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In this study, different hatch spacings were used to fabricate single layer and multiple layers, and its effect on porosity was investigated by using microcomputed tomography. The combination of laser power (100 W, 150 W, 175 W, and 195W) and scan speeds (600 mm/s, 800 mm/s, 1000 mm/s and 1200 mm/s) which resulted in the least number of pores were selected from the previous single-track experiment. Six levels of hatch spacings were selected based on the track width to form single and multiple layers: 60%, 70%, 80%, 90%, 120% and 150% of track widths. For the multilayer build, the variation in keyhole porosity within the given window of parameters were found to be attributed to the variation in the hatch spacing. In general, the pore number decreased with increase in hatch spacing from 60% to 90% but increased when hatch spacing further increased from 90% to 120%. 

The dynamic phenomenon of a melt pool during the laser powder bed fusion (LPBF) process is complex and sensitive to process parameters. As the energy density input exceeds a certain threshold, a huge vapor depression may form, known as the keyhole. This study focuses on understanding the keyhole behavior and related pore formation during the LPBF process through numerical analysis. For this purpose, a thermo-fluid model with discrete powder particles is developed. The powder distribution, obtained from a discrete element method (DEM), is incorporated into the computational domain to develop a 3D process physics model using flow-3d. The melt pool formation during the conduction mode and the keyhole mode of melting has been discerned and explained. The high energy density leads to the formation of a vapor column and consequently pores under the laser scan track. Further, the keyhole shape resulted from different laser powers and scan speeds is investigated. The numerical results indicated that the keyhole size increases with the increase in the laser power even with the same energy density. The keyhole becomes stable at a higher power, which may reduce the occurrence of pores during laser scanning.