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Researchers 3D print partially magnetic structure using one type of powder

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Iron filings stick to this mini chessboard with a partially magnetic structure, produced from a single type of steel power at different temperatures. Image: Empa

Researchers have succeeded in producing a partially magnetic structure from a single grade of metal powder using laser 3D printing.

The technique, published in Applied Materials Today, could aid in the design of more efficient electric motors, as well as be used for manufacturing structural components that react locally to different temperatures. 

The structure produced is a small metallic chessboard, 4mm long on either side, which features 16 squares, eight of which are magnetic, the other eight being non-magnetic. 

It was made using P2000, a special type of hard stainless steel containing around one per cent of nitrogen.

At first glance this alloy seems unsuitable for laser 3D printing. This is because in the melting zone of the laser beam, the temperature reaches upwards of 2,500°C in milliseconds, causing a large part of the nitrogen to evaporate, resulting in a change in the alloy’s properties.

The research team however, from the Swiss Federal Laboratories for Materials Science and Technology (EMPA), decided to use this effect to their advantage. With it they were able to create new alloys with novel properties and embed them in 3D-printed metallic work pieces, such as the chessboard, with micrometre precision.

They achieved this by modifying the scanning speed and intensity of the laser beam melting the particles in the powder bed, which varied the size and lifetime of the liquid melt pool in a specified manner. In the smallest case, the pool was 200μm in diameter and 50μm deep, in the largest case 350μm wide and 200μm deep. The larger melt pool allows much more nitrogen to evaporate from the alloy, causing the solidifying steel to crystallise with a high proportion of magnetisable ferrite. In the case of the smallest melt pool, the melted steel solidifies much faster, meaning the nitrogen remains in the alloy, with the steel crystallising mainly in the form of non-magnetic austenite.

The P2000 steel powder under an electron microscope (left). Due to the special spherical shape, the powder flows particularly well. Ariyan Arabi-Hashemi and Christian Leinenbach used a 3D laser printer to fine-tune the stainless steel alloy (right). (Image: EMPA)

The work could soon add a crucial tool to the methodology of metal production and processing, according to the researchers.

‘In 3D laser printing, we can easily reach temperatures of more than 2,500°C locally,’ said Christian Leinenbach, who with his colleague Aryan Arabi-Hashemi led the EMPA team. ‘This allows us to vaporise various components of an alloy in a targeted manner – e.g. manganese, alumnium, zinc, carbon and many more – and thus locally change the chemical composition of the alloy.’ 

The method is not limited to stainless steels, but can also be useful for many other alloys, for example certain nickel-titanium alloys also known as shape memory alloys. At what temperature these alloys ‘remember’ their programmed shape depends on just 0.1 per cent more or less nickel in the mixture. Therefore, using the researchers’ technique, structural components could be manufactured that react locally and in a staggered manner to different temperatures.

The ability to produce different alloy compositions with micrometre precision in a single component could also be helpful in the design of more efficient electric motors. ‘For the first time, it is now possible to build the stator and the rotor of the electric motor from magnetically finely structured materials and thus make better use of the geometry of the magnetic fields.’ EMPA stated in its news release discussing the research.

During 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 developed analytical methods available at EMPA were used for this purpose.

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