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Abstract

UV-cured 3D printing technology had become more and more popular due to its rapid printing speed and high molding accuracy. However, for ordinary Direct-ink-writing (DIW) printers, though there were widely materials can be used, but they cannot be quickly printed and molding accuracy was much low. Therefore, a simple method combined UV-cured and DIW was summarized in this paper, before DIW the mixed slurry (FeSiB and Epoxy resins soft magnetic composites without photoinitiator added) was pre-curing by UV for some time to a semi-cured state, the rheological properties of the slurry were investigated and the magnetic, mechanical properties and micromorphology of printed products were studied. Compared with the printed products without UV pre-curing, the coercive force (H_c) was reduced by 20.36%, the elastic modulus increased by 34.41%, and the insulating coating after UV pre-curing was more uniform, with no obvious defects.

Keywords

Direct-ink-writing ultraviolet soft magnetic composites

Introduction

As an advanced technology, material extrusion additive manufacturing includes Fused Deposition Modelling (FDM) and Direct Ink Writing (DIW).¹ Both of them need to generate digital models firstly, and then create 3D structures layer by layer. The difference is that the extrusion material of FDM is continuous wire, such as plastic wire, composite wire; the extusion material of DIW is slurry, also can be called as ink, the slurry is extrused through a nozzle by some presure (such as pneumatic, screw, piston) to fluent flow, which deposite and curing on the previous layer, repeat the process layer by layer to create a 3D part.^2,3 Direct ink writing (DIW) has been used to rapidly fabricate complex 3D shapes and high-precision objects in many areas as diverse as functional electronics,^4–6 living tissue,^7,8 smart materials,^9,10 ceramic materials,^11–14 and soft magnetic materials.^15–17 DIW depends on extruded slurry from the nozzle at controlled parameters, deposited layer-by-layer along designed model files without the need for expensive tooling, dies, or lithographic.^12,18,19

In order to get higher printing accuracy, there are some specific rheological requirements for slurry of DIW.¹² The slurry must not only have the characteristics of shear thinning to meet the fluxility through nozzle, but also have sufficient yield strength to ensure excellent self-support after printing. Epoxy resins (EP) are widely used as matrix materials for DIW technology of functional composites. In terms of fiber functional composites, Zhang²⁰ et al reported continuous carbon fiber/epoxy thermoset composites with special mechanical properties and low energy consumption prepared by DIW technology. Guo²¹ et al prepared a highly mechanically performant shape memory epoxy composite material by combining industrial grade epoxy resin E51 with short carbon fibers into slurry. Epoxy carbon nanotubes/GNPs conductive materials were prepared by A. Cortés²² using DIW in the field of anti-icing and de-icing. In term of magnetic functional composites, Brett Gibson Compton et al¹⁵ had successfully formulated a anisotropic NdFeB/EP bonded magnet by DIW at room temperature. Fang Yang²³ fabricated parts of NdFeB/SrFe₁₂O₁₉ mud deposition forming using DIW. In our previous work, the varying mass fraction FeSiB/EP composites parts FeSiB/EP soft magnetic composites were studied by DIW technology. The result showed that the vortex loss reduced and the magnetic performance of FeSiB/EP composite materials were improved.²⁴ However, due to the slow curing speed of EP, the use of DIW usually leads to poor printing accuracy.

In order to speed up the curing of EP, many scholars had used different grades of EP,^25,26 different curing agents,^27–29 and the production of epoxy photosensitive resins,^26,30,31 to study the curing kinetics of different epoxy resin systems. Recently, UV-assisted DIW technology has become more and more popular among many scholars due to its wide material selectivity, and room temperature-induced reaction,³² however ordinary DIW printers have not UV-assisted system, it is also not photosensitive to the use of a large amount of slurry in DIW, which causes difficulties to the promotion of UV-assisted DIW. Besides it is found that for the epoxy resin commonly used in DIW, the epoxy group itself can undergo a ring-opening polymerization reaction under the action of ultraviolet light to generate cross-linked products.³³ Thus, we have developed a simple way to combine UV curing and DIW printing, an additional UV light source was used to improve the curing efficiency of the epoxy resin matrix. Through UV light source pre-curing before printing, the self-supporting performance of the slurry is improved, which not only shortens the printing time, but also improves the comprehensive performance of the printed products.

In this paper, FeSiB/EP soft magnetic composite slurry for DIW printing was chosen as an example, the effects of UV light processing time and curing sequence on the rheological properties of FeSiB/EP soft magnetic composite slurries, as well as the microstructure, mechanical properties and magnetic properties of FeSiB/EP soft magnetic composites were systematically studied by using ultraviolet light assisted DIW technology.

Materials and experimental details

Materials

A bisphenol-A ER E-44 with an epoxy value of 0.41–0.47 was purchased from Shanghai Licheng Adhesive Co., Ltd., Shanghai, China. The curing agents were polyamide curing agent, which were supplied by Shanghai Licheng adhesive Co., Ltd., Shanghai, China. FeSiB amorphous alloy powder was provided by ourselves.

Experimental procedure

Different ratio of FeSiB/EP slurries without photoinitiator were pre-cured by UV to obtain a semi-solid slurry. Then the semi-solid slurry was used for DIW molding. Figure 1 was preparation process flow diagram.

Figure 1.

The schematic of processing composite parts by DIW.

The weight ratio of EP to polyamide curing agent was 1:1. The iron-based amorphous alloy powder (90.0 wt%) was added to mixture and stirred for 5 min, then put this slurry on PI mask with ultraviolet, the different ultraviolet pre-curing time was 0 min, 5 min, 8 min, 10 min, and 15 min. Then the prepared slurry was put into the printing cartridge, the nozzle diameter of cartridge was 0.6 mm. Processing parameters were set as follows: the printing speed was 1 mm/s; the printing pressure was 6–7 psi; the height of layer was 0.4 mm; and the spacing was 0.6 mm. The printed parts were absolutely cured after 12 h at a room temperature.

Characterization

The relationship between loss modulus and storage modulus was tested by rheometer (HAAKE MARSⅢ, Thermo Scientific Inc., USA)with a 20 mm flat plate and a gap of 1 mm at 30°C.The external and fracture surfaces of the 3D printed parts were also investigated by a field-emission scanning electron microscope (Phenom XL, Phenom China, Shanghai) with an operating voltage of 2–5 kV. The samples were further characterized by a vibrational sample magnetometer (LakeShore7404, Linkphysics Corporation, Shanghai). Mechanical properties tests included the tensile strength and tensile elastic modulus of the spline, with standard tensile strips ISO527. Splines were tested on a omnipotence tensile tester (CMT6104, Shenzhen Xinsanisi Metrology Technology Co., Ltd., Shenzhen) with five average for each ratio and rate of extension was 2 mm/min. Gel content test using immersion method with reference to ISO10147, repeats three times to take the average.

Results and discussions

Rheological behavior measurement

Rheological behaviors of FeSiB/epoxy compositing slurries with different ultraviolet pre-curing times were shown in Figure 2. From Figure 2(a), we could see that without any ultraviolet pre-curing, the magnetic slurry showed the property of high viscosity liquid, it could be printed with DIW, but the samples had not any self-support ability, so this slurry was not suitable for printing. After 5 or 8 min of UV pre-curing, Figure 2(b) and (c) showed that the values of the energy storage modulus (G′) and the loss modulus (G″) were similar, and the energy storage modulus was higher at low shear stress period, the elastic deformation was dominated; as the shear stress increased, there was an intersection point of G′ and G″, which was the yield point^21,33 of the slurry, after that point the viscosity was more significant, indicated that the slurry had a certain self-support ability and had a certain fluidity, which was suitable for printing. This phenomenon was mainly caused by UV pre curing, which leaded to the partial network structure through internal crosslinking of epoxy resin.³⁴ When the ultraviolet light pre-curing time was extended to 10 min or 15 min, the degree of magnetic slurry curing was high, the energy storage modulus was greater than the loss modulus, the elastic deformation was dominated, the slurry flow was difficult, and the printing paste appeared micellar slip phenomenon, resulting in holes, cracking, and other defects in the molded device, which greatly reduced the mechanical properties of the molded device.^35,36 Due to the high viscosity, it could even lead to epoxy resin accumulation, magnetic particles agglomeration, reducing the magnetic properties of the material.^37,38

Figure 2.

Storage modulus (G′) and loss modulus (G″) of epoxy-based slurry with varying ultraviolet cured time as a function of shear stress (a) UV-0min FeSiB (b) UV-5min FeSiB/EP (c) UV-8min FeSiB/EP (d) UV-10min FeSiB/EP (e)UV-15min FeSiB/EP.

In summary, the method of pre-curing without any photoinitiator could also play a role in the curing of epoxy slurry, thereby turning the slurry with bad printing performace into an excellent one. In addition, the gel contents of samples with pre-curing time of 0 min, 5 min, 8 min, 10 min, 15 min were studied, which were 55.15%, 69.26%, 69.68%, 74.4%, 99.5%. As the time of pre-curing was prolonged, the gel contents of samples increased, which meaned that the degree of epoxy resin crosslinking improved. Rheological and gel content results showed that when the gel content of the magnetic slurry was about 69% with the pre-curing time of 5 min and 8 min, the magnetic slurry had good printing performance.

Scanning electron microscopy (SEM)

Figure 3 showed secondary electron SEM images of the printed samples. We could clearly observe the surface morphology and spatial distribution of the particles in micron scale. The dark area (B) was corresponded to epoxy resin, and the bright area (A) was corresponded to FeSiB amorphous alloy. The (C) represented holes and defects. From Figure 3(a)–(e), it could be seen that with the pre-curing time increased, particles on the surface of the printed samples gradually became obvious, it was because that when the curing of printed products accelerated, the shape and composition distribution of the printed products were easier to retain, and the magnetic particles were less likely to settle. From Figure 3(f)–(j), we could see the sample pattern of cross-section. Compared with the samples of pre-curing time of 0 min, 5 min and 8 min, the samples with 10 min and 15 min pre-curing time performed more defects, which leaded to the poor printing performance. And as the pre-curing time increased, the size and number of holes gradually increased. For samples with pre-curing times of 10 min and 15 min, the holes size was about 30 μm to 60 μm. Therefore, we could conclude that as the pre-curing time became longer, the slurry mobility declined, the printing performance became worse. As noted before, the introduction of EP and UV light effectively increased the adhesion among FeSiB particles, and improved the interface strength of the printed parts.

Figure 3.

SEM images of surfaces and cross-sections of printed samples with different pre-curing times (a,f-0 min, b,g-5 min, c,h-8 min, d,i-0 min, e,j-15 min).

Mechanical properties

Figure 4 showed the mechanical properties of samples during different UV pre-curing time. Figure 4(a) was a line chart of tensile strength (σ), elongation at tensile strength (ε_B) and tensile fracture stress (σ_B) with varying of pre-curing time. As the pre-curing time increased, the tensile strength and tensile fracture stress increased firstly and then decreased, and the fracture elongation gradually decreased too. Generally pre-curing for 8 min contributed to the best strength. After pre-curing, EP was easier to form cross-linking network and during the DIW processing, less dimension variation appeared, so the mechanical strength would improve, although the toughness became worse. But if the pre-curing process took too long time, the slurry cross-linking degree increased too much (The gel contents of samples with pre-curing time of 0 min, 5 min, 8 min, 10 min, 15 min was 55.15%, 69.26%, 69.68%, 74.4% and 99.5%), as a result, it deteriorated the fluidity of slurry, increased the holes and defects of the printed samples, and finally caused an overall decline in mechanical properties. Figure 4(b) was a histogram of elastic modulus with the pre-curing time. The elastic modulus tended to obviously increase. The elastic modulus of 5 min and 8 min of pre-curing time was up to 724–740 MPa, much higher than that of the samples without UV pre-curing, the elastic modulus of 10 min and 15 min of pre-curing time was 583.99 MPa and 585.93 MPa, respectively, which was 23–25 MPa higher than that of the samples without UV pre-curing (550.28 MPa). This phenomenon also proved that the curing rate accelerated and linear size change declined during the printing process of the pre-curing slurry, which was helpful for improving the comprehensive mechanical properties of the composite material.

Figure 4.

(a) Tensile strength, tensile fracture strength and elongation at tensile strength of the samples with different UV pre-curing times (b) Elastic modulus of the samples with different UV pre-curing times.

In our work, the epoxy molecular chain firstly underwent thermal motion with increasing UV pre-curing time, and then the slurry was flowing dynamically. Over time, cross-linked network initiated, the slurry became semi-solid, and the viscosity increased, but still coordinated the printing conditions, and the print samples had a certain self-support ability and good accuracy, in this conditon the elastic modulus and tensile strength were all higher, which mainly due to the higher cross-linked networks.^36,39 With adding the pre-curing time, more cross-linked network reduced too large viscosity, as a result, the slurry flowing became difficult, the formed samples had many defective pores, resulting in a large reduction in mechanical properties.

In summary, when the curing time was 8 min, the mechanical properties of the spline were best. But this time, its tensile strength was 30.1 MPa, elongation at break was 6.43%, and elastic modulus was 724.69 MPa.

Magnetic performance test

This experiment tested the magnetic properties of the UV-FeSiB/EP composite, and Figure 5 was the vibrational sample magnetometer (VSM) map of the original composite powder with 3D printing devices in different UV pre-curing times. As could be seen from the Figure 5, parts all had excellent soft magnetic performance with high saturation magnetization strength (M_s). In the range of magnetic field strength of-1.5 T ∼ 1.5 T, the Figure 5 showed that as the M_s increased, the trend of coercive force (H_c) fluctuateed little overall. For soft magnetic composites, excellent soft magnetic performance requires high saturation magnetic induction strength M_s and low coercive force (H_c). The lower right corner of Figure 5 indicated the enlarged view of Ms from −20 Oe to 20 Oe.We could see that the introduction of the composite insulating coating did not greatly reduce the Ms of the magnetic powder.

Figure 5.

The hysteresis loops with different UV pre-curing times.

The experimental results showed that the pre-curing magnetic device M_s had a slightly less trend than the untreated magnetic powder, mainly because the coating of the composite powder was epoxy resin, which was a non-magnetic material, so it has an impact on the M_s value. The time of UV pre-curing had little effect on the M_s of this composite. When UV pre-curing for 8 min, the largest M_s value was 134.37 emu/g. When UV pre-curing for 15 min, the minimum M_s value was 129.22 emu/g. The coercive force of these sample was reduced by 3–4% compared with the original powder. This indicated that proper pre-curing had no effect on the magnetic properties of the composite, but when the pre-curing time was too long, the magnetic properties decreased as the internal defects of the printed sample increased.

Table 1 was the magnetic performance of the printed sample, it could be seen that the saturation magnetization strength (M_s) of the sample first slightly reduced and then slightly increased with the increase of pre-curing time from 0 min to 15 min. The coercive force of these samples was reduced by 3–4% compared with the original powder, indicating that the epoxy printed sample improved the ability to save the residual magnetic state. As mentioned above, we could find that the reduction of M_s and H_c were both in a small range, mainly because of the insulating effect of EP.^40,41

Table 1.

Magnetic parameters of printed samples of epoxy resin binders.

Sample	Magnetization strength (emu/g)	Coercive force (Oe)
Raw-powder	156.35	10.85
UV-0 min	133.67	7.058
UV-5 min	134.36	7.138
UV-8 min	134.37	8.64
UV-10 min	133.33	6.749
UV-15 min	129.22	7.618

Conclusion

FeSiB/EP magnetic slurries for DIW were successfully prepared via introducing epoxy resin as a binder. The specific conclusions were as follows:

(1) As UV pre-curing time becomes longer, print support performance is improved and fluidity becomes worse.

(2) During proper UV pre-curing time, the mechanical properties will increase with the increase of pre-curing time. However, when time is long enough, it will cause damages to the mechanical properties on account of the presence of defects, which is consistent with the results of scanning electron microscopy.

(3) The epoxy insulating layer reduces saturation magnetization and coercivity. However, UV pre-curing time has little effect on magnetic performance.

In summary, according to the analysis of rheological properties, surface morphology, mechanical properties and magnetic properties, the comprehensive performance of UV pre-curing for 8 min was the best.

Footnotes

Acknowledgments

This work is financially supported by National Natural Science Foundation of China (No. 52171149), Beijing Municipal Science and Technology Commission Project (No. Z211100004321004 and No. Z211100004321003).

Author contributions

Q.M.: Conceptualization, Writing-original draft, Writing-review & editing, Data curation, Methodology. X.F. and Y.S.: Supervision, Writing-review & editing. J.H.: Funding acquisition, Conceptualization, Supervision Writing-review & editing.

Declaration of conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation of China (52171149).

ORCID iD

Jing Hu

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Study on the FeSiB/EP soft magnetic composites performance with 3D direct-ink-writing and ultraviolet pre-curing

Abstract

Keywords

Introduction

Materials and experimental details

Materials

Experimental procedure

Characterization

Results and discussions

Rheological behavior measurement

Scanning electron microscopy (SEM)

Mechanical properties

Magnetic performance test

Conclusion

Footnotes

Acknowledgments

Author contributions

Declaration of conflicting Interests

Funding

ORCID iD

References