Hot Forging and Hot Pressing of AlSi Powder Compared to Conventional Powder Metallurgy Route

doi:10.4236/msa.2011.28152

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Materials Sciences and Applicatio n, 2011, 2, 1127-1133

doi:10.4236/msa.2011.28152 Published Online August 2011 (http://www.SciRP.org/journal/msa)

1127

Hot Forging and Hot Pressing of AlSi Powder

Compared to Conventional Powder Metallurgy

Route

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: *[email protected]

Received April 27th, 2011; revised May 21st, 2011; accepted June 12th, 2011

ABSTRACT

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

1128

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

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

800

35˚

200

mechanical sieving, and sieved powders with a specific

size range of 180 µm and 45 µm were chosen for this

investigation.

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

Technique.

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-

tained.

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-

Hot Forging and Hot Pressing of AlSi Powder Compared to Conventional Powder Metallurgy Route

1130

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

powder.

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

D10

D50

D90

0.94

1.16

106

180

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

Alloy

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

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

2.37

2.67

2.72

2.53

84.7

95.4

97.2

90.2

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

193

210

129

Hot Forging and Hot Pressing of AlSi Powder Compared to Conventional Powder Metallurgy Route

1132

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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