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					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
           a
            Department of Mechanical Engineering, Kakatiya Institute of Technology and Science, Warangal 506 015, India
              b
               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
      experimental alloys.

      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:chsastry@rediffmail.com)                   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
                              Sieve analysis
Aperture size           +150 µm         +45 µm          Pan
Retention               5% max         50% max        50%min
[




          Table 3— Particle size distribution of lead powder
                              Sieve analysis
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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