Materials Sciences and Applicatio n, 2011, 2, 1127-1133
doi:10.4236/msa.2011.28152 Published Online August 2011 (
Copyright © 2011 SciRes. MSA
Hot Forging and Hot Pressing of AlSi Powder
Compared to Conventional Powder Metallurgy
Sayed Moustafa1, Walid Daoush1*, Ahmed Ibrahim2, Erich Neubauer3
1Department of Powder Technology at Central Metallurgical. Research and Development Institute (CMRDI), Cairo, Egypt;
2Department of Mechanical Engineering, Faculty of Engineering at Helwan University, Cairo, Egypt; 3Department of Materials Re-
search at ARC Seibersdorf Research Center GmbH, Seibersdorf, Austria
E-mail: *
Received April 27th, 2011; revised May 21st, 2011; accepted June 12th, 2011
Aluminum silicon alloy of composition (Al-25%Si-3%Ni-1%Fe-2%Cu) was atomized using water atomization. The
powders were cold compacted in a die to produce green cylinder compacts. Four consolidation processes were applied,
namely; conventional sintering at 500
, sintering followed by hot forging to obtain pistons, one step hot forging into
pistons, and hot pressing. The microstructure of the sintered specimens showed inter-granular pores and oxide layers
on particle interfaces of 84% relative density. When the sintered specimens were hot forged, both the inter-granular
pores and oxide layers on particle interfaces almost disappeared and the relative densities increased up to about 95%.
The same microstructure is also obtained for the one step forged specimens, but the relative densities increased to
about 97%. However, the hot pressing specimens showed the presence of oxide layers on particle surfaces as well as
few isolated pores. The relative density of the hot pressed specimens was about 90%. Hardness and ultimate compres-
sion strength were measured. It is noted that the strongest bulk materials are those made by hot forging, followed by
those made by hot pressing and the weakest bulk materials are those made by conventional sintering.
Keywords: Aluminum-Silicon Alloy, Powder Metallurgy, Hot Forging, Hot Pressing, Hardness, Compression Test
1. Introduction
Aluminum alloys are not only light weight materials, but
also have excellent thermal conductivity, electrical con-
ductivity, corro sion resistance, and work ability. For these
previous reasons, aluminum is widely used in structural
components, electrical conductors, and consumer prod-
ucts. Among the newly developed aluminum alloys, rap-
idly solidified Al–Si alloys have superior mechanical
properties, such as high strength, modulus, wear resis-
tance and elevated temperature strength relative to con-
ventional alloys [1]. Also Al–Si alloys are well known as
typical high strength and lightweight materials, which are
widely used in automotive and aerospace industries due
to their higher strength; good wear resistance and low
thermal expansion co-efficient [2 ].
The early technological development of hypereutectic
Al-Si grew out of its application for internal combustion
engine blocks and pistons. This application uses all the
properties for which the material is well known: high
specific stiffness and strength, good hot strength, low
thermal expansion and excellent wear resistance. The
composition repr esen ts a co mpro mise between castab ility
and properties: it is close to the maximum silicon con tent
in which sound produc ts can cons istently b e obtained [3].
Al–Si alloys are manufactured by casting and powder
metallurgy methods. But, the relatively slower cooling
rate, associated with the conventional casting process,
produces coarse and segregated primary Si and/or eutec-
tic Si in the Al–Si alloys [4]. Yet, the distribution and
size of the primary silicon particles is more important
than the overall silicon content of the alloy. Therefore,
many efforts have been made in the microstructural
modification of casting Al–Si alloys in order to achieve
fine Si particles with the desired shapes and distributions.
For example, techniques such as modification [5], ternary
alloying [6], spray-deposition [2], or rapid solidification
processing [7] have been applied to refine the primary Si
*Associate Professor Walid. M. Daoush, Head of Powder Technology
Division at Central Metallurgical R&D Institute, Cairo, Egypt.
Hot Forging and Hot Pressing of AlSi Powder Compared to Conventional Powder Metallurgy Route
crystals as well as to achieve their homogeneous distri-
bution in hyper-eutectic Al–Si alloys. All the processes
mentioned above, rapid solidification of metallic melts
has been known to produce altered constitutional effects
such as formation of supersaturated solid solutions, me-
tastable intermetallic phases and even amorphous alloys
[8]. Besides, the microstructural features (grain size and
second phase/intermetallic inclusions) are also refined
and the segregation effects are significantly reduced. A
number of studies have been reported regarding fabrica-
tion of Al–Si alloys employing rapid solidification and
hot-extrusion processes [9]. The increase in strength and
wear resistance was achieved, as a consequence of rapid
solidification and/or incorporation of ternary alloying
transition elements.
The hypereutectic Al–Si alloys containing transition
metals such as Fe, Ni, Cr, Zr and Cu are unique materials
due to their particular properties at elevated temperatures
[10]. It is expected that alloys containing transition met-
als (Fe, Ni, Cr ) can precipitate fine intermetallic
compounds from rapidly solidified powder leading to a
high-strength as well as increased wear resistance at ele-
vated temperatures. The addition of Fe to rapidly solidi-
fied Al–Si alloys results in dispersion of fine intermetal-
lics and enhances rigidity, high temperature strength, and
wears resistance. Addition of Cr minimizes the decrease
of ductility and enhances forgibility [1 1].
The interfacial bonding between Al matrix and inter-
metallic compounds, and also the sharp corners of these
compounds make it difficult to use these materials in the
as-cast conditions. Several secondary processing steps
need to be employed for the refinement of the primary
phases and to get uniform distribution of intermetallic
phases in the Al matrix. These include homogenization
and controlled heat treatments along with further rolling
or extrusion. However, their secondary processing is also
a difficult proposition due to hard and coarse intermetal-
lic phases [12]. To avoid th ese difficulties, spray forming
processing route has been invariably used to produce
such alloys [13]. The process involves atomization of
liquid alloy, using a high energy inert gas jet, into a spray
of fine droplets and its subsequent deposition of a sub-
strate. The high cooling rate of the droplets during at-
omization and the unique mechanism of microstructural
evolution during deposition give rise to rapid solidifica-
tion effect in the microstructure and an equiaxed and
refined grain morphology, along with a uniform distribu-
tion of refined secondary phases.
The powder forging process of rapidly solidified Al
alloys was investigated in order to develop an inexpen-
sive alternative process to produce high strength parts
with complex shapes. It has been shown that the me-
chanical properties of powder forged parts ar e as good as
those produced by extrusion [9]. Additionally, since
powder forging produces a part in its final form directly
from starting powders, without machining being neces-
sary (near net shaping), the yield is high and less expen-
sive in comparison with powder extrusion. Generally, it
is necessary for powder consolidation to be performed by
solid-phase diffusion at temperatures far below the melt-
ing points of the raw material powders, to ensure that the
structural features obtained through rapid solidification
are not lost. However, the surfaces of Al alloy powder
are usually covered by an oxide layer approximately 10
nm thick. Unless this oxide film is ruptured and the fresh
powder particle surfaces are allowed to come into contact
with each other, it is not possible to obtain good bon ding
by diffusion. Therefore, the powder particles should be
bonded together by plastic deformation during powder
compaction step as well as forging step, and so under-
standing the deformation and compaction behaviors are
very important to achieve good quality parts. The defor-
mation behavior of powder metals during the compaction
step is different from that of porous materials during the
powder forging process. That is, the initial density is
much lower in the compaction process (the initial state is
powder) than in the powder forging process (the initial
state is a sintered metal), there are sidings among the
particles during the initial stage of compactio n. Therefore,
the yield function for powder metals should be different
from that of the porous materials. In addition, most stud-
ies of powder forming used the yield functions for porous
materials and ignored powder shape and size effects [14].
In this investigation, Al-25%Si-3%Ni-1%Fe-2%Cu
alloy has been atomized with water in order to refine the
primary silicon phase by rapid solidification. From these
refined powders, automotive pistons of small size were
fabricated using powd er metallurgy route combined with
hot forging and hot pressing processes.
2. Experimental
Aluminum silicon alloy of the chemical composition
Al-25%Si-3%Ni-1%Fe-2%Cu was prepared by induc-
tion melting in graphite crucibles in air. Powders from
this alloy were fabricated by superheated of the pro-
duced alloy in a graphite crucible up to 800˚C and bot-
tom pouring through a ceramic melt delivery nozzle of 6
mm diameter into a confined water atomizer operating at
a pressure of 20 MPa. The high pressure water jets wer e
directed against the molten stream. The melt flow rate,
estimated from the operating time and weight of the at-
omized melt, was about 4 kg/min. The water flow rate,
calculated from the water consumption rate, was about
200 l/min. Table 1 reports the atomization conditions
adapted for powder fabrication. The size distribution of
the alloy powder particles was measured by conventio nal
Copyright © 2011 SciRes. MSA
Hot Forging and Hot Pressing of AlSi Powder Compared to Conventional Powder Metallurgy Route 1129
Table 1 Water atomization parameters adapted for powder
fabrication of the investigated Al-25%Si-3%Ni-1%Fe-
2%Cu alloy.
Parameter Conditions
Pouring temperature, oC
Nozzle angle
Nozzle diameter, mm
Number of water jets
Molten stream flow rate, kg/min.
Water pressure, MPa
Water flow rate, l/min.
Water velocity, m/s
mechanical sieving, and sieved powders with a specific
size range of 180 µm and 45 µm were chosen for this
The produced powder was analyses by DSC at tem-
peratures up to 600˚C with he a ting rate o f 1 0˚C / m in i n
Ar atmosphere, in order to determine the solidus tem-
perature of the powdered alloy.
Four consolidation processes were applied to obtain
the bulk alloy, namely; conventional sintering, sintering
followed by hot forging , one step forging, and hot press-
ing. In the first method, the powder was cold compacted
into cylinder of 65 mm diameter followed by sintering at
500˚C under N2 atmosphere for one hour. The second
consolidation technique is hot forging of the sintered
specimens made by the first method into pistons. The
third process is carried out by heating the green cylinder
compacts in N2 atmosphere for 10 min. at 600˚C, and
then hot forged into pistons. The size of piston was 65
mm in diameter, and 60 mm in height. The hot pressing
process was carried out by filling the alloy powders into
a graphite die and pre-compacted at a pressure of 5 MPa,
placed inside a hot press from FCT Germany. After
evacuation the chamber was filled with nitrogen and
heated up with a heating rate of 10 K/min. At the same
time the mechanical pressure was increased to 30 MPa.
After achieving the temperatur e of 450˚C, a holding time
of 90 min was maintained. The samples were cooled
down to room temperature and removed from the graph-
ite die. Figure 1 illustrates the hot pressing technique.
Extensive metallographic investigations were carried
out for the atomized powders as well as the consolidated
bulk materials using both optical and SEM microscopy.
The density of the bulk materials was measured using
water as a floating liquid and the sintered density (ρ)
were calculated by the Archimedes method using the
following equation;
ρ = Wair / ( Wair – Wwater)
where, Wair and Wwater are the weight of the specimen in
air and water respectively.
The hardness of the consolidated alloy was measured
using Vickers hardness Tester under load of 1 kg. The
Figure 1. Schematic diagram illustrating the hot pressing
test was repeated five times at different points in each
sample, and the values were compiled by calculating the
average of the reported values of five sets of indentation
tests. Compression tests of the consolidated materials
obtained by the four methods were performed at room
temperature with a cross-head speed of 10–3 m/s and the
values of the ultimate compression strengths were ob-
3. Results and Discussion
3.1. Atomized Powders
Table 2 indicates the characteristics of the prepared at-
omized (Al-25%Si-3%Ni-1%Fe-2%Cu) alloy powders.
The particles have size from 45 to 250 m. Figure 2(a)
shows the typical optical morphology of the as atomized
(Al-25%Si-3%Ni-1%Fe-2%Cu) powder. The particle
shape of the as-solidified powders were mostly spherical
and had a smooth surface. Figures 2(b) and (c) show
SEM images with different magnifications of the cross-
sectional area of the produced powder. The ultrafine mi-
crostructure is due to the small particle size of the pro-
duced powder and a consequence of the high solidifica-
tion rate. Also there were no defects such as satellites or
pores on the surface of the prepared powders. The pri-
mary silicon of the atomized powders became very fine
and the silicon metal became very fine due to the high
supercooling effect during atomization process [15].
However, it was difficult to distinguish between the pri-
mary Si and the eutectic Si particles in th e microstructure
of the produced pow der .
DSC analysis was performed at temperature up to
600˚C in argon atmosphere to measure the solidus tem-
Copyright © 2011 SciRes. MSA
Hot Forging and Hot Pressing of AlSi Powder Compared to Conventional Powder Metallurgy Route
Figure 2 (a) Optical micrograph, (b) and (c) the cross
sectional SEM micrograph with different magnifications of
the produced atomized Al-25%Si-3%Ni-1%Fe-2%Cu alloy
Table 2 Characteriation of the produced Al-25%Si-3%Ni-
1%Fe-2%Cu alloy powder.
Apparent density, g/cm3
Tap density, g/cm3
Flow rate, s/50g
Mean grain size, µm
perature of the produced powder Al-25%Si - 3%Ni-1%Fe-
2%Cu alloy. Figure 3 shows the result of the DSC analy-
sis and the measured solidus temperature is 572.3˚C.
3.2. Microstructures and Densities of the Bulk
Four sintering processing methods were applied in this
Figure 3. DSC analysis of Al-25%Si-3%Ni-1%Fe-2%Cu
alloy powders.
investigation to obtain the bulk alloy. They are conven-
tional sintering, sintering followed by hot forging, on
step hot forging, and hot pressing. Figures 4 (a-d) illus-
trate the microstructures of the sintered alloys made by
the four methods.
Figure 4(a) shows the microstructure of Al-25%Si-
3%Ni-1%Fe-2%Cu alloy produced by conventional sin-
tering. It is noted that there are many inter-granular pores
as well as oxide layer on powder interfaces. In addition,
the sizes of the primary silicon increased. The eutectic
phase of Al-25%Si-3%Ni-1%Fe-2%Cu alloy and proba-
bly some other precipitates (e.g. Al2Cu, NiAl3, FeAl3,
etc.) are appeared and precipitated as several small parti-
cles distributed around the primary silicon.
Figure 4(b) illustrates the microstructure of the sin-
tered followed by hot forg ing alloy to obtain pistons. It is
clear that the inter-granular pores as well as oxide layer
on powder surfaces almost eliminated. The same results
are also obtained for the one step hot forging; see Figure
4(c). The only difference between the microstructures is
the grain size of primary silicon, which is much smaller
in case of one step hot forging. Th e reason for the disap-
pearance of both inter-granular pores and oxide layers by
hot forging is that the compacted materials was subjected
to high deformation accompanied with materials flow to
encompass with the new shape of pistons. The hot forg-
ing was carried out at temperature of 600˚C, which is
higher than the solidus temperature of the investigated
alloy in order to have some liquid phase to enhance the
flow of the material dur ing hot forging. It was found that
during preliminary experimental work for obtaining pis-
tons by hot forging, the material flow was very limited if
the temperature was lower than the solidus temperature.
The high deformation accompanied with material flow to
cope with the piston die cavity resulted in disrupted the
oxide layer and closed all pores under dynamic deforma-
tion effect of forging.
The microstructure of the hot pressing bulk alloy is in
Copyright © 2011 SciRes. MSA
Hot Forging and Hot Pressing of AlSi Powder Compared to Conventional Powder Metallurgy Route 1131
Figure 4. Microstructures of consolidated Al-25%Si-3%Ni-
1%Fe-2%Cu alloy with different processes, where, a) by
sintering, b) by sintering followed by hot forging, (c) by one
step forging and (d) by hot pressing.
dicated in Figure 4(d). Both isolated and/ or in-
ter-granular pores and the oxide layer on particle surfaces
are existed, but the volume and numbers of pores are
much less than those found in conventional sintering
materials. The reason of this disappointing result of the
hot pressing method may be attributed to the low holding
temperature at 450˚C. The pores and the oxide layer
could be eliminated if the hot pressing was carried out at
600˚C, i.e. above the solidus temperature.
The effect of consolidation process methods mani-
fested itself on densities of the bulk alloy materials. Ta-
ble 3 reports the relative densities of the four consolida-
tion methods. The hot forged materials showed the high-
est relative densities followed by the hot pressing one,
and the lowest relative density was recorded for the con-
ventional sintering materials.
3.3. Mechanical Properties
The room-temperature mechanical properties hardness
and ultimate compression strength (UCS) of the prepared
Al-25%Si-3%Ni-1%Fe-2%Cu are presented in Table 4.
It has been known that the addition of transition elements,
such as Fe, Ni and Cu increases the strength of the rap-
idly solidified Al alloys [7]. This combination of me-
chanical properties is directly related to the ultrafine
grain size of the Al grains and Si particles, as seen in
Figure 4. It was observed from the results of the UCS
and the hardness values of the four consolidated
Al-25%Si-3%Ni-1%Fe-2%Cu alloys as listed in Table 1
that, the strongest bulk materials are those made by hot
forging, followed by those made by hot pressing and the
weakest bulk materials are those made by conventional
sintering. The reason of that is mainly due to the highest
densities of the forged materials and the absence of the
oxide layer on powder interfaces and the presence of
oxide layer on powder interfaces decrease the bonding
strength between part i c le s [1 6, 17] .
3.4. Piston Forming
The automotive pistons of 65 mm diameter, and 60 mm
height could be fabricated by hot forging of either green
or sintered cylinders. The green cylinders or sintered
ones were heated at 600˚C for 10 min., in N2 atmosphere,
and quickly transferred into a die has the shape of the
required piston, and forged. The partially melted alloy
helps the materials to flow and fill the cavity of th e die to
Table 3. The density and relative density of the Al-25%Si-
3%Ni-1%Fe-2%Cu alloy sintered by different consolida-
tion processes.
Consolidation Process Density, g/cm3 Relative Density,%
Conventional sintering
Sintering and hot forging
One Step hot forging
Hot pressing
Table 4. Hardness and ultimate compression strengths of
Al-25%Si-3%Ni-1%Fe-2%Cu al lo y co n so l i da te d by di f fe r -
ent processes.
Consolidation Process Hardness, HV UCS, MPa
Conventional sintering
Sinterind and hot forging
One step hot forging
Hot pressing
Copyright © 2011 SciRes. MSA
Hot Forging and Hot Pressing of AlSi Powder Compared to Conventional Powder Metallurgy Route
Figure 5. Photo of a simple piston produced by hot forging.
form the shape of the piston. Also, the presence of liquid
phase disrupted the oxide film on the powders and in-
creased the consolidation effect of forging to produce
more sound bulk materials. Figure 5 illustrates the pro-
duced piston of simple shape. Hot forging process could
be used to produce automotive piston without limitation
in its chemical composition. Not only that, but also it is
possible to increase both the productivity and the mate-
rial yield of piston fabrication.
4. Conclusions
1) The atomization technique can be used to fabricate
aluminum silicon alloy powder includes small content of
transition metals such as nickel, iron and copper to en-
hance the mechanical properties. Very fine primary sili-
con particles were formed in the powder due to the cool-
ing effect of the atomization technique.
2) The microstructure and the mechanical properties of
the fabricated Al-25%Si-3%Ni-1%Fe-2%Cu alloy were
investigated as a function of the fabrication process. Four
consolidation processes were applied on the produced
powder. The hot forging processes of either sintered or
green compacts exhibited the largest densities and
strengths. The conventional sintering showed the small-
est densities and strength. But the hot pressed samples
indicated intermediate prop erties.
3) The hot forging process could be used to fabricate
automotive pistons with high productivity and without
limitation in the chemical compositions of the alloy.
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