Authors: Monica Keszler
Source: JuSER Publikationsportal (article, 2025-01-01)
Field: Materials Science — Electronic, Optical and Magnetic Materials

Field assisted sintering technology/spark plasma sintering in the direct recycling of hot-deformed Nd-Fe-B scrap and PM T15 steel swarf The push for a more circular economy worldwide has necessitated further research into direct recycling. There are many waste streams that currently do not have a possible direct recycling route developed. While many indirect recycling techniques may exist, they often come with the use of hazardous chemicals or a large consumption of energy. The aim of this work has been to investigate the use of field assisted sintering technology/spark plasma sintering (FAST/SPS) in the direct recycling of two waste streams that currently have no direct recycling route. The first stream of interest was scrap, out-of-spec hot deformed Nd-Fe-B magnets. These magnets, provided by an industry partner, WILO SE, have a desirable microstructure that would be destroyed by consolidation techniques like hot-deformation or sintering. A benefit of FAST/SPS is its capability to maintain and fine-tune microstructures due to its high heating rates and quick sinter times. Firstly, these scrap powders were partially densified into pre-forms via FAST/SPS. Afterwards, this work employed two different electric current assisted sintering (ECAS) techniques in the formation of new magnets from this scrap. The first was flash SPS (FSPS), a technique that uses a single power pulse and uniaxial pressure to deform a pre-form. The second was spark plasma texturing (SPT), a modified FAST/SPS technique that deforms a pre-form to a wider diameter. As the FAST/SPS and flash SPS devices allow for fine-tuned parameter control, different aspects such as deformation speed, applied pressure, pressure dwell time, and maximum temperature were adjusted across experiments. The ultimate goal was to generate magnets made from 100 wt% recycled hot-deformed Nd-Fe-B scrap for use in a demonstrator motor. The spark plasma texturing technique generated 100 wt% scrap magnets with a maximum energy product, (BH)max, of over 200 kJ/m3, which were successfully tested in a pump motor of the industrial partner, WILO. The second stream of interest was PM T15 high-speed steel swarf, provided by a second industry partner, Berghaus GmbH. This swarf was contaminated with lubricant oil and grinding medium, such as Al2O3. Because of the contamination, this sort of swarf is typically disposed of in landfill rather than recycled. After being cleaned of the lubricant oil, the swarf was sintered under various FAST/SPS conditions to optimize densification. Modifications to both the swarf and the FAST/SPS procedure were necessary to generate dense samples that would not break tools upon extraction. Samples were scaled up from  20 mm to  120 mm, leading to a demonstrator part of  120 mm made from 100 wt%PM T15 swarf that could be tested in a mock tunneling rig. Despite the two very different waste streams, FAST/SPS showed its strengths in microstructural control, densification, deformation, and utilization of unconventional powder morphology. Alongside this, energy measurements of the ECAS processes were taken for the discussion of practical industrial applications. Scrap ; Sintering ; Microstructure ; Spark plasma sintering ; Consolidation (business) ; Magnet ; Work (physics) Magnetic Properties of Alloys ; Bauxite Residue and Utilization ; Magnesium Alloys: Properties and Applications