Influences of powder characteristics and recoating conditions on surface morphology of powder bed in metal additive manufacturing

Metal additive manufacturing technology requires a real-time monitoring and feedback control system to assure the quality of the final products. In particular, it is essential to reveal the phenomena of recoating and melting-solidification processes in laser powder bed fusion using a real-time monitoring system because they influence strongly the occurrence of defects. This study was conducted to elucidate the correlation among the powder characteristics, recoating conditions, and surface morphology of a powder bed in the recoating process to determine the relationship between the surface morphology of the powder bed and the final product quality. To this end, a surface morphology measurement system composed of a powder recoating test bench and a layer surface morphology measurement apparatus was first designed and fabricated. Then, it was used to quantify the surface morphology of the powder bed. Specifically, the influences of the different powder characteristics and the recoating parameters of the powder supply ratio and recoating speed on the surface morphology of the powder bed were investigated using various powders of Al-10Si-0.4Mg (AlSi10Mg) alloy and Inconel 718 (IN718) alloy. The surface morphology of the powder bed was measured as the value of 2σ at a resolution of 30 μm in height. It was found that the angle of repose and the basic flow energy of the bulk powder are promising parameters for evaluating the surface morphology. The surface morphology was significantly affected by the powder characteristics and recoating speed. The value of 2σ for the AlSi10Mg powder increased rapidly over a recoating speed of 50 mm/s for all powders. The value of 2σ for the irregularly shaped AlSi10Mg powder was approximately 19 μm, and the 2σ values for the other powders were approximately 17 μm at a recoating speed of 15 mm/s. However, at a recoating speed greater than 300 mm/s, the irregularly shaped powder exhibited better surface morphology than did the spherical powder. The recycling process deteriorated the flowability of the new powder. However, the surface morphology of the spherical recycled powder was similar to that of the spherical powder. Consequently, the correlation among the powder characteristics, recoating conditions, and surface morphology of the powder bed was revealed by employing the surface morphology measurement system. Quantification of the surface morphology of the powder bed using the monitoring system facilitates control of the recoating process to prevent the occurrence of defects.