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Materials Science in Additive Manufacturing Functional materials for AM

SLS involves laying down a flat layer of polymer powder magnetization properties into hard magnetic materials,
particles and selectively sintering them with a laser into soft magnetic materials, and superparamagnetic materials.
the desired shape. After each layer is sintered, the powder Hard magnetic materials have high remanence and
bed is lowered, and another flat powder layer is laid on top, coercivity. Remanence (or residual magnetization) refers to
repeating the process until the entire object is fabricated. It the magnetization remaining in a material after the external
is important to note that SLS differs distinctly from selective magnetic field has been removed. This property renders
laser melting (SLM), as SLS does not fully melt the powder hard magnetic materials to be considered permanent
into a liquid state. The schematic of the SLS process is magnets. Coercivity is the strength of the external
shown in Figure 2D. SLS technology offers the capability to magnetic field required to demagnetize the magnetized
manufacture complex 3D shapes, enabling its application material. Hard magnetic materials can maintain significant
across a wide range of fields. In addition, its ability to residual magnetization even after the removal of the
mix various materials further extends its versatility. magnetic field following saturation, and they can also
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Particularly, noteworthy is the role of polymer powder retain notable residual magnetization when subjected to
in serving as support, enhancing design flexibility. From magnetic fields below their coercive strength. 35,36 Due to
prototype fabrication to final product manufacturing, SLS these characteristics, hard magnetic materials exhibit large
emerges as a promising manufacturing method extensively hysteresis (Figure 3A). Hence, composites containing hard
used across diverse industries. magnetic particles embedded in a polymer matrix retain
high residual properties after magnetization and exhibit
2.5. Material jetting independent behavior in response to applied magnetic
Material jetting is a manufacturing method where liquid fields below the coercive fields. Hard magnetic materials
materials are precisely sprayed through small nozzles to include ferrite-based substances such as barium ferrite
form layers. This technology offers exceptional fabrication (BaFe O ), strontium ferrite (SrFe O ), or neodymium
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speed, making it suitable for various industries and iron boron (Nd Fe B) and samarium cobalt (SmCo ,
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applications. In material jetting, materials are deposited Sm Co ) known as rare-earth magnets.
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at precise locations through small nozzles and then cured Soft magnetic materials, such as pure iron (Fe), nickel-iron
either by UV light or heat to form and bond layers. The alloy, and silicon-iron alloy, are known for their strong response to
referenced process schematic is shown in Figure 2E. This magnetic fields, characterized by high saturation magnetization.
process allows for the creation of models with complex However, they tend to retain a low residual magnetization
shapes and intricate details, providing high material after saturation and are easily demagnetized due to their low
selectivity and enabling the fabrication of parts with coercivity. That is, soft magnetic materials have narrow hysteresis
specific material properties tailored to the application’s (Figure 3B). In addition, these materials, utilized as fillers, possess
requirements. high relative permeability and swift responsiveness, making
them widely utilized in the fabrication of magnetic alignment
3. AM with functional materials and their composites. Ferromagnetic particles smaller than a certain
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applications critical size are referred to as superparamagnetic materials,
3.1. Magnetic powders behaving somewhat similarly to non-magnetic substances in
the absence of an external magnetic field, despite maintaining
Recently, there has been active research on magnetically relatively high magnetization (Figure 3C). Superparamagnetism
responsive soft materials among various types of polymer- is a property exhibited by nanometer-sized magnetic particles,
based stimuli-responsive soft materials. This interest where the continual thermal motion and Brownian motion
primarily stems from their advantage of faster response cause their magnetic structure to fluctuate, rendering them
compared to other operating modes, such as heat, light, essentially unaligned in the absence of an external magnetic field.
and electric fields. The magnetic field, which serves as Notably, Fe O nanoparticles are widely utilized in various fields,
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the actuating source for these magnetically responsive soft including biomedical applications and micro-robotics, due to
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materials, is non-contact and relatively easy to control, as their biocompatibility and relatively high magnetization. 37-39
its magnitude, phase, and frequency can be modulated Typically, Fe O undergoes a transition from a soft magnetic
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quickly and accurately, and the magnetic field is transparent phase to a superparamagnetic phase at a critical size of around
to most materials. 32-34 20 nm. 40
Magnetic filler particles generally consist of
ferromagnetic materials that exhibit significant 3.1.1. Magnetically responsive soft robots
magnetization under an external magnetic field. These Due to recent advances in 3D printing technology for soft
ferromagnetic materials can be classified according to their materials, current research is focused on developing soft

Volume 3 Issue 2 (2024) 5 doi: 10.36922/msam.3323