18
CHAPTER 2
LITERATURE REVIEW
2.1
INTRODUCTION
This research study pertains to the investigation of the behaviour of
ferrous P/M compacts with graphite and manganese additions during cold
upsetting. It is perceptible that P/M process is competing favorably with
machining, stamping, forging and castings. According to Narasimhan (2006),
the growths of ferrous powder metallurgy over the past four decades reflect
the advancements in materials, compaction, sintering technologies and other
related fields. However, there are many other fields in which ferrous P/M
parts are being used such as lawn and garden structural parts, hand tools,
hobby applications, household appliances, lock hardware, industrials motors,
controls, hydraulic applications and etc., and satisfy close dimensional
tolerance requirements for parts with complex geometries says James and
West (2002). Powder preform forging involves the fabrication of a preform by
the conventional P/M processing technique, followed by the conventional
forging of the preform to its final shape with substantial densification.
This chapter presents a comprehensive overview of the earlier
research work carried out in the area of forming of conventional and sintered
ferrous and ferrous based alloys. The literature that laid a platform for the
successful completion of the present investigation is mainly aimed to focus on
the studies involving workability, strain hardening of ferrous P/M preforms
during the cold upsetting condition.
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2.2
FERROUS POWDER METALLURGY
Out of the four principal alloying methods used in ferrous powder
metallurgy namely admixed alloys, partial alloys (diffusion alloys), pre-alloys
and hybrid alloys, the admixed powder method in which alloying additions, in
elemental form or as ferroalloys, such as Fe-Mn or Fe-Cr can be mixed with
an iron powder is the least expensive and most commonly used in ferrous
powder metallurgy as stated by James and West (2002). When the mix is
pressed, the additions are not alloyed with the iron powder. A admixed
materials, therefore, retain most of the compressibility of the base iron
powder. The degree of alloying during sintering is limited by the diffusivity
of the alloying elements in the iron at the sintering temperature.
Unalloyed iron based P/M parts are used for lightly loaded
structural applications and structural parts requiring self-lubrication where
strength is not critical. The combined carbon content of these P/M parts can
be from 0 to 0.3 wt%. Lall (2008), Iron parts that are essentially carbon free
are often used, particularly at higher part densities, for their soft magnetic
properties. P/M carbon steels with 0.3 to 0.6 wt% combined carbon possess
moderate strength and apparent hardness. They are used where such
properties combined with machinability are desired. Higher carbon P/M
steels, are more difficult to machine. In the as-sintered condition, carbon
steels have a ferrite / pearlite microstructure. Both medium and high carbon
steels can be heat - treated to increase tensile strength, improve apparent
hardness, and enhance wear resistance.
Few researches have been carried out on the iron and manganese
alloyed sintered steels. Elementally-substituted high Mn steels were studied as
potential low-activation replacements for austenitic stainless steels of the
types AISI 316, 320, 321 and FV548. The approach to the metallurgical
design of the compositions and prediction of the basic properties was also
20
investigated by Bott et al (1986). The mechanical property and forming limit
experiments were conducted on manganese TRIP/TWIP steel with manganese
of 18.8%. And a forming limit diagram was investigated. According to
Hao Ding et al (2011), the high manganese steel shows outstanding
mechanical properties combining high strength with good formability.
Deformation behaviour investigations were conducted on a rolled and a ascast conditions of austenitic manganese steel. Deformation, hardness and
microstructure have been investigated by Martin Schilke et al (2010). Microstructural investigation of cast medium-manganese steel was conducted by
Jingpei Xie et al (2008).
Monajati et al (2010), made an artificial neural network and it was
used to describe the effects of processing parameters on the evolution of
mechanical properties and formability of deep drawing quality steel sheets.
This model was a feed forward back-propagation neural network with a set of
19 parameters if included chemical composition, hot and cold rolling
parameters, and subsequent batch annealing process parameters to predict the
final properties, including yield strength, work hardening exponent, and
plastic strain ratio of sheets.
2.3
DEFORMATION BEHAVIOUR OF P/M PREFORMS
The rate of increase in the stress value with respect to strain is
greater than that which would be observed in a fully dense material of the
same composition under identical testing conditions, as the continued
reduction in the porosity level during upsetting increases the load bearing
cross-sectional area. This, in turn, increases the stress required for further
deformation, resulting in strain and work-hardening behaviour. Thus the total
work-hardening behaviour in a porous material is due to the combined effects
of densification and cold
Narayanasamy (2005).
working according to Selvakumar and
21
In plastic working of sintered powder material, the volume of
sintered powder material decreases, and the density of the material increases.
The density increases due to the plastic deformation of sintered powder
material called densification, and the essential reason plastic working can
improve the mechanical and physical properties of the sintered powder
material. Because densification occurs during plastic deformation of the
material, the plastic theory of traditional full density metal based on content
volume condition cannot be used to analyse the deformation of sintered
powder material. In the plastic deformation of sintered powder material, the
constant mass condition is obeyed as stated by Lin Huaa et al (2006).
When a solid cylinder is compressed axially between two flat-faced
parallel platens, the friction between the cylinder and the platens at their
surfaces of contact causes heterogeneous deformation, which in turn produces
“barrelling” of the cylinder as shown in Figure 1.18, Malayappan et al (2007)
investigated the barrelling of solid cylinders during cold upset forging with
constraint at ends. However, the use of lubricants reduces the degree of
barrelling and under the conditions of ideal lubrication the bulging or
barrelling can be reduced to zero as stated by Lin and Lin (2003).
In frictionless compression tests for sintered P/M preforms, the
deformation is uniform, showing no barrelling of the cylindrical surface.
However, reports on barrelling in P/M cylindrical preforms under cold upset
forging are described by Baskaran and Narayanasamy (2008a). The decrease
in aspect ratio along with an increase in friction at contact surfaces increases
the curvature of the bulged surface. According to Seymour Lowell et al
(2004), the non-uniform deformation in the presence of frictional forces
results in the existence of secondary tensile stresses in the circumferential
direction accompanying the axial compressive stresses. Since the primary
cause of fracture in upsetting is tensile stresses, it is therefore essential to
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investigate fracture during the deformation processing of sintered powder
materials with the help of axial upsetting tests.
The plastic deformation of porous material is similar to that of pore
free materials, but is complicated by the effect of substantial volume fraction
of voids in the material. Different theories and methods of analysis have been
developed for analyzing problems in conventional metal forming processes.
Yet they cannot be applied as they are to P/M working. Because change of
volume and yielding of porous metals are not completely insensitive to the
hydrostatic stress imposed. In fact, the mode of deformation is quite different
in porous preform materials in comparison to wrought materials and is a
function of both density and the hydrostatic stress, which have been promoted
due to induced strain during powder preform forging as stated by Rajesh
kannan et al (2008).
In cold working, the porous P/M material, apart from experiencing
the usual strain hardening, also experiences ‘geometrical work-hardening’,
due to a continued increase in density leading to the enhancement in area of
the cross-section. Thus, according to Narayanasamy et al (2008), the total
work hardening in P/M preforms is due to densification as well as cold
working of the base material surrounding the pores. During the elastic
deformation of fully dense material, Poisson’s ratio remains constant and is a
property of the material. During the plastic deformation of conventional
materials this ratio is 0.5 for all materials that confirm to volume constancy.
However, in the plastic deformation of sintered P/M preforms, density
changes occur resulting in Poisson’s ratio remaining less than 0.5 and only
approaching to 0.5 in the near vicinity of the theoretical density. It has been
well established that increase in the volume of the voids decreases the relative
critical pressure and vice-versa. However, the closing and opening of voids
occur with tri-axial compression and tension respectively, with the absolute
23
values of the relative critical pressure remaining the same in each case.
Whenever the relative critical pressure is exceeded, a void of given geometry
begins to open faster than it would close under tri-axial compression of the
same magnitude. The factors determining the geometry change of the pore are
the pattern and the level of the plastic deformation. Thus, Baskaran and
Narayanasamy (2008) found that, it is obvious that the beginning and the
continuation of pore closure can be accomplished at a comparatively lower
pressure when the material is subjected to plastic deformation.
In any upsetting operation, the induced height strain would result in
creating subsequent lateral flow of the P/M preform material. However, a
spherical pore would undergo flattening and simultaneous elongation in the
direction of the lateral flow. This leads to the situation where a relative
motion between the opposite sides of the collapsed pore, due to the presence
of shear stress, becomes feasible and the mechanical rupturing of the oxide
film takes place. Virgin metal is now exposed for bond formation across the
collapsing pore surfaces. It has been established by Miroslav Plancak et al
(2009), that the flow of material during upsetting and densification produces
fibering of inclusions in the lateral direction.
2.4
DENSIFICATION BEHAVIOUR
The present study is integral to these approaches, and is concerned
with the structural changes accompanying the densification of a porous
preform. Densification behaviour and forming limits of sintered iron–0.35%
carbon steel preforms with different aspect ratios were investigated
experimentally by Narayan and Rajeshkannan (2010) in a previous study. It
has shown that mechanical behaviour and densification can be related to
particle and pore morphology.
24
Various stress and stain parameters and their relation were studied
at the densification process of sintered plain carbon steel by Rajeshkannan
et al (2012). From the relationship between densification and flow, it was
possible to develop a yield criterion and plasticity equation for porous
materials fracture behaviour was described in terms of a simple fracture
criterion. Limited data on toughness as a function of known and controlled
forging conditions were also determined. On the basis of the macroscopic
flow data, and various physical models, an analytical model was developed
for porous metal bodies which accurately reflect the local mechanics of
deformation and densification.
Biner et al (1990) studied the effects of hydrostatic pressure up to
1104 MPa on densification of porous iron containing 0.3 -11.1% porosity. For
the porosities studied, densification as a result of pressurization increased
with hydrostatic pressure and initial porosity. The 0.3% porosity iron was the
only one, whose density did not increase with pressurization up to 1104 MPa.
Current deformation models of ductile porous materials are based on
Gurson’s yield criterion and rigid-plastic Finite Element Method analysis
which predicted much faster densification with pressurization than observed
for porosity contents of 6.2% or less.
Chandramouli et al (2007) conducted experimental investigations
on sintered cylindrical preforms of Fe and Fe–1%C to understand the
mechanism of densification and deformation during cold and hot upset
forming operations. The densification rate is found to depend on the flow
stress as well as the deformation rate of the preforms. Further, the
densification rate increases monotonically up to an axial deformation level of
about 0.5 (true strain) and thereafter slightly reduced in both Fe and Fe–1%C
alloy. The applied stress during cold upsetting increases with densification.
But the rate of increase is not uniform. The rate of densification is also
25
observed to be on the higher side with the addition of 1%C with Fe during
both cold and hot forging.
2.5
COMPACTION AND SINTERING
The design and fabricational aspects of a tooling set consisting of a
die, top punch, bottom punch, core rod and coil springs to make powder
metallurgical gears were discussed in detail by Venkatraman and Senthilvelan
(2000). Lubricants are used in P/M for minimizing die wear and aiding
powder handling and a performance evaluation was conducted using a
statistical experimental design with three premixed lubricants by German et al
(1987). A numerical simulation study undertaken to explore the effect of
variations in fill density on the final-density distribution achieved within a
pressed part and the associated effect on tool stresses reported that the
changes in density and stress level were dependent on the powder type and
the initial die-fill distribution assigned at the initiation of compression as
found by Korachkin et al (2008).
In the sintering of P/M parts several different types of problems can
be encountered leading to poor product quality and higher costs. Whittaker
(1994) examined these problems and has given significant tips to identify and
avoid them. Successful post-sintering heat treatment of P/M parts requires
proper selection of material, heat treating parameters and heat processing
equipment. The factors which must be considered such as part density,
material composition, quenching or cooling method, process factors, and
equipment related variables were discussed in detail by Herring and Hansen
(1998). Reducing the weight of the moving parts in engines and transmissions
is also important in increasing efficiency and lowering fuel consumption. Due
to the manufacturing process, sintered steel parts have a porosity of 5 to 15%,
making them lighter than solid cast or forged parts of the same geometry and
thus particularly suitable for weight reduction. Tailored parts can be produced
26
with high-density zones to cope with high stresses, and low-density zones for
weight reduction. In addition, a maximized strength-to-weight ratio can be
achieved by optimized design for sintering as referred by Rau and Krehl
(1999).
Desorption of gases, reduction of surface oxides, and neck
formations have been studied by Danninger and Gierl (2001). The dissolution
behaviour of graphite during sintering of Fe-0.8%C and the resulting
properties were studied using standard high quality natural graphite and an
artificial graphite grade for ferrous powder metallurgy by Danninger et al
(2001). The results of experiments were presented to give an overview of
apparent harness and carbon penetration as they relate to porous powder
metallurgy products by Prucher (2003). A study to assess the effect of a
variation in processing parameters on mechanical properties was performed
using design of experiments methods by Thankur et al (2004). The model was
used to find possible interaction effects between the processing variables, and
their influence on mechanical properties and to optimize mechanical
properties.
The effect of density on the tensile strength of sintered alloys is
analyzed by Straffelini and Molinari (2002) using the concept of effective
load-bearing section which depends on the porosity content and the sintering
degree, i.e. the sintering temperature and time and the eventual addition of
elements, which activate the sintering processes. Added to this, some peculiar
characteristics of sintered alloys relevant to their ductility were also
examined. A report on work relating microstructure to properties in the
development of a manganese high-strength P/M steel in which strength is not
sensitive to cooling rate is presented by Sulowski and Cias (2002).
A new technological approach to the fabrication of high density
P/M parts via single pressing sintering, allowing cold compaction to be
27
performed without admixed lubricants, has been studied by Mamedov and
Mamedov (2004). The influence of in pore gas on the compacts green density
and their sintered properties was evaluated. A mathematical expression
relating in pore gas pressure in the compacts to the green density was also
developed. Cold upsetting experiments were carried out on sintered Fe-0.8C
steel preforms in order to evaluate the technical relationship that exists
between the applied stresses against continuous deformation and densification
by Kannan et al (2008).
Elemental powders of atomized iron, graphite, manganese, copper
and titanium were mixed in suitable proportions using a ball mill, then
compacted and the sintered preforms were subjected to cold upsetting, hot
upsetting and cold repressing in order to study the plastic deformation and
densification characteristics of the low alloy P/M preforms. This was studed
by kandavel et al (2009). The role of microstructure on mechanical properties
of sintered ferrous materials was studied using a method based on electrical
conductivity measurement. The mechanical properties of the sintered
compacts were also evaluated to establish a relationship between
conductivity, total porosity, pore morphology, and mechanical behaviour by
Simchi et al (2000). Moreover, the relationship between Mo dissolution,
formation of sintered contacts and mechanical properties were demonstrated
to assess the viability of the conductivity measurement method for studying
the sintering behaviour of P/M materials and its influence on physical and
mechanical properties.
2.6
WORKABILITY OF P/M MATERIAL
Kuhn
and
Downey
(1971)
investigated
the
deformation
characteristics and the plasticity theory of sintered powder materials and
studied the basic deformation behaviour of sintered iron powder performing a
simple homogeneous compression test and also proposed a plasticity theory
28
relating yield stress and Poisson’s ratio to the density. A new yield criterion is
also proposed for the prediction of forming stresses in repressing and in plane
strain compression. Al-Qureshia et al (2008) investigated the green density
and porosity of the compact as a function of deformation process parameters.
Comparison between experimental and theoretical results of the green density
and the total porosity distributions demonstrated remarkable agreement for all
the tested conditions for different work hardening behaviour of the particles.
Workability refers to the relative ease with which a material can be
shaped through plastic deformation and it is a function of the material as well
as the process according to Venugopal et al (2003). However, understanding
the influence of the process related parameters such as friction and die
geometry on the limits of deformation in metalworking has been a difficult
problem all along. To understand the workability criteria of any material, a
clear concept of fracture criterion for ductile fracture must be established.
Workability of a material mostly depends upon the character (ductility) of the
material, and the deformation processes. The initiation of ductile fracture is a
major factor influencing the limit to workability in many metalworking
operations. Although, the macro-defects set a visible limit to workability, a
limit defined and based on plastic deformation is considered better.
2.6.1
Yield Behaviour of Composites
A number of compaction models for the rate independent
densification of porous materials have been reviewed by Doraivelu et al
(1984). These models assume an elliptical yield surface in deviatoric stress Vs
mean stress space and are calibrated against a limited set of experimental
data. Doraivelu et al (1984) and Kuhn and Downey (1971) calibrated their
models using uniaxial unconstrained compression tests on sintered aluminium
alloy. Kim et al (1990) conducted combined tension-torsion tests on sintered
iron specimens for the calibration of their yield function.
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Micro-mechanical models for the Stage I isostatic compaction of
powders have been developed by taking into account the increase in the
particle contact number and the growth of contact area with relative density.
These models have been extended by Fleck et al (1992) to non-isostatic
deformation. Yield surfaces arising from hydrostatic and closed-die
compaction are taken from Fleck (1995). The yield surfaces are shown for
axisymmetric loading, with axis of mean stress and deviatoric stress. Values
of stress have been normalised by the macroscopic yield pressure for isostatic
compaction.
Sridhar and Fleck (2000) investigated a triaxial test rig to study the
axi-symmetric cold compaction behaviour of powder composites comprising
aluminium with SiC reinforcement and lead shot with steel reinforcement.
Under hydrostatic loading, the pressure-density response shows an increase in
strength with increasing volume fraction of reinforcement. For a given
volume fraction of inclusions, the compaction pressure to achieve a given
relative density increases with diminishing size of reinforcement. The yield
surfaces are measured after iso-static and closed-die compaction. It is found
that the shape depends upon the deformation path, with the greatest hardening
along the loading direction. The effect of reinforcement on the overall shape
of the yield surface is found to be minor.
The yield function allows the possibility of describing a transition
between the shapes of a yield surface typical of a class of materials to that
typical of another class of materials. This is a fundamental key to model the
behaviour of materials which become cohesive during hardening (so that the
shape of the yield surface evolves from that typical of a granular material to
that typical of a dense material), or which decrease cohesion due to damage
accumulation as stated by Davide bigoni and Andrea piccolroaz (2004).
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2.6.2
Strain Hardening Behaviour
Plastic deformation of sintered porous materials in-compression is
always accompanied by an increase in density. The continued increase in bulk
density under compression for porous materials causes an increase in flow
stress, the latter being a function of porosity content, which is the resistance
offered by the deforming material continues to increase. This resistance, in
general, increases due to enhancement of density and to work-hardening
effects. The work hardening in sintered iron powder is different from that
shown by wrought irons and the work-hardening exponent ‘n’ is always
constant for wrought iron whereas, for sintered iron billets it is a function of
the initial compact density.
Inigoraj et al (1998) report the experimental data against the
existence of empirical relationships between the material parameters namely
strain hardening values (n) and strength coefficient values (k) and the ratio of
the initial preform densities to the theoretical density. Further, it was found
that ‘n’ consists of two segments, one representing the work hardening of the
matrix material and the other due to densification. It has also been reported
that a power-law relationship exists between ‘k’ and the present fractional
theoretical density.
Further, Narayanasamy and Pandey (1998) state the strength
coefficient (k), increases with decreasing iron particle-size range in the
aluminium matrix. Both ‘n’ and ‘k’ values were found to decrease when the
dispersed iron-particle range was greater. Further, it was been found that the
rate of change of ‘n’ and ‘k’ values were not the same for both of the aspect
ratios of the preforms tested: indeed, a pronounced difference existed. This
has established that the initial geometry of the P/M preforms plays a
predominant role influencing both ‘n’ and ‘k’. In general, as the iron content
31
in the aluminium matrix was increased, ‘k’ was found to increase also,
irrespective of the iron particle-size and the initial aspect ratio.
2.6.3
Uniaxial compression test
In metal working processes, the large plastic deformations are
achieved. In such processes, the strain path, the strain rate, the flow stress, the
stress state and the temperature can vary. Metal working processes achieve
shape changes by either plastic deformation or a combination of plastic
deformation and cracking. In forging processes, the onset of ductile fracture is
a major limitation and consequently fracture must be avoided. Metal working
processes can also be classified according to the forces applied to the work
piece direct compression, indirect compression and tension, bending and
shearing. Shear stresses are responsible for the shape change in the work
piece, whilst the hydrostatic stress influences the material ductility or
workability. A hydrostatic pressure enhances material ductility by suppression
of void nucleation and growth and conversely a tensile hydrostatic stress
promotes material fracture as stated by Abdel-Rahman (1995).
A simple approximate determination of workability limits was
carried out by Abdel-Rahman (1995) using two mechanical tests: uniaxial
compression and uniaxial tension. The two tests were chosen for their
simplicity and also since the corresponding state of stress for each test is
clear. A linear relationship between the workability function (strain to
fracture) and the stress formability was proposed. The equation of the
proposed workability limit was being easily formulated. From these
observations it can be concluded that the free-surface forming limit in
upsetting and related processes coincides with a transition to a plane-stain
stress as observed by Abdel-Rahman (1995). The fracture limit locus for
surface cracking approximates to a straight line.
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The oblique cracks on the free surface are the main failure mode
and the crack pattern depends on the friction between the die and work piece.
The formation of shear band is also one of the vital reasons for failure. Flow
and heat treatment also affects the workability. Jiang and Dodd (1995) studied
on Al-Sic heat treated composite. From the compression deformations they
observed that the high density of dislocations were generated in the matrix
close to the particle in cooling while there occurred de-bonding of the
interface between the particle and the matrix. Further, it was noticed that, the
stress is the major factor leading to failure of composites during tension and
compression. The fractures resemble the conventional ductile materials.
2.6.4
Effect of Strain
The hoop strain ( ), the axial strain ( z), and strain path from the
collar specimens were not always completely linear throughout the test.
Sowerby et al (1984) described a series of experiments and some associated
theoretical work, which would assist in assessing the suitability of certain
steels designated for cold forging operations and also found that the ductile
materials yields in excessive high loads before surface cracking occured and
recommended the collar test for studying the workability of ductile materials.
They performed three distinct upsetting tests and analyzed the prediction of
surface fracture using finite element methods and by comparing the
experimental data. They concluded that the hoop and the axial strain values at
the free surface of an upsetting specimen were employed to obtain the
associated stress value for plane stress state condition using simple plasticity
theory. However, as an expedient a linear strain path was found to be assumed
when the stress history was calculated using the simple theory of plasticity.
Abdel-Rahman and El-Sheikh (1995) investigated the effect of the
relative density on the forming limit of P/M compacts during upsetting. They
presented a workability factor describing the effect of the mean stress and the
33
effective stress-which later is a function of the relative density and then
discussed the effect of the relative density using two theories for P/M
characterization in deformation and fracture. Further, they suggested that the
formability index for porous compact is expected to be less than that of solid
metals (pore-free) because porous specimens yield and fracture at strain are
less than those for pore-free metals for the same aspect ratio and frictional
conditions and presented a more negative value means greater strain-tofracture values. Negative values of formability index correspond to the
addition of hydrostatic compression, while positive value of the same
corresponds to the addition of hydrostatic tension: the first delays fracture,
while the second increases the susceptibility to it.
2.7
LIMITATIONS OF EXISTING WORK
Some of the limitations identified based on the literature survey of
the topics such as ferrous powder metallurgy, compaction and sintering,
workability and strain hardening of pores materials applied to powder metal
forming processes are listed below:
Reported results in literatures have shown that only limited
amount of work has been carried out on three dimension stress
analysis of ferrous powder metallurgy compacts.
Workability study of Fe-C-Mn ferrous powder metallurgy
compacts with alloying elements under triaxial stress state
condition during cold upsetting has not been done previously.
Effect of carbon addition on the densification and workability
during cold upsetting conditions has not been discussed in any
of the work verified in the literature.
34
Effect of carbon and manganese addition on the strain hardening
and strength coefficient behaviour of Fe-C-Mn preforms during
cold upsetting conditions has not been discussed.
The effect of the relative density with respect to the formability
stress index has not been revealed so for of ferrous P/M
preforms.