Influence of the Milling Cutter Diameter on the Cutting tool Life when Machining Inconel 718.
Zetek, Miroslav ; Vozar, Vojtech ; Baksa, Tomas 等
Influence of the Milling Cutter Diameter on the Cutting tool Life when Machining Inconel 718.
1. Introduction
Inconel 718 belongs to a group of difficult-to-machine materials.
It offers excellent strength at low and high temperatures, corrosion
resistance under demanding conditions and low thermal diffusivity.
Therefore, it found use in components that operate under extreme loads
in heavy-duty applications. Their examples include parts of combustion
turbines, thin-walled aircraft components and parts of space shuttles.
Its use in production has called for operation sequences which deliver
optimum machining performance. Today, conventional machining of Inconel
718 poses a major difficulty in terms of the process economy. The
motivation for this study arose from a search for appropriate machining
parameters and from efforts to improve machining productivity of this
material. For this purpose, information from worldwide databases was
gathered. One of the key factors which govern the economy of cutting
nickel alloys is the short durability of cutting tools. Numerous studies
have been devoted to this aspect. One of them explored the effects of
cutting conditions on tool durability. This study found that the
dominant type of wear, regardless of cutting conditions, is the built-up
edge which eventually leads to chipping of the cutting part of tool.
Furthermore, if an inadequate thin film is chosen, the intensity of
abrasive wear increases very rapidly. [1] This was confirmed by another
study. Its authors were dealing with thin film optimization--which can
improve overall durability by several per cent. [2] Cutting conditions
affect the type and progress of wear very strongly. This issue was
explored by authors of [3]. They monitored changes in the wear type
depending on cutting speed, feed and cutting depth. They also correlated
these aspects with the machined surface quality and monitored the impact
of cutting conditions on changes in the material. It is clear that the
greatest impact on durability results from changes in the cutting speed,
whereas the quality of machined surface is dictated predominantly by
changes in feed. Surface integrity in machining of Inconel 718 is the
topic of additional studies. One of them [4] mapped defects in the
machined surface in relation to the occurrence of wear and built-up
edge. Similar results were reported by authors of [5] who correlated
them with cryogenic machining. They found that better outcome is
obtained with standard cooling. Another study describes industrial
machining of turbine blades of this material. Its authors dealt with
optimizing the production process while taking into account the
occurrence of wear and maintaining the quality of machined surface and
its accuracy. [6] They found that the profile shape affected the
resultant roughness and accuracy. They used the findings in another
publication [7] in which they added investigation of the influence of
wear on roughness at various points along the turbine blade profile
during machining with a ball mill. The dominant factor in the resultant
quality of surface was the location of engagement of the ball mill along
the blade profile. It was confirmed that roughness becomes less
favourable with decreasing engagement radius, and therefore with
decreasing cutting speed. The similar results gave authors which were
focused on the turning with the different cutting tool radius [8]. One
of the many available options for productive machining of Inconel 718 is
High Speed Machining (HSC). Yet, the cutting part wear is a major factor
in this process as well. This was the subject of the detailed study [9]
which monitored the amount of wear of a cemented carbide cutting part
vs. the cutting speed used. The results suggested that the amount of
wear increased with cutting speed. The material, which is very
appropriate for HSC machining of Inconel 718, is cubic boron nitride
(PCB). The study [10] found that in this case, too, wear is governed by
the cutting speed but also by the microstructure of the work and by the
exact chemical composition of the cutting part's material. A very
novel method of controlling the durability of CBN tools involves
creating a defined surface texture on the cutting part of the tool. This
is the focus of the study [11] which describes the texture shapes,
cutting conditions and the benefits of this approach. Since Inconel 718
is difficult to machine, there are other studies which seek productive
cutting and machining methods for this material. Authors of [12]
explored the economy and environmental impacts of various waterjet
cutting processes. The last article [13] deal with possibilities of
evaluation oscillation absorption and damping factor is increased due to
the large dissipative forces resulting from the resistance of the tool
holder material that has texture with different metal deformation
conditioned orientations. Clearly, this field offers many options for
optimizing the machining process--and any improvement in tool durability
counts.
On the base of these studies, the article will be deal with
influence of the milling cutter diameter on the cutting tool life when
machining Inconel 718. The main aim is to describe how the different
cutting tool diameter influence the tool life, which is depending on
side and rear plane and connected angels. For the evaluation, the
different engaging conditions were used with the constant machining
conditions.
2. Experiment
The objective of this experiment was to identify the relationship
between the diameter and durability of a milling cutter. The diameters
under test included 50, 100 and 125 (121) mm. The limit flank wear was
set at 0.15 mm. During analysis of the static geometry and conversion
into the kinematic geometry, it was found that the geometric set-up of
these tools would differ. They were therefore adjusted to ensure that
the resulting kinematic geometries of engagement were identical for all
tool variants. Circular cemented-carbide exchangeable cutting inserts
(below referred to as VBD) were used which had a thin film which had
been specially developed for Inconel 718. No defects in this film were
found upon a thorough inspection. All experimental runs were carried out
at constant cutting speed [v.sub.c]=40m/min, feed per tooth
[f.sub.z]=0.2mm and depth of cut [a.sub.p]=2 mm. A constant width of
engagement was chosen for the first variant, [a.sub.e]=30% D of the
milling cutter, as recommended by the manufacturer.
In this case, the other parameters were set as follows:
Only a single VBD was employed in the experiment in each case,
which can lead to lower durability of the tool due to instability which
occurs as a single VBD enters the work. To verify the findings, the
experiment will be repeated with a fully-fitted milling head.
The entry into the workpiece was modified by pre-milling the work
at 30% D for the tool diameter. Therefore, the VBD was not affected by
the entry and exit variations, as illustrated in Fig. 2.
It is important to check the material prior to testing: the
chemical composition, microstructure, hardness and other mechanical
properties if relevant. With regard to machinability and the effects on
tool durability, the important aspects include the chemical composition,
heat treatment history and the resultant microstructure and hardness.
To check the homogeneity of microstructure of the workpiece,
samples were taken from its surface, from a location 20 mm below the
surface and from the workpiece centre.
The images show that the grain shapes and sizes depend on the
location within the workpiece. On the other hand, this is a common
occurrence in the industrial practice. Therefore, no steps were taken to
modify the microstructure. Instead, hardness was measured in these
locations. Its values were in the range of 34-39 HRC, depending on the
location.
Machining was carried out in a 5-axis milling centre. The workpiece
was clamped in a hydraulic vice. In order to ensure that identical
material (microstructure) is removed by tools of each diameter, they
were all applied to a single layer of the workpiece in each case. The
effect of the microstructure variation was thus eliminated.
The results are plotted in Graph 1.
The graph indicates that durability increases with the tool
diameter. Constant [a.sub.e] means that the arc of engagement is the
same for all tools, although the width of engagement increases with the
tool diameter. Hence, the smaller is the tool diameter, the more entries
the tool must complete, which means that it sustains more impacts. Upon
conversion per diameter basis, the following results were obtained:
One can therefore assume that with the number of impacts on the
workpiece, the tool wear will increase more rapidly, reaching the limit
value of 0.15 mm much earlier, as reflected in the durability.
For further comparison and verification, an experiment was carried
out under identical cutting conditions on a workpiece which had not been
pre-milled.
Durability was higher than with the pre-milled workpiece but it no
longer increased with the tool diameter. This set- up will therefore be
analysed further because the results represent mean values from repeated
runs.
The last experiment involved a comparison with a milling head with
all VBDs fitted, under identical conditions. While the tools with 100 mm
and 121 mm diameters had the same numbers of cutting inserts (VBDs) and
an identical arc of engagement, the stability condition was fulfilled,
i.e. more than one VBD being engaged at a time. For these reasons, only
these two diameters were used for the comparison.
The graphs clearly show that the use of a single VBD in the
experiment affects the actual durability of the tool. It is mainly due
to increased instability, as the tool suffers stronger vibration than a
tool on which more than 1 VBD is engaged and which remains stable
throughout the cutting operation. On the other hand, this has no impact
on the plausibility of the results. Machining with a single VBD led to
uniform abrasive wear, same as with the milling head with all positions
occupied. No significant differences were found. The wear profile is
shown in Fig. 5.
3. Conclusion
The results show the effects of the tool diameter and cutting
conditions on tool durability. While the basic principle for defining
cutting conditions was observed, i.e. a constant width of engagement, it
was found that durability increases with the tool diameter. This may be
of major importance in selecting the machining strategy and while
considering the diameter for a particular size of machined surface. With
Inconel 718, it is appropriate to use multiple tools of different
diameters, instead of choosing a single diameter for removing material
without the need for tool replacement. In many cases, this variant may
prove very costly due to lower durability, and therefore the need for
more frequent replacement of VBDs. These experiments also showed the
effects of the stability of the cutting process on tool durability. When
a single VBD is used, durability decreases. This means that in the
choice of engagement conditions made in the industrial practice, it is
important to ensure that more than a single point of the tool is engaged
at a time. This may prove difficult or even unfeasible to meet with
tools of small diameters. The above analyses provide underlying
knowledge for future investigations which will focus on various
engagement conditions.
DOI: 10.2507/28th.daaam.proceedings.058
4. Acknowledgments
This paper was supported by the programme of applied research,
experimental development and innovation GAMA, No. TG02010011--Support of
UWB commercial opportunities.
5. References
[1] H. Abdul, J. A. Ghani, H. Che, and M. S. Kasim. (2016). Effect
of cutting speed on the carbide cutting tool in milling Inconel 718
alloy, J. Mater. Res., Vol. 31, No. 13, pp. 1885-1892, DOI:
10.1557/jmr.2015.380.
[2] M. Sortino, S. Belfio, G. Totis, E. Kuljanic, and G. Fadelli.
(2015). Innovative tool coatings for increasing tool life in milling
Nickel-coated Nickel-Silver alloy, Energy Procedia, Vol. 100, pp.
946-952. DOI: 10.1016/j.proeng.2015.01.453
[3] M. A. Xavior, M. Manohar, P. Jeyapandiarajan, and P. M.
Madhukar. (2017). Tool Wear Assessment During Machining of Inconel 718,
Procedia Eng., Vol. 174, pp. 1000-1008, DOI:
10.1016/j.proeng.2017.01.252.
[4] C. Liu, C. Ren, G. Wang, Y. Yang, and L. Zhang. (2015). Study
on surface defects in milling Inconel 718 super alloy, J. Mech. Sci.
Technol., Vol. 29, No. 4, pp. 1723-1730, DOI: 10.1007/s12206-015-0345-1.
[5] A. Iturbe, E. Hormaetxe, A. Garay, and P. J. Arrazola. (2016).
Surface Integrity Analysis when Machining Inconel 718 with Conventional
and Cryogenic Cooling, Procedia CIRP, Vol. 45, pp. 67-70, DOI:
10.1016/j.procir.2016.02.095.
[6] J. Kyncl, L. Beranek, K. Kolarik, and Z. Pala. (2014). The
research of the surface profile after profiling of inconel 738LC,
Procedia Engineering, Vol. 69, pp. 974-979. DOI:
0.1016/j.proeng.2014.03.078.
[7] J. Kyncl and A. Molotovnik. (2015). The research of the surface
profile after profiling of superalloys, Energy Procedia, Vol. 100, pp.
853-860. DOI: 10.1016/j.proeng.2015.01.441.
[8] N. Qehaja, K. Jakupi, A. Bunjaku, M. Brugi, a H. Osmani.
(2015). Effect of Machining Parameters and Machining Time on Surface
Roughness in Dry Turning Process, Procedia Eng., roc. 100, s. 135-140,
DOI: 10.1016/j.proeng.2015.01.351
[9] D. M. D'Addona, S. J. Raykar, and M. M. Narke. (2017).
High Speed Machining of Inconel 718: Tool Wear and Surface Roughness
Analysis, Procedia CIRP, Vol. 62, pp. 269-274, DOI:
10.1016/j.procir.2017.03.004.
[10] H. Tanaka, T. Sugihara, and T. Enomoto. (2016). High Speed
Machining of Inconel 718 Focusing on Wear Behaviors of PCBN Cutting
Tool, Procedia CIRP, Vol. 46, pp. 545-548, DOI:
10.1016/j.procir.2016.03.120.
[11] T. Sugihara, H. Tanaka, and T. Enomoto. (2017). Development of
Novel CBN Cutting Tool for High Speed Machining of Inconel 718 Focusing
on Coolant Behaviors, Procedia Manuf., Vol. 10, pp. 436-442, DOI:
10.1016/j.promfg.2017.07.021.
[12] G. S. Kadam and R. S. Pawade. (2017). Surface integrity and
sustainability assessment in high-speed machining of Inconel 718--An
eco-friendly green approach, J. Clean. Prod., Vol. 147, pp. 273-283,
DOI: 10.1016/j.jclepro.2017.01.104.
[13] J. Olt a V. V. Maksarov. (2015). Development of
chatter-resistant system of cutting tool, Annals of DAAAM and
Proceedings of the International DAAAM Symposium, 2015 p. 223-226. DOI:
10.2507/26th.daaam.proceedings.031
Caption: Fig. 1. Visualization of engagement conditions
Caption: Fig. 2. Visualization of the pre-milled workpiece
Caption: Fig. 4. Microstructure of the Inconel 718 workpiece (from
left: surface, 20 mm below the surface, centre)
Caption: Fig. 5. Visualization of wear (left: 3D-scanned image of
wear; right: 2D optical micrograph)
Caption: Graph 3. Effects of stability on tool durability in
cutting with a single and multiple VBDs
Table 1. Cutting conditions at constant ae
D = 50 mm D = 100 mm D = 121.76
vc [m/min] 40 40 40
vf [m/min] 51 25 21
ap [mm] 2 2 2
ae [mm] 15 30 36.53
fz [mm] 0.2 0.2 0.2
n [-] 255 127 105
Arc of engagement
[PSI] [[degrees]] 66.42 66.42 66.42
Q [cm3/min] 1.53 1.53 1.53
Table 2. Number of impacts of tool on the workpiece at the
end of durability
[??]50 mm [??]100 mm [??]121.76 mm
Alternative No. 2 2700 2375 2075
Graph 1. Tool durability vs. tool diameter in machining
a pre-milled workpiece
ae=30%D
Cutting tool diameter [??]D Cutting tool life T [min]
[??]50mm 10,6
[??]100mm 14,92
[??]121,76mm 19,84
Note: Table made from bar graph.
Graph 2. Tool durability vs. tool diameter in
machining a non-pre-milled workpiece
ae=30%D--without milling surface
Cutting tool diameter [mm] Cutting tool life T(min]
[??]50mm 11,31
[??]100mm 16,96
[??]121,76mm 13,77
Note: Table made from bar graph.
Fig. 3. Chemical composition of workpiece
C Mn Si P S Ca Cr
0,017 0,10 0,07 0,008 0,0005 <0,0003 18,23
Al Cu Mo Ni N Ti Fe Nb To
0,51 0,04 2,98 53,3 0,006 0,98 18,22 5,09 0,31
B Pb Ta O Other
0,0024 <0,0003 <0,020 <0,00003 Mg = 0,0010; Sn = 0,0011;
Se = <0,0003
COPYRIGHT 2018 DAAAM International Vienna
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2018 Gale, Cengage Learning. All rights reserved.