Damascus steel is a famous type of steel, which was easily recognized by the wavy or watery alternating dark and light pattern on the metal. The pattern often looked like flowing water, a ladder pattern, or a teardrop pattern. Swords forged from Damascus steel were not only beautiful but resistant to shattering, tough, and able to be honed to a sharp, resilient edge. Weapons made from iron were inferior to those made with Damascus steel, giving those an edge to those fighting with a sword made from Damascus steel.
Today, we are not able to replicate the original method for making Damascus steel since it was cast from wootz. Wootz was a type of steel made in India more than two thousand years ago. Melting iron and steel together with charcoal in an atmosphere with little to no oxygen was the process used to create cast wootz steel. In these specific conditions, the metal soaked up the carbon from the charcoal. Slowly cooling the alloy of iron and steel produced a crystalline material that contained carbide. Damascus steel was produced by forging the resulting wootz into various objects, especially swords. Modern attempts at reproducing the steel have failed as the raw materials available to us today are different from those available two thousand years ago, and manufacturing techniques have naturally changed dramatically over the years.
In objects made from true Damascus steel, the pattern on the metal runs all the way through the medal. The pattern on replicas is often simply etched on the surface to produce the typical light and dark pattern found on Damascus steel. However, this pattern will wear away over time, which will not happen with the original material.
Philipp Kürnsteiner, Markus Benjamin Wilms, Andreas Weisheit, Baptiste Gault, Eric Aimé Jägle, and Dierk Raabe, designed a new type of steel, which copies the layered pattern that Damascus steels are famous for. Their findings are published in the article “High Strength Damascus Steel by Additive Manufacturing”.
In creating the new steel, the researchers utilized the strengths of directed energy deposition. Directed energy deposition is a complex three-dimensional printing process often used to add additional material to already existing components. Using focused energy force like a laser or electron beam, the directed energy deposition process melted the compound. As the compound is melted, it is simultaneously deposited by a nozzle.
Utilizing the inherent heat treatment in directed energy deposition, a common form of additive manufacturing, allows manufacturers to lower their costs and increase efficiency in producing their products. This is due to the fact that a post-process heat treatment is no longer required.
The research group designed a nickel and titanium based steel, which is ideal for additive manufacturing processes like directed energy deposition. The researchers digitally controlled the microstructure of the alloy layer by layer by controlling both the temperature and cooling rates. The result is a steel with adjustable properties, according to Kürnsteiner.
While Damascus steel was created through a folding and forging process, the new steel is formed using digital controls. Directed energy deposition makes the layer by layer construction possible, copying the structure formed in the original Damascus steel via the folding and forging process. Using cyclic reheating, the researchers directed the phase transformations that are needed to create a strong and ductile steel. In the first phase, nickel rich martensite is formed by transforming austenite. During the second transformation, nickel-titanium precipitate is formed, causing the steel to harden.
Controlling the cooling down process allowed researchers to regulate the sequencing of the two transformations, creating an alternating pattern between hardened and non-hardened regions.
Researchers studied the intricate hierarchical microstructure of steels manufactured with the additive manufacturing process, using a combination of analytical techniques such as atom probe tomography and electron backscatter diffraction. As a result, the researchers were able to put together a comprehensive picture of the microstructure down to the nanometre.
The research team continues to test different possibilities for controlling the heat treatment. This would allow for the possibility of being able to locally adjust the microstructure. Locally adjusting the microstructure works best in the alloys that are able to react optimally to the specific conditions of the additive manufacturing process, which is a transformative approach in industrial production, allowing for the formation of lighter, stronger parts and systems.
In addition, the research team plans to design other alloys that would work well within the process. Manufacturing across multiple industries stands to benefit from the results. Tools, construction parts, etc. could be built with soft cores covered by a hard outer protecting without adding additional coatings or putting the products through another type of hardening treatment.
Further information: Philipp Kürnsteiner et al. High-strength Damascus steel by additive manufacturing, Nature (2020). DOI: 10.1038/s41586-020-2409-3