Characterization and Microstructure of Recycled Eroded Particles from Die-Sink Electro Discharge Machining of H11 Alloy for Applicability in Additive Manufacturing

Different waste streams from electro discharge machining (EDM) were investigated for an upcycled usage in processes for additive manufacturing (AM). These erosion sludges accumulate in filter cartridges and at the bottom of machining basins. The enclosed particles were extracted, sieved and investigated via laser diffraction, dynamic image analysis, scanning electron microscopy, optical emission spectroscopy, elemental analysis and flowability measurements. Additionally, thermal, crystallographic and metallographic investigations as well as X-ray micro-computed tomography (mu-CT) were utilized for the characterization of particle and material properties. In general, eroded powders fulfill the requirements for AM regarding particle size and shape very well, which is confirmed in morphological investigations and powder flow characteristics showing similar properties as the H11 AM reference material. The chemical composition of the powders is equal to the machined H11 alloy, except for the high carbon content. Carbon is entrapped in the iron lattice originating from pyrolysis of the present dielectric fluid and the graphite electrode during rapid solidification, which leads to a transition from martensite to cementite structures. This change is observed in the microstructure of powders, in which acicular primary cementite and austenite are present. After remelting with slow heating and cooling rates the microstructure changed to ledeburite II with retained austenite and martensitic phases. The pore size and shape distributions obtained by mu-CT measurements showed a pore formation in the compact sample. These results provide a fundament of major properties as well as handling and recycling suggestions for eroded particles enclosed in waste sludges.