Indian Journal of Engineering & Materials Sciences
Vol. 17, February 2010, pp. 56-60
Densification behaviour of Al-Pb alloys –A study of effect
of certain process parameters
ChV S H S R Sastrya* & G Ranga Janardhanab
Department of Mechanical Engineering, Kakatiya Institute of Technology and Science, Warangal 506 015, India
Department of Mechanical Engineering, JNT University College of Engineering, Kakinada 533 003, India
Received 14 October 2008; accepted 25 November 2009
The recent research into the alternate sliding bearing materials unequivocally point to the beneficial role of lead in
aluminum. But, these alloys offer a manufacturing challenge, due to wide immiscibility gap. For sliding bearing
applications, controlled porosity (size, distribution and nature) is an important consideration, as it influences the tribological
performance through mechanical properties and spreading of lead in aluminum matrix. In the present investigation, the
effects of alloy composition, ball to charge ratio and mixing/milling route on densification behaviour of Al-Pb alloys
processed through conventional ball milling and attrition milling routes, using XRD, SEM and compressibility test are
studied. The resulting morphological changes of powder complexes are examined, on five compositions of alloys to
determine compaction response. The study concludes that attrition milling is an effective method for densification of
Keywords: Al-Pb alloys, Powder metallurgy, Compressibility test, Densification behaviour
Aluminum-lead alloys are considered as bearing not been reported on the relation which exists
materials of 21st century1. The manufacturing of between densification behaviour and wear behaviour,
leaded aluminum alloys had been difficult owing to in respect of Al-Pb alloys.
the big difference in specific gravity and wide In the manufacturing technologies of P/M products,
immiscibility gap2. As a result, Al-Pb alloys were die compaction is widely used. However, P/M parts
found to exhibit very strong sedimentary tendency formed by die compaction have inhomogeneous
upon solidification; hence, the conventional methods density distributions due to the friction between the
of melting and casting are inadequate for the powder and the die wall. Inhomogeneity in density
production of an Al-Pb system3. In view of the above, leads to non-uniform shrinkage or distortion during
various researchers have developed different sintering process, and thus makes it difficult to control
processing techniques such as vortex casting, bottom the shape of final P/M parts7. In order to control the
discharge chill casting, continuous casting and roll shape of final P/M parts, the appropriate models are
bonding, roll compaction and sintering, atomization necessary for densification of composite powders to
and powder rolling, rapid solidification processing4,5, simulate cold compaction responses.
stir casting6, and hot extrusion4 for Al-Pb alloys. In A number of yield functions have been developed
most of these studies the main focus had been on for densification behaviour of metal powder, so far.
manufacturability with homogeneous microstructure. Whereas Bruhns and Sluzalec8 analyzed density
For sliding bearing applications, apart from changes with the help of a theoretical and numerical
homogeneous microstructure, porosity control (size, analysis. Kuhn and Downey9, Shima and Oyane10 and
distribution and nature) is also an equally important Doraivelue et al.11 proposed yield functions based on
consideration, as it influences the tribological uniaxial tests on sintered powder compacts. Brown
performance through mechanical properties and and Weber12, however, showed that densification
spreading of lead in aluminum matrix. The choice of behaviour of loose powder is different from that of
porosity is one of the major advantages of the P/M sintered powder in uniaxial tests. The discrepancy in
method of manufacturing. But, significant work has densification behaviour is more important, in the early
stage of compaction, because the inter-particle
*Corresponding author (E-mail:firstname.lastname@example.org) cohesion of loose powder compact is much lower than
SASTRY & JANARDHANA : DENSIFICATION BEHAVIOUR OF Al-Pb ALLOYS 57
that of a sintered powder compact. Thus, these occurs as a function of applied pressure is technically
models9-11 may not be appropriate for the early stage evaluated through the compressibility test. Further,
of compaction response. the test data is taken as a dependable basis for
A macroscopic constitutive model from particle determination of compaction pressure for new metal
deformations was proposed by Fleck et al.13 However, powder mixes. Under these circumstances, the
this yield function did not agree well with optimum compaction pressure to be applied, on
experimental data of soft metal powder during die particular powders, should be determined by taking in
compaction14. to consideration, the effect of process parameters.
A number of researchers also adopted models, for
densification of metal powder from soil mechanics. Experimental Procedure
These models include Drucker-prager model10, cam- The alloys were prepared by powder metallurgy
clay model11 and cap models12,13. Out of their results it technique with the compositions presented in Table 1.
can be concluded that a more appropriate way to The particle size distribution of aluminum powder and
predict densification behaviour of loose soft metal lead powder used are presented in Tables 2 and 3
powder in all stages of compaction must be based on respectively.
experimental data of loose powder and should The Al and Pb powders of various compositions
incorporate densification due to inter-particle are mixed in conventional ball mill and attrition mill
movements as well as particle’s plastic deformation. separately (each operated at 200 RPM). Samples are
The degree of porosity in sintered or unsintered withdrawn from these mills after 45 min of
metal powder parts depends on the extent of continuous run. They are analysed by powder X-ray
densification. The porosity in an unsintered powder diffractometer of BRUKER-axs D-8-Advance model,
metallurgy item is determined indirectly by making equipped with Lynx eye counter, Graphite crystal
density measurements through compressibility test. monochromator and working with Cu-Kα radiation.
However, the density in case of a particular powder The resulting X-ray diffractograms are presented in
mix depends upon the compaction pressure applied. Figs 1 and 2. The morphology of powder samples
And the optimum compaction pressure for a metal milled by both the methods is examined under
powder mix, in turn, depends upon a variety of factors scanning electron microscope and the details are
like composition, morphology of starting powders, presented in Figs 3 and 4.
particle distribution, speed of operation of mixing
mill, time for mixing, energy input in to mixing
process, ball to charge ratio, etc. Hence, relatively
little direct experimental data are available for various
combinations of metal powders being experimented.
As such, the densification in metal powders which
Table 1— Composition of metal powders
Sl. No. Al (wt %) Pb (wt %)
1 95 5
2 90 10 Fig. 1— X-Ray diffractogram of Al-Pb alloys (Ball Milled)
3 85 15
4 80 20
5 75 25
Table 2— Particle size distribution of atomized aluminum powder
Aperture size +150 µm +45 µm Pan
Retention 5% max 50% max 50%min
Table 3— Particle size distribution of lead powder
Aperture size + 150 µm +75 µm - 45 µm
Retention 1 % max. ... 80 % max Fig. 2— X-Ray diffractogram of Al-Pb alloys (Attrition Milled)
58 INDIAN J. ENG. MATER. SCI., FEBRUARY 2010
The schematic of the set up used for green compact. The weight of the compact is
compressibility test is presented in Fig. 5. For the determined to the nearest 0.05 g, to calculate the
purpose of compressibility test, the lower punch is green density of the compact. These steps are
inserted into the die and the assembly is placed on the repeated, using appropriately larger amounts of
lower platen of the hydraulic press. Using supporting powder and final compacting forces of 1,000, 1,500,
spacers which may be wooden blocks, the die is 2,000, 2,500, 3,000, 3,500, 4,000, 4500, 5,000, 5,500,
raised so that the depth of powder fill will be proper 6,000, 6,500 and 7,000 kg. The variation of density
to give a green compact height equal to the compact with pressure, for various compositions of Al-Pb
diameter + or – 10%. The powder depth varies with alloys is presented in Figs 6-10. Also, the effects of
the particular powder used and was determined on the powder mixing route (ball milling and attrition
basis of a number of trial tests. The alloy powder is milling) and ball to charge ratio (BTCR) on the
poured into the die cavity to fill it to overflowing. density are clearly revealed in these figures.
Then, the powder is levelled off flush with the top of
the die by passing the straight edge of the spatula Results and Discussion
horizontally across the die top. The die is tapped a The basic processes which occur during
little to make the powder level settle slightly below compaction of metal powders, under pressure, are
the die top. The upper punch is positioned so that it bulk movement, deformation and fracture of particles.
rests in the die and on top of the powder. A Bulk movement and rearrangement of particles will
preliminary force of 0.2 kg is applied and then result in a more efficient packing of the powder; that
removed, after which the spacers are removed. Then, is known as densification.
a force of 500 kg is applied, building up to this The X-ray diffractograms in Figs 1 and 2 reveal
amount at a constant rate of loading. The applied various elements present in the powder mix. All the
force is gradually withdrawn and the green compact is diffractograms belonging to various compositions of
ejected by use of a die push-down spacer placed
between the die top and the bottom of the platen
above it. Subsequently, the height of the green
compact is measured, to calculate the volume of the
Fig. 5— Set- up used for compressibility test
Fig. 3– SEM photographs of Al-Pb alloys (ball milled)
Fig. 4– SEM photographs of Al-Pb alloys (produced by attrition
milling) Fig. 6— Compressibility characteristics of 95Al-5Pb alloys
SASTRY & JANARDHANA : DENSIFICATION BEHAVIOUR OF Al-Pb ALLOYS 59
the alloys under consideration, have recorded peaks at balls/container leading to collisions that increases the
the same value of 2θ, denoting the presence of lead level of microstructural strain, enhancing its
and aluminum only. However, at higher contents of mechanical properties.
lead and traces of lead oxide were observed. Thus, the mechanical alloying process refines
Comparing between the XRDs obtained for ball microstructural features by introducing extensive cold
milled alloys and attrition milled alloys, it can be work (leading to high dislocation density). By this
observed that there is a shift in the Bragg angles at process, high strengths can be achieved by refining
which diffractions occurred and also broadening of particle size and homogenizing its distribution. By
the base took place. These two effects serve as an mechanical alloying (MA) process, there is increased
evidence to state that attrition milling has resulted in scope for solid solution strengthening as even
relatively higher degree of cold work, and lattice insoluble alloying elements may be fragmented into
strain, and confirm the formation of disorder type fine dispersion. The dissolution of the solute during
structures of alloys. Comparing between the SEM MA is facilitated by three important factors, namely:
photographs shown in Figs 3 and 4 for ball milled and local heating, lattice defects and shortened diffusion
attrition milled samples, one can clearly observe the distances. The severe cold working resulting from
uniform distribution of finely divided lead particles MA also aids in diffusion by providing many sites for
and cold welding of aluminum and lead particles in pipe diffusion, by making powder particles thinner,
mechanical alloying process. Besides, the powder decreasing the diffusion distances, thereby enhancing
particles are continuously trapped between balls and the solution of the solute in the solvent, aiding
homogenization. It is likely that apart from the finely
Fig. 7— Compressibility characteristics of 90Al-10Pb alloys Fig. 9 — Compressibility characteristics of 80Al-20Pb alloys
Fig. 8— Compressibility characteristics of 85Al-15Pb alloys Fig. 10— Compressibility characteristics of 75Al-25Pb alloys
60 INDIAN J. ENG. MATER. SCI., FEBRUARY 2010
dispersed dispersoid, MA powder will have observed. The flattening can be attributed to the
microstructures that will show signs of extended solid greater deformation in the direction parallel to the
solutions and microcrystallinity. In view of this, compaction direction. An examination of a cross-
mechanical alloying process resulted in uniform section of a sample compact in planes parallel and
distribution of powder mixture with superior degree perpendicular to the pressing direction revealed this
of densification. difference in pore shape; in the plane perpendicular to
It can be observed from Figs 6-10 that for the the pressing direction the pores are more equiaxed. The
particle size distribution considered, in this work, the study concludes that attrition milling is an effective
density of Al-Pb alloy complex increases with method for densification of experimental alloys.
increase in compaction pressure and ball to charge
ratio. However, attrition milled complexes are found Acknowledgement
to exhibit greater densities, compared to ball milled The authors are thankful to Sri A Dayanand Reddy,
powder complexes. This can be attributed to the fact Managing Director Vasantha Tool Crafts Pvt. Ltd.,
that attrition milling can result in finely divided lead Hyderabad, for helping in designing and fabrication
particles and cold welding of Aluminum and lead of the attrition mill as well as the dies and punches.
particles. The small particles move relatively greater The authors are highly indebted to Dr Ahmed Hussain
distances because of their ability to pass through the and Dr Shakuntala for the support extended in XRD
small channels among the particles. and SEM analysis of the samples at Indian Institute of
The extent of densification, achieved in these alloys, Chemical Technology, Hyderabad.
however, decreases with increase in compaction
pressure. This can be attributed to the fact that the References
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