|
October 2004
Putting on the squeeze to improve porous metal density
A group working at Hunan University's School of Materials
Science and Engineering in Changsha, China, has developed
a novel rolling process to improve the densification and workability
of porous spray-deposited metal…
The principle of spray deposition (SD) was firstly proposed
by A.Singer at the University of Swansea in the UK during
the 1970s [1]. This process can prepare near net-shape products
with less oxide contamination and fine grained, equiaxed and
cellular final structure [2-4]. The spray deposited preform
is not fully dense, usually containing 10-15 per cent porosity,
dependent on the thermal and solidification conditions of
the liquid droplets during deposition [5]. The tensile strength
of the preform, which depends on the density and metallurgical
bond of its constituent particles, is very low, and the elongation
is virtually zero.
In order to prepare a fully dense metal strip by the spray
deposition route, a green preform is first prepared by spray
deposition, followed by forging, extrusion or rolling using
the appropriate processing techniques to achieve the required
mechanical properties as well as eliminating porosity. Understanding
the densification, deformation and fracture behaviour of porous
metals during the forming operation is of great importance.
Densification of porous metal preforms by hot working requires
shearing the pores so that they collapse and the porosity
is eliminated [6]. Unlike the densification of porous metal
by close-die forge or extrusion via a high hydrostatic stress
field, densification of porous metal during rolling occurs
under a generalised stress field having both hydrostatic and
deviatoric components.
The application of such a stress field on porous materials
causes flattening of pores and brings about rapid densification
by collapsing the pores at a faster rate. Rolling porous materials
is an effective fabrication process for obtaining a fully-density
product from a spray deposited porous preform. However, elongation
and spreading in longitudinal and transverse directions is
a disadvantage - forming surface and side cracks during rolling
when the materials lack sound metallurgical bonds.
An optimal design of such a process requires a proper understanding
of the effect of different variables on the deformation behaviour
of porous materials. In hot densification rolling of porous
metal strip, densification is achieved essentially by the
longitudinal flow occurring in the strip that, in return,
is related to the thickness deformation given to the strip
and its instantaneous relative density. In the case of hot
rolling of a fully dense material, since the constancy of
volume is maintained, a decrease in thickness results in the
increase in length and spreading in width. However, in the
case of the porous materials, volume changes take place during
rolling and the longitudinal and transverse flow is significantly
lower than that produced in a fully dense material when given
the same amount of thickness deformation.
Although theoretical investigations for understanding the
rolling behaviour of fully dense materials have been carried
out, there are comparatively few studies of rolling porous
metals [7, 8]. Shima and Yamada [7] predicted the density
and thickness of strips obtained by the roll compaction of
loose metal powder using the upper-bound approach. The plane-strain
hot rolling of sintered porous plates was modeled by Mori
and Osakada, using the finite-element method [8]. However,
a considerable gap still exists in the understanding of the
effect of rolling parameters on deformation and fracture behaviours.
In the present work, an attempt has been made to analyse
the strains in deformation zone and the effect of rolling
parameters (packing materials, original width of workpiece
and thickness reduction) on deformation behaviours (the spreading,
elongation and changes in volume of porous workpiece) and
fracture behaviors. A novel rolling processing named Frame-Pack
rolling is developed which can improve the workability of
porous workpiece effectively during rolling.
The spray deposited material used in this investigation was
4032, as given in Table 1, with a relative density of about
90 per cent. The relative density (r) is defined as the ratio
of apparent density to theoretical density. The spray deposition
process parameters are listed in Table 2.
Test 4032 preforms, 26mm in thickness, 50mm in width and
90 mm in length, were used to investigate the deformation
behaviour (spreading, elongation and reduction in thickness)
in the rolling deformation zone of porous metal. A schematic
representation of the rolling deformation zone is shown in
Figure 1. A test preform was packed with stainless steel on
both surfaces. The hot rolling was performed on a two-high
mill with rolls of 360mm in diameter and 600mm in length.
The nominal rolling speed was 0.43m/s. The workpiece was preheated
for approximately one hour in a furnace operated at a fixed
temperature about 733ºK. Thickness was reduced by 30
per cent in one pass.
More samples were rolled with different packing materials.
Materials with three different friction factors were used.
They were aluminum, stainless steel and stainless steel with
a rough surface - artificially made nicks on the surface of
stainless steel gave a higher friction factor. Rolling comprised
preheating (approximately one hour) and reheating (approximately
30 minutes) between each pass in a furnace operated at a fixed
temperature about 733ºK. Packed preforms were hot rolled
for three to five passes in which the nominal reduction per
pass was approximately 10 per cent.
A 4032 test preform, 16mm in thickness, 49.1 mm in width
and 88.4mm in length was used to test frame-packed rolling.
To investigate the effects of the frame on the workability
and fracture of a porous workpiece, the 4032 preform was placed
in a frame made from porous 8009 alloy with stainless steel
on top and bottom surfaces. A schematic representation of
the frame pack technique is shown in Figure 2. The relative
density of 8009 alloy is 80~85 per cent which was less than
that of the 4032 preform. The size of the frame was 16mm in
thickness, 49.5 mm in width and 88.8mm in length. The framed
preform was hot rolled for three passes in which the nominal
reduction per pass was approximately 10 per cent.
Fractional thickness deformation, fractional longitudinal
elongation, fractional lateral spreading and fractional changes
in volume were calculated from the dimensions of samples before
and after hot rolling. An average of four to five measurements
was taken for each parameter.
The results of this work are divided into three sections:
deformation characteristics of porous metal workpieces in
the deformation zone; effects of packing materials and original
width on deformation behaviours of porous workpieces, effects
of frame on the deformation and fracture behaviours.
The deformation characteristics of porous metal workpieces
in the deformation zone are presented in Figure 3. From Figure
3(a)-(c), it is observed that the transverse strain near to
the surface is lower than that near to the centre, while the
thickness strain is characterised by an opposite trend. The
longitudinal strain behaviour near to the surface and centre
of the deformation zone were found to be two curves crossed
at neutral point which occurs closer to the exit. In the backward
slip zone, the longitudinal strain near to the surface is
higher than that of near to the centre. In the forward slip
zone, the longitudinal strain near to the surface was found
to be lower than that of near to the centre. This is explained
easily in terms of the friction between roller and workpiece
that hinders the metal flow. The effect of friction is decreased
from the surface to the centre. However, friction accelerates
longitudinal flow of metal in backward zone while blocking
it in the forward zone.
Figure 4 shows the effects of thickness reduction on strain
variation across the thickness of porous workpieces. It is
clear that up to the thickness reduction of about 40 per cent,
the strain variation across the workpiece thickness is symmetrical
around the centre of the workpiece, and the strain is higher
in near-surface areas relative to the centre of the workpiece,
which is different from the fully dense material. As for fully
dense materials, when the thickness reduction is small, compressive
deformation mainly takes place near to the surface. Strains
across the workpiece thickness are uneven. Strain hardening
in metals near the surface makes deformation in the centre
possible with the increase of thickness reduction, which results
in a relatively even strain across the workpiece thickness.
However, as regards porous materials, the presence of pores
in the matrix will result a lower work hardening effect during
deformation relative to fully dense materials. Therefore the
compressive deformation still occurs mainly in the metals
near to the surface even where the thickness reduction is
up to about 30 per cent. Friction between roller and workpiece
makes it reduction difficult near the surface of fully dense
metal, but porous metal can be compacted more easily near
the surface due to the pores.
The effects of packing materials on the spreading, elongation
and changes in volume of porous workpieces are shown in Figure
5. Different packing materials give different friction condition
at the roll/workpiece interface. From the graphs shown in
Figure 5, the spreading and elongation are observed to decrease
with the friction factor, while the reduction in volume is
increased with it, i.e. the densification rate is accelerated
with the friction factor.
In materials processing, a workpiece of simple shape is deformed
plastically into a final complex form, hopefully without defects.
If the workpiece contains voids initially, the task of processing
is considerably more difficult. Since voids form as an intermediate
step in the development of ductile fracture in fully dense
materials, their pre-existence at the initiation of plastic
deformation seriously limits the amount of metal flow that
can be achieved before fracture [10]. The presence of highly
dispersed porosity makes the preform crack easily.
It is noted that the performs are liable to cracking or tearing
when the thickness reduction is up to a critical value, other
than at the initial stage during rolling. This may be because
that during the initial stage of rolling, rolling serves mainly
to compact the strip without effecting much extension, and
overall stresses acting on the strip, compressive in thickness
and tensile in the rolling direction, are too small to cause
any plastic deformation of the material [11]. It may be assumed
that any longitudinal stress component tending to produce
cracking is small. However, though metal particles are not
plastically deformed, it is possible that stress concentration
near pores may cross the yield point of the material thereby
causing the sliding of the particles in the direction of rolling
and restacking themselves in void regions available in their
vicinity. Transitional stacking of particles results in densification.
So most of the compressive strain given to the preform in
the thickness direction is consumed without plastically deforming
the material. Very little elongation in the rolling direction
observed at early stage of rolling could thus be understood.
The efficiency of densification is higher in the initial
stage of rolling. With increasing thickness reduction, after
attaining a relative density of about 0.95, the density and
compressive strength of material increases rapidly. The plastic
deformation becomes the predominant densification mechanism,
the spreading and elongation in rolling rapidly increase and
some of the flow goes into elongation without further closing
the pores. The efficiency of densification becomes low and
the material still lacks a sound metallurgical structure.
Once the tensile stress in the rolling direction exceeds the
fracture stress, microcracks on the surface perpendicular
to the rolling direction occur. In addition, the reduction
of surface temperature of the workpiece during rolling due
to the contact of roller and workpiece results in decreased
working ability and the appearance of microcracks. Two approaches
are effective in improving the workability of porous materials
during rolling - keeping a uniform temperature across the
thickness of the workpiece and restraining longitudinal and
transverse metal flow during early stages to speed up densification
and obtain sound metallurgical bonds.
It is clear from Table 3 that fracture strains increase with
friction, which can be induced by different packing materials.
For difficult-to-roll fully dense materials, hot pack rolling,
usually packing the workpiece in a ductile and sacrificial
material (cans), is an effective method to improve the rolling
ability. These cans minimise heat losses, as well as oxidation
or other contamination of the workpiece during hot rolling.
As for porous materials, they play another important role
increasing the friction between roller and workpiece, providing
high friction on the flow of metal during rolling and more
effective densification.
In this experiment, two packing materials are used: aluminium
(soft can, whose yield stress is lower than workpiece) and
stainless steel (hard can, whose yield stress is higher than
workpiece). The results show that the fracture strain of aluminium-packed
workpiece is lower than that of the stainless steel-packed.
This trend may be because that the softer aluminium layer
undergoes a larger elongation during rolling, resulting in
a tensile stress on the surface of workpiece surface which
is liable to give birth to the cracks on surface. However,
a hard can provides an inverse effect on the flow of the workpiece
during rolling and the restraining effect is intensified with
a rough surface.
In order to impose a more intense restrain on the elongation
and spreading on the metal during rolling, another packing
process has been developed, i.e. the Frame-Pack method. There
is a gap only of about 0.4mm between the frame and workpiece
to allow the workpiece into the frame.
The mechanism of a porous frame in restraining the flow of
the workpiece both in transverse and longitudinal direction
is shown in Figure 6. During rolling, the reduction in thickness
of frame is the same as that of the workpiece. The spreading
and elongation of frame and workpiece are shown in Figure
6 designated Db1, DL1 and Db2, DL2, respectively. It is clear
that Db1 acts as an restrain to Db2. the elongation of frame
(DL1) will be less than that of the workpiece (DL2) when given
to the same thickness reduction due to a relatively low density.
The frame imposes an effective restraint on the elongation
of the workpiece, while at the same time, the frame around
the sides proves to be of benefit in maintaining a uniform
temperature in the workpiece during the rolling process. From
the results of the investigations reported in this paper,
the following conclusions may be drawn.
(1) Friction, caused by different pack materials, affects
the deformation behaviour by a decrease of lateral and longitudinal
flow, resulting in accelerated densification.
(2) Lateral flow of metal decreases with the original width
while the densification speed increases.
(3) A porous frame around the sides of porous workpiece can
improve the workability effectively by imposing restraints
to the lateral and longitudinal flow that results in an effective
densification.
The team
This feature was abstracted from Deformation and fracture
behaviour of a porous 4032 alloy prepared by spray deposition
during hot rolling, prepared by Chen Zhen-hua, Zhan Mei-yan
and Xia Wei-jun.
Acknowledgements
The authors would like to thank Chen Gang and Teng Jie for
the preparation of porous preforms. Thanks are also due to
Li Jian-jian and Wu Heng for their careful preparation of
rolling workpieces.
References
1. Singer A R E. Met. Matter., 4 (1970) 246
2. U Prakash, T Raghu, A A Gokhale, S V Kamat: J. Mater. Sci.,
34(1999)5061
3. E J Lavernia and N J Grant: Mater.Sci.Eng., 98(1988)381
4. D J Skinner, R L Bye, D Raybould and A M Brown: Scipta
Metall., 20(1986)867
5. S Hariprasad, S M L Sastry, K L Jerina et al. : Metall.Trans.,
24 A (1993) 865
6. S Bhargava, R K Dube. Metall.Trans., 19A(1988)1205
7. S Shima, M Yamada. powder. Metal., 27 (1984):39
8. K Mori and K Osakada. Inter Mech Sci 29(1987)229
9. CHEN Zhen-hua, in Multi-layer spray deposition technology
& applications (Hunan University publishing, Chinese,
2003) p.192
10. Howard A Kuhn. applied mechanics division, AMD, 16(1976)171
11. R K Dube. Inter. Mater. Rev., 35 (1990) 253
|
