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8 Platform one Researchers in Germany have developed a technique to produce Damascus steel using additive manufacturing for the first time (writes Nick Flaherty). Damascus steel is hard yet tough because it consists of layers of different iron alloys. The team, from the Max- Planck-Institut fur Eisenforschung (MPIE) in Dusseldorf and the Fraunhofer Institute for Laser Technology in Aachen, has developed a process that allows the steel to be produced layer by layer in a 3D printer. The key to the process is adjusting the hardness of each of the steel’s alternating hard and ductile layers of an alloy of iron, nickel and titanium using a laser. Such composites are of interest for aerospace components or tools made using 3D printers. “We have succeeded in specifically modifying the microstructure of the individual layers during 3D printing so that the final component has the desired properties – and all this without subsequent heat treatment of the steel,” said Philipp Kurnsteiner, post-doctoral researcher at the MPIE. The laser beam melts the material as well as the top layer of the already solidified metal, changing the crystal structure of the steel in individual metal layers and altering the mechanical properties without changing the chemical composition. Pauses in the printing process allow the formation of hardening precipitates. “Under certain conditions, small nickel–titanium microstructures form,” said Kurnsteiner. “When subjected to mechanical stress, they hinder the movement of dislocations within the crystal lattice that is characteristic of plastic deformation.” In order to be able to create the nickel– titanium structures, the researchers interrupt the printing process for a certain time after each newly deposited layer, during which the metal cools down to below 195 C. “Below this temperature, a transformation of the crystal structure occurs in the steel,” explained Eric Jagle, head of the Alloys for Additive Manufacturing group at MPIE. “The so-called martensite phase is formed, and only in this phase can the nickel-titanium microstructures be created.” However, in order to allow precipitates to form, reheating is necessary, which is where the laser energy from the subsequent layer is used. Layers that have been directly covered with the next layer without interruption remain softer because they are not yet present as martensite at this point. There are numerous key process parameters for influencing the microstructures during 3D printing. Jagle said that in addition to or instead of the pause time – which varies in this study – martensite formation and subsequent precipitation hardening could also be controlled by varying the laser beam’s energy, focus or printing speed as well as external heating and cooling techniques. In experiments, the researchers produced cube-shaped or cuboid steel pieces with side lengths of a few centimetres as models for objects with more complex geometries. “This was achieved in a single manufacturing step, without the additional process steps previously required for surface hardening such as nitriding,” said Jagle. “The technology opens new doors for adjusting the local microstructures in a defined manner during the additive production of even complex workpieces and making post-treatment unnecessary,” Kurnsteiner said. “Until now, it has been common practice to use conventional alloys in 3D printing. However, many steels are not optimally suited for additive manufacturing. Our approach is to develop new alloys that can exploit the full potential of 3D printing.” Materials October/November 2020 | Unmanned Systems Technology Key to manufacturing the Damascus steel was to adjust the hardness of each layer First for 3D-printed steel