A legend reborn: Additive manufacturing creates Wootz-Damascus steel
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rtisans in ancient India melted iron with woodchips as sources of carbon in clay crucibles and produced a hypereutectoid steel (C content ~1.5% by weight) called Wootz steel. Their technique, perfected over centuries, resulted in steels of high strength and ductility. These steels were traded in Damascus and spread to many countries, which turned them into swords of amazing performance with aesthetically pleasing wavy patterns on the surface. Combining the expertise of researchers at the Max Planck Institute for Iron Research in alloy design with that in laser processing by a team at the Fraunhofer Institute for Laser Technology, a new Damascuslike steel has now been created by additive manufacturing (AM), commonly known as three-dimensional (3D) printing. The key to the success of this work, as reported in a recent issue of Nature (doi:10.1038/s41586-020-2409-3), is the innovative use of the digital capabilities of AM to introduce a controlled hierarchical structure without the need for post-treatment. Directed energy deposition (DED) is an AM technique that uses alloy powder fed by a carrier gas and a computer-aided design file to produce a near-net component in a single step. The alloy powder is injected into a melting pool of the same composition, heated by a laser beam. Then a subsequent layer is deposited and melted and thus the process is continued layer by layer. Interestingly, while a new layer is being deposited, the previously synthesized layer below gets heated up as well. The researchers call this effect “intrinsic heat treatment” (IHT).
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A legend reborn: Additive manufacturing creates Wootz-Damascus steel
“We exploit this IHT to trigger 2 mm precipitation hardening reactions already in situ during the AM process. Via a precise control of the IHT we can digitally control the microstructure layer by layer in order to produce a steel with locally adjustable properties,” says Philipp Kürnsteiner, the lead author of the study. A time- and energyconsuming heat-treatment step is thus avoided. By designing new alSample Height (mm) loys with fast precipitation kinetics, one can use the control of the timeOptical micrograph showing a layered Damascus-steeltemperature profile induced by the like structure consisting of a sequence of soft and hard laser beam to obtain the desired bands. The precipitates are confined to the thin, dark layers by a precise control of the intrinsic heat treatment during properties after printing. the additive manufacturing process. The red line shows the The researchers designed an increase in hardness in the dark layers due to precipitation Fe19Ni5Ti (wt%) alloy where hardening. Credit: P. Kürnsteiner and M. Wilms. IHT can bring about the layerwise formation of two distinct structures that, in combination, enhance a pattern that resembles that of Damascus the mechanical performance of the alsteels. More importantly, the hard layers loy. “We use the digital access to the AM gave the strength and the soft layers ductilprocess parameters to pre
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