Effect of Pulsed Magnetic Field on Spark Plasma Sintering

of Iron-Based Powders

Xiaoqiang Li*, Yongquan Ye, Yu Tang and Shengguan Qu

National Metallic Materials Net-shape Forming Engineering Research Center,South China University of Technology, Guangzhou 510640, P. R. China

Iron-based powders were sintered by spark plasma sintering coupled with different pulsed magnetic field strength ranging from 0 to3.93MA�m�1. The effects of pulsed magnetic field on the sintering behavior of the powders as well as the microstructure and mechanicalproperties of sintered alloys were investigated. The results showed that the sintering temperature field on the cross section of sample was moreuniform via coupling a pulsed magnetic field. The density, hardness and bending strength of the alloy sintered by coupling an appropriate pulsedmagnetic field, arose to 7.75 g�cm�3, 55 HRC and 1235MPa, respectively. There was no remarkable change of sintered density with a furtherincrease of pulsed magnetic field strength, while the hardness and bending strength of sintered alloys adversely decreased. The roles of pulsedmagnetic field coupled with electric field are explained to accelerate the diffusion and reaction of alloying elements by raising sinteringtemperature, facilitate powders rearrangement, intensify sparking among powders, improve the growth of sintering neck and the formation ofnew sintering neck, and reduce the sintering temperature gradient on cross section. [doi:10.2320/matertrans.M2010057]

(Received February 17, 2010; Accepted April 2, 2010; Published May 26, 2010)

Keywords: iron-based powders, spark plasma sintering, pulsed magnetic field, coupling

1. Introduction

Iron-based powder metallurgy (PM) materials are usedwidely in automobile, shipbuilding, chemistry, aviation,medical appliance and so on.1–3) However, the traditionalforming and sintering techniques of iron-based powders oftenproduce relatively low sintered density, coarsen microstruc-ture and consequently lead to low mechanical properties,which have been difficult to meet some special requirementsof modern industry. Thus, the development and applicationof new powder forming and sintering technique is necessaryto produce good performance iron-based materials.

In the past decade, a novel sintering technique, sparkplasma sintering (SPS), has been developed rapidly.4,5)

SPSing is characterized by applying a certain pressure andpassing intense pulsed electric current into the powdercompact or/and die. Essentially, the sintering process can bedefined as a three-field coupling sintering of electric, stressand temperature fields. It is now successfully used to prepareadvanced alloys, ceramics, composites and even functionalmaterials with high mechanical properties mainly because ofobtaining near full densities and ultrafine microstructure.6–9)

However, K. Vanmeensel et al. proved that the sinteringtemperature field within the sample, especially on the crosssection, is inhomogeneous during SPSing.10,11) In recentstudy, L. Guo and his coworkers proposed a new method thatan axial alternating magnetic field is coupled with SPSing,and testified by numerical simulation that the homogeneity ofsintering temperature field becomes better, due to the skineffect of the induced current from external alternatingmagnetic field.12) In their study, however, the experimentsand the other roles of magnetic field in sintering were notstudied.

Here, we present a study of the sintering of iron based alloyby SPSing coupled with a pulsed magnetic field. The main

aim of this study is to clarify the roles of coupled pulsedmagnetic field in SPSing and the feasibility of the four-fieldsintering technique including electric, magnetic, tress, andtemperature fields.

2. Experimental Procedure

Commercial grade Fe, Cu, Ni, Mo and C powders weremixed with a nominal composition of Fe-2Cu-2Ni-1Mo-0.8C(mass%) for 24 h in a low energy mixer. Then, 8 g powdersevery time were moved into an Al2O3 ceramic floating mouldof 20mm inside diameter. To sinter the mixed powders, afour-field sintering method was employed by coupling anexternal pulsed magnetic field with SPSing. A schematic ofthe sintering system is shown in Fig. 1. In the present work,the peak, base, duty ratio and repetition frequency of pulsedcurrent used for sintering were set up as 2700A, 360A, 50%and 50Hz, respectively. The frequency of pulsed magneticfield kept 5Hz, with various strengths of 0, 0.79MA�m�1,2.36MA�m�1 and 3.93MA�m�1. The sintering time waswithin the range of 1–4min. A constant pressure of 30MPawas applied throughout the sintering process.

Upper punchelectrode

Lower punchelectrode

Powder

Graphite upper punch

Al2O3mould

P

P

pressurizing system

DC

pul

se a

nd

cons

tant

cur

rent

ge

nera

tor

Position measurement systemWater-cooling system Temperature measurement system

Con

trol

ler

Wire coil Mag

netiz

ing

and

dem

agne

tizin

g

Graphite lowerpunch

Fig. 1 Schematics of sintering system.*Corresponding author, E-mail: Lixq@scut.edu.cn

Materials Transactions, Vol. 51, No. 7 (2010) pp. 1308 to 1312#2010 The Japan Institute of Metals EXPRESS REGULAR ARTICLE

The microstructure of sintered samples was observed by anOLYMPUS PME3 optical microscope and a LEO 1530 VPscanning electron microscope (SEM). The sintered densitywas calculated by Archimedes’ method using water. Thehardness was measured by a HDI-1875 Rockwell hardnesstester. The mechanical property was examined by a Xinsansi-CMT5051 mechanical property testing apparatus.

3. Results and Discussion

3.1 Sintered densityThe effect of pulsed magnetic field strength on sintered

density is shown in Fig. 2. With increasing pulsed magneticfield strength, the density of the as-sintered sample markedlyincreased, and the maximum reached 7.75 g�cm�3 with amagnetic field strength of 2.36MA�m�1 after sintering for4min. The fractographs of the sintered samples proved that

the extra application of pulsed magnetic field decreased theporosity. As we know, the main driving force of sinteringis temperature and consequently sintered density usuallyincreases with sintering temperature. Thus, the increase ofthe sintered density in the case of the same sintering timeseems to be explained by a higher sintering temperature,which is obtained owing to the effect of the induced current inthe powders caused by the pulsed magnetic field superposedon SPSing, as described in Ref. 12). However, too intensemagnetic field adversely resulted in a slight decrease insintered density. This may be also attributable to the effect ofthe coupled magnetic field. Except that the pulsed magneticfield is helpful to elevate sintering temperature by theeffect of its induced current, it facilitates the pileup andrearrangement of powder particles by the pulsed electro-magnetic force produced during the forming and sintering.The pulsed electromagnetic force comes from the interactionof the pulsed magnetic field and the electric current flowingthrough powders. It induces vibration effect imposed onpowder particles. Intensive pulsed magnetic field commonlymeans big pulsed electromagnetic force on powders andsubsequently yields more sufficient powder arrangementand higher stacking density. Nevertheless, over-big pulsedelectromagnetic force produced by an over-intensive pulsedmagnetic field somewhat aggravates the pileup and arrange-ment of powders and even destroys the formation of stablesintering necks.

3.2 Microstructural evolutionFigure 3 shows the microstructure of the samples sintered

for 3min by coupling various pulsed magnetic field strengths.By increasing the pulsed magnetic field strength, the porosityof the as-sintered samples decreased significantly. After3min sintering without any external pulsed magnetic field,pores in the sintered sample did not completely eliminateand even some large pores are still observed as shown in

Sintering time, t/min

Den

sity

,ρ/g

·cm

-3

1 32 44

5

6

7

8

magnetic field strength0 0.79MA·m-1

2.36MA·m-1

3.93MA·m-1

Fig. 2 The effect of pulsed magnetic field strength on the sintered density

for different sintering time.

20µm

(b)

20µm

(a)

(d)

20µm20µm

(c)

Fig. 3 The microstructure of samples sintered by coupling various pulsed magnetic field strengths of (a) 0, (b) 0.79MA�m�1,

(c) 2.36MA�m�1 and (d) 3.93MA�m�1.

Effect of Pulsed Magnetic Field on Spark Plasma Sintering of Iron-Based Powders 1309

Fig. 3(a), suggesting a relatively low sintering temperatureand consequently an incomplete sintering. By coupling afield of 0.79MA�m�1, the amount and size of the pores in theas-sintered samples decreased markedly although the inter-faces between the powder particles can still be distinguishedin Fig. 3(b). When the magnetic field strength increased to2.36MA�m�1, the microstructure in the sintered alloys washomogeneously fine with only few pores. Further increasingthe pulsed magnetic field to 3.93MA�m�1 made the sinteringtemperature rise too high and in turn led to a substantial graingrowth, seen in Fig. 3(d).

Figure 4 shows the back scattered electron images (BEI)of the samples sintered for 3min without and with an externalpulsed magnetic field. As the alloying elements in the studiedmaterial system have higher atomic numbers, the alloyingelement-rich areas are brighter than the iron base in the BEIimages. In the sample sintered only by SPSing (Fig. 4(a)), thealloying elements, including Ni, Cu and particularly Mo,segregated along powder particle and grain boundaries, andrarely diffused into the iron. After sintering by coupling apulsed magnetic field of 2.36MA�m�1, a more uniformand dispersive alloying element distribution was attained(Fig. 4(b)). This also further confirms that sintering temper-ature rises with the coupled pulsed magnetic field strengthincreasing, as the temperature greatly determines the diffu-sion coefficients of alloying elements. Because the sinteringlacked vacuum and inert gas protection, the higher sinteringtemperature even made part of iron somewhat oxidate.

3.3 Mechanical propertyFigure 5 presents the effect of pulsed magnetic field

strength on the hardness of sample sintered for 3min. Withthe increase of the magnetic field strength from 0 to 2.36MA�m�1, the hardness of the as sintered sample increased ingeneral, especially in the position away from the sample’scentre. However, the further increase of magnetic fieldranging from 2.36 to 3.93MA�m�1 resulted in a decrease in

the hardness. The effect of pulsed magnetic field on thehardness of sintered sample was similar to that on the sintereddensity shown in Fig. 2. The hardness of sintered samplemainly depends on the sintering sufficiency, and its relativedensity and microstructure. By coupling pulsed magneticfield, as described above, the sintering temperature increased,and insultingly the sintering became more sufficient, thesintered density rose and the microstructure of sinteredsample was more homogeneous. So, the hardness alsoincreased. The decrease of hardness with more intensivepulsed magnetic field was attributed to over-high sinteringtemperature. Figure 5 also shows the uniformity of hardnessin the radial direction of sintered sample. Especially for thesample sintered only by SPSing, the hardness at the centrewas substantially higher than that at the rim on the same crosssection, due to the heat loss produced by heat conduction,radiation and convection at the outside surface of samplein sintering. Coupling a pulsed magnetic field also canimprove the homogeneity of hardness because of facilitatinga uniform sintering temperature distribution in the radialdirection by the ‘‘skin effect’’ of the induced electric current.When the magnetic field strength increased from 0 to2.36MA�m�1, the bending strength was maximum, being1235MPa (see Fig. 6), which is similar to the change trend ofthe hardness. Obviously, a pulsed magnetic field coupled

Mo

Cu

Ni

(a)

Pore

Pore

20µm

(b)

Mo Mo

Iron oxide

Iron oxide

Pore Pore 20µm

Fig. 4 BEI micrographs of samples sintered for 3min by coupling various

pulsed magnetic fields of (a) 0 and (b) 2.36MA�m�1.

Har

dnes

s, H

RC

Distance from sample centre, d/mm

0 42 6 8 10

0

20

40

60

magnetic field strength

0 0.79MA·m-1

2.36MA·m-1 3.93MA·m-1

Fig. 5 The effect of pulsed magnetic field strength on the hardness of

sample sintered for 3min.

Pulsed magnetic field, H/MA·m-1

Ben

ding

str

engt

h, σ

/MPa

0 2 31 4800

900

1000

1100

1200

1300

Fig. 6 The change of bending strength with respect to pulsed magnetic

field strength while sintering time being 3min.

1310 X. Li, Y. Ye, Y. Tang and S. Qu

with SPSing helps to produce a good performance of iron-based alloys.

3.4 Analysis on the effect of pulsed magnetic field informing and sintering

On the basis of the above, a conclusion is drawn that apulsed magnetic field coupled with SPSing is helpful tosintering and improves the mechanical properties of sinteredmaterials. By analyzing, the effect of pulsed magnetic fieldon SPSing is primarily clarified, mainly including thefollowing four aspects. (1) Many ringshaped electric currentsare induced in the electric conducting powders if metalpowders are sintered under such a varying magnetic field aspulsed type. The induced currents can raise the sinteringtemperature by Joule heat effect and resultingly promotesintering. Additionally, due to the heat loss produced by heatconduction, radiation and convection at the outside surfaceof sample in SPSing, the sintering temperature distributionalong the radial direction is uneven and commonly decreasesgradually from the center to the outside. Coupling a pulsedmagnetic field can also improve the sintering temperaturehomogeneity, because the ‘‘skin effect’’ of the inducedcurrent caused by it led to a reverse temperature distributionalong the radial direction. (2) When electric current, includ-ing induced current and noninduced current from the electricfield in SPSing, meets a pulsed magnetic field, a pulsedelectromagnetic force will occurs, if only there is differencein their directions. As shown in Fig. 7(a), there is a directiondifference between pulsed magnetic field H and current I2flowing through the sintering neck between powders Ap andCp. Commonly, the sintering neck is favorably oriented withthe center line of Ap and Cp powders in SPSing. Once thepulsed magnetic field H is superposed, an electromagneticforce Fy, perpendicular toH, will be imposed on the sintering

neck. If the sintering neck is liquid, its axial line deviatesfrom the starting direction and an inclination angle � occurs.After the magnetic field H disappears, the axial line willreturn to its starting direction. Especially under externalpulsed magnetic field, the liquid sintering neck starts to swingintermittently because of the pulsed electromagnetic force,then the cross-section area of the sintering neck grows andmore fresh surfaces of the two powders are involved in thesintering neck. So the efficiency of mass transfer betweenpowders is heightened and the sintering is accelerated. (3) Inthe case of that the directions between magnetic field andcurrent flowing through a powder particle are different, anelectromagnetic force also occurs on the powder particle bytheir interaction. The occurrence of Lorentz force will helpto overcome the friction force and break the bridge jointsbetween the particles, that is, facilitate the rearrangement ofpowders in densification. With the increment of the Lorentzforce by increasing pulsed magnetic field strength, the motionof particles becomes more violent, which means powderrearrangement intensifies. Meanwhile, it also improves theuniformity of sintering temperature field, because thepowders that no electric current first flows through areelectrified by contacting the electrified powders around themunder Lorentz force, as shown in Fig. 7(b). (4) As known,SPSing differs from traditional sintering techniques, such ashot pressing and hot isostatic pressing, because of usingpulsed current combined with pressure application, which isthought to generate spark discharges or even plasma betweenpowder particles to be sintered. The spark discharges mayclean the powder particle surfaces, and thus enhancesintering. During SPSing, the value of the sintering temper-ature also depends strongly on the scale of sparking.Particularly, the transient higher sintering temperature atinterparticle contacts are the result of the influence of

(a)

(b)

Fig. 7 Schematic illustration of sintering (a) at sintering neck and (b) of powder particles. Left: only SPSing; Right: SPSing coupled with

an external pulsed magnetic field.

Effect of Pulsed Magnetic Field on Spark Plasma Sintering of Iron-Based Powders 1311

discharging caused by pulsed electric current, which alsohelp sintering. When an external pulsed magnetic field isapplied to couple with SPSing, it will intensify the currentdensity in sintering neck by adding an induced current, and soenhance the sparking and raise the sintering temperature atsintering neck. Consequently, the SPSing of powders can beimproved by coupling a superposed variable magnetic field.Moreover, following M. A. Verzhakovskaya et al.,13) anexternal pulsed magnetic field affects the diffusion coefficientof alloying elements and the dislocation density in ironduring conventional sintering, when the sintering temper-ature is slightly less than Curie point. Application of a pulsedmagnetic field leads to magnetization reversal to the sample,which is accompanied by a change in the period of domainwalls and their shift. Domain walls moving during magnet-ization can actively interact with dislocations, as a resultof which mass transfer becomes more efficient. Althoughwe do not know whether this effect still exists above Curietemperature, pulsed magnetic field at least can enhanceatomic diffusion at low temperature stage of sinteringprocess.

4. Conclusions

(1) Fe-2Cu-2Ni-1Mo-0.8C (mass%) elemental mixedpowders were sintered by SPSing coupled with a pulsedmagnetic field ranging from 0 to 3.93MA�m�1. The density,hardness and bending strength of the alloy sintered bycoupling 2.36MA�m�1 pulsed magnetic field, reached up to7.75 g�cm�3, 55 HRC and 1235MPa, respectively. A furtherincrement of pulsed magnetic field strength inversely madethe hardness and bending strength decrease. By coupling aproper pulsed magnetic field with SPSing, the microstructureand mechanical properties of sintered sample became morehomogeneous.

(2) During the SPSing of iron based powders, the super-posed pulsed magnetic field can promotes the forming and

sintering of powders because it accelerates the diffusion andreaction of alloying elements by raising sintering temper-ature, facilitates powders rearrangement, intensifies sparkingamong powders, improves the growth of sintering neck andthe formation of new sintering neck, and reduces the sinteringtemperature gradient on cross section.

Acknowledgements

This topic of research was financed by the NationalHigh Technology Research and Development of China(No. 2007AA03Z112), the Guangdong Nature ScienceFoundation (No. 8151064101000026), the Advanced Re-search Fund of DOD (No. 9140A18040709JW1601) and theFundamental Research Funds for the Central Universities(No. 2009ZZ0019).

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