An investigation into melting modes in selective laser melting of Inconel 625 powder: single track geometry and porosity

The laser process parameters used in selective laser melting (SLM) affect the mode of melting and the process characteristics such as the porosity. Some parameters may have a higher effect on the single-track characteristics than the others. This study aims at determining both the individual and combined effects of laser parameters on the single-track geometry and porosity for Inconel 625 alloy powder. The single tracks with linear energy densities (LED) ranging from 0.16 to 0.98 J/mm are fabricated to explore the effect of LED on melting modes and the keyhole porosity. A micro-CT scanner is used to measure the pores formed inside the tracks, and the results showed that the porosity increased with an increase in LED in the keyhole mode. Besides, metallography is performed to reveal the transverse melt pool boundary and to differentiate the melting modes. The transverse melt pool boundary shows that the depth of the melt pool increases significantly compared to the width with an increase in LED, which signifies the change in the melting from the conduction to keyhole mode. Further, another set of experiments is performed by using different combinations of laser power and scan speed from the same LEDs of 0.48 J/mm and 0.75 J/mm. The melt pool boundary shows that within the same LED, the depth of the melt pool increases significantly with an increase in the laser power. However, for the same LED, the single-track porosity increased up to a certain level; then, the porosity decreased with the further increase in the power and speed. The physics behind such phenomenon is explained by the particle-level SLM process simulations developed using FLOW-3D. The simulation results show that the change in the melting mode from the conduction to keyhole for the same LED is due to the increased vaporization with an increase in the laser power, which enhanced the vapor pressure.