Research on warping and dross formation of overhang structure manufactured by Laser Powder Bed Fusion

Abstract Warping and dross formation are the main defects of overhang structure formed by Laser Powder Bed Fusion. In order to study the process of warping and dross formation, the “7” shape overhang structure with different lengths and heights of overhang was printed. The influence of temperature field and stress field on the forming quality of overhang structure was analysed by numerical simulation. The results of experiment and simulation showed that there were significant differences in the forming process of temperature field between solid support zone and powder support zone. Due to the poor thermal conductivity of powder, the molten pool in the powder support zone was much larger than that in the solid support zone. On one hand, the molten pool sank due to the action of gravity and capillary force, which lead to the melting of the powder outside its original shape and contour, formed a droplet like dross formation on the lower surface. On the other hand, the temperature difference between regions led to large thermal stress. When the thermal stress exceeded the material strength, warping deformation occurred on the top area and affected the subsequent powder laying process. The powder could not be spread on the warping zone so it was remelted when the next layer is processed. As the number of forming layers increased, the original powder area became solid after fusion and solidification, so the thermal conductivity and stiffness increased continuously, the variation of temperature tended to be stable and the deformation of the top area gradually decreases. The study of warping and dross formation process was helpful to understand the defect change process in overhang manufacturing process.