Engineers from the Max-Planck-Institut für Eisenforschung (MPIE) in Düsseldorf and the Fraunhofer Institute for Laser Technology in Aachen have combined ancient and modern technology by developing a way to 3D print Damascus steel.
If you want to make a far-away look of longing come into a medieval sword enthusiast's eyes, just mention Damascus steel. Originally referring to a kind of steel made from ingots of Wootz steel that came from India over two thousand years ago and was manufactured or traded in Damascus, it now refers to a whole class of steel marked by sinuous, wavy, light and dark banding patterns that resemble flowing water.
Because Wootz steel is no longer available, making true Damascus steel is now a lost art, but not for the want of many scientists and craftsmen trying to reverse-engineer the existing examples. However, the basic idea behind it is very well understood and if you go to a modern Renaissance faire you're likely to find many reproduction blades of surprisingly high quality for sale at the swordsmith's booth.
A Damascus steel blade is made by taking bands of iron and steel, heating them to red hot, and twisting them together. Then the smith hammers them out, reheats, retwists, and rehammers until the intricate, flowing pattern emerges. The result is a worked steel that the smith can control the properties of by controlling the carbon content, creating a tough, flexible steel for a sword's core and then welding on another steel that's been worked to be stiff and hard and can be sharpened to form the blade edges.
Today, Damascus steel is commonly made using two different grades of steel alloy, but it's still very much a craft that's more art than science. Now, researchers are bringing Damascus steel into the 21st century using 3D printers and lasers.
Instead of using two different materials and working them to form a new alloy, the new technique uses only one material – an alloy powder of iron, nickel, and titanium. This is put down layer after layer with a laser melting and fusing the powder to form the desired shape. The excess powder is then removed to reveal the final product.
That's basic 3D metal printing, but where the new technique differs is that the laser is used to alter the crystalline structure of the metal to form alternating layers of hard and ductile steel – a sort of printed Damascus steel.
"We have succeeded in specifically modifying the micro-structure 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." says Philipp Kürnsteiner, a post-doctoral researcher at the MPIE. "Under certain conditions, small nickel-titanium micro-structures form. These so-called precipitates, harden the material. When subjected to mechanical stress, they hinder the movement of dislocations within the crystal lattice that is characteristic of plastic deformation."
How the laser can make this alteration is a matter of timing. As each layer is added, the metal is allowed to cool to below 195 °C (383 °F). This leaves a soft layer. To achieve a hard layer on top, a second layer of metal is added, allowed to cool, and the laser is brought to bear, altering the structure and hardening it. The result is a steel that is a combination of strength and ductility. According to the team, by varying the laser's energy, the speed of the printing process, and other factors, the properties of the metal can be controlled with considerable precision.
"The technology opens new doors for adjusting the local micro-structures in a defined manner during the additive production of even complex workpieces and making post-treatment unnecessary," says Kürnsteiner. "Until now, it has been common practice to use conventional alloys in 3D printing. However, many known steels are not optimally suited for additive manufacturing. Our approach is to develop new alloys that can exploit the full potential of 3D printing."
Source: Max Planck Institute
