1. Introduction
During recent years, interest in the study of the soft magnetic composites (SMCs) has increased in the magnetic and electric industries. SMC materials are basically pure iron powder particles coated with very thin electrically insulating layers. As
Figure 1 shows, the insulating layer restricts the eddy current, which is generated by an alternating magnetic field to mainly travel within each individual particle. Due to eddy current restriction, SMC materials provide new opportunities to design compact, light, and cost-efficient solutions for high-performance motors [
1,
2,
3,
4].
In conventional powder metallurgy processing, the parts are sintered after compaction. Sintering is a high-temperature heat treatment which develops the bonds between the powder particles and enhances their mechanical performance. However, SMC powders cannot be sintered, because they reduce the eddy current. If the magnetic flux changes in the magnetic substance, an electromotive force is generated—a swirl of current flows through the magnetron. This is called an eddy current, which consumes the mechanical energy of the driving part in Joule heating, thus reducing efficiency. To reduce this, the particles should not be bonded to each other, as shown in
Figure 1. The temperatures involved would cause the insulation coating to break down, giving a poor electrical performance as the particles would bond to each other as shown in
Figure 2 [
5,
6]. Therefore, in order to form an SMC, only the compaction method is applied.
It is well known that high-frequency vibrations, such as ultrasonic vibration, reduce particle friction [
7,
8,
9]. Researchers proposed the usage of ultrasonic vibration during the powder compaction process to increase the density and reduce the internal friction without the increase in compaction pressure. J. Tsujino et al. reported the ultrasonic vibration press of copper and piezoelectric ceramic powder in a vacuum condition [
10,
11], and O.L. Khasanov et al. reported the ultrasonic compaction of ceramic nano-powder [
12]. Both studies made the following conclusions—the ultrasonic compaction during the ceramic nano-powder compaction (a) lowered lateral pressure to a greater degree in a low compaction pressure condition, (b) more effectively decreased an elastic after-effect in a high-pressure environment, and (c) improved the green compact quality [
13,
14,
15].
According to the published literature, one of the effective methods for powder compaction is ultrasonic-assisted compaction under a pressure of 250 MPa, which may potentially be applicable to massive powder metallurgy manufacture processes [
16].
However, the ultrasonic compaction method faces a problem that limits its wider application. Due to an unknown mechanism, the high densification phenomenon is generally accepted without clear explanations. The aim of this research, therefore, is to investigate the relationship between low internal friction forces and the high densification of powders using a force balance model. We expect that our proposed model will contribute to the wider usage of ultrasonic compaction and its successful industrialization.
In this paper, the reduction in friction and the improvement in resultant density are studied as functions of compression pressure, vibration amplitude/frequency, and input power. Through this experiment, fundamental information such as the densification mechanism under ultrasonic action and the optimum design parameter for high densification is determined. It is found that the input power needed to drive the ultrasonic vibration system is the governing parameter for obtaining the high density results of an SMC specimen.
2. Materials and Methods
The shape and dimensions of the die were set, as shown in
Figure 3; the thickness was 50 mm, the outer diameter was 160 mm, the inner diameter was 20 mm, and the flattening curve was 40 mm for use at 19.8 kHz.
The ultrasonic compaction variables are displayed in
Table 1. The material used for this research was limited to SMCs termed SMC550 and SMC500. The coefficient of the relation equation of determining density of SMCs was firstly taken.
Table 2 shows the coefficient of the equation for determining the density of SMCs without ultrasonic action. Through
Table 1 and
Table 2, the resultant density with ultrasonic action may consist of two portions, as shown in Equations (1) and (2); one is the density increase by the ultrasonic effect and the other is the conventional compaction effect, without the ultrasonic effect.
To observe the voids or porosity among the particles, a microscopic analysis of the SMC powders was needed. In
Figure 4, the SEM (Scanning Electron Microscope) pictures of SMC500 are shown, which were used for the analysis.
The equipment installed and its setup for the experiment are shown in
Figure 5, and were also published in our previous work [
16]. In this research, a Universal Test Machine (UH-F500) (Shimadzu, Kyoto, Japan) was used to apply the pressure. The ultrasonic wave was produced by a signal generator (WF1943A) (NF corporation, Yokohama, Japan) and enlarged through a high-power amplifier (HSA4101) (NF corporation, Yokohama, Japan). The amplified wave was applied to each vibrator. The input current and voltage were measured by CT (Current Transformer) and PT (Potential Transformer) at the point where the ultrasonic transducer was connected and the values were displayed using an oscilloscope. Each amplifier generated a maximum voltage of 200 V and a maximum current of 1A. Also, in order to consider the effect of the driving frequency on the densification of SMCs, the change in density with different driving frequencies was measured. Here, vibration compacting die was designed for operating at 20 kHz half wavelength longitudinal vibration and the press frame had a hydraulic static compacting pressure source. The experiment was conducted for 3 seconds, so as not to destroy the insulation layer due to the temperature increase.
The ultrasonic compaction system was set up as shown in
Figure 6. A soft magnetic composite (SMC) was chosen among various types of powder. The SMC has a very high potential for practical use due to its outstanding magnetic characteristics, but its compact density cannot increase beyond a certain level. Hence, its mechanical strength is limited up to a certain point using a conventional compaction method. If the mechanical strength of the SMC could be improved through an ultrasonic compaction process, then more advanced materials that are made of SMCs could be produced. For this reason, the SMC was chosen to be tested among other powders.
Figure 7 shows the assembled ultrasonic compaction mold system with the supporting fixture. A total of six ultrasonic transducers were connected to the die and arranged in a radial direction. From the cross-sectional view (upper right picture in
Figure 7), there was a hole where the specimen was compacted and structured. In the lower right picture of
Figure 7, the connection method between the ultrasonic transducer and the fixture was also described. In order to prevent the production of an electrical shock, the ground line (green cable) was set up at the die and fixture opposite each other.
Figure 8 shows the measured results of the vibration distribution of the interior area of a hole where the SMC was compacted. It was measured using a mirror because there was no margin in the interior area of the hole. The measured vibration amplitude distribution agreed well with the FEM (Finite Element Method) simulation results. The maximum vibration points were ① and ⑥, and the minimum vibration points were ③ and ④. However, the deviation of the vibration amplitude between the maximum and minimum points was almost the same at each point with a deviation of 1.5 μm. The vibration amplitude of position No.1 was 5.5 μm.
As the ultrasonic compaction is taking place, the proposed system is expected to undergo densification, as described below. In the first stage, pressure is applied to the SMC sample in the die where the density is determined by the equilibrium force, as expressed in Equation (3). The applied force contributes to the densification of the SMC as shown in
Figure 9a.
The friction between the die wall and powders as well as between the particles themselves prevent the further densification of the SMC. Densification is also affected by the formation force. Therefore, the resultant density is achieved when the applied and resistance force reach an equilibrium state.
where
: Applied force;
: Barrier friction force;
: Inter-particle friction;
: Formation force.
: Friction coefficient between barrier and powder;
: Friction coefficient among particles themselves;
In the second stage, the ultrasonic vibration is applied to the die through BLT (Bolt Clamped Langevin Type) -type ultrasonic transducers while the pressure is applied, as shown in the
Figure 9b. Ultrasonic vibration is an effective way to reduce the internal friction [
11], because it has a small amplitude and high frequency. Vibration of the die wall, therefore, is expected to reduce the internal friction between the die wall and particles, as well as the inter-particle friction in a system. The SMC powder then rearranges itself to go into empty spots due to the lowered internal friction, and the further densification of SMC is achieved as shown in
Figure 9c.
To observe the voids or porosity among the particles after compaction, a microscopic analysis was carried out. In
Figure 10, the microstructures of compacts with and without ultrasonic vibration are shown with a compressive pressure of 300 Mpa. Because of the low pressure applied, the experiment was in the “repacking” stage. As shown, there was higher porosity exhibited after the conventional compaction method than after ultrasonic vibration. Even with the same compressive pressure, the density increment by the reduction of porosity between the particles was clearly observed in the microstructure.
4. Conclusions
There have been many cases reported in academic societies indicating that ultrasonic compaction contributes to a higher densification of given samples. Ultrasonic compaction technology has received great attention recently since the low mechanical strength of given samples can be improved when it is used to develop the motor core. It is important to note, however, that the reasoning for this phenomenon and the densification mechanism of an ultrasonic compaction has not been reported yet. Due to this unknown mechanism, quantitative analysis of ultrasonic compaction could be done with limitations.
Based on the proposed force balance model, the experiments were designed to test and measure the densification of SMC samples with both ultrasonic and non-ultrasonic effects in the compaction process. The density profile of the SMC sample with an ultrasonic effect was higher than that with the non-ultrasonic effect. Ultrasonic compaction lowered the internal friction between particles and the die wall, as well as the inter-particle friction inside the die wall. Thanks to this lowered internal friction, particles can rearrange and go into empty spots, making a more compacted SMC sample. In order to verify this high densification phenomenon, the mixture of SMC and lubricant was made with different mixing ratios. As more lubricant was added to the SMC sample with the non-ultrasonic compaction effect, a higher densification was achieved due to the lowered internal friction among the particles themselves. Thus, it was experimentally proved that the ultrasonic compaction effect plays the same role as a lubricant in the compaction process—lowering the internal friction in the system.
With the densification mechanism studied in this paper, a motor made of an SMC core, which was fabricated using an ultrasonic compaction, showed an enhanced efficiency by up to 5% compared to that of a motor made via a conventional method [
19,
20]. With the proposed mechanism, various applications and modified technologies, such as the example given above, can be achieved and the successful industrialization of ultrasonic compaction in various sectors is expected.