General relationship of porosity and selected features of the pore ensemble in Laser Powder Bed Fusion of metallic materials

Porosity is the most important defect in laser powder bed fusion (LPBF) of metallic materials. Achieving a minimum value is the key task in process development, and retaining a constant low value is required for quality assurance. Different parameters affect porosity, such as LPBF machine set up, adjusted process parameterization, and used material. Porosity as integral value is determined by the individual pores of the part, which create in their entirety the pore ensemble. Pores are in general distinguished by their formation mechanism into lack of fusion pores, keyhole pores and gas pores, but their proper classification based on their geometrical features is challenging. Besides porosity the maximum defect size and general occurrence of defects exceeding a critical size might be considered as they can affect the performance of the part negatively. We present general relationships of selected features of the pore ensemble and the total porosity, which holds for different LPBF machines, processing conditions and materials. The maximum detected individual defect become larger as porosity increases. This behavior is not random and follows a clear trend. Additionally, the fraction of large defects, i.e. their number per area in microstructural images, is strongly correlated to the total porosity. Both relationships, for maximum size and fraction, do not primarily depend on the pore formation mechanism and are proven for different well established and new LPBF-materials.