INTRODUCTION
Nickel based alloys has high strength to weight ratio,
excellent erosion resistance, excellent corrosion resistance. However
nickel based alloys have poor machinability, this may be due to their
high chemical reactivity with most cutting tools and therefore, have a
tendency to weld to the cutting tools during the machining, thus leading
to generation of Build Up Edge (BUE), chipping and premature tool failure.
Its low thermal conductivity increases the temperature at the tool-workpiece
interface, which effects on tool life. Nickel alloy are being milled increasingly
to make critical components for aerospace, medical and chemical processing
industries. These materials are required to have good surface integrity
and geometrical accuracy. It is necessary to investigative machining process
to approach to ideal results and reduce the process time and improve the
surface quality. Current researches focus mostly on the machinability
of nickel alloy (Rahman, 1997; Kura, 1999). In the milling process the
end of tool life is more frequently caused by chipping, cracks and breakage
of the edge, (rather than regular tool wear) than in other machining process,
such as turning and drilling. This occurs because milling is an interrupted
operation, where tool cutting edge enters and exits the workpiece several
times per second. In addition, chip thickness varies as the edge penetrates
the workpiece. Regular tool wear mechanisms will be predominating only
if the tool is tough enough to resist the mechanical and thermal shocks
of the process (Chandrasekaran, 1985). The major cause of tool failure
at high cutting speeds is cracking of thermal origin. This occurs because
the edges are exposed to a high level of thermal shock due to the high
temperatures caused by high speeds and high degree of temperature variation
typical of the process (Bahatia et al., 1978). At low cutting speed
cracks of a mechanical origin are mainly responsible for tool failure
as in his situation cutting forces are higher and temperatures are lower.
Cracks of a mechanical origin may occur due to shocks either at the entrance
of the cutting edge or during the exit of the edge from the workpiece.
Problems due to shocks at the entrance of the cutting edge can be worsened
by the tendency of the chip to adhere to the tool rake face (Kabaldin,
1980). One of the causes of the excessive chipping of the carbide tools
used in milling operations is a phenomenon called (foot forming). When
the tool edge is ready to exit the workpiece, it causes a rotation of
the primary shear plane making its angle negative and instantaneously
increasing the force on the edge. In face milling and machining with tools
of carbide indexable inserts the entrance of the cutting edge into the
workpiece is more critical for the chipping of the cutting edge than the
its exite (Caldeirani Felho, 1998). An in other cutting processes cutting
speed is the most influential cutting condition on tool life followed
by feed and then by depth of cut (Ferraresi, 1972). In this study the
milling operation on a machining center is considered as a more importance
for machining of nickel alloy. The feasibility of using of CNC milling
machine and other facilities like FEA, SEM and roughness tester are assessed
by observing the wear, breakage, cutting temperature and surface roughness
values of the cutting tools.
Experimental Condition
An age hardening nickel alloy 242 as a workpiece material since it
is the most commonly used grade in the aerospace and gas turbine industries
in size of 170x110x20 mm are used. The experiments were carried out in
a very rigid CNC milling machine Okuma MX-45VA with speed ranging from
25 to 100 m min-1 (Fig. 1 ).
The chemical compositions and physical properties are
shown in Table 1 and 2.
|
| Fig. 1: |
Okuma MX-45-VA-CNC milling machine |
| Table 1: |
Chemical composition |
|
| Table 2: |
Physical properties |
|
| Table 3: |
Thickness of coating layers |
|
Up to 1 mm thickness of the top surface of workpiece was removed prior to
actual machining in order to eliminate any surface defects that can adversely
affect machining results. The cutting tools as carbide class, TiAlN, TiCN/TiN,
TiAlN/TiN and TiCN/Al
2O
3 are used. The tool was coated
with multi layers 3 to 7 μm (
Table 3).
The tool had diameter 50 mm. Tool flank wear on tools
was measured by an optical microscope and scanner electronic microscope
SEM, considered a main parameter used in this study to compare performance
of tools therefore flank wear limit value (vB) = 0.3 mm was
selected as reference with respect to ISO 3685, surface roughness (Ra)
values of all passes are recorded and compared. The surface roughness
of the workpiece was measured by a portable R200 instrument. The conducted
operation was a fixed milling passes 90.5 mm lengths with an extra beginning
distance 5 mm for each pass to decrease of tool shock when interfacing
with workpiece. Table 4 shows the cutting parameters,
cutting speed, feed rate, axial depth and radial depth.
Several times the experiments were interrupted in order
to measure the flank wear of the tool and the surface roughness of the
workpiece. FEA ThirdWave AdvantEdge is used to analysis and simulate of
cutting forces and cutting temperatures.
RESULTS AND DISCUSSION
Wear Mechanism and Tool Failure Mode
The development of flank wear curve obtained for four cutting tools
operated when kept cutting speed constant are shown in Fig.
2-4, respectively. It was observed that during the
using of TiAlN, TiCN/TiN, TiAlN/TiN and TiCN/Al2O3 cutting
tool, flank wear was observed an acceptable result with lower cutting
depth of 0.4 mm cutting speed 100 m min-1, were employed. At
high cutting depth 1 mm with cutting speed 100 m min-1. The
tool worn quite rapidly, resulting in a shorter tool life. Generally all
cutting tool worn rapidly at high cutting speed tests and when applied
high cutting depth, cutting tools experienced different modes of failure
throughout the trails. These were non-uniform flank wear, chipping and
catastrophic failure (Fig. 5-8) show
images of the failure modes at various cutting parameters for all cutting
tools employing. Chipping of the cutting edges was common at high cutting
speeds and high cutting depth, perhaps due to when interfacing of cutting
tool with workpiece.
|
| Fig. 2: |
Flank wear curve when machining at speed of 100 m min-1
and feed rate 0.2 mm rev-1 with axial depth 0.4 mm |
|
| Fig. 3: |
Flank wear curve when machining at speed of 100 m min-1
and feed rate 0.3 mm rev-1 with axial depth 0.7 mm |
|
| Fig. 4: |
Flank wear curve when machining at speed of 100 m min-1
and feed rate 0.2 mm rev-1 with axial depth 1.0 mm |
|
| Fig. 5: |
Catastrophic failure (TiAlN) at cutting depth 1 mm with speed
100 m min-1 |
|
| Fig. 6: |
Non uniform flank wear and BUE, (TiAlN), cutting depth 1
mm speed 100 m min-1 |
|
| Fig. 7: |
Uniform flank wear, (TiAlN),cutting depth 0.4 mm speed 100
m min-1 |
|
| Fig. 8: |
Chipping, (TiAlN), cutting depth 1.0 mm speed 100 m min-1 |
|
| Fig. 9: |
Catastrophic failure-TiCN/Al2O3 -cutting
tool |
Tool Life
Results in term of tool life when machining of nickel alloy by using
of TiCN/Al
2O
3 cutting tool are given a clear vision
(
Fig. 9) this tool recorded very short tool lives with
catastrophic failures when carried out of experiments, the maximum tool
life for this tool are recorded at medium and low cutting depth, these results
concern of cutting forces which played as a significant role in shortest
of the tool life for TiCN/Al
2O
3 tools. the results
revealed the outstanding performance of all cutting tools at low and medium
cutting depth and cutting speed. The longest tool life was recorded for
(TiAlN) cutting tool at medium cutting depth when with lower cutting speed.
Surface Roughness
Among several CNC industrial machining processes, milling is a fundamental
machining operation. End milling is the most common metal removal operation
encountered. It is widely used in a variety of manufacturing industries including
the aerospace and automotive sectors, where quality is an important factor in
the production of slots, pocket, precision molds and dies. The quality of the
surface plays a very important role in the performance of milling as a good-quality
milled surface significantly improves fatigue strength, corrosion resistance,
or creep life. Surface roughness also affects several functional attributes
of parts, such as contact causing surface friction, wearing, light reflection,
heat transmission, ability of distributing and holding a lubricant, coating,
or resisting fatigue. Therefore, the desired finish surface is usually specified
and the appropriate processes are selected to reach the required quality (Mike
et al., 1999). The influence of cutting parameters on surface finish
results first group recorded superior results when applied of high cutting speed
with moderate feed rate at lower cutting depth, second group with moderate cutting
depth the results are changed whereas produced butter surface finish with increase
of cutting speed and feed rate, third group recorded butter surface finish when
applied moderate cutting speed with high feed rate. Surface finish results explained
butter surface finish when applied high cutting speed and moderate feed rate
with lower cutting depth (Table 5-7).
Cutting Temperatures
Figure 10, 11 and 12
shows the distribution of temperature on the cutting edges boundaries with various
valued and cutting tools deformations, by using ThirdWave AdvantEdge simulation
software obtained on these figures and results.
| Table 5: |
Surface roughness values under various cutting conditions |
|
| Ra: Raughness, Cu: Cutting depth |
| Table 6: |
Surface roughness values under various cutting conditions |
|
| Ra: Raughness, Cu: Cutting depth |
| Table 7: |
Surface roughness values under various cutting conditions
|
|
| Ra: Raughness, Cu: Cutting depth |
|
| Fig. 10: |
Cutting temperature and tool deformation with cutting depth
0.4 mm |
|
| Fig. 11: |
Cutting temperature and tool deformation with cutting depth
0.7 mm |
|
| Fig. 12: |
Cutting temperature and tool deformation with cutting depth
1 mm |
Observed the influence of cutting depth on cutting temperature in term of tool
deformation and the final result for cutting temperature, whereas when applied
low cutting depth the deformation and temperature are acceptable with low values
these result arise depending on cutting depth, at (Fig. 12)
noticed generation and increasing of temperature and higher deformation when
interfacing of cutting edge with workpiece.
CONCLUSIONS
According to the results obtained in this study, the
following conclusions could be drawn as follows:
| • |
Flank wear modes are noticed as acceptable results at lowest cutting
depth with high cutting speed and moderate feed rate. |
| • |
Highest flank wear results recorded with increasing of cutting depth
and cutting depth. |
| • |
The best tool life obtained with PVD cutting tools at low and moderate
cutting depth. |
| • |
Generally performance of PVD butter than CVD cutting tools. |
| • |
Catastrophic failure mode observed with increasing of cutting speed
and cutting depth. |
| • |
Optimum surface roughness results are recorded with decreasing of
cutting depth. |
| • |
Lowest cutting temperature noticed at low and moderate cutting depth. |
| • |
ool deformation at low value noticed on cutting edge when applied
of lowest cutting depth. |