Damascus steel enjoys a legendary reputation. It is strong and ductile at the same time because it consists of layers of different iron alloys. In ancient times, this made it the material of choice for sword blades. Now a team from the Max Planck Institute for Iron Research in Düsseldorf and the Fraunhofer Institute for Laser Technology in Aachen has developed a process that allows steel to be produced layer by layer in a 3D printer, with the hardness of each layer being specifically adjusted. Such composite materials could be interesting for 3D printing of aerospace components or tools.
Even Celtic smiths combined various iron alloys to produce the material that later became known as Damascus steel. It owes its name to the trading centre through which the composite material of oriental origin came to Europe. However, while Indian and Arabic damascus were created by a sophisticated smelting process, European smiths developed the art of folding two alloys into many thin layers. The layered structure of Damascus steel can usually also be recognized visually by a characteristic stripe pattern.
Although there are iron alloys available today that are both hard and ductile, they are often not easy to process with 3D printers, the means of choice for many complex or individually designed components. For this reason, scientists at the Max Planck Institute and the Fraunhofer Institute have developed a technique that allows a single starting material to be used directly in 3D printing to produce a steel that is composed alternately of hard and ductile, i.e. soft layers - a kind of Damascus steel.
They used the well-known technique of laser cladding. But the laser beam not only makes it possible to melt the material in question. It can also be used to heat the top layer of the already resolidified metal. The research team used this to specifically change the crystal structure of the steel in individual metal layers - and thus influence the mechanical properties without changing the chemical composition.
To do this, they developed an alloy of iron, nickel and titanium. Initially, this alloy is relatively soft. But under certain conditions, small nickel-titanium microstructures are formed, which then provide a special hardness.
To be able to create the nickel-titanium structures, the researchers interrupted the printing process after each newly applied layer and allowed the metal to cool to below 195 degrees Celsius. This is when the crystal structure begins to change. They then heated the material again. To do this, the researchers use the laser energy with which the next layer is printed.
Layers that have been directly coated with the next layer without a break remain softer because they are not yet present as martensite at this point. The tests confirmed an excellent combination of strength and ductility, according to the researchers.
A number of process adjustment screws are suitable for influencing the microstructures during 3D printing. Instead of alternating between heating and cooling, the crystal formation and subsequent hardening can also be controlled by varying the laser energy, laser focus or printing speed or by using external heating and cooling techniques.
In their experiments, the researchers produce cube-shaped or cuboidal steel pieces with side lengths of a few centimeters. The knowledge gained can also be transferred to objects with more complex geometries, for which computer-controlled 3D printing is of interest. In addition, the damascene-like steel with its periodically changing layers is just one example of the possibility of locally influencing the microstructure of an alloy during the manufacturing process. For example, it is just as well possible to create tool components with a continuous soft core, which are then surrounded by a hard, abrasion-resistant outer layer. It is also conceivable, according to the researchers, that the technology could be used to adjust not only the hardness but also other properties such as corrosion resistance locally.
This new approach heralds a paradigm shift in the design of alloys, explains Philipp Kürnsteiner, a postdoctoral fellow at the Max Planck Institute for Iron Research and one of the scientists involved. "Many known steels are not optimally suited for additive manufacturing. Our approach now is to develop alloys just enough to exploit the full potential of 3D printing.
Original publication: https://www.nature.com/articles/s41586-020-2409-3#citeas