Materials Sciences and Applicatio ns, 2011, 2, 1708-1718
doi:10.4236/msa.2011.212228 Published Online December 2011 (http://www.SciRP.org/journal/msa)
Powder Metallurgical Fabrication and
Microstructural Investigations of
Aluminum/Steel Functionally Graded Material
Mahmoud M. Nemat-Alla1, Moataz H. Ata2, Mohamed R. Bayoumi3, Wael Khair-Eldeen4
1Mechanical Engineering Department, Assiut University, Assiut, Egypt; 2Faculty of Industrial Education, Sohag University, Sohag,
Egypt; 3Mechanical Engineering Department, Assiut University, Assiut, Egypt; 4Mechanical Engineering Department, Assiut Uni-
versity, Assiut, Egypt.
Received October 8th, 2011; revised November 24th, 2011; accepted December 3rd, 2011.
ABSTRACT
Aluminum/steel electric transition jo ints (ETJs) are used in aluminum reduction cell for the purpose of welding alumi-
num rod and steel bracket components. Solid state welding process used for joining aluminum and steel at the electric
transition joints have the drawbacks of cracking and separation at the interface surfaces. Cracking and separation at
the electric transition joints are caused by the stress singularities that developed due to the mismatch in thermal and
mechanical properties of each material. To overcome the drawback of electric transition joints, aluminum/steel func-
tionally graded may be used as electric transition joints or proposed. Therefore manufacturing and investigation of
aluminum/steel functionally graded materials fabricated by powder metallurgy process were carried out through the
current work. Different samples with different layers of aluminum/steel functionally graded materials were compacted
using steel die and punch at the same compacted pressure and sintered temperature. After investigating the different
samples of aluminu m/steel functio nally grad ed materials u nder differen t fabrication condition s, the suitable fabrica tion
regime was determined with the aid of microscopic observations.
Keywords: Powder Metallurgy, Functionally Graded Materials, Aluminum/Steel Electric Transition Joint,
Microstructural Investigations
1. Introduction
Electrolytic reduction cells of aluminum requ ire high cu-
rrent density that passed through the electrical connection
between an aluminum rod and a steel bracket. Bolted
connection between the aluminum rod and the steel bracket
exhibit high electrical resistance, in addition to deterioration
over the time due to oxide buildup, corrosion, and arcing.
Therefore, welding processes may be the suitable way for
joining the aluminum and steel m aterials. Unfortunately, all
the various permutations of aluminum and steel are non-
welded by traditional fusion welding processes. The
difficulties in the welding of aluminum with steel by
fusion welding processes have been a great challenge for
engineering, because they result from hard and brittle
intermetallic phases that are formed between aluminum
and steel at elevated temperatures (Fe3Al, FeAl, FeAl2,
Fe2Al5, FeAl3) [1]. Solid state welding process, such as
explosion welding, cold roll bonding, and friction w eld in g,
provide a means for making a strong, ductile metallurgical
bond between these various metal combinations [2-6].
However, none of these technologies are suitable for
traditional equipment fabrications. The concept of an ele-
ctrical transition joint (ETJ) was introduced as a practical
solut ion for joining th e aluminum and steel materia ls in the
electrolytic reduction cells of aluminum. ETJ’s are small
bi-metallic inserts between alum inum rod and st ee l br ac ket .
The upper surface of the bi-metallic insert (aluminum) is
welded by the aluminum rod while the lower surface
(steel) is welded by the steel bracket. ETJ’s are manu-
factured using one of the solid state welding processes,
such as explosion welding , cold ro ll bond ing, and friction
welding. In the cold roll-bonding process, alu minum and
steel plates are passed through a rolling mill with su-
fficient pressure and reduction to break-up surface oxides
on the mating surfaces and create bonding between the
dissimilar metals [7]. The main drawback of the bi-
metallic materials is cracks initiation and propagations or

Powder Metallurgical Fabrication and Microstructural Investigations of 1709
Aluminum/Steel Functionally Graded Material
separation at the interface surfaces under thermal and
mechanical loads. This may be attributed to the stress
singularities at the interface surfaces due to the mismatch
in thermal and mechanical properties of each material
component.
Recently, functionally graded material (FGM) concept
is proposed to overcome the above drawbacks [8-12].
FGM is a mixture of two different distinct materials
fabricated in such a way that the volume fractions of the
constituents are varied gradually in a pr edetermined com-
position prof ile. The profile of the co mposition is starting
with 100% of one material at a surface of the plate and
varied gradually with intermediate composition through
the thickness of the plate, where the microstructure and
properties are smoothly varied, ending with 100% of the
other material at the other surface. It is worthy note that
the gradual variation in composition in FGM does not
have the internal boundaries found in multilayer mate-
rials, and hence it exhibit better resistance to thermal and
mechanical loads [13]. Therefore, FGM overcomes the
drawback of bi-metallic plates, such as ETJ’s, due to
gradual variation of thermal and mechanical properties.
In the current investigations FGM concept was adopted
in order to overcome the drawbacks of using aluminum/
steel ETJ’s. The aluminum/steel ETJ will be replaced by
aluminum/steel functionally graded layers plate, where
composition are smoothly varied through the th ickness of
the plate from 100% aluminum at one surface to 100%
steel at the other surface.
Various techniques have been employed to fabricate
FGM such as chemical and physical vapor deposition
(CVD/PVD), plasma spraying, electroplating and combus-
tion synthesis, self propagating high-temperature synthe-
sis (SHS), centrifugal casting, controlled mold fillin g and
powder metallurgical processing [14]. Powder metallur-
gical processing is one of most viable routes for manu-
facturing of FGM [15]. Production of FGM by powder
metallurgical (PM) processing involves rapid solidifica-
tion that offers unique advantages that are important to
the ductility of the material. For example, segregation in
the powdered material can be minimized, very fine grains
can be produced and solid solu bility of allo ying elements
can be increased [15,16]. PM major advantages are; cost
effectiveness in producing certain parts as compared to
other manufacturing processes, high production rates,
production of complex shapes, bimetallic and laminated
special purpose parts can be made from mould layers and
different metallic powders, certain types of parts can be
made only by PM by mixing different metals, non-metals,
metals and ceramics etc., to achieve the desired properties of
the component such as, production of cermets (ceramic +
metals), mechanically alloyed super alloys and copper
welding electrodes with dispersed alumina (Al2O3) [17].
Because all of these advantages, PM processes are ide-
ally suited for fabricating aluminum/steel Functionally
Graded materials that can be effectively used as electric
transition joints which is the purpose of the current in-
vestigations.
2. Eexperimental Work and Investigations
2.1. Powder Characteristics
The raw powders used in the current investigations were
aluminum powder with 95.07% purity and steel powder
with 99.01% purity. Scanning electron microscopy of
aluminum and steel raw powders used in the PM produc-
tion operation, as received from the supplier, at magnify-
cation of ×100 are shown in Figure 1. The aluminum
particles have irregularly sh aped with rough surface pro-
jecttions while the steel particles have relatively sph e rical
shaped with rough surface projections. The apparent den-
sity of Aluminum powder and Steel Powder were found
to be 1176.5 Kg/m3 and 3658.5 Kg/m3 respectively. The
particle size distribution of aluminum and steel powders
were performed and the obtained results are listed in Ta-
ble 1. It can be noticed that the aluminum powder and
the steel powder are nearly of the same size distribution
which will give homogenous distribution of the rein-
forcement through the matrix. Also, the chemical com-
positions of the two adopted powders are listed in Table
2.
2.2. FGM Processing by Powder Metallurgy
2.2.1. Mixing and Blending O pe ration
Six cylindrical specimens of aluminum/steel FGM were
produced with different number of layers (2, 3, 6, 9, 15
and 21 layers). The compositions of the specimens were
gradually varied from 100% of aluminum powder in one
side to 100% of steel powder in other side with interme-
diate graded composition between the two sides. The six
(a) (b)
Figure 1. Scanning electron microscopy of aluminum and
steel powders at magnification of ×100. (a) Aluminum pow-
der; (b) Steel powder.

Powder Metallurgical Fabrication and Microstructural Investigations of
1710 Aluminum/Steel Functionally Graded Material
cylindrical specimens of aluminum/steel FGM were used
to investigate the effect of the graded compositions and
the number of layers on the microstructure. The weight
percent and volume fraction variations through each layer
in the specimens are listed in Table 3. The composition
of each layer was calculated and the mixture was blended
in dental amalgamator (YDM China) blender for 5 min-
utes to reach a homogenous distribution of the rein-
forcement in the mixture.
Table 1. Particle size distribution of aluminum and steel pow-
ders.
Aluminum powder Steel Powder
180 m 0.12% 180 m 0.815%
125 m 18.3% 125 m 15.36%
90 m 38.96% 90 m 27.52%
63 m 25.05% 63 m 23.66%
45 m 8.8% 45 m 4.96%
Fines 8.38% max Fines 27.3% max
Table 2. Chemical composition of aluminum and steel pow-
ders.
Composition analysis of pure
aluminum in weight percent
(wt%)
Composition analysis of pure
steel in weight percent
(wt%)
Al 95.07% Fe 99.01%
Si 3% Ni 0.51%
S 0% Cu 0.19%
Cl 0.15% Al 0.14%
Fe 0.7% S 0.07%
Ni 0.11% Cl 0.08%
Cu 0.57%
Zn 0.4%
Table 3. Aluminum and steel w eight and volume per centage
in each layer.
Specimen and layers Aluminum powder Steel powder
CodesLayer No. wt% vol% wt% vol%
1 0 0 100 100
S1 2 100 100 0 0
1 0 0 100 100
2 50.01 74.43 50.06 25.56 S2 3 100 100 0 0
1 0 0 100 100
2 20.01 42.16 79.99 57.83
3 39.98 66.02 59.99 33.98
4 59.99 81.38 40 18.61
5 79.98 92.1 19.99 7.89
S3
6 100 100 0 0
1 0 0 100 100
2 12.51 29.41 87.52 70.59
3 24.99 49.26 75.01 50.73
4 37.49 63.62 62.5 36.38
5 49.99 74.45 50.01 25.55
6 62.49 82.93 37.51 17.07
7 75 89.74 24.99 10.26
8 87.51 95.33 12.51 4.67
S4
9 100 100 0 0
1 0 0 100 100
2 7.15 18.34 92.81 81.65
3 14.31 32.73 85.69 67.27
4 21.41 44.26 78.61 55.74
5 28.49 53.74 71.49 46.26
6 35.69 61.79 64.31 38.2
7 42.77 68.43 57.49 31.56
8 49.98 74.43 50.03 25.57
9 57.39 79.61 42.85 20.39
10 64.17 83.95 35.77 16.45
11 71.39 87.96 28.49 12.04
12 78.55 91.44 21.43 8.56
13 85.68 94.58 14.3 5.42
14 92.76 97.42 7.15 2.58
S5
15 100 100 0 0
1 0 0 100 100
2 5.01 13.3 94.99 86.69
3 10.01 24.49 90 75.5
4 14.98 33.94 84.99 66.05
5 20.01 42.17 80 57.82
6 24.99 49.26 75.01 50.73
7 29.97 55.51 70.02 44.48
8 34.98 61.06 65 38.93
9 40.01 66.01 60.04 33.98
10 45.02 70.46 55.02 29.54
11 49.99 74.45 50.01 25.55
12 54.99 78.08 44.99 21.92
13 59.98 8138 40 18.62
14 65.01 84.41 35.01 15.59
15 69.99 87.18 30.01 12.82
16 74.95 89.73 24.99 10.27
17 80.02 92.11 19.97 7.88
18 84.94 94.29 14.97 5.7
19 89.95 96.33 9.98 3.67
20 94.98 98.21 5.03 1.78
S6
21 100 100 0 0

Powder Metallurgical Fabrication and Microstructural Investigations of 1711
Aluminum/Steel Functionally Graded Material
2.2.2. C ompaction
The die, upper and lower punches were lubricated using
Zinc stearate fine powder, to prevent adhesion of powder
with die surface and decrease the coefficient of friction
between die bore surface and powder, then the lower
punch was assembled with the die cavity. Two different
techniques of powder compaction were used in the current
study. First one is, after mixing and blending process for
the composition of each layer it was pre-compacted un-
der low pressure before stacking the next layer. After that
all layers compacted at 990.694 MPa ( correspond to ma-
chine load of 35 ton) at a press of 500 ton capacity pro-
duced by (werkstoffprufmaschinen WPM Germany).
Three layers (S2) and 6-layers (S3) specimens were fab-
ricated using this technique. After compaction it was
found that separation occurred between the green speci-
men layers, this may be attributed to the separating sur-
face formed through the layer interface as shown in Fig-
ures 2(a) and (b). To avoid separating surface occurred
when using the first technique, the mixed powders were
sequentially stacked in the die, layer by layer, with a
stepwise compositional distributions. Also, the specimen
was compacted at 990.7 MPa w ithout pre-compaction pres-
sure. This was the second technique. The compositional
of the specimen changes from 100% steel to 100% alu-
minum through the different layers. Using the second
technique it was found that no separation occurred be-
tween layers in green specimens.
Also, it seems that good mechanical interlock between
the different layers of the specimen is achiev ed. New six
specimens (S1, S2, S3, S4, S5 and S6) were compacted
successfully using the second technique.
2.2.3. Sintering Process
Green compact specimen cannot be used because it has
many drawbacks. In order to overcome such drawbacks
sintering process is needed. The sintering temperature for
powder varies in the range from 0.7 to 0.9 of the melting
point. Due to variation of the melting temperature of the
basic components of the specimen, aluminum and steel,
two sets of sintering temperature were adopted, 800˚C
and 600˚C. The adopted sets of sintering temperature
were above and below the melting point of aluminum.
Usually, sintering time depends upon the specimen di-
mensions and type of the metal. However for Alnico
magnets, it is 2 h. [18]. After sintering process it was
found that failure occurred in specimens that sintered at
800˚C. This failure can be attributed to a new compound
that has formed between aluminum and steel at elevated
temperature. Therefore, the sintering temperature should
be less than the melting point of both powders consti-
tutes.
All six specimens with different number of layers and
compositions, as listed in Tab le 3, that sintered at 600˚C
for 2 h will be adopted through the current investigations .
Finally, the fabrication scheme, of aluminum/steel func-
tionally graded materials, that used in the current invest-
tigations can be schematically summarized as shown in
Figure 3.
3. Results and Discussion
Six cylindrical aluminum/steel FGM specimens were fa-
bricated with 21 mm diameter and 28 mm approximately
height. Specimens for microstructural inspection were
sliced with a diamond saw perpendicular to layer surface,
and their surfaces were ground, polished carefully and ex-
amined using standard metallographic techniques. Macro-
and Microstructural features were characterized by opti-
cal microscopy (OM) and scanning electron microscopy
(SEM) with (EDS) analysis using (JEDL-JSM 5400 LV)
microscope.
(a) (b)
Figure 2. Compacted specimens with separating surfaces.
Figure 3. Flow chart of PM fabrication of functionally gra-
ded material .

Powder Metallurgical Fabrication and Microstructural Investigations of
Aluminum/Steel Functionally Graded Material
1712
Figure 4 shows a general view and details of (SEM)
with (EDS) analysis, where the aluminum phase appears
dark and steel phase appears light.
Figure 5 shows the microstructures of the two layers
specimen, S1. Figure 5(a) shows the microstructure in
the layer that has a composition of 100% steel and Fig-
ure 5(c) shows the microstructure in the layer that has a
composition of 100% aluminum while Figure 5(b) shows
the microstructure at the interface between the 100%
aluminum layer and 100% steel layer. One can see that a
large only crack exist in the 10 0% aluminum. This crack
is almost parallel to the interface between the steel and
aluminum layers. The cause of the appearance of such
crack is the high tensile thermal stresses that generated
during cooling process, from sintering temperature to
room temperature, due to the high difference between the
coefficients of thermal expansion of steel and alumi-
num. The low mechanical strength of the aluminum leads
to crack initiatio n and propagation in aluminum layer not
in steel layer. Figure 6 shows SEM micrographs for the
same specimen, S1, at two different magnifications, ×15
and ×200.
From Figure 6(a) it can be observed that two long
cracks in aluminum phase and a sharp interface between
the two layers are clearly appears. Also, Figure 6(b).
shows SEM micrographs at ×200 for crack in aluminum
layer. White zone around crack is aluminum oxide. Fi-
nally, the appearance of cracks in two layers specimen is
evident on the effectiveness of using FGM in the design
of electric transition joint.
Figure 4. General details of aluminum/steel composite layer using scanning electron microscopy (SEM) at magnification of
×500 and EDS analysis.
(a) (b) (c) (a) (b)
Figure 6. SEM micrographs at different magnifications (a)
×15 and (b) ×200.
Figure 5. Optical micrographs at magnification of ×200 for
the two layers specimen, S1.

Powder Metallurgical Fabrication and Microstructural Investigations of 1713
Aluminum/Steel Functionally Graded Material
Figure 7(a) shows the macrophotograph of micro-
structure for the steel/aluminum graded three layers. It is
interesting to note that crack-free steel/aluminum graded
three layers specimen was successfully fabricated. The
sharp interface composition was replaced with interme-
diate composition layer, where the microstructure and
properties are smoothly varied from steel to aluminum
through the height of the specimen. The weight percent
variation of steel along the height of the specimen is
shown in Figure 7(b), from100 wt% of steel on the left
side to 100 wt% aluminum on the right side.
SEM micrographs of steel/aluminum graded three lay-
ers, for specimen S2, and X-ray energy dispersive spec-
trum (EDS) analysis are shown in Figure 8. It is clear
that good agreement between EDS analysis and the
composition of each layer which gives confidence in the
obtained re sults.
Figure 9 shows the optical microstructures of the three
(a)
(b)
Figure 7. (a) Macrophotograph of microstructures for steel/
aluminum graded three layers, S2; (b) The variation of steel
weight percent along the height of steel/aluminum graded
three layers from the lower surface layer, that has 100 wt%
steel.
Figure 8. SEM micrographs at magnification of ×200 and
EDS analysis for steel/aluminum FGM specimen, S2, that
show the variation of the compositions through the layers.
Figure 9. The optical microstructures of 3 layers graded
specimen, S2. (a) 100 w% steel layer; (b) 50 w% steel layer;
(c) 100 w% aluminum layer; (d) The interface between 100
w% steel layer and 50 w% steel layer; (e) The interface
between 100 w% aluminum layer and 50 w% steel layer.

Powder Metallurgical Fabrication and Microstructural Investigations of
1714 Aluminum/Steel Functionally Graded Material
layers aluminum/steel graded specimen, S2. Where Fig-
ure 9(a) shows 100 W% steel layer, Figure 9(b) shows
50 W% steel layer, Figure 9(c) shows 100 W% alumi-
num layer, Figure 9(d) shows the interface between 100
W% steel layer and 50 W% steel layer and Figure 9(e)
shows the interface between 100 W% aluminum layer
and 50 W% steel layer. Generally from Figure 9 it is
clear that gradual change of the layers composition or at
the layers interface is quietly achieved.
Figure 10 shows the macrophotograph of microstruc-
ture for steel/aluminum 6 graded layers specimen, S3. It
is clear that both aluminum and steel components are
continuous graded through the microstructure, this good
continuity of microstructure can eliminate cracks that
appear at the interface and reflects the design idea from
using functionally grad ed materials.
Figure 11 shows macrophotograph of microstructure
for steel/aluminum 6 graded layers specimen, S3, and the
corresponding steel weight percent of each layer from the
height of the lower surface according to EDS analysis.
Figure 12 shows optical micr ographs at magnificatio n of
Figure 10. Macrophotograph of of micro structure for steel/
aluminum 6 graded layers specimen, S3, and the corre-
sponding steel weight percent of each layer from the height
of the lower surface according to EDS analysis.
Figure 11. SEM micrographs at magnification of ×200 for
steel/aluminum 6 graded layers specimen, S3.
Figure 12. Optical micrographs at magnification of ×200 at
the interfaces of 6 graded layers specimen, S3. (a) Interface
between 1st layer, 100 w% steel and 2nd layer; (b) Interface
between 2nd layer and 3rd layer; (c) Interface between 3rd
layer and 4th layer; (d) Interface between 4th layer and 5th
layer; (e) Interface between 5th layer and 6th layer.

Powder Metallurgical Fabrication and Microstructural Investigations of 1715
Aluminum/Steel Functionally Graded Material
×200 at the interfaces of 6 graded layers specimen, S3. It
is worth to note that the optical micrographs at magnify-
cation of ×200 for the different layers of 6 graded layers
specimen, S3, are obtained but not shown. From Figures
10-12 it is very interesting to note that increasing the
number of layers in specimen increased the interlock
between steel and aluminum. Also, it can be seen that
there are no defects observed, and smooth gradient dis-
tribution from steel to aluminum is successfully obtained
Figure 13 shows macrophotograph of microstructures
of steel/aluminum 9 graded layers specimen and the cor-
responding steel weight percent along the height from the
lower surface, 100% steel. It is clear that as the number
of layers increases the difference of the composition be-
tween layers decreases. Figure 14 shows SEM micro-
graphs at magnification of ×200 for steel/aluminum 9
graded layers specimen, S4. Also, from Figure 14 it can
be seen that very good gradual change of the basic con-
stituents, steel and aluminum, through the layers from
100% steel to 100% aluminum.
Figure 15 shows Macrophotograph of microstructures
for steel/aluminum 15 graded layers specimen and the
corresponding steel weight percent according to EDS
analysis along the height, from the lower surface, 100%
steel.
Figure 13. Macrophotograph of microstructures of steel/
aluminum 9 graded layers specimen and the corresponding
steel weight percent according to EDS analysis along the
height from the lower surface, 100% steel.
Figure 14. SEM micrographs at magnification of ×200 for
steel/aluminum 9 graded layers specimen, S4.
Also, SEM micrographs at magnification of ×200 for
steel/aluminum 15 graded layers specimen, S5 was ob-
tained, not shown here. From the above result, it is clear
that increasing number of layers will reduce the interface
between layers and produce functionally graded material
not functionally graded layers. Therefore, steel/aluminum
21 graded layers w ill be adopted as steel/aluminu m func-
tionally graded material.
Figure 16 shows macrophotograph of microstructures
for steel/aluminum graded material specimen, S6, and the
corresponding steel weight percent according to EDS

Powder Metallurgical Fabrication and Microstructural Investigations of
Aluminum/Steel Functionally Graded Material
1716
Figure 15. Macrophotograph of microstructures for steel/ Figure 16. Macrophotograph of microstructures for steel/
aluminum graded material specimen, S6, and the corre-
sponding steel weight percent according to EDS analysis
along the specimen height from the lower surface, 100%.
aluminum 15 graded layers specimen and the correspond-
ing steel weight percent along the height, from the lower
surface, 100% steel.

Powder Metallurgical Fabrication and Microstructural Investigations of 1717
Aluminum/Steel Functionally Graded Material
Figure 17. SEM micrographs at magnification of ×200 for steel/aluminum functionally graded specimen.
analysis along the specimen height from the lower sur-
face, 100%. Figure 17 Shows SEM micrographs at mag-
nification of ×200 for steel/aluminum functionally graded
specimen, S6. From Figures 16 and 17 it is clear that
gradual variation of the basic components was achieved
and steel/
rocess w
y
mpact them together without pre-
e is the successful way in the con-
si
and produce FGM instead of functionally graded layers
material.
5) Smooth gradual change of the composition in the
steel/aluminum FGM can eliminate the microscopic in-
terface such hat traditional steel-alu- minum
“Aluminum-Steel Electric Tran-
dbook, Welding, Brazing & Soldering, Vol. 6, 1993,
pp. 160-164.
[5] J. G. Banker plosion Welding,”
aluminum graded material specimen fabrication
as successful. The fabricated steel/aluminum
joint.
p
graded material specimen with very smooth transition
will leads to disappearing of the thermal stresses singu-
larities and minimizing the stress concentration values.
4. Conclusions
1) A functionally graded steel/aluminum material was
successfully fabricated by PM processing with com- po-
sition changing from 100% steel in one side to 100%
aluminum in the other side.
2) The fabrication process of FGM by stacked layer b
layer and finally co
compacting pressur
dered fabricatio n process.
3) For fabricated steel/alu minum FGM by PM, th e sin-
tering temperature should not increase above 600˚C
where a new compound was formed when sintering tem-
perature above 600˚C.
4) Increasing the number of layers in steel/aluminum
FGM can decrease the sharp interface between the layers
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