Magnetic Forming

 

(Electro-)magentic forming of ceramics

Duration: 08/2022 – 08/2023

Motivation

Due to their persistent plasticity, wet-extruded ceramic masses cannot be printed in every geometry by the extrusion-based additive manufacturing process LDM (Liquid Deposition Modelling). Perforations and material overhangs for example pose great difficulties. Often, a high proportion of support structure - which must be subsequently removed at great expense - is necessary to produce digitally developed components. In this context, the digital manufacturing process LDM of plastic clay masses in industry and construction enables a new look at established and material-related design and shaping. Precise manufacturing processes using digital models are comparatively fast, cost-effective and sustainable. However, material-specific properties and operator influence play a decisive role in the conversion to the material world. The printed component does not correspond to the digital CAD model!

Methods

In order to control the classic deformation behavior in the LDM process, stainless steel particles that can be attracted via a magnetic field were added to the clay matrix. The aim was to develop a ceramic composite material that can be compacted, stabilized and formed in the LDM through plant modification and the influence of electromagnetic forces. A test extension of the processable materials was carried out for the typical construction ceramic Al₂O₃.

Results

In the clay-steel material system, the following conclusions could be drawn:

• The integration of corrosion-resistant steel particles into a liquid clay mass enabled the controlled deformation of printed walls by magnetic fields.
• The corrosion of steel particles in the liquid was limited by a chromium content above 15 wt.%. No difference in the thickness of the formed chromia scale was observed after 3 days, as demonstrated by microsections of the sintered composite.

  Copyright: © IWM

Figure 1: Light microscopic images of air-sintered composites consisting of clay with (a) iron particles or (b,c) 430L steel particles. Samples were dried and sintered immediately (a,b) or after 3 days (c).

 

• The sintering of the composite material could be carried out in the same way that pure clay and crack-free bending bars were obtained. No additional reactions between steel and clay were observed until the steel particles in the liquid mass reached a content of 40 wt.%.
• The determined four-point bending strength of 7 MPa represented a significant decreased in strength after the integration of steel particles. The corresponding strength of the pure clay amounted to 66 MPa.
• The printing pattern influenced the achieved strength, as demonstrated with pure sintered clay.

  Copyright: © IWM

Figure 2: (a) Printing patterns 1, 2, and 3 of the different tested groups of bending bars. (b) Weibull plot of pure clay printed in the different printing patterns. (c) Weibull plot of pure clay and clay with 17.5 wt.% AISI 630 particles, both printed in pattern 2.

 

Findings from forming in the (electro-) magnetic field are as follows:

• Despite the almost 10-fold higher holding force of the electromagnet (40 kg) in comparison to the permanent magnet (4.4 kg), the focus of the magnetic field on the printed wall could not be improved. To stop the attractive force of the permanent magnet on the wall, a linear travel unit was successfully integrated into the experiment set-up.
• The forming of additively manufactured labile structures in ceramics by magnetic fields bears the risk of a collapse between the magnet and the printed object. To control the distance, an optical sensor was integrated. Progressively decreasing distances could predict a possible collapse and were interpreted as a signal for the retraction of the permanent magnet.
• A good deformation strategy was found by combining several phases of approach and retraction of the permanent magnet to allow for a relaxation of the liquid material.

  Copyright: © K:G

Figure 3: Deformation of the wall: (a) by a permanent magnet and pulsating movement (without laser sensor); (b) schematic comparison to the initial position.

 

In contrast to the clay (+steel particles) system, liquid deposited alumina paste exhibits no green strength after drying and parts cannot be transferred into a sintering furnace. Therefore, the addition of binder and dispersing agents is neccessary. In total 17 different recipes were tested with varying contents of Al2O3 particles, deionized water, disperging agent Dolapix, and binder agent Aquazol. For each slurry the extrusion performance through a syringe, the shape stability during drying and the homogeneity was evaluated.

• The optimized paste consisted of 84 wt.-% alumina, 12,4 wt.-% water, 1,75 wt.-% Dolapix and 1.75 wt.-% Aquazol. To distribute the aquazol is was soluted in 60 °C (1:1 weight parts) deionized water.
• Due to the limited water absorption of the Al2O3, any proportion could be replaced by steel particles without qualitative changes in viscosity.
• Bending bars made from pure alumina and alumina + steel particles were printed and sintered at 1400 °C in air.
• The characteristic strength of the composite material (99 MPa) is as high as the strength of the pure alumina (90 MPa). In contrast conventionally sintered material the strength is low. However, it is seen as first success that the particle integration did not reduce the strength as it was the case in the system clay+steel particles.

Partner

• Lehrstuhl für künstlerische Gestaltung (K:G), RWTH Aachen University

Publications

Klug, C.; Herzog, S.; Kaletsch, A.; Broeckmann, C.; Schmitz, T.H. Forming of Additively Manufactured Ceramics by Magnetic Fields. Ceramics 2022, 5, 947-960. https://doi.org/10.3390/ceramics5040068

Funding information

• Funded by the Bundesministerium für Bildung und Forschung (BMBF) and the Ministerium für Kultur und Wissenschaft des Landes Nordrhein-Westfalen (MKW) im Rahmen der Exzellenzstrategie von Bund und Ländern

Funding number: G:(DE-82)EXS-SF-OPSF647