Materials Sciences and Applicatio ns, 2011, 2, 609-614
doi:10.4236/msa.2011.26082 Published Online June 2011 (
Copyright © 2011 SciRes. MSA
Consolidation of MA W-Ni-Fe Alloyed Powder by
Microwave-Assisted Sintering
Wensheng Liu, Yunzhu Ma, Qingshan Cai
State Key Laboratory for Powder Metallurgy, Central South University, Changsha, China.
Received February 17th, 2011; revised March 21st, 2011; accepted April 12th, 2011.
MA W-Ni-Fe alloyed powder compact was sintered by microwave technology, and the influence of microwave sintering
on consolidation of W-Ni-Fe alloy was studied. The fracture morphology and microstructure of alloys were measured
by SEM and metallurgical microscope. The experimental results showed that microwave sintering promoted the densi-
fication of MA W-Ni-Fe alloyed powder quickly with the higher heating rate. The density of the sintered samples in-
creased with the increase of sintering temperature, and significant densification shrinkage occured at 1300˚C ~ 1400˚C.
The tungsten grain grew rapidly at 1450˚C. When the alloy was microwave sintered at 1550˚C, the inner structure of
alloy is more homogeneous, the average W grain size is about 15 μm, and the relative density of sintered specimen is
Keywords: W-Ni-Fe Alloy, Microwave Sintering, Consolidation
1. Introduction
Tungsten heavy alloys (WHAs) have a series of excellent
physical and mechanical properties, such as high density,
high strength, good electrical and thermal conductivity,
good radiation absorption capacity, low linear expansion
coefficient, good weldability, good mechining properties,
etc. Therefore, WHAs are widely used in high technol-
ogy fields, especially in the military industry. With the
rapid development of science and technology, the more
and more requirements to the properties of WHAs are put
forward. So new research on WHAs considered by mate-
rial scientists has become a hotspot in recent years.
Tungsten heavy alloys are usually consolidated through
liquid phase sintering. However, during liquid sintering,
solid/liquid phase easily produce viscous flow, segrega-
tion, and even cause collapse and deformation because of
the large difference in density between W phase and
binder phase. In recent years, in order to control liquid
sintering collapse and gain high proportion of tungsten
heavy alloys, solid-phase sintering, two-stage sintering
and activated sintering of tungsten-based alloys have
been studied deeply [1-5].
Microwave sintering technology, with the heat gener-
ated by a combination of the special wave band and the
basic fine structure of the material, is a method to
achieve densification of the material which is heated to
the required temperature by means of the dielectric loss.
Compared with the traditional heating process, the heat-
ing efficiency of microwave is mainly come from the
polarization of the alternating electromagnetic field,
which makes the dipoles of inner material turning around
repeatedly, resulting in a stronger vibration and friction,
thereby heating the material [6]. Microwave sintering has
a lot of merits, such as energy and time saving [7-10],
and reducing production cost and energy consumption is
of great significance, so that it has considerable prospec-
tive potential when it is introduced into production man-
ufactures of powder metallurgy materials. But the tradi-
tional views indicated that the metal reflects microwave
as a conductor, so the microwave could not be used to
heat the metal. Until 1999, it was first reported using
microwave sintering technology to achieve the sintering
of iron-base powder metallurgy parts [11]. In recent
years, the microwave sintering of metal and alloy have
been researched as a popular problem. G. Prabhu [12]
have studied the sinterability of tungsten powder in mi-
crowave and the density of 93% of TD was gained. Si-
milarly, tungsten heavy alloys (92.5W-6.4Ni-1.1Fe) have
also been sintered in microwave and were found to have
better hardness, tensile strength and elongation when
compared to conventionally sintered heavy alloy [13].
The main objective of the present study is to investigate
Consolidation of MA W-Ni-Fe Alloyed Powder by Microwave-Assisted Sintering
the consolidation of MA W-Ni-Fe alloyed powder and
microstructure response during microwave assisted sin-
2. Experimental
Powders (as shown in Table 1) were mechanically al-
loyed in a high energy ball mill for 20 h with the compo-
sition of 93W-4.9Ni-2.1Fe (wt%).
The mechanically alloyed composite powders were
pressed into green tensile compacts by hydraulic press
(YH41-25C). After pretreatment, the green compacts
were sintered in a microwave furnace with frequency of
2.45 GHz (HAMiLab-V6). At the heating rate of 30˚C/
min, the whole sintering process was carried out at vari-
ous sintering temperatures ranging from 1250 to 1500˚C
for 5 min in a flowing reducing atmosphere (10 vol.% H2
and 90 vol.% N2). The density of the sintered specimens
was characterized by the Archimedes principle. The ten-
sile fracture morphology was observed by SEM (JSM-
6360LV). The sintered specimens were polished by au-
tomatic polishing machine, and then, etched. The optical
microstructure was observed by optical microscopy
3. Results and Discussion
3.1. Densification Behaviour
In the sintering process of powder metallurgy materials,
sintering temperature and sintering time are the key fac-
tors to the densification of sintered compact. Figure 1
shows the influence of sintering temperature on the den-
sification behavior of 93W-Ni-Fe sintered compact. It is
found that the densities of the sintered samples increase
with the increase of sintering temperature, and significant
densification shrinkage occurs at 1300˚C ~ 1400˚C. Be-
cause W-Ni-Fe alloy has an eutectic temperature of
1460˚C, it is solid-phase sintering when the sintering
temperature is below 1460˚C. By solid phase sintered at
1450˚C by microwave, the relative density of the sample
sintered could reach 98.6%, and the effect of high densi-
fication of solid phase sintering is realized. Jain [14] re-
vealed that high density materials could be obtained for
pure W by microwave sintering in short time, which in-
dicated that microwave had great effect on W particle
re-arrangement and W atom diffusion. Microwave sin-
tering can accelerate atomic diffusion significantly [15],
and achieve the rapid densification of samples. During
sintering process from 1400˚C to 1450˚C, the connected
skeleton forms and compact densification occur rapidly,
and formation of γ-(Fe,Ni) phase, the diffusion of W in
γ-(Fe,Ni) phase, the self-diffusion of W, and the void and
defect diffusion occur quickly because of microwave
Table 1. Performance parameter of raw powder.
PowderPowder shape Particle size (μm) Powder purity (%)
W Irregular 2.0 99.9
Ni Spherical 5 ~ 8 99.5
Fe Irregular 5 ~ 8 99.5
Figure 1. Effects of microwave-sintering temperature on
densification of 93W-Ni-Fe alloys.
field activation. When the temperature reach 1500, the
sintering process is liquid phase sintering, which increase
the dissolution and precipitation process of W in binder
phase. The formation of a lot of liquid phase and the ef-
fect of capillary force lead to the combination and growth
of grains, the liquid phase infiltrations in W grain boun-
dary and the void is filled, which make the sintered
compact further densification.
3.2. Microstructure Analysis
3.2.1. SEM Morphology Features
Figure 2 shows the microstructure of 93W-Ni-Fe alloys
sintered at 1250˚C ~ 1550˚C for 5 min via microwave sin-
tering. As can be seen from the graph, when the tem-
perature is lower (Figures 2(a) and (b)), there are big
porosities between powder compacts, and the sintered
compact is in the early stage of solid-phase sintering, at
that moment, the diffusion process is carried out, grains
start to contact, and the sintering neck begin to form.
With the sintering temperature increase, the sintering
neck grow, the big porosities between powder compacts
eliminate, γ-(Fe,Ni,W) phase is precipitated and
generated constantly, binder phase increase and W grains
are separated gradually. When the temperature reach
liquid sintering temperature (Figure 2(f)), a large num-
ber of liquid phases are generated and W grains are
sphericized. Homogeneous microstructure of sintered
Copyright © 2011 SciRes. MSA
Consolidation of MA W-Ni-Fe Alloyed Powder by Microwave-Assisted Sintering
Copyright © 2011 SciRes. MSA
(a) (b)
(c) (d)
(e) (f)
Figure 2. SEM image of alloy specimens microwave sintered at different temperatures: (a) 1250˚C; (b) 1300˚C; (c) 1350˚C; (d)
400˚C; (e) 1450˚C; (f) 1500˚C; and (g) 1550˚C. 1
Consolidation of MA W-Ni-Fe Alloyed Powder by Microwave-Assisted Sintering
samples with fine grains are obtained by microwave sin-
tering at 1500˚C. This is reason why the microwave sin-
tering process is the material as a whole coupled with
microwave heating, is a “volume heating”, is a more
uniform temperature distribution, leads to the more
evenly sintering of the grains [6]. On the other hand, the
microwave sintering holding time is only 5 min, the pore
of samples diffusion time is short, which generates re-
sults in fine grains in alloys. When the sintering tem-
perature is 1550˚C (Figure 2(g)), as can be seen that the
grain size of microwave sintered samples grows signifi-
cantly, while relative density of specimen reaches 99.0%.
3.2.2. Meta l l o gra ph i c Structure Analysis
Figure 3 shows the metallurgical microstructure of
93W-Ni-Fe alloys sintered at 1250˚C ~ 1550˚C for 5 min
via microwave sintering. As can be seen from the figure,
when the sintering temperature (1250˚C ~ 1300˚C) is low,
the powder particle is small, with complex shape and
irregular arrangement. When the temperature rises from
1300˚C to 1400˚C, significant densification shrinkage
occurs, the relative density of sintered samples increases
from 90.2% to 98%. According to the modern sintering
theory, sintering densification of tungsten heavy alloys in
solid-state sintering stage occurs under the comprehen-
sive effect factors, such as the interaction of defects and
defect expanding, the reduction of surface energy caused
by diffusion, the formation of diffusion driving force and
pore migration, mutual diffusion of elements, formation
of the connected W skeleton, and so on.
When microwave sintering temperature is at 1450˚C,
the densification of sintered samples reaches a high value
and the density is 98.6%. There is an obvious phenome-
non of growth on W grains. W grain growth is realized
through the solid-state diffusion of W atoms because of
microwave field activation. Microwave sintering can
effectively reduce the energy barrier of W atom diffusion,
and can significantly increase the speed of atomic diffu-
sion which accelerates the grain growth [15]. In addition,
microwave sintering is in order to realize the internal and
external material uniform heating, which depends on the
transforming microwave energy absorbed by material
itself into kinetic energy and potential energy of the inner
material. Under the action of microwave electromagnetic
energy, atomic diffusion accelerates, which promoting
the activation of sample grain, decreasing the reaction
activation energy, reducing the reaction temperature and
speeding up the reaction. That results in a small amount
of liquid formation of the microwave sintered samples
below the liquid temperature, the formation of liquid-
-phase and the role of capillary force will lead to the
consolidation and growth of grain.
When the sintering temperature is at 1500˚C ~ 1550˚C,
the sintered body is in the state of liquid phase sintering.
The generation of liquid and the role of capillary force
lead to liquid infiltrating in W grain boundary and
growth of W grains, so that pores are filled and densifi-
cation is further improved. From Figure 3(f), it can be
seen that the distribution of tungsten grains in the matrix
phase is not uniform, because liquid flowing and particle
rearrangement are carried out insufficiently due to short
sintering time. And the particle shape is irregular, with a
large number of angular-shaped particles, and particle
size and binder distributing are all not uniform. When the
temperature rises to 1550˚C (Figure 3(g)), the tungsten
grains and matrix phase are distributed more evenly, and
the contact between the tungsten grains is closer and the
grain size is more uniform, the average W grain size is
15 μm, the shape of grain is nearly spherical, the relative
density of the alloy is up to 99%. This is because the sin-
tering temperature increases, and the formation of a lot of
liquid phase, the migration and rearrangement of W
grains, as well as the late solution and precipitation are
quite sufficient. In the dissolution-precipitation process,
small particles solve first, the solubility of the sharp cor-
ners part and the convex part are greater than the solubil-
ity of the concave part and the rounded part in large par-
ticle, which results in the preferential solution of these
parts. When the solubility in the liquid is saturated, the
precipitation will occur at the concave part, so that W
grains grow into a nearly circular structure. On the other
hand, W element is an isometric system with a body-
centered cubic crystal structure, it has the same dissolu-
tion-precipitation rate in all directions, so spherical
structure grains are easy to form.
4. Conclusions
1) Microwave sintering technology can also be used to
prepare the W-Ni-Fe material, when compared with
conventional sintering, it has a significant advantage with
the faster heating rate and the shorter sintering cycle.
2) The microwave sintered 93W-Ni-Fe samples have
significant densification shrinkage at 1300˚C ~ 1400˚C.
After solid state sintering by microwave processing at
1450˚C, the relative density of sintered samples is up to
98.6%, but there is an obvious phenomenon of grain
3) The inner structure of microwave liquid phase sin-
tered samples is more homogeneous at 1550˚C, and the
average grain size of the nearly spherical tungsten is
about 15 μm, the relative density of the sample sintered
can reach 99%.
5. Acknowledgements
The author thank National Natural Science Foundation of
China (No. 50774098) and Creative research group of
Copyright © 2011 SciRes. MSA
Consolidation of MA W-Ni-Fe Alloyed Powder by Microwave-Assisted Sintering613
(a) (b)
(c) (d)
(e) (f)
Figure 3. Optical photographs of alloy specimens microwave sintered at different temperatures: (a) 1250˚C; (b) 1300˚C; (c)
350˚C; (d) 1400˚C; (e) 1450˚C; (f) 1500˚C; and (g) 1550˚C. 1
Copyright © 2011 SciRes. MSA
Consolidation of MA W-Ni-Fe Alloyed Powder by Microwave-Assisted Sintering
National Natural Science Foundation of China (Grant No.
50721003) for financial support.
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