Freshly printed magnets
During metal processing in the 3D laser printer, temperatures of more than 2500 degrees Celsius are reached within milliseconds, at which some components evaporate from the alloys. Empa researchers recognized an opportunity in this problem - and are now using the effect to create new alloys with different properties during the printing process and embed them with micrometer precision in 3D-printed metallic workpieces.
The innovation is hard to appreciate with the naked eye: a small metallic chessboard with an edge length of four millimeters. At first glance, it shines like polished steel; at second glance, minute differences in color are visible: the tiny chessboard has 16 surfaces, eight appear slightly darker, eight lighter. The inconspicuous material sample proves that 3D printing using laser beams and metal powder is not only suitable for creating new geometric shapes, but also for producing new materials with completely new functionalities. The small chessboard is a particularly obvious example: Eight surfaces are magnetic, eight non-magnetic - yet the entire workpiece has been 3D printed from a single type of metal powder. Only the strength and duration of the irradiated laser light were varied.
As a starting point, an Empa team led by Ariyan Arabi-Hashemi and Christian Leinenbach used a special type of stainless steel developed some 20 years ago by Hempel Special Metals in Dübendorf, among others. The so-called P2000 steel contains no nickel, but about one percent nitrogen. P2000 steel does not cause allergies and is well suited for medical purposes. It is particularly hard, which makes conventional machining by milling more difficult. Unfortunately, it also seems unsuitable as a base material for 3D laser printing at first glance: It quickly becomes very hot in the melting zone of the laser beam. Therefore, a large part of the nitrogen contained normally evaporates and the P2000 steel changes its properties.
Turning the problem into an advantage
Arabi-Hashemi and Leinenbach succeeded in turning this disadvantage into an advantage. They modified the scanning speed of the laser and the intensity of the laser light, which melts the individual particles in the metal powder bed, and thus specifically varied the size and lifetime of the liquid melt pool. This was 200 micrometers in diameter and 50 micrometers deep in the smallest case, and 350 micrometers wide and 200 micrometers deep in the largest case. The large melt pool allows a lot of nitrogen to evaporate from the alloy; the solidifying steel crystallizes with a high proportion of magnetizable ferrite. With the smallest melt pool, the melt solidifies much faster. The nitrogen remains in the alloy; the steel then crystallizes primarily in the form of non-magnetic austenite. As part of the experiment, the researchers had to determine the nitrogen content in tiny, millimetre-sized metal samples very precisely and measure the local magnetisation to within a few micrometres, as well as the volume ratio of austenitic and ferritic steel. A number of highly sophisticated analytical methods available at Empa were used for this purpose.
Targeted metal production
The experiment, which seems like a gimmick, could soon add a crucial tool to the methodology of metal production and processing. "With 3D printing, we can easily reach temperatures of more than 2500 degrees Celsius locally," says Leinenbach. "This allows us to selectively vaporize different components of an alloy - for example, manganese, aluminum, zinc, carbon and more - and thus locally change the chemical composition." The method is not limited to stainless steels, but can also be useful for many other alloys.
Leinenbach is thinking, for example, of nickel-titanium alloys, which are known as "shape memory alloys". The temperature at which the alloy "remembers" its given shape depends on just 0.1 percent more or less nickel in the mixture. With the aid of a 3D laser printer, it was possible to create components that react to different temperatures in a locally staggered manner.
Fine structures for electric motors of tomorrow
The ability to produce alloys with micrometer precision in a component could also be helpful in building new, more efficient electric motors. For the first time, this offers the possibility of building the electric motor's stator and rotor from magnetically finely structured materials in order to better exploit the geometry of the magnetic fields. The decisive factor in the discovery of the correlation between the laser power and the size of the melt pool and the material properties was the know-how in the field of "Additive Manufacturing" that has been built up at Empa for around nine years. Since then, Christian Leinenbach's team has been one of the world's leading working groups dedicated to the material science issues surrounding 3D laser printing processes. At the same time, the Empa researchers have gained experience in process monitoring, especially the measurement of melt pools, whose size and service life are crucial for the targeted modification of alloys.
