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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 8 ( 2 0 0 8 ) 41–47
journal homepage: www.elsevier.com/locate/jmatprotec
Characterisation of different lubricants concerning the
friction coefficient in forging of AA2618
B. Buchner ∗ , G. Maderthoner, B. Buchmayr
University of Leoben, Department of Product Engineering, Franz-Josef-Strasse 18, A-8700 Leoben, Austria
a r t i c l e
i n f o
a b s t r a c t
Article history:
In this paper a number of aluminium forging lubricants were tested with respect to their
Received 27 February 2007
lubricating effect and their applicability in the production of parts with long flow paths. The
Received in revised form
tests were conducted on a custom built testing machine that simulates the parameters of
13 June 2007
industrial production. The lubricants were assessed considering the change of the coeffi-
Accepted 21 June 2007
cient of friction during the test and the influence on tool and workpiece surface. From the
presented analysis, the most appropriate lubricant for the investigated process was found.
© 2007 Elsevier B.V. All rights reserved.
PACS:
06.60.Mr
06.60.Vz
Keywords:
Friction
Forging
Aluminium
1.
Introduction
Aluminium alloys are important construction materials in the
automotive industry, in racing and aerospace due to their low
specific weight, their corrosion resistance and their ability to
achieve high strength with certain alloying additions. AA2618,
e.g. is a heat treatable Al–Cu–Mg–Fe–Ni forging alloy developed for high temperature applications, especially for aircraft
engine components (Oguocha, 1999). Due to the desired
mechanical properties of aerospace components, parts made
of AA2618 are typically forged. In the production of complex
aluminium components by means of drop forging, besides the
workpiece material, the lubricant is the most important factor
to obtain a successful process.
∗
1.1.
There were two major tasks in this investigation: on the one
hand, a testing method for characterising lubricants with
respect to their lubricating effect and their applicability in
the production of parts with long flow paths had to be developed. On the other hand, experiments with a representative
selection of commercial lubricants had to be performed.
1.2.
State of the art
In the experimental determination of friction in metal forming, direct and indirect methods are distinguished: in direct
testing methods, the friction stresses are acquired by using
measurement pins locally in the tool–workpiece interface,
Corresponding author. Tel.: +43 3842 402 5604; fax: +43 3842 402 5602.
E-mail address: [email protected] (B. Buchner).
URL: http://www.metalforming.at (B. Buchner).
0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmatprotec.2007.06.057
Problem statement
42
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 8 ( 2 0 0 8 ) 41–47
Fig. 1 – Friction tests, where friction is determined from the
change of the specimen’s shape (the specimens are marked
gray): (a) ring-compression test, (b) spike test, (c)
double-cup-extrusion test.
whereas friction is determined via a deduced quantity such as
force or deformation in indirect experiments. For the characterisation of lubricants, usually indirect methods are utilised
because in indirect measurements the friction stress is averaged on the entire tool–workpiece interface and thus local
inhomogeneities are compensated. Indirect testing methods,
where friction is determined from specimen deformation, are
very popular in industrial practice. The tests are easy to perform and represent the real process very well. However, most
of the parameters cannot be controlled independently from
each other, and no stationary state is present. Due to this
these kind of tests are usually used in comparative investigations. The most popular friction test in this category is the
ring-compression test developed by Kunogi (1954) and Male
and Cockcroft (1965) (see Fig. 1(a)). This method was lately
used for the evaluation of lubricants by Meiners and Hornhardt
(2003) and Petrov (2007). In Fig. 1(b) the spike test utilized for lubricant characterization, e.g. by Sheljaskow (2001)
is presented, and Fig. 1(c) shows the double-cup-extrusion
test as depicted by Kim et al. (2004). All these experiments
have in common that their parameters can be varied only
in very limited ranges and the sliding distances are quite
short.
Indirect experiments, in which friction is determined from
force or torque measurements, allow a more detailed investigation of the tribological interactions in the tool–workpiece
interface. Hansen and Bay (1986) performed backward-canextrusion followed by a rotation of the container, where the
torque transferred from the cup to the punch was measured (see Fig. 2(a)). Groche and Kappes (2004) utilise a
sliding-upsetting test, in which at first a cylindrical specimen is compressed with a special tool in order to obtain
a homogenous surface expansion on the bottom side of
the specimen and then the lower tool is moved and the
sliding force is measured (Fig. 2(b)). Doege et al. (2002b)
developed a model experiment to determine friction and
heat transfer in backward-can-extrusion by means of a
lower punch equipped with strain gauges and thermocouples (Fig. 2(c)). Lately, two different research teams (Ngaile
et al., 2007; Daouben et al., 2007) evaluated lubricants by
performing a sliding-upsetting test where relative motion
is applied on an indenter penetrating a cylindrical billet
(Fig. 2(d)).
Fig. 2 – Friction tests, where friction is determined from
force (or torque) measurements (the specimens are marked
gray): (a) backward-can-extrusion test performed by
Hansen and Bay (1986), (b) sliding-upsetting test proposed
by Groche and Kappes (2004), (c) backward-can-extrusion
test utilised by Doege et al. (2002b), (d) sliding-upsetting test
introduced by Daouben et al. (2007) and Ngaile et al. (2007).
2.
Experimental work
The experiments were performed on a rotational forging tribometer (RFT) that was originally designed for ring-on-disc
tests. This device had to be adapted to the actual conditions
(long flow paths, sliding on virgin lubricant layers) using a
special toolkit.
2.1.
Testing device
Fig. 3 shows the RFT. The circular motion is supplied from
the bottom side by means of an asynchronous motor and an
angular gear box, whereas the compression force is applied by
a hydraulic cylinder from the top side. The specimen (workpiece) is mounted on a rotary disc and transmits the frictional
torque to the pivot-mounted ring-shaped tool. The tool is supported by a load cell via a lever arm which enables an accurate
measurement of the frictional torque. The acquisition of the
compression force is realised by another load cell, the rotational speed of the specimen is calculated from the speed of
the asynchronous motor. Frictional speed as well as interface
pressure depend on tool geometry. The specimen is heated
to forging temperature using an inductive heating system
whereas the tool is heated by a heating sleeve. The lubricant
is sprayed onto the tool with a handgun. The testing device
is controlled by a programmable logic control unit (PLC), the
data acquisition is realised by a measurement amplifier that
is connected to a commercial personal computer.
The tribometer has a maximum upsetting force of approximately 100 kN and provides a maximum torque of more then
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 8 ( 2 0 0 8 ) 41–47
43
Fig. 3 – The rotational forging tribometer testing device.
Fig. 5 – Toolkit for the presented experiments.
800 Nm on the rotary disc. The rotational speed can be up to
1.7 s−1 , whereas the acceleration proceeds as shown in Fig. 4.
Under this consideration, the influence of the inertia of the
engine on the sliding velocity is significant and can lead to
wrong measurements, especially at short sliding distances.
The sliding distance itself is not constrained by the testing
device. The maximum temperature of the specimen is 1200 ◦ C,
the maximum temperature of the tool is 450 ◦ C. The temperatures can be kept constant in an interval of ±2.5 ◦ C.
2.2.
Toolkit and specimen geometry
As the ring-on-disc test, for which the RFT was originally
designed, does not meet the demands on long flow paths and
on the sliding at virgin lubricant layers, a new toolkit had to
be developed. The sliding at virgin lubricant layers can only
be realised when tool and specimen are not in contact over
the entire sliding surface at the same time. This eliminates
a ring-shaped geometry. Furthermore, the active surface of
the tool has to be plane in order to avoid additional plastic
specimen deformation leading to wrong measurements during sliding. Long sliding distances can be achieved by adjusting
the distance between contact area(s) and axis of rotation.
The only possibility to meet these demands, is to perform a
kind of ring-on-disc test as usually used to investigate tribolog-
Fig. 4 – Acceleration curves of the rotary disc for several
nominal values.
ical interactions at low normal pressures. However, Attanasio
et al. (2007) have performed pin-on-disc experiments under
forming conditions and Vergne et al. (2001) performed pin-ondisc tests to measure friction under hot rolling conditions. In
both investigations, the pin represented the tool and the disc
represented the workpiece. In the present analysis, the role
of pin and disc was exchanged to avoid additional tangential stresses due to plastic interlocking. Furthermore, two pins
were used instead of one in order to load the device symmetrically.
Fig. 5 shows the toolkit used in the reported investigation,
where tool and specimen were made of hot work tool steel
1.2344 (hardened to 52 HRC) and AA2618, respectively. A very
important point is that the tool–workpiece contact area should
remain constant during the analysis independent of the compression force. This demand was met by placing a ring around
the specimen, making the contact area constant to 154 mm2 .
2.3.
Execution of the tests
First, the tool was brought to “operating temperature” and well
lubricated. When the testing sequence was started by the PLC,
the specimens were heated, and the final temperature was
kept constant for a predefined time in order to allow temperature equalisation. After this, the execution of the test itself
was started automatically. In order to avoid an interaction of
different influences, the specimens were compressed in a first
step and thereafter the rotation began. The data acquisition
was started by a mechanical trigger. After the test, tool and
specimen were inspected.
Three tests were executed for each parameter set. Each
experiment was performed with virgin specimens, whereas
the tool was changed only when a new lubricant was applied.
In order to maximise the effects on a tool surface modification, all tests were started at the same tool position. The four
investigated lubricants are listed in Table 1 and the parameter
matrix is shown in Table 2.
It has to be stated that the specimen’s surface is enlarged
during upsetting in dependence of the compression force,
44
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 8 ( 2 0 0 8 ) 41–47
Table 1 – Lubricant test matrix
Lubricant type
A
B
C
Dispersion of graphite in water
Dispersion of graphite in water
Emulsion of a dispersion of graphite
in water and mineral oil
Dispersion of graphite in mineral oil
D
Table 2 – Parameter matrix
Parameter
Value
Sliding velocity, v (mm/s)
Normal stress, (MPa)
Tool temperature, TT (◦ C)
Specimen temperature, TS (◦ C)
150
210–330
250
450
e.g. different forces lead to different amounts of surface
expansion. However, the influence of surface enlargement is
restricted to the start of the test, thereafter virgin lubricant
layers are present independently of the normal force.
2.4.
Evaluation of the tests
As already mentioned, the main focus of the investigations
was the characterisation of different lubricants with respect
to their lubricating effect and their applicability in the production of parts with long flow paths. Hence, an adequate
assessment was performed on the basis of the acquired measurement curves and on the basis of the inspection of the
sliding surfaces after the tests.
2.4.1.
Evaluation of the measurement curves
During each experiment, the normal force, Fn (compression
force), the tangential force, Ft (friction force), and the rotational speed, v, were measured (see Section 2.1). By means of
the known contact area, A, both the normal stress, , and the
friction stress, , can be calculated:
=
Fn
,
A
(1)
=
Ft
.
A
(2)
The evaluation of the measurements was carried out in
several steps (see Fig. 6):
(1) All data were acquired with a fixed sampling rate, i.e. the
time intervals between the sampling were kept constant.
However, for the comparison of measurements, a constant
stroke interval is preferable. Thus, in a first step, all the
data were transferred from equal time spacing to equal
stroke spacing.
(2) For all three tests representing the same parameter set,
average measurement curves were determined by calculating the mean -, -, and v-values for each stroke
increment, i
¯ i =
i1 + i2 + i3
,
3
(3)
Fig. 6 – Evaluation of the measurement curves. The mean
friction stress divided by with the mean normal stress
gives the mean friction coefficient.
¯ i =
i1 + i2 + i3
,
3
(4)
v̄i =
vi1 + vi2 + vi3
.
3
(5)
(3) The begin of the stationary region of the experiment was
identified as the stroke si at which the average velocity
became equal to or greater than the reference velocity (v̄i ≥
vref ).
(4) For each average curve, the regression line was determined
in the interval corresponding to the stationary region of
the experiment.
(5) From these regression lines, the values characterising the
lubricants were derived. The friction coefficient, , which
is calculated in the centre of the regression line, is wellestablished in depicting the friction-reducing effect of
lubricants
=
Ft
,
Fn
(6)
where Fn is the normal force and Ft is the tangential force.
The characterisation of the physical stability of lubricant
layers in dependence of the sliding distance is a less common task. In the scope of the present work, the following
two values have been considered suitable to describe the
durability of lubricant layers: the slope, ˛, of the regression
line and the standard deviation, S.D., of the mean curve.
2.4.2.
Surface inspection
After each test, the tool and the specimen surface were
assessed optically. Furthermore, after the completion of the
test series, the surface roughness of all tools were measured
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 8 ( 2 0 0 8 ) 41–47
Fig. 7 – Average friction stress at one parameter set. Each
curve is derived from three measurements and represents a
lubricant.
in order to get further information on the tribological interactions in the tool–workpiece interface. This was done by
measuring roughness profiles in and perpendicular to the sliding direction at 20, 50 and 80 mm sliding distance and by
calculating the averaged values.
3.
Results and discussion
Fig. 7 presents mean curves of the four lubricants listed in
Table 1 at the same testing conditions. Lubricant A seems to
be not affected by the surface expansion in the upset area
(corresponding to the first 14 mm of the sliding track). The
sticking friction stress is high (about twice) compared to the
other lubricants and it is larger than the sliding friction stress
which decreases slightly with increasing distance. Regarding
the lubricants B–D, the sliding friction stress exceeds the static
friction stress and increases until it reaches a maximum at
a distance of about 14–20 mm. However, the maximum sliding friction stress of lubricants B and C is in the range of that
of lubricant A, but the sliding friction stress of lubricant D is
about twice this value. At sliding distances larger than 20 mm,
the friction stress reaches a steady state for lubricants B and
C. Only in the case where lubricant C was tested in undiluted condition at high pressure (corresponding to the curve in
Fig. 7), lubricant break-down occurred. Using lubricant D, the
friction stress decreases from its maximum value and reaches
a steady state at a sliding distance of approximately 50 mm.
In order to show the three quantities characterising the
lubricants (friction coefficient, , slope of regression line, ˛,
and standard deviation of the mean curve, S.D.) in a single diagram, a new quantity combining ˛ and S.D., the progression of
friction coefficient, pfc, was considered
pfc =
˛
·
|˛|
45
Fig. 8 – Results of the investigation of four lubricants.
coefficient is slightly decreasing with lubricant A and it is
slightly increasing with lubricants B and C during the test.
However, the outermost point of C indicates lubrication breakdown at the corresponding testing parameters (compare with
Fig. 7). A different behaviour is shown by the oil-based lubricant D. On the one hand, the friction coefficients tend to higher
values than those of the lubricants on water basis, and on the
other hand, the pfc is strongly negative what indicates a strong
decrease of the friction coefficient during the test.
Fig. 9 shows the sliding tracks on the four tools. These sliding tracks were assessed visually in terms of compactness of
the lubricant layer. It was found that the oil-free lubricants
A and B formed compact layers that were not worn off during the test and prevented aluminium pick-up. However, after
the test, parts of the lubricant layers flaked off. In contrast,
the oil-containing lubricants showed another behaviour. The
water-oil-based lubricant C formed a very homogeneous layer,
but it was worn off at high pressures and pick-up occurred in
small areas. The oil-based lubricant D was completely abraded
2
˛2 + (10 · S.D.) .
(7)
The factor 10 was introduced to make ˛ and S.D. of the same
order of magnitude (to cause a balanced weighting).
Fig. 8 shows the result of the investigation on the four lubricants. Each point in the diagram corresponds to the result of
a single testing condition. The lubricants A–C show a stable
behaviour (a smooth curve progression), whereas the friction
Fig. 9 – Sliding tracks for different lubricants (sliding was
performed clockwise).
46
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 8 ( 2 0 0 8 ) 41–47
Table 3 – Tool roughness values
Rz (␮m)
Lubricant
Sliding direction
A
B
C
D
7.58
3.81
3.72
5.03
Perpendicular
14.19
7.29
6.54
9.82
and heavy pick-up was present in the beginning of the sliding
track (near the upset area). When sliding progressed, further
pick-up occurred in small areas.
Comparing the roughness values of the tools (see Table 3)
the following is found: lubricant A gives the highest tool roughness, the roughness when applying B or C is approximately
the half, and the roughness when applying lubricant D is inbetween the roughness values of A and B, respectively.
The intention of this investigation was the assessment of
lubricants with respect to their lubricating effect and their
applicability in the production of parts with long flow paths.
This means in terms of the analysed quantities that the values of the friction coefficient, , the slope of regression line,
˛, and the standard deviation of the mean curve, S.D., should
be small. The claim for a small friction coefficient in the die
cavity is evident in order to achieve die filling, a small slope
of regression line indicates steady state sliding conditions (no
lubrication breakdown, etc.) and a small standard deviation
ensures process stability.
Due to these considerations, lubricant A or B would be
the right choice under the investigated conditions. However,
the surface roughness is much lower with lubricant B what
leads to lower friction coefficients and a smoother workpiece
surface. The poor result of the oil-based lubricant D can be
explained by regarding Fig. 9. It can be clearly seen that the tool
temperature was too low to evaporate the carrying agent. As
a result, the lubricant was squeezed out of the tool–workpiece
interface in the upsetting step, and in the beginning of the
rotation, pick-up occurred. In further sliding, hydrodynamic
lubrication was present and the friction coefficient decreased
rapidly. However, the lubricating effect was not strong enough
to prevent further pick-up completely. It is assumed, that the
oil-containing lubricants C and D would give better results at
higher tool temperatures.
4.
Conclusion
A new approach for the characterisation of lubricants under
aluminium forging conditions has been presented. Compared
with other tests, the demands on the lubricants are obviously
less severe in the presented setup. At the beginning of the
rotation, new lubricant gets into the tool–workpiece interface,
and after a sliding distance of approximately 14 mm, tool
and specimen are separated by a compact lubrication layer
even under high initial surface expansion. Furthermore, each
point of the lubricant layer is loaded just for a short time.
In contrast to other experiments (see e.g. Hansen and Bay,
1986), the new test does not investigate the expansibility or
the chemical stability of lubricants, but it analyses the abra-
sion resistance of lubricant layers. As the lubricant is applied
to the tool in hot die forging and the workpiece is in sliding
contact with the tool surface (usually superimposed by surface
expansion), this behaviour should be of interest in tribological
analyses.
Concerning the main features, the presented setup is similar to the sliding-upsetting test (Groche and Kappes, 2004),
however, the new setup works without an adjustability of surface expansion, because its influence is only present in the
beginning of the test. In contrast to the sliding-upsetting test,
in which the sliding distance is short compared to the upset
area, the new method allows the development of a – also under
tribological aspects – stationary region.
From the test results a clear formation of lubricant groups
was decided (see Fig. 8), which allowed to determine the applicability of the lubricants for the chosen process parameters
and materials. However, an additional investigation of these
influences is advisable because of the interactions of sliding,
surface enlargement and thermo-chemical exposure in the
real process.
Acknowledgements
The authors want to thank the federal province of Styria
(“Zukunftsfonds Steiermark”, Project 19) for financing the
project and Fuchs Schmiermittel GmbH and Acheson Industries for the provision of the lubricants.
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