80 max planckresearch Knowledge from ·2 | 2020 traditional way, often here, used is seen traditional that such as of its characteristic pattern. in contrast, producing contrast, in pattern. of its characteristic material design: damascus steel produced in the the produced steel in damascus design: material a material of this kind in a3d in involves printer kind of this a material varying the properties of materials that can can that of materials properties the varying for decorative purposes nowadays because nowadays because for purposes decorative be processed using this technology. this using processed be

photo: BigtUnAonline/AlAmY StoCK foto materialS & technology

A legend 81 from the 3D printer

Text: Karl Hübner

In ancient times, it was the material of choice for sword blades. Now, a kind of Damascus steel can be produced in a 3D printer using a technique developed by a team from the Max-Planck- Institut für Eisenforschung in Duesseldorf and the Fraunhofer Institute for Laser Technology in Aachen. Composite materials of this kind could be of interest for aerospace components or toolmaking.

Max Planck Research · 2 | 2020 Knowledge from H B M G

NG U H SC FOR EN S I E

R Ü F

I MP / INER TE S RN . KÜ P : PHOTO

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Forging materials with light: when manufactur- ing metals with a 3D printer, laser energy can be used not only to melt the powdered starting materials and work them into complex components, but also to bring about transfor- mations in the metal’s structure.

Born out of necessity and destined to Celtic smiths combined various iron smiths developed the art of folding become a legend. In the past, black- alloys – perhaps initially only to re- two alloys into many thin layers. smiths were able to influence the cycle the valuable iron – and thus The layered structure of Damascus properties of iron alloys only by ad- obtained the material that later be- steel can usually be recognized by a justing their carbon content. They came known as Damascus steel or characteristic stripe pattern. obtained either a soft yet tough or a damask. It owes its name to the trad- hard yet brittle steel. Especially for ing center through which the com- Although there are currently ferrous swords, a tough and hard material posite material of oriental origin en- alloys that are both hard and tough, was needed so that the blades would tered Europe. While Indian and Ara- they are often not specifically made not break or have to be straightened bic damask was created by a sophis- for the 3D printing process and in the middle of a battle. ticated smelting process, European therefore do not fully exploit the ad-

Max Planck Research · 2 | 2020 materialS & technology

Summary

A team from the Max-Planck-Institut für Eisenforschung and the have become standard in many in- Fraunhofer Institute for Laser dustrial sectors in the space of just a Technology is developing alloys for use in 3D printing. few years. They are able to produce plastic as well as metallic compo- The researchers recently nents. The respective alloy is added presented a technique as a fine powder, melted by a laser whereby 3D printing can beam, and repeatedly applied layer directly convert a single by layer to form the target compo- starting material into a kind of Damascus steel with nent. For the last few years, fuel in- alternating hard and jection nozzles for aircraft engines, tough layers. for example, have been produced 83 using this additive manufacturing By fine-tuning various method, which is also known as laser parameters, such as the cladding. printer’s pause times, the laser energy, or the speed at which the metal is 3D printed, it is The laser beam makes it possible to not possible to make localized only melt the respective material but adjustments not only to the also to heat the top layer of the al- hardness but potentially also ready resolidified metal.T his is ex- to other properties. actly what the team of the Max Planck researchers in Duesseldorf used to specifically change the crys- tal structure of the steel in individ- ual metal layers. In this way, they can influence the mechanical prop- erties without changing the chemi- cal composition.

They developed an alloy consisting of vantages of this manufacturing tech- ture of the individual layers during iron, nickel, and titanium. At first, nique. Using 3D printing, scientists 3 D printing so that the final compo- this alloy is relatively soft. “Under at the Max-Planck-Institut für Ei- nent has the desired properties – and certain conditions, small nickel-tita- senforschung and the Fraunhofer all this without subsequent heat nium micro-structures form. These Institute for Laser Technology have treatment of the steel,” says Philipp so-called precipitates harden the now developed a steel that consists of Kürnsteiner, post-doctoral re- material,” explains Kürnsteiner. just one single starting material, but searcher at the Max-Planck-Institut “When subjected to mechanical is made up of alternating hard and für Eisenforschung. stress, they hinder the movement of ductile layers – a kind of Damascus dislocations within the crystal lat- steel. “We have succeeded in specif- 3D printers for additive manufacturing tice that is characteristic of plastic ically modifying the micro-struc- (the technical term for this process) deformation.”

Max Planck Research · 2 | 2020 Knowledge from

Freshly printed metal: Philipp Kürnsteiner inspects a cube-shaped sample of steel comprised of alternating tough and hard layers. The researcher created the material using a sophisticated method that allowed him to control the 3D printing process. WB D / EN K IN K V N A R F : PHOTO

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In order to be able to create the nickel- ture, a transformation of the crystal 2020, professor at the University of titanium structures, the researchers structure occurs in the steel,” ex- the Bundeswehr Munich. “A ‘mar- interrupt the printing process for a plains Eric Jägle, head of the “Alloys tensite’ phase is formed. It is only certain time after each newly deposit- for Additive Manufacturing” group during this phase that the nickel- ed layer. The metal cools down to at the Max-Planck-Institut für Ei- titanium microstructures can per- below 195°C. “Below this tempera- senforschung and, since January cipitate.” However, in order to allow

Max Planck Research · 2 | 2020 materialS & technology

GLOSSARy

precipitates to form, reheating is abrasion-resistant outer layer. 3D printing necessary. To achieve this reheating “Thanks to our concept of local con- Also known as additive manufacturing, this technique allows components the researchers exploit the laser en- trol, this was achieved in a single with complex shapes or custom ergy used to deposit the subsequent manufacturing step – without the designs to be built up layer by layer. layer. additional process steps previously The method was originally developed required for surface hardening such for processing plastics but can now This additional effect caused by the la- as nitriding,” stresses Jägle. Accord- also be used for metals and other mate- ser beam of the 3D printer is re- ing to the researchers, it might also rials. ferred to as intrinsic heat treatment. be possible to use the technology to Composite Layers that have been directly cov- locally adjust other properties such Composites are a combination of ered with the next layer without in- as corrosion resistance. materials with various properties. terruption remain softer because Packaging made of plastic-coated they are not yet present as marten- “The technology opens new doors for cardboard is one typical example. site at this point. adjusting the local micro-structures Damascus steel is made up of iron alloys with differing degrees of in a defined manner during the ad- hardness and is therefore both hard For the first time, the researchers are ditive production of even complex and tough at the same time. able to directly create a composite work pieces and making post-treat- material consisting of layers with dif- ment unnecessary,” says Kürn- ferent properties from a single start- steiner. The researcher also sug- ing material during the production gests a paradigm shift: “Until now, process. Kürnsteiner is impressed by it has been common practice to use the mechanical properties of the ma- conventional alloys in 3D printing. terial produced in this way: “The However, many known steels are not tests confirm an excellent combina- optimally suited for additive manu- tion of strength and ductility.” facturing. Our approach is to de- velop new alloys that can exploit the 85 Many different process parameters are full potential of 3D printing.” suitable for influencing the mi- cro-structures during 3D printing. Jägle explains that in addition to or instead of the pause time, which is varied in this study, martensite for- For alloys designed using additive manufacturing, the Max Planck researchers begin by preparing a mation and subsequent precipita- powder bed. A laser then produces the desired material tion hardening could also be con- directly from the powder – layer by layer – in order trolled by varying the laser energy, to manufacture the components. laser focus, or printing speed as well as external heating and cooling

techniques. H B M G

In their experiments, the researchers NG U produced cube-shaped or cuboid H SC

steel pieces with side lengths of a FOR EN

few centimeters as models for ob- S I E

jects with more complex geometries R Ü F

for which computer-controlled 3D I MP printing is of interest. They also em- / M LE

phasize that the Damascus-like A S

steel with its periodically changing ED

layers is only one example of how HM A the micro-structure of an alloy can N MI be locally influenced during the S manufacturing process. For exam- : YA

ple, it is equally possible to create PHOTO tool components with a continuous soft core surrounded by a hard,

Max Planck Research · 2 | 2020