Materials Sciences and Applicatio n, 2011, 2, 870-877
doi:10.4236/msa.2011.27117 Published Online July 2011 (http://www.SciRP.org/journal/msa)
Effect of Slow Cooling in Reducing Pore Size in a
Sintered Powder Metallurgical 6061 Aluminium
Alloy
S. Solay Anand1*, B. Mohan2, T. R. Parthasarathy3
1Department of Mechanical Engineering, Adhiparasakthi Engineering College, Melmaruvathur, Tamil Nadu, India; 2Department of
Production Technology, MIT Campus, Anna University, Chennai, India; 3Metallurgist, MetMech Engineers, Chennai, India.
Received December 15th, 2010; revised January 24th, 2011; accepted May 25th, 2011.
ABSTRACT
The usage of powder metallurgy aluminium compacts in lieu of ferrous components in automotives helps to lower vehi-
cle weight. The major drawback in the commercially available press sintered aluminium alloy is porosity which is
mainly dependent on the powder metallurgical process parameters such as compaction pressure, sintering temperature
and cooling rate a fter sin tering. In th is paper the effect o f pa rticle size and fu rnace con trolled co o ling after sin tering on
porosity level and micro hardness of an elemental 6061 aluminium alloy has been investigated. Aluminium particle
sizes of 20 µm a nd 150 µm were used. The elemental 6061 aluminium alloy powders are warm compacted at 175 MPa.
After sintering for about one hour at 600˚C, the aluminium compacts were furnace cooled at the rate of 1˚C/min to dif-
ferent temperatures of 500˚C, 400˚C, 300˚C and 200˚C. When the cooling temperature after sintering inside the furnace
is effected at various temperatures from 600˚C to 200˚C, for a precipitate hardened aluminium compacts with alumin-
ium particle size of 20 µm, the porosity level reduced by 26% and that for aluminium particle size of 150 µm, the poros-
ity level reduced by 23%. Marked improvement in micro hardness value is also observed correspondingly.
Keywords: Powder Metallurgy, Particle Size, Cooling Rate, Precipitation Hardening, Porosity
1. Introduction
Due to revolution in automobile industry towards light
weight metals, there is a particular interest in aluminium
matrix composites [1,2], especially through powder met-
allurgy (P/M), as it is a means by which complex, net
shape light weight components can be produced cost
effectively [3]. The main drawback in P/M components
is porosity. Sintered materials are typically characterized
by residual porosity after sintering, which is quite detri-
mental to the mechanical properties of these materials [4,
5]. The nature of porosity can be controlled by several
processing variables such as compaction pressure, sin-
tering temperature and time, alloying ad ditions and parti-
cle size of initial powders [4]. In particular, the fraction,
size, distribution and morphology of the porosity have a
profound impact on mechanical behavior of P/M com-
ponents [5-8]. Porosity in sintered alloys are mainly due
to 1) primary pores carried over from green state and
arising from the removal of the lubricant wax which are
not entirely eliminated by shrinkage phenomena; 2) sec-
ondary pores generated by the diffusion of alloying ele-
ments into the major phase, leaving residual porosity
located at the sites of the origin al alloy particles; 3) bub-
bles generated by the vaporization of a volatile phase [9].
In the early works [10,11] on various aluminium alloy
powder compacts it is clearly shown that the progression
of sintering and the final porosity in the system is also
dependant on the process variables such as additive par-
ticle size, heating rate and the final sintering temperature.
Current commercial aluminium powder metallurgy al-
loys are based on the wrought 6xxx series (Al-Mg-Si)
and 2xxx series (Al-Cu-Mg) of alloys, where as little
research has been conducted using elemental powders.
This work was therefore undertaken to ascertain whether
the data on porosity lev el of artificial hardened elemental
6061aluminium P/M alloy has any adverse effect on fur-
nace controlled cooling from sintering temperature after
sintering. During furnace controlled cooling from sinter-
ing temperature, the levels of porosity alo ng with density

Effect of Slow Cooling in Reducing Pore Size in a Sintered Powder Metallurgical 6061 Aluminium Alloy871
and micro hardness were also examined for two different
particle size of aluminium powders.
2. Experimental Methods
The composition of the elemental 6061Al alloy considered
for this experimental study is shown in Table 1. Two dif-
ferent particle sizes of aluminium powders of 20µm and
150µm are selected for this study. Aluminium powders of
irregular shapes are produced by atomization process as
received from Metal Powder Company. SEM images of
aluminium powders used are shown in Figure 1.
Utmost care has been taken in order to maintain uni-
formity in alloy composition for all compacts. The sche-
matic of the powder metallurgical process is shown in
Figure 2. For 500 grams of aluminium powder, each
alloy powder of the 6061 alloy composition is weighed to
an accuracy of three decimals of a gram, with the help of
microbalance. Each alloying element was mixed with
aluminium powd er in an electric mixer runnin g at 50 rp m
for 10min. According to density calculations for 6061
aluminium alloy used, 96.15grams of blend is taken for
each compact of size 70 × 50 × 10 mm3. The blend pre-
pared is poured out in a rectangular die of size 70 × 50 ×
30 mm3 and warm compacted at 175 MPa in a uniaxial
hydraulic press of 100 Tonnes capacity. The die and
plunger temperature is maintained at 150˚C and 170˚C
respectively. Paraffin wax is used as die wall lubricant
for each compact. The green compacts obtained are de-
waxed at 300˚C for about 20min. and sintered at 600˚C
in a high purity nitrogen atmosphere (dew poin t < –60˚C)
for one hour. Nitrogen gas is passed at the rate of 3
lit/min. After sintering, the sintered compacts are cooled
inside the furnace at a rate of 1˚C/min and taken out at
various temperatures of 600˚C, 500˚C, 400˚C, 300˚C,
200˚C and air cooled. The sintered compacts were solu-
tion heat treated (SHT) at 530˚C for 90 min, water
quenched and precipitate hardened (artificial aging) at
170˚C for 6 hrs.
Aluminium compacts were tested for Vickers micro
hardness by using Wilson’s micro hardness tester at 0.5
kgf load. Percentage of volume porosity is found out at
various regions according to ASTM B276 standards by
using dewinter Material plus software coupled online
with optical microscope of 1000X magnification. Density
measurements were done according to ASTM standard
B328.
3. Results and Discussion
The effects of furnace cooling temperature after sintering
of 6061 aluminium compacts with aluminium particle
size of 20 µm and 150 µm on porosity, density and micro
hardness have been discussed below.
3.1. Porosity
Figures 3 and 4 show microstructure of a precipitate
hardened 6061 aluminium compact with aluminium par-
ticle size of 20 µm and 150 µm, for a furnace cooled
temperature of 600˚C, 500˚C, 400˚C, 300˚C and 200˚C
after sintering. From these microstructures it is observed
that the pore size get reduced as furnace cooled tempera-
ture after sintering reduces from 600˚C to 200˚C irre-
spective of the aluminium particle size used. It is also
observed that the size of the phases formed during sin-
tering alsogets reduced, as the furnace controlled cooling
Table 1. Composition of 6061 Aluminium alloy used in the present work.
Material Mg Si Cu Fe Zn Sn Mn Al
Composition in wt.% 1.2 0.6 0.3 0.7 0.25 0.3 0.3 96.75
(a) (b)
Figure 1. SEM images of aluminium powders of particle size (a) 20 µm and (b) 150 µm.

Effect of Slow Cooling in Reducing Pore Size in a Sintered Powder Metallurgical 6061 Aluminium Alloy 872
Figure 2. Schematics of Specimen pr e par ation.
(a)
(b)
(c)
(d)
(e)
Figure 3. SEM images of 6061 precipitate hardened alu-
minium compact with aluminium particle size of 20 µm, at a
fumace cooled temperature of (a) 600˚C, (b) 500˚C, (c)
400˚C, (d) 300˚C and (e) 200˚C.
after sintering is effected from 600˚C to 200˚C which is
mainly due to the precipitation of these phases in to the
aluminium matrix.
Furthermore these phases get dispersed into the alu-
minium matrix dur ing solution heat treatment followed by
precipitation hardening. Figure 5 shows the EDAX at
aluminium matrix of sintered 6061 aluminium compact,
from which it is observed that the phases formed are
composed of silicon, magnesium copper and traces of tin-

Effect of Slow Cooling in Reducing Pore Size in a Sintered Powder Metallurgical 6061 Aluminium Alloy 873
(a) (e)
Figure 4. SEM image of 6061 precipitate aluminium com-
pact with aluminium particle size of 150 µm, at a fumace
cooled temperature of (a) 600˚C, (b) 500˚C,(c) 400˚C,(d)
300˚C and (e) 200˚C.
and zinc. Form this it is speculated that the phases mainly
formed are Mg2Si, Al-Si, Al2Cu and Mg2Zn [12].
Figure 6 and Figure 7 shows the optical microstruc-
ture of precipitation hardened 6061 aluminium compact
with an aluminium particle size of 20 µm and 150 µm,
furnace cooled after sintering at 600˚C and 200˚C. The
phases of Mg2Si particles (which are dark and tiny)
which are dispersed inside the grain boundaries are visi-
ble in the microstructure shown in Figure 6. The finer
particles of Mg2Si (dispersed inside the aluminum grains
as visible in Figure 7) are due to precipitation after solu-
tionising.
(b)
Very low AlSi eutectics could be seen as the silicon
has combined with Magnesium to form Mg2Si. In com-
parison with cooling from 600˚C after sintering, more
effective fusion of the grains were observed at different
fields, as the cooling is effected from 200˚C after sinter-
ing. Due to the precipitation of these phases during the
slow furnace controlled cooling at 200˚C the grain size of
the aluminium particle increased.
Figure 8 show the gain size measurement of precipi-
tate hardened 6061 aluminium compact with aluminium
particle size of 150 µm, at a furnace cooled temperature
of 600˚C and 200˚C after sintering, by using dewinter
software. The grain size of the aluminium matrix
measured optically(curved line length along grain
boundary Figure 8), seem to be changed as the cooling is
effected upto 200˚C. Higher grain size for a furnace
cooled temperature of 200˚C indicates continous grain
growth which in turn leading to decrease in pore size.
The porosity level as measured according to ASTM
B276 by the dewinter material plus software for alumi-
nium compacts furnace cooled at various temperature
with aluminium particle size of 20 µm and 150 µm, after
sintering, solution heat treatment and precipitate hardened
(c)
(d)

Effect of Slow Cooling in Reducing Pore Size in a Sintered Powder Metallurgical 6061 Aluminium Alloy 874
Figure 5. EDAX of the sintered 6061 aluminium compact showing the various compositions present in the phases formed.
20
µm
20
µm
(a) (b)
Figure 6. Optical microstructure of precipitation hardened 6061 aluminium compact with aluminium grain size of 20 µm at a
furnace cooled temperature of (a) 600˚C and (b) 200˚C after sintering , at a magnification of 500 X.
20
µm
20
µm
(a) (b)
Figure 7. Optical microstructure of precipitation hardened 6061 aluminium compact with aluminium grain size of 150 µm at
a furnace cooled temperature of (a) 600˚C and (b) 200˚C after sintering , at a magnification of 500 X.

Effect of Slow Cooling in Reducing Pore Size in a Sintered Powder Metallurgical 6061 Aluminium Alloy 875
20
µm
20
µm
(a) (b)
Figure 8. Grain Size measurement for precipitate hardened6061 aluminium compact with aluminium grain size of 150 µm, a
furnace cooled temperature of (a) 600˚C and (b) 200˚C after sintering.
is shown in Figure 9. For a precipitate hardened 6061
aluminium compact furnace cooled after sintering at
200˚C with aluminium particle size 20 µm the porosity
level is apparently equal to 6.14 vol% and that for an
aluminium particle size of 150 µm the porosity level is
apparently equal to 7.82 vol%. This variation in porosity
level for the various aluminium particle size us ed may be
related to compressibility [13], which is good for finer
aluminium particle size of 20 µm when compared with
aluminium particle size of 150 µm. Similar results of
reduced porosity level during slow cooling have been
reported, in the work by Kent et al., [14], on age harden-
ing of sintered Al-Cu-Mg-Si-Sn alloy systems. Precipita-
tion induced densification due to furnace controlled
cooling after sintering observed here, is also similar to
that of the work reported by Lumley et al., [9], on pre-
cipitation induced densificatio n due to increase in copper
content in a sintered Al-Zn-Mg-Cu alloy.
3.2. Density
The density of the sintered, solution heat treated and arti-
ficial hardened specimens are measured according to
ASTM B238 standards. The various density values mea-
sured are shown in Figure 10.
The increase in density for precipitate hardened com-
pacts is mainly due to the precipitation of various phases
formed during sintering, into the aluminium matrix dur-
ing solution heat treatment and artificial hardening proc-
ess, which further leads to increased aluminium grain
size as discussed earlier. The result obtained is similar to
that of the precipitation induced densification due to in-
crease in copper content in a sintered Al-Zn-Mg -Cu alloy,
reported by Lumely et al., [9]. Maximum density value
obtained for precipitation hardened aluminium compacts
are 2.62 g·cm–3 and 2.59 g·cm–3 for an aluminium parti-
cle size of 20 µm and 150 µm respectively. The observed
density values are lower as that reported by Showaiter et
al, [15] on their work on “Compaction, sintering and
mechanical properties of elemental 6061 Al powder with
and without sintering aids” may be due to the decrease in
compaction pressure.
3.3. Micro Hardness
Variations in micro hardness for various furnace cooled
temperature after sintering for 6061 aluminium compacts
with aluminium particle size of 20 µm and 150 µm are
shown in Figure 11. Irrespective of particle size, the
densification of aluminium grains taken place during
furnace controlled cooling up to 200˚C, has proved
enhanced hardness. The ability to measure micro hard-
ness increases within the grain improved, without inter-
ception of the pores which retards hardness measure-
ment. It is presumed that the dissolution of alloy po wders
taken place in aluminium solid solution. The slow cool-
ing enhances the grain growth, which could be seen and
measured through the microscope. The measurement of
micro hardness clearly showed the enhanced hardness at
200˚C. It is evident that increase in density as the cooling
is effected from 200˚C after sintering, increases the
hardness for a given matrix. Irrespective of aluminium
particle size, the hardness value increases for 6061 alu-
minium compacts sintered, solution heat treated and pre-
cipitate hardened, as the furnace cooled temperature after
sintering decreases to 200˚C. For precipitate hardened
6061 aluminium compact furnace cooled at 200˚C, the
micro hardness reaches a value of ~57HV0.5 for alumin-
ium particle size of 20 µm and ~ 46HV0.5 for aluminium
particle size of 150 µm. The increase in hardness value on

Effect of Slow Cooling in Reducing Pore Size in a Sintered Powder Metallurgical 6061 Aluminium Alloy 876
(a) (b)
Figure 9. Porosity level of a aintered, solution heat treated and precipitate hardened 6061 aluminium compacts for various
fumace controlled temperatures after sintering for an aluminium partocle size of (a) 20 µm (b) 150 µm.
(a) (b)
Figure 10. Various density of 6061 aluminium compacts, fumace cooled after sintering at various cooling temperature, with
aluminium particle size of (a) 20 µm (b) 150 µm.
(a) (b)
Figure 11. Various micro hardness of 6061 aluminium compacts, fumace cooled after sintering at various cooling tempera-
ture, with aluminium particle size of (a) 20 µm (b) 150 µm.
furnace con trol led c oolin g aft er si nteri ng is simil ar to the
results reported by Kent et al., [14] in the age hardening
of a sintered Al-Cu-Mg-Si-(Sn) alloy. The time taken to
achieve peak hardness increased at low furnace cooled
temperature of 200˚C after sintering is due to fact that the
rate of precipitation is largely controlled by diffusion of
solute elements which is highly dependent on cooling
temperature [16]. The prolonged furnace controlled
cooling to reach peak hardness was also observed in near
dense wrought 2014 Al alloys [17,18].

Effect of Slow Cooling in Reducing Pore Size in a Sintered Powder Metallurgical 6061 Aluminium Alloy877
4. Conclusions
The following points have been concluded from the
work.
1) Furnace controlled cooling after sintering reduces
porosity level due to densification of various phases
formed irrespective of aluminium grain size.
2) Due to densification of the phases, grain size
enlargement takes place which results in higher density
values.
3) Pores are mainly formed at the aluminium particle
boundaries and at boundaries of various undissolved
phases formed by the alloying elements and these phases
formed precipitates during solution heat treatment due to
which grain size enlargement and pore size reduction
takes place.
Irrespective of aluminium particle sizes, the porosity
level may be much more reduced if the compaction
pressure is further raised.
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