Numerical simulation of porosity formation in a novel hybrid additive manufacturing approach

A novel type of hybrid additive manufacturing (HAM) for future space applications has recently been developed in collaboration between the Institute of Metals and Technology (IMT), European Space Agency (ESA) and industrial partners. This novel approach combines the two existing technologies of powder bed fusion (PBF) and direct energy deposition (DED) in order to reap the benefits of both. While PBF is known for its unmatched capacity to produce geometrically complex objects with high precision and excellent mechanical properties, their size is limited by the size of the chamber and their production time is long. DED on the other hand imposes no limitations on the size of the objects at the price of lower dimensional accuracy and poorer mechanical properties. In the scope of the forementioned collaboration, HAM process chains were developed for Inconel 718, Inconel 625 and Ti6Al4V. Samples were manufactured to showcase the feasibility of the developed technology and mechanical tests confirmed that excellent mechanical properties can be obtained under the right conditions. The latter was however not achieved for all of the manufactured samples, as a strong porosity was often observed to develop at the PBF/DED interface. While the interface porosity represents a major problem for the further use of the developed technology, the mechanism of its formation and the circumstances which lead to it remain unknown. This project focuses on resolving the interface porosity issues by exploring the process of porosity formation and a usable range of manufacturing parameters that ensure the structural integrity of fabricated objects. To achieve these goals, a computational modelling approach is proposed, where elements of different state-of-the-art numerical modelling approaches will be combined to form a comprehensive simulation of heat transfer and free surface evolution during the use of the developed HAM technology. Volume of Fluid method will be employed, using Eulerian approach and finite volume discretization scheme to model dynamics of two-phase flow of gaseous and metallic phase in order to simulate the solidified surface that develops during the use of HAM. The micro CT measurements of porosity that were already performed for the manufactured samples will provide the necessary data for model validation. Series of simulations well then be run while systematically varying the manufacturing parameters within predefined ranges to explore a usable process window for Inconel 718 as one of the key materials for the space industry. Pressure, velocity and temperature fields from the simulation results will be meticulously analysed to obtain a deeper understanding of the porosity formation. While resolving the existing problems with the HAM technology will have immediate implications for the space industry, important implications in other industrial branches can also be expected in the near future due to the increased manufacturing capabilities of HAM.