- Email: [email protected]
A parametric study of improving tool life by electrospark deposition
Wear, 138 (1990)
137 - 151
137
A PARAMETRIC STUDY OF IMPROVING TOOL LIFE BY ELECTROSPARK DEPOSITION* E. ALLEN BROWN Hewlett Packard Corporation,
Corvallis, OR 97330
(U.S.A.)
GARY L. SHELDON Department of Mechanical and Materials Engineering, Vancouver, WA 99663 (U.S.A.)
Washington State University,
ABDEL E. BAYOUMI Department of Mechanical and Materials Engineering, Pullman, WA 99164-2920 (U.S.A.)
Washington State University,
Summary Electrospark alloying (ESA) is a pulsed microwelding surfacing process for alloying a hard wear resistant electrode with a metallic substrate. This process was evaluated as a means of increasing metal cutting tool life in machining operations. A number of ESA electrode material parameters were first screened with a crossed cylinder wear testing machine (ASTM G-83). Full hard high speed steel (HSS) cylinders were run against an ESA-coated ‘stationary HSS cylinder. Three superior coatings from these tests were then applied to the flank and face surfaces of high speed steel tools. Machining test results showed that the dominant wear mode of crater wear was significantly reduced by ESA coating. ESA coating of the tool face only was shown to increase wear resistance significantly more than coating both face and flank surfaces. In addition, programming the electrode motion from side to side rather than front to back over the tool face increased tool life. Comparisons are made with uncoated HSS and TiN-coated (PVD technique) HSS tools. The TiN-coated tool has a similar tool life to failure. The best ESA coated tool had a life better than the HSS tool by roughly 2000%.
1. Introduction 1.1. The metal cuttingprocess Even with the development of carbide and other advanced cutting tool materials, HSS still comprises roughly 50% of all the cutting tools used by the machine tool industry. Some of the advantages of high speed steel (HSS) *Paper presented at the International CO, U.S.A., April 8 - 14, 1989. 0043-1648/90/$3.50
Conference
on Wear of Materials, Denver,
@ Elsevier Sequoia/Printed
in The Netherlands
138
over the other commonly used cutting tool materials are: toughness, ease of sharpening and lower cost. The major drawback of HSS tools compared with tools composed of materials such as Tic, WC, TIN, etc., is a lower hot hardness. This means that HSS has to be used at lower cutting speeds in order to keep the tool temperature below the softening point. HSS is also softer than the other materials mentioned and less chemically stable than some of these materials. The metal cutting process subjects the tool to an extreme environment with both high temperatures and severe strain. The cutting process is extremely complex and there is little agreement as to the exact nature of the interaction that takes place between tool and workpiece. Under normal conditions, a cutting tool must satisfy three basic requirements. It must have a wear resistant surface, be of a fracture resistant material and resist gross plastic deformation [ 11. HSS, a standard cutting tool material, excels in certain situations because of its toughness. When compared with other common cutting tool materials, HSS is the toughest (only carbon steel is superior). Tool wear is such a complex and varied process that it cannot be described by a single simple mechanism. The locations and extent of wear are different, changing with tool material, operation, cutting condition, workpiece material and so forth. Different areas of the same tool may involve a different wear mechanism because the temperature, sliding velocity and stress are different [2, 31. The two variables having the major effect on the wear rate of the tool are the temperature and the normal pressure on the phase of the tool [4]. Understanding the metal cutting process is simplified by reducing the situation to two dimensions. This can be simulated by orthogonal cutting which involves a two surfaced tool with the cutting edge moving perpendicular to the cutting direction. The tool surface which is in contact with the chip is called the tool face (rake surface, rake face). As the tool removes the chip a new work piece surface is exposed to the lower surface of the tool. This lower tool surface is called the tool flank (relief or clearance surface, flank face). The power required at the tool is the product of the cutting force and velocity. With an orthogonal cutting tool the flank is the tool surface next to the work piece. This means that the rubbing that takes place on the flank will be between the tool flank and the newly machined surface. A term used to refer to the wear on the flank surface is wear land. Crater wear takes place on the tool face and is caused by the chip flowing along the tool face. The depth, width or tool edge to crater edge are all ways of measuring the amount of crater wear that has taken place. Temperature, pressure, normal force and real contact area all are more severe on the face than on the flank [5]. It is accepted that because of the severe conditions on the tool face, the apparent and real contact areas in the crater are almost equal [2]. As the crater increases, the normal stress shifts further back in the crater area. When the crater is first forming the maximum wear takes place at the point of maximum temperature. After the crater is formed the temperature
139
is not the sole controlling factor; the wear rate is highest at the back of the crater and the temperature is at its peak ahead of this position [ 51.
1.2. The ESA coating process and past results on cutting tools The coating of cutting tools started in the mid-1960s, although it may have been earlier in the U.S.S.R. The reasons for coating the cutting tool in a production situation are three-fold, i.e. to increase the life of the tool, to improve the surface quality of the product and to increase the production rate. Today there are coated carbide and HSS tools; some multilayered coatings have recently come on the market. When a tool is coated the subsurface material retains its desirable strength characteristics while the coating can act as a buffer zone to surface interactions. The tool wear process is predominantly a surface phenomenon and thus is well suited to the use of coatings. Coating selection can be matched to the particular type of wear that is dominant in a given situation. Coatings have also been shown to reduce friction, and increase abrasion and adhesion resistance [6]. In tests it was found that the improved wear characteristics last even after the coating has worn through [ 51. Many types of coatings for cutting tools have been used. The most widely used commercial coatings materials are Tic, TiN and A1,03, others are TaC, WC and TiB2. These materials all have good hot hardness and chemical stability to varying degrees, with the best coating being dictated by the wear conditions. The selection of the best coating for a given situation is complex. The prevalent commercial coating of HSS cutting tools is accomplished by physical vapor deposition (PVD) [7]. One other coating technique which has shown promise is electrospark alloying (ESA) but little work has been done to evaluate its performance. ESA is a coating process that could be described as pulsed microwelding. The process uses a short duration high current electrical pulse to alloy an electrode material with the substrate. The alloy that is formed on the tool is not simply a surface coating but is fused into the subsurface, and alloying between the surface deposited and substrate occurs. One major advantage of ESA is the ability to use any electrical conductor as the electrode, thus a variety of surface types can be formed [8]. The specific ESA coating process represented in this paper was developed by one of us (G.L.S.) at Washington State University (WSU) and involves a moving electrode, a power source that can be repetitively pulsed, and a means of causing relative motion between the workpiece and electrode. There is no standard power or frequency for ESA coatings so it is best if the frequency, voltage and pulse energy can all be varied depending on the application. The electrode is in light continuous contact with the substrate and continuous motion of the electrode is necessary to prevent welding. It is sometimes desirable for a gas atmosphere to be supplied around the spark area. A change in any one of these parameters can affect the quality of the coating [ 9, lo].
140
Although the spark is of short duration it is enough to vaporize a portion of the electrode. Globules of the electrode are accelerated through the plasma that has been formed by the spark and, along with some vapor, impacted on the tool surface. The short duration of the spark allows the small liquid pools formed of electrode mixed with tool material to solidify rapidly. The time and energy involved are both small enough to cause minimal change in the substrate material [ 91. Even though the ESA process has been around since the late 1950s little testing has been done to compare some of the various coatings possible by ESA with other commercially available coatings to reduce tool wear of HSS. Kahlon et al. [lo] tested HSS tools with a WC coating and the results showed an increase of 400% in tool life over uncoated tools. Another interesting result from their tests was a slight decrease in the cutting force with distance cut until the tool started to fail. This trend in the cutting force was not changed by coating the tool. In a different set of tests done by Shemegon [ll], HSS drills showed a 30 - 100% increase in the tool life with an ESA coating on the drill flank. In another series of tests machining steel (BHN 190), Vaidyanathan [12] was able to increase tool life by a factor of 4 using very large capacitors for energy storage and a vibrator-type applicator device. 2. Experimental
work
2.1. Screening procedure ESA coatings do not lend themselves well to any of the various wear prediction techniques because the electrode material is not applied just on the surface but actually alloys with the subsurface; the chemical composition of an ESA coating is a mixture of electrode and base material varying with depth [8]. Thus ESA coatings are not simple compounds, and a surface analysis will not give a representation of the wear characteristics as the coating wears through. To overcome this situation a simple, accurate and inexpensive screening technique was needed. The technique that was selected is the crossed cylinder test using cylinders of diameter 0.5 in as detailed in ASTM Standard G-83. The objection to this simple screening test is that a freshly cut surface is not continuously exposed to the stationary coated cylinder. Olsson et al. [13] have overcome this objection with modified cross-cylinder equipment, but this modification was not used in this work. The rotating cylinder represents the workpiece and should ideally be the same as the workpiece, namely 41L40 steel. Using this material it was found that very random and inconsistent wear scars resulted which were not suitable for coating evaluation. The decision was made to use HSS (M2 grade) as the rotating cylinder. The final test conditions were: the rotating cylinder HSS (R, = 61 - 63) cylinder representing the workpiece, the stationary cylinder representing the cutting tool, ESA coated HSS-MB (R, * 61 - 63). A normal load of 44.5 N (10 lbf) and a rotation rate of 100 rev min-’ were used.
141
In order to select the best coating material and other coating conditions a preliminary screening was done with the crossed cylinder wear test using electrode materials listed in Table 1. The possible tool coating materials were tested when applied with different pulse energy combinations and a number of coating layers. Total pulse energy is a function of capacitor, voltage and repetition rate settings; a base point for this parameter was taken from values used in previous ESA coating tests done at WSU. Electrode A had shown good wear resistance in these tests and it was chosen as the coating material to use in the first few tests. The wear scar area on the coated stationary samples was evaluated relative to the revolutions for several different coatings. As the testing progressed the coatings parameters were adjusted to optimize coating conditions for minimum wear. The first variable to screen was the cover gas. An atmosphere of CO2 showed slightly better results over argon and N2 and, thus, was used in all of the future ESA coatings. Next the voltage, capacitance and number of coating layers were evaluated. With a base of data established to evaluate the variables of voltage, capacitance and the number of coats, different coating materials could be tested. These ESA coating parameters were then adjusted until the crossed cylinder wear test results showed minimum wear. From the screening test results, the best three ESA coatings were selected to apply to the cutting tools. Two control samples were also employed to compare the ESA coating wear resistance. The lower control was uncoated HSS and the upper control was TiN PVD. Both were used as rotating specimens against the same stationary steel specimen. The TiNcoated HSS cylinders and cutting tools were graciously supplied by Vermont Tool and Die Co. 2.2. Cutting tests In all of the cutting tests the same selected conditions of speed, feed and tool geometry were used (Table 2). No lubrication was used in any test. The best three ESA coatings selected were applied to ground then polished HSS cutting tools from a set of prepared tools. The remaining tools from the set were used as upper or lower control samples and either used for commercial TiN coating by PVD or uncoated. A typical tool and ESA coating process is shown in Fig. 1, with the tool nested in a heat sink to prevent burning of the sharp tool edge. In coating a surface by ESA, options exist as to the linear path direction the electrode TABLE 1 ESA electrode material composition Code
Composition
A B C
71.5% WC, 12% TIC, 10.5% TaC, 6.5% Co 69% WC, 20% Tic, 6% Ni, 4% TaC titanium diboride with unknown binder _
142 TABLE 2 Tool geometry Rake angle 15” Clearance angle 6” Cutting speed 170 ft min-’ Feed rate 0.011 in rev-’
Fig. 1. ESA applied on the tool face.
FACE
ADHESION)
Fig. 2. Cutting tool nomenclature
and optional directions of electrode motion [ 3 1.
takes in progressively coating the part; for cutting tools this direction may be side to side or front to back as shown in Fig. 2. A third option would be more or less random in nature as with a hand held applicator. For the first set of experiments to be described, the electrode motion was programmed in a side to side direction.
143
With a calibrated two component force dynamometer, the two control tools and a set of three ESA coated tools were tested in a lathe cutting operation using a cylindrical workpiece (Fig. 3). The cylinders were 0.25 in thick with an average diameter of 2.13 in made of fully annealed 41L40 steel with a hardness of 150 R,. The test times and forces both were recorded with a data logger-computer setup. Wear measurements were taken at prearranged time intervals by stopping the cut and removing the tool for inspection. A dial indicator was used for crater depth and a measuring microscope was used to measure the flank land. The tests were generally run until each tool showed signs of failure by either collapse of the tool tip from crater wear or flank failure with a severe groove forming on the flank.
Fig. 3. Lathe cutting setup showing force dynamometer,
tool and cylinder.
3. Results and discussion 3.1. Crossed cylinder screening A plot of the crossed cylinder test results is presented in Fig. 4. The coated test sample data show coatings A, B and C are closely grouped with much less wear than the uncoated HSS sample and roughly comparable to the TiN wear scar. These electrode compositions were then used in the actual cutting tests. Coatings A and B were applied at a relatively high energy level (0.25 J) and a 600 Hz pulse rate while coating C was applied at a lower energy level (0.1 J) and at a 1200 Hz pulse rate. The electrode motion was from side to side over the tool face at a speed of 15 mm s-l. The deposited weights are given in Table 3.
0
TON PVD
/
-
I
. HSS OA B AC
.
/ 9 y
04-
01
0
I
2
,
4
REVOLUTIONS,
6
thousands
Fig. 4. Wear scar area us. revolutions for selected ESA coatings on HSS, TiN PVD on HSS and uncoated HSS.
TABLE 3 ESA coating code for tools Test tool
ESA coating
Surface
Weight gain (mg cmw2)
1
A A
Face Flank
28.44 25.58
2
B B
Face Flank
34.31 34.16
3
C C
Face Flank
2.86 1.28
3.2. Cutting
tests - side to side ESA application
of face and flank surface
The first cutting tests were run with one of the three selected ESA coatings applied on both surfaces of the cutting tools. The uncoated HSS and the commercial TiN PVD coated HSS tools were also tested to compare with these ESA-coated tools. The crater and flank wear data us. distance cut is shown in Figs. 5 and 6. All the tools failed after 60 m of distance cut except for the TiN PVD coating which showed minimal signs of wear at this distance. Figure 7 is data from ESA coating of both tool surfaces and shows the change in cutting power us. distance cut. These three plots display a number of important test results. First, ESA coating of both tool surfaces does not improve the tool life significantly beyond the tool life available with an uncoated HSS tool under these test conditions. Composition A was the best of the ESA coatings with an improvement in tool life of about 300%. This result would seem to be a
0
TiN
PVD
0 20
0
40
SO
0
DISTANCE, m
20
40
SO
DISTANCE, m
Fig. 5. Crater depth us. distance cut, both tool surfaces coated. Fig. 6. Flank wear us. distance cut, both tool surfaces coated.
I
/
I
2.8 -
2?1:
u
:
2.0 -
x _ IXE 3
+
-
^
1.2 -
h
_ o
TIN
PVD
. HSS q A O’Si 0.4
O0
mB AC
J
I 20
1 40
1 SO
DISTANCE, m
Fig. 7. Power vs. distance cut, both tool surfaces coated.
great improvement, but the TiN commercial coating results were clearly far superior. The two wear plots also show the two ESA-coated tools that failed first did so with rapid flank wear. Even the uncoated HSS tool showed far less flank wear than both B- or C-coated tools. This result may be anticipated because of the impact on the tool tip by the energy involved in the ESA coating process, but without further testing concrete conclusions cannot be drawn. The power consumption data with both tool surfaces coated by ESA is presented in Fig. 7. It shows a very interesting phenomenon noted in the relation between cutting power and distance. This phenomenon is the fact that the electrode C-coated tool used cutting power that was significantly
146
lower than the other cutting tools. TIN coating is known to significantly reduce friction and should reduce the cutting force below that of HSS, as it is shown to do in Fig. 7. Electrode C coating reduced the cutting force well below even the TiN PVD coatings value, but did not significantly increase the tool life. Although ESA coating of both tool surfaces did not give reduced wear equivalent to the TiN PVD coating, as indicated by the crossed cylinder screening results, the crater wear rate was greatly reduced by all three ESA coatings. 3.3. Cutting tests - side to side ESA application on face surface Even though the uncoated HSS tool had shown rapid cratering rates as the dominant wear mode, it was accompanied by low flank wear rates (Figs. 4 and 5); the ESA-coated tools (A, B, C) had the opposite wear response under the same cutting conditions. From these observations the decision was made to carry out the rest of the cutting tests with tools having only their face surfaces coated by the ESA method. Figures 8 and 9 show the test results for all five tools tested to failure with the ESA coating only on the tool face. The ESA-coated tools all performed much better with only the face coated than with both surfaces coated. Composition B coating showed the best effect on the tool life of the three ESA tools and was roughly an equivalent distance cut to failure as the TiN PVD coated tool. They both showed an increase of approximately 1000%. over the uncoated HSS tool. The other ESA-coated tools had improvements over the uncoated tool of 800% with type A and 275% with type C. Figure 10 shows the power relative to the distance cut. It can be seen that the less power needed for cut the longer the tool life. This holds for the
0 0
40
60
0
120
DISTANCE,
160
200
240
0
m
Fig. 8. Crater depth us. distance cut, tool face coated. Fig. 9. Flank wear vs. distance cut, tool face coated.
40
80
120
DISTANCE.
160 m
200
240
147
a’
1.2 -
s
o
g
0.6-
0
PVD
Da AC
0.4 -
0
TIN
. HSS q *
““““1”‘. 40
60
120
DISTANCE,
160
200
240
m
Fig. 10. Power us. distance cut, tool face coated.
TiN PVD coating as well as the ESA coatings. It is interesting to note that all of the tools had a trend of increasing power with increasing distance cut except the C electrode ESA-coated tool. By examining the two tool wear US. distance cut plots and the failed tools, the tool failure wear area could be ascertained. The two ESA-coated tools, composition A and C, lasted the longest distance and failed by crater wear in a similar fashion to the uncoated HSS tool. The electrode B coated HSS tool failed on the flank with a massive groove formed. The C-coated tool failure may have been of a similar nature to the TiN PVD tool, as noted by a similar crater depth at failure. These results are supported by the photographs shown in Fig. 11 for the four different tool materials.
3.4. Cutting tests - front to back ESA application on Face Surface The failure mode of the ESA-coated tool, composition B, was by breaking through the coating at a particular spot, then failing by rapid wear of the HSS. This would indicate a thin spot in the coating at this particular location. This non-uniformity could possibly be improved by coating with a front to back motion rather than a side to side motion (Fig. 2). This would also expose the sharp cutting edge to the beat of the spark for a shorter time period, as compared to a continuous pass down the edge. This comparison was made with composition B coating on HSS, using exactly the same experimental setup previously described, coating only the face of the tool. The results were very positive. Figure 12 shows the face of the two coated tool surfaces. Figure 12(a) is a retest of the tool with coating
148
(a)
(b)
Cd)
(e) Fig. 11. Failed tool face: (a) coated with TiN PVD, (b) HSS, (c) coating A, (d) coating B, (e) coating C.
(a)
(b)
Fig. 12. Failed tool surfaces using eelectrode composition B: (a) tool face -coating applied side to side, (b) tool face - coating applied front to back.
432
E o.26 i
0.24
z r
a20
;
0.16
4 I& 0.12 0.08
0 04060
I20
160
200 240 260 DISTANCE, m
320
360
400
440
1 0
Fig. 13. Flank wear us. distance cut, tool face coated; electrode composition and motion shown.
applied side to side. Failure is the same as occurred earlier with the formation of a breakthrough in the coat at a weak spot and subsequent formation of a deep groove in the flank. Failure occurred after 220 m of coating. Figure 12(b) is an identical coating parameter, but with the coating pattern applied in a front to back manner. After 460 m of cutting no coating breakthrough has occurred. The coating itself has taken on a more polished appearance but is still intact on the cutting edge. This result is further illustrated in Fig. 13, showing the increasing wear of the flank and as a function of cutting distance. The data for side to side application and for TiN (PVD) from Fig. 9 are also shown. The front to back method is clearly superior to the commercial TiN after about 150 m of cut. It is still serviceable after 450 m and is at least 2000% better than the uncoated tool.
150
4. Conclusions
The cutting tests have shown that ESA coating of HSS cutting tools can significantly reduce wear and greatly increase the tool life. Under the specific cutting conditions used, the best ESA coating (B) applied only on the tool face increased the tool life approximately equal to the commercially produced TiN PVD coated HSS tool when applied with side to side motion and at least twice as good when applied with front to back motion. Preliminary cross-cylinder screening did not predict the wide spreads in tool life of the different selected coatings or the particularly poor performance of coating material C (TiB2). The poor results of the TiBz coating may be partially attributed to the low weight gain of the tool coated with TiB2. The general differences between the screening and the cutting results could be partly caused by coating two different shapes, one round and the other flat, and the fact that fresh HSS surfaces are continually exposed to the coating in the cutting test. The power at the tool when compared with distance cut and wear gave some interesting results, not all of which can be explained. All of the tools tested gave results that were expected with an increase in power with an increase in wear except one. The face only coated TiBz had a decreasing power with wear, and power lower than even TiN when both surfaces of the tool were coated with TiB2. The general conclusion from the testing done is that ESA coating with selected materials and preferred electrode motion has great potential as a cutting tool coating. Different electrode materials and ESA coating conditions make a vast difference in the coatings’ performance.
Acknowledgments
The authors wish to acknowledge with thanks the assistance provided by the personnel of the Washington State University’s Mechanical Engineering Machine Shop and the Design Laboratory.
References 1 M. C. Shaw, Metal Cutting Principles, Clarendon hem, Oxford, 1984. 2 E. M. Trent, Metal Cutting, Butterworths, London, 1984. 3 V. A. Tipnis, Cutting tool wear, Wear Control Handbook, ASME, New York, 1980, p. 891. 4 G. Boothroyd, Fundamentals of Metal Machining and Machine Tools, Scripta Book co., 1984. 5 S. Soderberg and M. Hogmark, Wear mechanisms and tool life of high speed steels related to microstructure, Int. J. Wear, 110 (1986) 315. 6 W. Topinka, Wear resistant surface coatings deposited on Ti-6Al-4V by electrospark alloying, Maeters Thesis, Washington State University, 1988.
151
7 N. P. Suh, Tribophysics,
Prentice-Hall, Englewood Cliffs, NJ, 1986. 8 G. L. Sheldon and R. N. Johnson, in K. C. Ludema (ea.), Electrospark deposition - a technique for producing wear resistant coatings, ASME Wear of Materials, ASME, New York, 1985, pp. 388 - 396. 9 R. N. Johnson and G. L. Sheldon, Advances in the electro-spark deposition coating process, J. Vacuum Sci. Technol., 4 (1986) 2740 - 2746. 10 C. S. Kahlon, H. J. Baker, C. F. Noble and F. Koenigrberger, Electric spark toughening of cutting tools and steel components, Int. J. Mach. Tool Des. Res., 10 (1970) 95. 11 V. I. Shemegon, Hardening edge-type tools by electric-spark alloying, Stankii Instrum., 57 (4) (1986) 19. 12 S. Vaidyanathan, Reducing wear of high-speed steel cutting tools by spark hardening, Znt. J. Wear, 19 (1970) 255. 13 M. Olsson, S. Soderberg, S. Jacobson and S. Hogmark, A new equipment for friction and wear testing of cutting tool materials, UPTEC 8641 R, May 1986 (Uppsala University Teknikum).
137 - 151
137
A PARAMETRIC STUDY OF IMPROVING TOOL LIFE BY ELECTROSPARK DEPOSITION* E. ALLEN BROWN Hewlett Packard Corporation,
Corvallis, OR 97330
(U.S.A.)
GARY L. SHELDON Department of Mechanical and Materials Engineering, Vancouver, WA 99663 (U.S.A.)
Washington State University,
ABDEL E. BAYOUMI Department of Mechanical and Materials Engineering, Pullman, WA 99164-2920 (U.S.A.)
Washington State University,
Summary Electrospark alloying (ESA) is a pulsed microwelding surfacing process for alloying a hard wear resistant electrode with a metallic substrate. This process was evaluated as a means of increasing metal cutting tool life in machining operations. A number of ESA electrode material parameters were first screened with a crossed cylinder wear testing machine (ASTM G-83). Full hard high speed steel (HSS) cylinders were run against an ESA-coated ‘stationary HSS cylinder. Three superior coatings from these tests were then applied to the flank and face surfaces of high speed steel tools. Machining test results showed that the dominant wear mode of crater wear was significantly reduced by ESA coating. ESA coating of the tool face only was shown to increase wear resistance significantly more than coating both face and flank surfaces. In addition, programming the electrode motion from side to side rather than front to back over the tool face increased tool life. Comparisons are made with uncoated HSS and TiN-coated (PVD technique) HSS tools. The TiN-coated tool has a similar tool life to failure. The best ESA coated tool had a life better than the HSS tool by roughly 2000%.
1. Introduction 1.1. The metal cuttingprocess Even with the development of carbide and other advanced cutting tool materials, HSS still comprises roughly 50% of all the cutting tools used by the machine tool industry. Some of the advantages of high speed steel (HSS) *Paper presented at the International CO, U.S.A., April 8 - 14, 1989. 0043-1648/90/$3.50
Conference
on Wear of Materials, Denver,
@ Elsevier Sequoia/Printed
in The Netherlands
138
over the other commonly used cutting tool materials are: toughness, ease of sharpening and lower cost. The major drawback of HSS tools compared with tools composed of materials such as Tic, WC, TIN, etc., is a lower hot hardness. This means that HSS has to be used at lower cutting speeds in order to keep the tool temperature below the softening point. HSS is also softer than the other materials mentioned and less chemically stable than some of these materials. The metal cutting process subjects the tool to an extreme environment with both high temperatures and severe strain. The cutting process is extremely complex and there is little agreement as to the exact nature of the interaction that takes place between tool and workpiece. Under normal conditions, a cutting tool must satisfy three basic requirements. It must have a wear resistant surface, be of a fracture resistant material and resist gross plastic deformation [ 11. HSS, a standard cutting tool material, excels in certain situations because of its toughness. When compared with other common cutting tool materials, HSS is the toughest (only carbon steel is superior). Tool wear is such a complex and varied process that it cannot be described by a single simple mechanism. The locations and extent of wear are different, changing with tool material, operation, cutting condition, workpiece material and so forth. Different areas of the same tool may involve a different wear mechanism because the temperature, sliding velocity and stress are different [2, 31. The two variables having the major effect on the wear rate of the tool are the temperature and the normal pressure on the phase of the tool [4]. Understanding the metal cutting process is simplified by reducing the situation to two dimensions. This can be simulated by orthogonal cutting which involves a two surfaced tool with the cutting edge moving perpendicular to the cutting direction. The tool surface which is in contact with the chip is called the tool face (rake surface, rake face). As the tool removes the chip a new work piece surface is exposed to the lower surface of the tool. This lower tool surface is called the tool flank (relief or clearance surface, flank face). The power required at the tool is the product of the cutting force and velocity. With an orthogonal cutting tool the flank is the tool surface next to the work piece. This means that the rubbing that takes place on the flank will be between the tool flank and the newly machined surface. A term used to refer to the wear on the flank surface is wear land. Crater wear takes place on the tool face and is caused by the chip flowing along the tool face. The depth, width or tool edge to crater edge are all ways of measuring the amount of crater wear that has taken place. Temperature, pressure, normal force and real contact area all are more severe on the face than on the flank [5]. It is accepted that because of the severe conditions on the tool face, the apparent and real contact areas in the crater are almost equal [2]. As the crater increases, the normal stress shifts further back in the crater area. When the crater is first forming the maximum wear takes place at the point of maximum temperature. After the crater is formed the temperature
139
is not the sole controlling factor; the wear rate is highest at the back of the crater and the temperature is at its peak ahead of this position [ 51.
1.2. The ESA coating process and past results on cutting tools The coating of cutting tools started in the mid-1960s, although it may have been earlier in the U.S.S.R. The reasons for coating the cutting tool in a production situation are three-fold, i.e. to increase the life of the tool, to improve the surface quality of the product and to increase the production rate. Today there are coated carbide and HSS tools; some multilayered coatings have recently come on the market. When a tool is coated the subsurface material retains its desirable strength characteristics while the coating can act as a buffer zone to surface interactions. The tool wear process is predominantly a surface phenomenon and thus is well suited to the use of coatings. Coating selection can be matched to the particular type of wear that is dominant in a given situation. Coatings have also been shown to reduce friction, and increase abrasion and adhesion resistance [6]. In tests it was found that the improved wear characteristics last even after the coating has worn through [ 51. Many types of coatings for cutting tools have been used. The most widely used commercial coatings materials are Tic, TiN and A1,03, others are TaC, WC and TiB2. These materials all have good hot hardness and chemical stability to varying degrees, with the best coating being dictated by the wear conditions. The selection of the best coating for a given situation is complex. The prevalent commercial coating of HSS cutting tools is accomplished by physical vapor deposition (PVD) [7]. One other coating technique which has shown promise is electrospark alloying (ESA) but little work has been done to evaluate its performance. ESA is a coating process that could be described as pulsed microwelding. The process uses a short duration high current electrical pulse to alloy an electrode material with the substrate. The alloy that is formed on the tool is not simply a surface coating but is fused into the subsurface, and alloying between the surface deposited and substrate occurs. One major advantage of ESA is the ability to use any electrical conductor as the electrode, thus a variety of surface types can be formed [8]. The specific ESA coating process represented in this paper was developed by one of us (G.L.S.) at Washington State University (WSU) and involves a moving electrode, a power source that can be repetitively pulsed, and a means of causing relative motion between the workpiece and electrode. There is no standard power or frequency for ESA coatings so it is best if the frequency, voltage and pulse energy can all be varied depending on the application. The electrode is in light continuous contact with the substrate and continuous motion of the electrode is necessary to prevent welding. It is sometimes desirable for a gas atmosphere to be supplied around the spark area. A change in any one of these parameters can affect the quality of the coating [ 9, lo].
140
Although the spark is of short duration it is enough to vaporize a portion of the electrode. Globules of the electrode are accelerated through the plasma that has been formed by the spark and, along with some vapor, impacted on the tool surface. The short duration of the spark allows the small liquid pools formed of electrode mixed with tool material to solidify rapidly. The time and energy involved are both small enough to cause minimal change in the substrate material [ 91. Even though the ESA process has been around since the late 1950s little testing has been done to compare some of the various coatings possible by ESA with other commercially available coatings to reduce tool wear of HSS. Kahlon et al. [lo] tested HSS tools with a WC coating and the results showed an increase of 400% in tool life over uncoated tools. Another interesting result from their tests was a slight decrease in the cutting force with distance cut until the tool started to fail. This trend in the cutting force was not changed by coating the tool. In a different set of tests done by Shemegon [ll], HSS drills showed a 30 - 100% increase in the tool life with an ESA coating on the drill flank. In another series of tests machining steel (BHN 190), Vaidyanathan [12] was able to increase tool life by a factor of 4 using very large capacitors for energy storage and a vibrator-type applicator device. 2. Experimental
work
2.1. Screening procedure ESA coatings do not lend themselves well to any of the various wear prediction techniques because the electrode material is not applied just on the surface but actually alloys with the subsurface; the chemical composition of an ESA coating is a mixture of electrode and base material varying with depth [8]. Thus ESA coatings are not simple compounds, and a surface analysis will not give a representation of the wear characteristics as the coating wears through. To overcome this situation a simple, accurate and inexpensive screening technique was needed. The technique that was selected is the crossed cylinder test using cylinders of diameter 0.5 in as detailed in ASTM Standard G-83. The objection to this simple screening test is that a freshly cut surface is not continuously exposed to the stationary coated cylinder. Olsson et al. [13] have overcome this objection with modified cross-cylinder equipment, but this modification was not used in this work. The rotating cylinder represents the workpiece and should ideally be the same as the workpiece, namely 41L40 steel. Using this material it was found that very random and inconsistent wear scars resulted which were not suitable for coating evaluation. The decision was made to use HSS (M2 grade) as the rotating cylinder. The final test conditions were: the rotating cylinder HSS (R, = 61 - 63) cylinder representing the workpiece, the stationary cylinder representing the cutting tool, ESA coated HSS-MB (R, * 61 - 63). A normal load of 44.5 N (10 lbf) and a rotation rate of 100 rev min-’ were used.
141
In order to select the best coating material and other coating conditions a preliminary screening was done with the crossed cylinder wear test using electrode materials listed in Table 1. The possible tool coating materials were tested when applied with different pulse energy combinations and a number of coating layers. Total pulse energy is a function of capacitor, voltage and repetition rate settings; a base point for this parameter was taken from values used in previous ESA coating tests done at WSU. Electrode A had shown good wear resistance in these tests and it was chosen as the coating material to use in the first few tests. The wear scar area on the coated stationary samples was evaluated relative to the revolutions for several different coatings. As the testing progressed the coatings parameters were adjusted to optimize coating conditions for minimum wear. The first variable to screen was the cover gas. An atmosphere of CO2 showed slightly better results over argon and N2 and, thus, was used in all of the future ESA coatings. Next the voltage, capacitance and number of coating layers were evaluated. With a base of data established to evaluate the variables of voltage, capacitance and the number of coats, different coating materials could be tested. These ESA coating parameters were then adjusted until the crossed cylinder wear test results showed minimum wear. From the screening test results, the best three ESA coatings were selected to apply to the cutting tools. Two control samples were also employed to compare the ESA coating wear resistance. The lower control was uncoated HSS and the upper control was TiN PVD. Both were used as rotating specimens against the same stationary steel specimen. The TiNcoated HSS cylinders and cutting tools were graciously supplied by Vermont Tool and Die Co. 2.2. Cutting tests In all of the cutting tests the same selected conditions of speed, feed and tool geometry were used (Table 2). No lubrication was used in any test. The best three ESA coatings selected were applied to ground then polished HSS cutting tools from a set of prepared tools. The remaining tools from the set were used as upper or lower control samples and either used for commercial TiN coating by PVD or uncoated. A typical tool and ESA coating process is shown in Fig. 1, with the tool nested in a heat sink to prevent burning of the sharp tool edge. In coating a surface by ESA, options exist as to the linear path direction the electrode TABLE 1 ESA electrode material composition Code
Composition
A B C
71.5% WC, 12% TIC, 10.5% TaC, 6.5% Co 69% WC, 20% Tic, 6% Ni, 4% TaC titanium diboride with unknown binder _
142 TABLE 2 Tool geometry Rake angle 15” Clearance angle 6” Cutting speed 170 ft min-’ Feed rate 0.011 in rev-’
Fig. 1. ESA applied on the tool face.
FACE
ADHESION)
Fig. 2. Cutting tool nomenclature
and optional directions of electrode motion [ 3 1.
takes in progressively coating the part; for cutting tools this direction may be side to side or front to back as shown in Fig. 2. A third option would be more or less random in nature as with a hand held applicator. For the first set of experiments to be described, the electrode motion was programmed in a side to side direction.
143
With a calibrated two component force dynamometer, the two control tools and a set of three ESA coated tools were tested in a lathe cutting operation using a cylindrical workpiece (Fig. 3). The cylinders were 0.25 in thick with an average diameter of 2.13 in made of fully annealed 41L40 steel with a hardness of 150 R,. The test times and forces both were recorded with a data logger-computer setup. Wear measurements were taken at prearranged time intervals by stopping the cut and removing the tool for inspection. A dial indicator was used for crater depth and a measuring microscope was used to measure the flank land. The tests were generally run until each tool showed signs of failure by either collapse of the tool tip from crater wear or flank failure with a severe groove forming on the flank.
Fig. 3. Lathe cutting setup showing force dynamometer,
tool and cylinder.
3. Results and discussion 3.1. Crossed cylinder screening A plot of the crossed cylinder test results is presented in Fig. 4. The coated test sample data show coatings A, B and C are closely grouped with much less wear than the uncoated HSS sample and roughly comparable to the TiN wear scar. These electrode compositions were then used in the actual cutting tests. Coatings A and B were applied at a relatively high energy level (0.25 J) and a 600 Hz pulse rate while coating C was applied at a lower energy level (0.1 J) and at a 1200 Hz pulse rate. The electrode motion was from side to side over the tool face at a speed of 15 mm s-l. The deposited weights are given in Table 3.
0
TON PVD
/
-
I
. HSS OA B AC
.
/ 9 y
04-
01
0
I
2
,
4
REVOLUTIONS,
6
thousands
Fig. 4. Wear scar area us. revolutions for selected ESA coatings on HSS, TiN PVD on HSS and uncoated HSS.
TABLE 3 ESA coating code for tools Test tool
ESA coating
Surface
Weight gain (mg cmw2)
1
A A
Face Flank
28.44 25.58
2
B B
Face Flank
34.31 34.16
3
C C
Face Flank
2.86 1.28
3.2. Cutting
tests - side to side ESA application
of face and flank surface
The first cutting tests were run with one of the three selected ESA coatings applied on both surfaces of the cutting tools. The uncoated HSS and the commercial TiN PVD coated HSS tools were also tested to compare with these ESA-coated tools. The crater and flank wear data us. distance cut is shown in Figs. 5 and 6. All the tools failed after 60 m of distance cut except for the TiN PVD coating which showed minimal signs of wear at this distance. Figure 7 is data from ESA coating of both tool surfaces and shows the change in cutting power us. distance cut. These three plots display a number of important test results. First, ESA coating of both tool surfaces does not improve the tool life significantly beyond the tool life available with an uncoated HSS tool under these test conditions. Composition A was the best of the ESA coatings with an improvement in tool life of about 300%. This result would seem to be a
0
TiN
PVD
0 20
0
40
SO
0
DISTANCE, m
20
40
SO
DISTANCE, m
Fig. 5. Crater depth us. distance cut, both tool surfaces coated. Fig. 6. Flank wear us. distance cut, both tool surfaces coated.
I
/
I
2.8 -
2?1:
u
:
2.0 -
x _ IXE 3
+
-
^
1.2 -
h
_ o
TIN
PVD
. HSS q A O’Si 0.4
O0
mB AC
J
I 20
1 40
1 SO
DISTANCE, m
Fig. 7. Power vs. distance cut, both tool surfaces coated.
great improvement, but the TiN commercial coating results were clearly far superior. The two wear plots also show the two ESA-coated tools that failed first did so with rapid flank wear. Even the uncoated HSS tool showed far less flank wear than both B- or C-coated tools. This result may be anticipated because of the impact on the tool tip by the energy involved in the ESA coating process, but without further testing concrete conclusions cannot be drawn. The power consumption data with both tool surfaces coated by ESA is presented in Fig. 7. It shows a very interesting phenomenon noted in the relation between cutting power and distance. This phenomenon is the fact that the electrode C-coated tool used cutting power that was significantly
146
lower than the other cutting tools. TIN coating is known to significantly reduce friction and should reduce the cutting force below that of HSS, as it is shown to do in Fig. 7. Electrode C coating reduced the cutting force well below even the TiN PVD coatings value, but did not significantly increase the tool life. Although ESA coating of both tool surfaces did not give reduced wear equivalent to the TiN PVD coating, as indicated by the crossed cylinder screening results, the crater wear rate was greatly reduced by all three ESA coatings. 3.3. Cutting tests - side to side ESA application on face surface Even though the uncoated HSS tool had shown rapid cratering rates as the dominant wear mode, it was accompanied by low flank wear rates (Figs. 4 and 5); the ESA-coated tools (A, B, C) had the opposite wear response under the same cutting conditions. From these observations the decision was made to carry out the rest of the cutting tests with tools having only their face surfaces coated by the ESA method. Figures 8 and 9 show the test results for all five tools tested to failure with the ESA coating only on the tool face. The ESA-coated tools all performed much better with only the face coated than with both surfaces coated. Composition B coating showed the best effect on the tool life of the three ESA tools and was roughly an equivalent distance cut to failure as the TiN PVD coated tool. They both showed an increase of approximately 1000%. over the uncoated HSS tool. The other ESA-coated tools had improvements over the uncoated tool of 800% with type A and 275% with type C. Figure 10 shows the power relative to the distance cut. It can be seen that the less power needed for cut the longer the tool life. This holds for the
0 0
40
60
0
120
DISTANCE,
160
200
240
0
m
Fig. 8. Crater depth us. distance cut, tool face coated. Fig. 9. Flank wear vs. distance cut, tool face coated.
40
80
120
DISTANCE.
160 m
200
240
147
a’
1.2 -
s
o
g
0.6-
0
PVD
Da AC
0.4 -
0
TIN
. HSS q *
““““1”‘. 40
60
120
DISTANCE,
160
200
240
m
Fig. 10. Power us. distance cut, tool face coated.
TiN PVD coating as well as the ESA coatings. It is interesting to note that all of the tools had a trend of increasing power with increasing distance cut except the C electrode ESA-coated tool. By examining the two tool wear US. distance cut plots and the failed tools, the tool failure wear area could be ascertained. The two ESA-coated tools, composition A and C, lasted the longest distance and failed by crater wear in a similar fashion to the uncoated HSS tool. The electrode B coated HSS tool failed on the flank with a massive groove formed. The C-coated tool failure may have been of a similar nature to the TiN PVD tool, as noted by a similar crater depth at failure. These results are supported by the photographs shown in Fig. 11 for the four different tool materials.
3.4. Cutting tests - front to back ESA application on Face Surface The failure mode of the ESA-coated tool, composition B, was by breaking through the coating at a particular spot, then failing by rapid wear of the HSS. This would indicate a thin spot in the coating at this particular location. This non-uniformity could possibly be improved by coating with a front to back motion rather than a side to side motion (Fig. 2). This would also expose the sharp cutting edge to the beat of the spark for a shorter time period, as compared to a continuous pass down the edge. This comparison was made with composition B coating on HSS, using exactly the same experimental setup previously described, coating only the face of the tool. The results were very positive. Figure 12 shows the face of the two coated tool surfaces. Figure 12(a) is a retest of the tool with coating
148
(a)
(b)
Cd)
(e) Fig. 11. Failed tool face: (a) coated with TiN PVD, (b) HSS, (c) coating A, (d) coating B, (e) coating C.
(a)
(b)
Fig. 12. Failed tool surfaces using eelectrode composition B: (a) tool face -coating applied side to side, (b) tool face - coating applied front to back.
432
E o.26 i
0.24
z r
a20
;
0.16
4 I& 0.12 0.08
0 04060
I20
160
200 240 260 DISTANCE, m
320
360
400
440
1 0
Fig. 13. Flank wear us. distance cut, tool face coated; electrode composition and motion shown.
applied side to side. Failure is the same as occurred earlier with the formation of a breakthrough in the coat at a weak spot and subsequent formation of a deep groove in the flank. Failure occurred after 220 m of coating. Figure 12(b) is an identical coating parameter, but with the coating pattern applied in a front to back manner. After 460 m of cutting no coating breakthrough has occurred. The coating itself has taken on a more polished appearance but is still intact on the cutting edge. This result is further illustrated in Fig. 13, showing the increasing wear of the flank and as a function of cutting distance. The data for side to side application and for TiN (PVD) from Fig. 9 are also shown. The front to back method is clearly superior to the commercial TiN after about 150 m of cut. It is still serviceable after 450 m and is at least 2000% better than the uncoated tool.
150
4. Conclusions
The cutting tests have shown that ESA coating of HSS cutting tools can significantly reduce wear and greatly increase the tool life. Under the specific cutting conditions used, the best ESA coating (B) applied only on the tool face increased the tool life approximately equal to the commercially produced TiN PVD coated HSS tool when applied with side to side motion and at least twice as good when applied with front to back motion. Preliminary cross-cylinder screening did not predict the wide spreads in tool life of the different selected coatings or the particularly poor performance of coating material C (TiB2). The poor results of the TiBz coating may be partially attributed to the low weight gain of the tool coated with TiB2. The general differences between the screening and the cutting results could be partly caused by coating two different shapes, one round and the other flat, and the fact that fresh HSS surfaces are continually exposed to the coating in the cutting test. The power at the tool when compared with distance cut and wear gave some interesting results, not all of which can be explained. All of the tools tested gave results that were expected with an increase in power with an increase in wear except one. The face only coated TiBz had a decreasing power with wear, and power lower than even TiN when both surfaces of the tool were coated with TiB2. The general conclusion from the testing done is that ESA coating with selected materials and preferred electrode motion has great potential as a cutting tool coating. Different electrode materials and ESA coating conditions make a vast difference in the coatings’ performance.
Acknowledgments
The authors wish to acknowledge with thanks the assistance provided by the personnel of the Washington State University’s Mechanical Engineering Machine Shop and the Design Laboratory.
References 1 M. C. Shaw, Metal Cutting Principles, Clarendon hem, Oxford, 1984. 2 E. M. Trent, Metal Cutting, Butterworths, London, 1984. 3 V. A. Tipnis, Cutting tool wear, Wear Control Handbook, ASME, New York, 1980, p. 891. 4 G. Boothroyd, Fundamentals of Metal Machining and Machine Tools, Scripta Book co., 1984. 5 S. Soderberg and M. Hogmark, Wear mechanisms and tool life of high speed steels related to microstructure, Int. J. Wear, 110 (1986) 315. 6 W. Topinka, Wear resistant surface coatings deposited on Ti-6Al-4V by electrospark alloying, Maeters Thesis, Washington State University, 1988.
151
7 N. P. Suh, Tribophysics,
Prentice-Hall, Englewood Cliffs, NJ, 1986. 8 G. L. Sheldon and R. N. Johnson, in K. C. Ludema (ea.), Electrospark deposition - a technique for producing wear resistant coatings, ASME Wear of Materials, ASME, New York, 1985, pp. 388 - 396. 9 R. N. Johnson and G. L. Sheldon, Advances in the electro-spark deposition coating process, J. Vacuum Sci. Technol., 4 (1986) 2740 - 2746. 10 C. S. Kahlon, H. J. Baker, C. F. Noble and F. Koenigrberger, Electric spark toughening of cutting tools and steel components, Int. J. Mach. Tool Des. Res., 10 (1970) 95. 11 V. I. Shemegon, Hardening edge-type tools by electric-spark alloying, Stankii Instrum., 57 (4) (1986) 19. 12 S. Vaidyanathan, Reducing wear of high-speed steel cutting tools by spark hardening, Znt. J. Wear, 19 (1970) 255. 13 M. Olsson, S. Soderberg, S. Jacobson and S. Hogmark, A new equipment for friction and wear testing of cutting tool materials, UPTEC 8641 R, May 1986 (Uppsala University Teknikum).