The use of vegetable oils in the industrial sector is not a new idea. In Ancient
Egypt, vegetable oil and animal fats were used in the construction of monuments
(Nosonovsky, 2007). In the early 16th century, European
sailors made soap from palm oil. In the 19th century, the people of France and
England used palm oil to make candles. They also used palm oil as a gas for
lighting. Before the development of petroleum, palm oil was used to grease the
axle boxes of railway carriages. In the tin plate industry, palm oil was initially
used to prevent the oxidation of iron and as a flux before tinning (Henderson
and Osborne, 2000).
In Malaysia, palm oil shows great potential for production as a lubricant.
Palm oil is environmentally friendly and has a high biodegradability in comparison
to mineral oil. Palm oil is also the most efficient oil-bearing crop in terms
of land utilization and productivity. One hectare of palm oil could produce
almost ten times more oil than other oilseeds, such as soybean and sunflower.
Palm oil could produce an average of 3.74 tons of oil per hectare every year,
compared to 0.38 tons and 0.48 tons of oil per hectare per year for soybean
and sunflower, respectively (Ming and Chandramohan, 2002).
For these reasons, palm oil could fulfil the demand for a vegetable-based lubricating
oil. Studies on the feasibility of the use of palm oil in diesel engines (Bari
et al., 2002) and as a hydraulic fluid (Wan Nik
et al., 2000) showed satisfactory results in comparison to petroleum-based
Plastic flow of metal in the bulk metal forming process is influenced by many
factors, such as the lubricants condition, friction, die shape and billet
temperature (Li et al., 2008). From the point
of view of lubrication, the plastic flow of a metal in the bulk metal forming
process could be used to predict the lubricants performance and the friction
constraint behavior between the tool and work piece (Kamitani
et al., 2008).
In this study, the performance of RBD palm olein as a lubricant in the cold-work plane strain extrusion process was investigated. The evaluations focused on the forming load, surface roughness and plasticity behavior. The results show that the work piece that was extruded with RBD palm olein as a lubricant has a low extrusion force and better plasticity behavior than those extruded with additive-free paraffinic mineral oil.
In a previous study by Syahrullail et al. (2005)
the plane strain extrusion apparatus consisted of a taper die, billet and plane
plate tool; the billet was not symmetrical. In the current investigation, a
symmetrical billet and a pair of taper dies were used. The symmetrical billet
performs similarly to the design of the tool in the actual extrusion process
in the industrial sector. The reduced extrusion force in the present investigation
is obvious when compared to the previous investigation and gives a good impression
of the ability of palm oil when it is used as a metal forming lubricant.
MATERIALS AND METHODS
Experimental apparatus: Figure 1a shows a schematic sketch of the plane strain extrusion apparatus that was used in the experiments. The main components are the container wall, taper die and work piece (billet). The taper die has a 45-degree die half angle. The taper die is made from tool steel SKD11 and the necessary heat treatments were done before the experiments. The experimental surface of the taper die (surface that contacts the billet) was polished with an abrasive paper and has a surface roughness, Ra, of approximately 0.15 μm. A specified amount of lubricant was applied to this surface before the experiments. The same type of test lubricant was applied to the other surfaces of the experimental apparatus. The taper die has a Vickers hardness of 650 Hv.
Figure 1b shows a schematic sketch of the billets that were used in the experiments. The billet material is pure aluminium, A1100. The billet shape was produced using an NC wire cut electric discharge machining device. Two similar billets were stacked and used as one billet unit. One side of the contact surface of the combined billets was the plastic flow observation plane during plane strain extrusion. The observation plane is not affected by the frictional constraint of the parallel side walls. A squre grid pattern that measures the material flow during the extrusion process was scribed by the NC milling machine onto the observation plane of the billet. The grid lines were V-shaped grooves with a 0.5 mm deep, 0.2 mm wide and 1.0 mm interval length. The billets were annealed before the experiments. The experimental surface of the billet (surface that contacts the taper die) has a surface roughness of approximately 2.5 μm. The Vickers hardness of the billet is 38 Hv.
Testing lubricants: The testing lubricant was RBD palm olein (marked
as PO). The RBD is an abbreviation for Refined, Bleached and Deodorized. Palm
olein is the liquid fraction that is obtained by the fractionation of palm oil
after crystallization at a controlled temperature. In these experiments, a standard
grade of RBD palm olein, which was incorporated in the Malaysian Standard MS
816:1991, was used (Pantzaris, 2000). The results obtained
from experiments using RBD palm olein were compared with those from experiments
using additive-free paraffinic mineral oil (VG30) (marked as PF).
||(a) A schematic sketch of the plane strain extrusion apparatus
and (b) the combination of the billets
||Viscosity of RBD palm olein and the paraffinic mineral oil
properties for RBD palm olein and the additive-free paraffinic mineral are shown in Table
Experimental procedure: Lubricants were applied onto the experimental surface of the taper die. The amount of lubricant was approximately 25 mg. The mass measurement was done with a digital balance that had a tolerance of 0.1 mg. The billets were cleaned with acetone. After that, the taper die and the billet were assembled as shown in Fig. 1. The plane strain extrusion apparatus was assembled and placed onto the hydraulic press machine. The load cell and displacement sensor were used to record the extrusion force and the ram displacement; the data was saved in a computer. The experiments were carried out at room temperature. The extrusion was stopped at a piston stroke of 35 mm, where the extrusion process is in a steady-state condition. The ram speed is constant at 0.85 mm sec-1. After the experiment, the partially-extruded billets were taken out from the plane strain extrusion apparatus and the combined billets were separated for the surface roughness measurement and the plasticity analysis.
Figure 2 shows the pattern of grid lines on the observation
plane of the billet after the experiment. Horizontal grid lines (straight line
parallel to the extrusion direction) on the observation plane of the billet
show the plastic flow lines that appeared during the steady-state extrusion
process after the experiment.
||Picture of the grid line pattern on the observation plane
of the billet after the experiment
||Coordinate system that was used in the analysis
The horizontal lines were traced with tracing
software, according to the coordinate system that is shown in Fig.
3. After tracing, the digital data were prepared as raw data input for the visioplasticity
Figure 3 shows a schematic diagram of the x-y orthogonal coordinates system that was used to analyze the deformation zone; the plastic flow lines that appeared during the steady-state extrusion condition were used. Figure 3 also shows some of the variables that were used in the analysis and calculations and the position that was established in the same coordinates system in the observation plane of the billet.
The plastic flow velocity in the deformation zone, the effective strain rate
and the effective strain were calculated using Eq. 1-5.
Since, the analytical calculation procedure is explained in an earlier publication,
it is omitted here (Syahrullail et al., 2005).
Velocity component (velocity in the x-direction, u; velocity in the y-direction, v)
Strain rate component (sec-1):
Effective strain rate (sec-1):
Effective strain (time integration value of the effective strain rate along the flow line):
In the equations, Vo is the velocity of the press ram in mm sec-1 and Xi is the distance in mm from the y-coordinate axis (X = 0) of the i-th flow line in the region where deformation does not occur.
RESULTS AND DISCUSSION
Extrusion force: Figure 4 shows the extrusion force
with the piston stroke curves. Figure 4 shows that the extrusion
force reached a constant level during the process and that the extrusion process
became a steady-state condition at a piston stroke of more than 30 mm. The extrusion
force difference at steady-state conditions (at a piston stroke of 35 mm is
about 6 kN. A comparison of the extrusion forces in the presence of these lubricants
shows that the extrusion force was lower for RBD palm olein, as compared to
the paraffinic mineral oil (VG30). This is because the fatty acids in the palm
oil reduced the frictional constraint between the tool and billet surfaces.
Previous research also shows that the palm oil has a low friction coefficient
(Abdulquadir and Adeyemi, 2008).
Surface roughness: The values of the arithmetic mean surface roughness,
Ra, along the experimental surface of the billet were measured with a Mitutoyo
Formtracer profilometer device. The measured direction is perpendicular to the
||Extrusion force-piston stroke curves
||Surface roughness (Ra) of the experimental surface of the
The experimental surface of the billet is the surface of
the billet that contacts the taper die and the container. The experimental surface of the billet is labelled as the X-axis. The distribution
of the arithmetic mean surface roughness, Ra, is shown in Fig.
5. As a result, the surface roughness for the product area of the billet
that was extruded with RBD palm olein is smaller as compared to those that were
extruded with paraffinic mineral oil (VG30). The experimental surface of the
billet at the location X = -8 mm is connected to the taper die surface. When
the experimental surface of the billet slides on this area, the wedge effect
starts to occur and creates a thin layer of lubricant (Seiji,
2002). The condition between the material and the tool constituted mixed
lubrication by a thin lubricant film (boundary lubrication); the adsorption
of fatty acids from the palm oil played the role of maintaining the thin lubricant
(Bowden and Tabor, 2001).
||CCD pictures of the product surface condition at the location
X = 4 mm
This would reduce the ratio
of metal-to-metal contact between the tool and billet surface for RBD palm olein
(as compared to the paraffinic mineral oil (VG30)) and reduce the extrusion
force. Since, the lubricant thickness is very thin, the product will reflect the
surface roughness of the tool and make the billets surface product of
RBD palm olein smaller in comparison to the billet that was extruded with paraffinic
mineral oil (VG30).
Figure 6a and b show CCD pictures of the
product surface condition at the location X = 4 mm with 85 times magnification.
From the observed surface of the experimental billet, we can confirm that there
is no severe wear. Both of the lubricants show satisfactory lubrication performances.
In application, both of the lubricants could be used; however, RBD palm olein
shows a reduction in the forming load, which could save the energy consumption.
Velocity at the experimental surface of the billet: From the digital
tracing data, the velocity component of the billet that slides on the taper
dies surface was calculated using the visioplasticity method, see Eq.
2. A comparison of the v- (horizontal) and u-components (vertical) of the
plastic flow velocity along the experimental surface of the billet (the velocity
condition of the billet sliding on the taper dies surface) are shown in
Fig. 7 and 8, respectively. The results
show that the velocity distribution in the billet that was extruded with RBD
palm olein is clearly different in comparison to those that were extruded with
the paraffinic mineral oil (VG30).
||Distribution of the v-component (horizontal) of the velocity
along the experimental surface of the billet
||Distribution of the u-component (vertical) of the velocity
along the experimental surface of the billet
The reduced extrusion force of RBD palm olein shows a reduced friction between
the taper die and the billets. Due to the low value of RBD palm olein friction
coefficient and the RBD palm oleins capability to stick very well onto
the taper dies surface, which makes the metal-to-metal contact ratio decrease,
the sliding velocity on the taper die of the billet that was extruded with RBD
palm olein is higher in comparison to those that were extruded with the paraffinic
mineral oil (VG30).
||Mutual comparisons of the horizontal flowlines in the deformation
Flow lines observation: From the digital tracing data, the flow lines (horizontal grid lines) of the billet, which was extruded with RBD palm olein, were compared with the billet that was extruded with the paraffinic mineral oil. For a better comparison of the results, the digital tracing data were recalculated and repositioned at a constant distance of Xi (Fig. 3) for both of the lubrication conditions. Figure 9 shows a mutual comparison of the horizontal grid lines. A comparison of the flow lines of the billets show the plasticity flow behavior of the billet, while it was extruded through the taper die at a steady-state condition during the extrusion process. For the billet that was extruded with RBD palm olein, due to the low friction condition between the tool (taper die) and billet, the flow lines (plasticity flow) are slightly influenced in comparison to those that were extruded with the paraffinic mineral oil (VG30).
The plastic flow in pure aluminium was evaluated with RBD palm olein as a lubricant in a plane strain extrusion apparatus. The results of the experiment and the analysis show that RBD palm olein could reduce the extrusion force, lower the value of the surface roughness, Ra and increase the sliding velocity in comparison to the paraffinic mineral oil (VG30). From the results, we confirm that RBD palm olein showed a satisfactory lubrication performance in comparison to the paraffinic mineral oil (VG30).
The authors would like to thank the Mechanical Engineering Faculty at the Universiti Teknologi Malaysia for their cooperation during the preparation of this study. The authors also wish to thank the Universiti Teknologi Malaysia for financial support through the grant vote 77024.