During metal processing in 3D laser printers, 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 in 3D-printed metallic workpieces with micrometer precision.
For example, magnetic areas can be created in non-magnetic surfaces, even though 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, the Empa team used a special type of stainless steel, known as P2000 steel, which contains no nickel but around one percent nitrogen. It is particularly hard, which makes conventional machining by milling more difficult. Unfortunately, it also appears to be unsuitable as a base material for 3D laser printing at first glance, as it quickly becomes very hot in the melting zone of the laser beam. As a result, a large proportion of the nitrogen it contains normally evaporates and the P2000 steel changes its properties.
The researchers have now 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 selectively varied the size and service life of the melt pool. A large melt pool allows a lot of nitrogen to evaporate from the alloy and the solidifying steel crystallizes with a high proportion of magnetizable ferrite. With a small 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 temperatures of more than 2500 degrees Celsius can be reached locally during 3D printing, not only stainless steels but also many other alloys can be treated in a targeted manner and various components of an alloy can be vaporized, e.g. manganese, aluminium, zinc, carbon and more. This allows the chemical composition to be changed locally. Applications of the new process can be seen, for example, in shape memory alloys or the fine structures of electric motors.
