Balling phenomenon and cracks in alumina ceramics prepared by direct selective laser melting assisted with pressure treatment

Abstract The manipulation of defects during selective laser melting plays a key role in forming dense ceramic parts with complex shapes. In this study, selective laser melting of alumina with pressure assistance was performed on powder bed, and the defects during selective laser melting, including balling and cracks, were investigated. It was found that, unlike the competition mechanism between spreading and solidification in the metal system, the balling phenomenon of alumina was dominated by the spreading process. According to the calculation, the spreading time of alumina melt was less than 1/5 of the solidification time. Even the alumina powder was fully melted without splashing, the balling phenomenon could still occur. Although lower scanning speed could increase laser dwell time and improve the stability of the molten pool, the efficiency and surface roughness were sacrificed. Severe balling occurred in this condition. When pressure treatment was applied, the balling phenomenon was greatly alleviated. However, surface cracks were observed. There are mainly two reasons for the formation of surface cracks: the shrinkage of the melt and the shear stress in the edge between partially melted zone and fully melted zone. To alleviate the surface cracks, the distribution of the density of powder bed, the laser parameters, and the particle size should be well controlled. This paper provided an experimental reference for the manipulation of balling phenomenon and surface cracks of alumina melt in the SLM process performed on powder bed.