defect root cause and better understand a whole Manufacturing process to reduce the chance of critical defect formation by varying a process parameter. A shielding gas has rarely been considered as a parameter and was mainly utilized as an inert atmosphere necessary to protect the melt pool against the oxygen and to carry a post product away. Laser welding is a similar technology to L-PBF where the community has an extensive understanding of the effect of shielding gas on the welding process. This work aims to leverage the benefits offered by several shielding gases from welding to L-PBF to achieve better process stability. Argon, Helium, Nitrogen, and their mixes were used to form various processing atmospheres in L-PBF and AISI 316L was chosen as a material in this research. This study is focused to better understand the melt pool behavior and estimate its effect on real parts, a high-speed camera was used to monitor plasma intensity and fluctuations under Argon and Nitrogen processing gases that were further diluted with different amounts of Helium. Overall, plasma intensity and its fluctuations were quantified for various shielding gas compositions. To prove the melt pool stability, another approach was carried out. melt pool cross sections were metallographically obtained to estimate the plasma effect on their dimensional fluctuations. Further study focuses on real parts production under various processing atmospheres. X-ray CT analysis was conducted to quantify the defects and surface quality for Argon, Nitrogen, Argon-Helium and Nitrogen-Helium processing atmospheres. Samples were produced using various Laser Velocities to enter conduction, transition, and keyhole modes to fully study the whole range of L-PBF defects under different shielding gases. A process parameter shift was adjusted for each atmosphere to prove the change of effective energy density change. Finally, high-density tensile samples were manufactured to verify the cooling rate effect of processing atmosphere on the microstructure and mechanical properties. EBSD analysis was performed to deeply characterize microstructure while tensile tensing was carried out to find a connection between microstructure and mechanical properties of 316L. Overall, the addition of Helium to the processing atmosphere showed a significant increase of melt pool stability that dramatically reduced the number of defects in 316L. It also expanded a process parameter at which high density parts can be manufactured. Enhances cooling rate also affected the microstructure that improved the strength of As-Build parts and ductility after Solution Annealing.