Intensified sintering of iron powders under the action of an electric field: Effect of technologic parameter on sintering densification

Intensified sintering of iron powders under the action of an electric field: Effect of technologic parameter on sintering densification

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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 8 ( 2 0 0 8 ) 264–269

journal homepage: www.elsevier.com/locate/jmatprotec

Intensified sintering of iron powders under the action of an electric field: Effect of technologic parameter on sintering densification Keqin Feng a,b,∗ , Yi Yang a , Mei Hong b , Jinling Wu a , Shanshan Lan a a b

School of Manufacturing Science and Engineering, Sichuan University, Chengdu, Sichuan 610065, PR China Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616, USA

a r t i c l e

i n f o

a b s t r a c t

Article history:

In the process of intensified sintering of iron powders under the action of an electric field

Received 6 June 2007

with low voltage and high current, the effect of sintering temperature and sintering time

Received in revised form

on the densification process were investigated by means of thermal simulation. Both the

24 November 2007

metalloscope results and the statistic analysis of the distribution of pores in sintered com-

Accepted 29 December 2007

pacts show that an electric field is not only providing the Joule heat for a compact, but also accelerating the diffusion among iron atoms in the sintering process, which leads to intensified sintering. The relative density of the sintered compact can be over 95% at relatively

Keywords:

low sintering temperature (800 ◦ C) or within relatively short sintering time (2 min) at high

Electric field

temperature. Both sintering temperature and sintering time have effect on the process of

Iron powders

iron powder densification. In this study, a sintering temperature of 1100 ◦ C and a sintering

Sintering

time of 4 min lead to over 97% consolidation for iron powder compact.

Technologic parameter

1.

Introduction

Sintering, as one of the most basic and important processes in powder metallurgy, may play the crucial role in the properties and cost of final products. However, conventional sintering process takes long time and needs high temperature. Therefore, there is a very high demand to improve the sintering technique in reducing sintering temperature and shortening sintering time, and thus reducing the energy cost. Intensified sintering is put forward mainly for economic reasons (Guo, 1998). At present, several intensified sintering methods, such as hot-press sintering (Feng and Jia, 2004; Zhu and Mei, 2003), activator-diffusion intensified sintering (Gao, 1992), liquid-phase intensified sintering (Gao, 1992), self-propagating reaction sintering (Salamon and Eriksson,

© 2008 Elsevier B.V. All rights reserved.

2007), microwave sintering (Wang and Gypta, 2007), and spark plasma sintering (Grosdidier and Ji, 2007; Frage and Cohen, 2007; Munir and Anselmi-Tamburini, 2006), have been used to achieve lower sintering temperature, or higher sintering rate, or improved properties of sintered materials. However, these methods have their own disadvantages and limit. Among them, spark plasma sintering has received more and more attention worldwide, which used to sinter powders under the simultaneous influence of a current and pressure. Powders are placed in a die and heating is affected by passing a current through the die and the sample while a press is applied on the powders. The significant features of spark plasma sintering include high heating rate (as high as 1000 ◦ C/min), the application of pressure, and the effect of the current (Munir and Anselmi-Tamburini, 2006).

∗ Corresponding author at: School of Manufacturing Science and Engineering, Sichuan University, Chengdu, Sichuan 610065, PR China. Tel.: +86 28 85404898. E-mail address: kq [email protected] (K.Q. Feng). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.12.117

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Table 1 – Technologic parameter of the experiment Number of sample

1# 2# 3# 4# 5#

Fig. 1 – Schematic representation of Gleeble equipment.

Recently, our research group has proposed a new intensified sintering technique under the action of an electrical field (Feng and Yang, 2005). Similar to spark plasma sintering, the advantages over conventional methods include lower sintering temperature, shorter holding time, and marked improvements in properties of materials consolidated by this method. However, this method differs from spark plasma sintering in that the powders are firstly compressed to form a green compact with a certain relative density, and then heating is effected by passing a current through the sample while no pressure or low pressure is applied on the powders. Especially, the significant differences between this method and spark plasma sintering are the heating rates and electric current. In the former heating rate as high as 105 ◦ C/s and electric current as high as 105 A can be achieved, which are greatly higher than spark plasma sintering. Higher heating rate have been shown to enhance densification by passing the nonidentifying mechanism of surface diffusion and by creating an additional driving force due to large thermal gradients (German, 1996). Therefore, it is possible to sinter nanometric powders to near theoretical values with little grain growth under pressureless sintering by our method. In this paper, by using the thermal-simulation method, the effects of sintering temperature and sintering time on the densification process for iron powders were investigated, and the role of electric field in sintering process was discussed too.

2.

Technologic parameter Sintering temperature (◦ C)

Sintering time (min)

800 1100 1100 1100 1350

6 2 4 6 6

tain sintering temperature rapidly at a preset heating rate of 600 ◦ C/s in vacuum (<10−4 Pa) by the high electric current passing through it, and it was held several minutes (sintering time) at sintering temperature. Meanwhile, the actual temperature data of the compact during the experiment were recorded at a frequency of 5 Hz. The detailed technologic parameters of the experiment are shown in Table 1. The sintered compact’s density was measured by Archimedes’ technique. Moreover, as shown in Fig. 2, the sintered compact was cut into half and divided into 6 spots in diameter and 45 spots in axis. The pore size distributions were observed by cross-section observation using metalloscope, and analyzed statistically according to the standard of ISO4505-1978 (Huang, 2001).

3.

Results

The metallographic photographs of sintered compacts are shown in Fig. 3. The pores in sintered compacts are not all closed circles, but are of various shapes including straight lines, curves and disordered clouds, which indicate that the pores are not sphericized completely.

Experimental procedures

Iron powders (98% purity) with an average particle size of 60 ␮m, were cold pressed in a steel-mould to form green compacts with a diameter of 12.8 mm, a height of 13.6 mm, and a relative density of 73%. Fig. 1 shows the schematic of the equipment used in the experiments: a Gleeble thermalsimulation instrument made by Dynamic System Inc., USA. It controls heating process by a computer-controlled system according to the value of the preset heating rate, and the accuracy of controlled temperature is ±3 ◦ C. Moreover, the electric field has low voltage and high current. In the experiments, a green compact of iron powders was heated up to a cer-

Fig. 2 – The distributed pattern sketch map of metallographic examination.

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Fig. 3 – The metallographic photographs of sintered compacts 100×. (a) 1# (800 ◦ C, 6 min), (b) 2# (1100 ◦ C, 2 min), (c) 3# (1100 ◦ C, 4 min), (d) 4# (1100 ◦ C, 6 min), (e) 5# (1350 ◦ C, 6 min).

3.1. Effect of sintering temperature on sintering densification Comparing the micrographs of 1# , 4# and 5# compacts in Fig. 3, the compact sintered at 800 ◦ C has more big pores (diameter d > 30 ␮m) than those sintered at 1100 ◦ C and 1350 ◦ C, and the pore distribution of the compact sintered at 1100 ◦ C is similar to that of the compact sintered at 1350 ◦ C. The effect of sintering temperature on the densification of compacts are shown in Fig. 4, which indicate that the iron powder compact can be sintered to relative density over 95% at a relatively low temperature (800 ◦ C) under the electric field, compared to conventional sintering methods. However, the porosity of the sintered compact is not increasing with the increase of sintering temperature monotonously. The density of the compact sintered at 1100 ◦ C is the highest, and the den-

sity is lower when the sintering temperature reaches 1350 ◦ C. As a result, the best sintering temperature is 1100 ◦ C in this experiment.

3.2.

Effect of sintering time on sintering densification

The micrographs of 2# , 3# and 4# compacts in Fig. 3, together with the effect of sintering time on the densification on sintered compacts shown in Fig. 5, prove that iron powder compact can be sintered to a relative density over 95% in a relatively short holding time (2 min) under the electric field. Within a sintering time (2–4 min), the densification degree increases with the increase of sintering time, but it decreases when the sintering process is too long (6 min). As a result, the best sintering time is 4 min in this research.

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Fig. 4 – The effect of sintering temperature on porosity of sintered compacts.

4.

Discussion

4.1. The role of an electric field in the process of intensified sintering Under the typical current activated sintering, the temperature and current are not independent and thus thermal effect of the current (Joule heating) cannot unambiguously be separated from the intrinsic role of the current. The observed current’s influence on mass transport can be attributed to one of the several intrinsic effects including flux (electromigration), an increase in point defect concentration, or a reduction in mobility activation energy for defects. The degree of sintering densification of powder compact is greatly depended on mass transport, and the major benefit of intensified sintering is promoting the diffusion of atoms. In this process of intensified sintering, the role of an electric field is not only providing the Joule heat for a compact, but also accelerating the diffusion among iron atoms. As a compact is porous, it contains a lot of interfaces. When a current passes through a compact, Joule heat will be generated at an interface due to the contact resistance, which raises the temperature of the compact. However, under the electric field, electrons move regularly against their crystal lattice and form a current.

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Fig. 6 – The effect of sintering temperature on the pore distribution of sintered compacts.

The resistance to electron movement results from interactions between electrons and atoms on crystal lattice, that is, collision. Having frequently collided with electrons, atoms on the crystal lattice obtain a lot of energy and atom vibration become stronger, raising the atom temperature (Mrowee, 1980; Binling and Xikun, 1989). In this way, atoms easily gain enough energy to separate from their equilibrium places on the crystal lattice, which leads to atom diffusing at a lower temperature. Munir and his coworkers suggest that the effect of an electric field is dependent on its strength and the value of the effective charge of the diffusing atoms. The results of his studies have been explained in terms of electromigration, where the field modifies the diffusion flux equation by a field-related term (Munir and Anselmi-Tamburini, 2006; Frei and Munir, 2007). Based on the theory of Solid-State Physics, the higher the current, the faster the electrons move (Binling and Xikun, 1989). That is, the higher the current, the more easily atoms diffuse. Previous research results have been shown that a high heating rate (which meant large current density) can enhance the sintering process by promoting the diffusion of atoms (Kim and Johnson, 1983; Young and Mcpherson, 1989; Bengisu and Inal, 1991). In this experiment, the output electric current increases with the increase of the preset heating rate, and the preset heating rate is as high as 600 ◦ C/s [corresponding to 16500A (Feng and Yang, 2005)], which can enhance the sintering process of iron powders. In other words, the compact can easily gain enough energy to accelerate iron atom diffusion at a lower temperature under the action of an electric field. As a result, the sintering process can take place at a lower temperature and finish in a shorter time, which differs from the conventional sintering process remarkably.

4.2. Effect of sintering temperature on sintering densification

Fig. 5 – The effect of sintering time on porosity of sintered compacts.

As shown in Fig. 4, the densification degree of the compact sintered at 1350 ◦ C is lower than that of the compact sintered at 1100 ◦ C. And from the effect of sintering temperature on the distribution of pores in sintered compacts shown in Fig. 6, most of the pores in the compact sintered at 1100 ◦ C are small (d = 10–30 ␮m) or very small (d < 10 ␮m), and the number

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of pores with d ≤ 30 ␮m is 99.7% of the total number. Similar to the compact sintered at 1100 ◦ C, the compact sintered at 1350 ◦ C has 98.8% of pores with d ≤ 30 ␮m. However, the number of big pores with d > 30 ␮m in the compact sintered at 800 ◦ C increases obviously, and the number of pores with d ≤ 30 ␮m is only 97.9%. According to the sintering kinetics, the higher the sintering temperature, the higher the densification degree (Mrowee, 1980), but too high a sintering temperature is unfavorable to enhance the densification degree. The main reason of it is that the plastic flow is accelerated at high temperature, which enhances the densification process as indicated by Bingham model. As a result, parts of atomic group in the powders fill into the vicinal pores by plastic flow (Sagel-Ransijin and Winnubst, 1996). However, a certain small pores formed at grain boundaries through vacancy diffusion can deform and even gather together to form big pores because of the effect of plastic deformation. When the sizes of these big pores are greater than a critical size, they will grow up and merge each other, and then their irregular shape change into spherical shape. Therefore, comparing the compact sintered at 1100 ◦ C, the number and size of pores in the compact sintered at 1350 ◦ C increase in this experimental condition.

4.3.

Effect of sintering time on sintering densification

From Fig. 5, it can be seen that the sintering time is relatively short by using this intensified sintering technology. The densification degree of compacts increases with the increase of sintering time, but it is reduced slightly when the sintering time is more than 4 min. As shown in Fig. 7, the diameter of all pores in the compact sintered for 4 min are in the range of 10–30 ␮m; in the compact sintered for 2 min and 6 min, 99.5% and 98.5% of the pores have diameter no larger than 30 ␮m, respectively. The pores change with the sintering time. When the sintering time is not long enough (2–4 min), the spheroidize shrinkage of porosity continues although the process of forming isolated and closed pores gradually from porosity nets has been completed. With the increasing of sintering time, the number of big pores (d > 30 ␮m) reduces due to the growth of

crystals, the increase of plastic deformation, and the disappearance of grain boundaries. With prolonged sintering time (4–6 min), the total number of pores decreases and the smaller pores disappear, but the pores with diameters bigger than a certain critical size grow and merge. Some researchers believe the growth of small and closed pores are caused by the accumulation of vacancy at grain boundaries through diffusion. Too long a sintering time can lead to the growth of accumulative crystals, reducing the compact’s density slightly.

5.

Conclusions

Iron powders can be well sintered under the action of an electric field with low voltage and high current, as well as high heating rate. Because the electric filed can accelerate the diffusion of atoms, lower temperature sintering or shorter time sintering can be achieved. Iron powder compacts reach over 95% of their theory density when they were sintered at relatively low sintering temperature (800 ◦ C) or within relatively short sintering time (2 min) at high temperature. Sintering temperature has significant effect on the densification degree of iron powder compact during the intensified sintering. This study indicates that there is an optimum sintering temperature at which the sintered compact can reach the highest densification degree, and this best sintering temperature in this experiment is 1100 ◦ C. Sintering time has some effect on the densification degree of iron powder compact in the intensified sintering, and that there is also an optimum sintering time in which the sintered compact can reach the highest densification degree. Under this experimental condition at a sintering temperature of 1100 ◦ C, the best sintering time is 4 min.

Acknowledgements This work was supported by the National Nature Science Foundation of China (No. 50404014) and China Postdoctoral Science Foundation (No. 20060390177).

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Fig. 7 – The effect of sintering time on the pore distribution of sintered compacts.

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