Hendrick Niemann, Eu-gene Ng, Hue Loftus, Adrian Sharman, Richard Dewes and David Aspinwall*
1. INTRODUCTION
Titanium alloys are used extensively in the aerospace industry for turbine and compressor blades in the cooler parts of the engine and for cases, etc. They have excellent strength to weight ratios and corrosion resistance together with good elevated temperature properties and an oxidation limit of - 600°C. The a-13 alloy, Ti-6Al-4V is the most common and accounts for over halfofthe world's sales of titanium alloys1•
Numerous studies2•7
have shown titanium and its alloys to be difficult to machine, regardless of the criteria used to measure machinability (tool life, productivity, cutting temperatures, etc). This has been attributed to their low thermal conductivity which concentrates heat in the cutting zone (typically less than 25% that of steel), retention of strength at elevated temperatures and high chemical affinity for all cutting tool materials.
Cutting temperatures of 800 to 1100°C have been recorded when turning at relatively low cutting speeds5• 8
• 9 and temperature gradients in the tool are much steeper than for more conventional workpiece materials. In addition, the heat is concentrated at the tool edge, whereas for steel the maximum rake face temperature occurs at some point away from the cutting edge2• 10
• Although the cutting forces generated are not excessively high (similar
• H.Niemann, E. Ng, A. Shannan, R. Dewes and D. Aspinwall, University of Birmingham, Edgbaston, Birmingham, BIS 2TT, UK. H. Loftus, Technicut Ltd, Attercliffe, Sheffield, S9 3WP, UK.
Metal Cutting and High Speed Machining, edited by
D. Dudzinski et al., Kluwer Academic/Plenum Publishers, 2002 181
182 H. NIEMANN ET AL
to those with steel), they are confined to a small area due to the short chip contact length, leading to high stresses. The combination of high stress and temperature, is such that tool plastic deformation is a characteristic wear mechanism. Depth of cut notching can also be a problem with intermittent cutting operations.
In order to achieve long tool life, the surface of a cutting tool needs to possess high hot hardness in order to minimise the effects of abrasion/deformation and the ability to resist diffusion/dissolution wear mechanisms. In addition, the tool must have sufficient toughness in order to withstand chipping/fracture. In the absence of a single tool material able to meet all cutting tool requirements, the development and use of advanced coatings provides a workable solution. Currently, it is estimated that over 75% of all carbide tools are sold as coated products11• The composition/range of coatings and their associated mechanical/physical properties is extremely diverse, see Table I. Figure I shows ball crater micrographs of sample coatings. Despite the wide range of products available, the consensus of research opinion suggests that when machining (predominately turning) titanium alloys, coated carbides provide no significant advantage, indeed a number of papers suggest an increased susceptibility to wear over comparable uncoated grades2• 5
• 1 2.
13• Titanium's characteristic as a universal solvent is the principal reason for this and despite extensive ongoing research effort there appears to be no obvious composition to resolve the problem although it has been suggested that borides are potential candidates5.
Another consideration in critical applications such as in the aerospace industry is the possible diffusion of elements of the tool coating into the workpiece surface.
In contrast to turning, there are relatively few publications detailing the high speed end/ball end milling of titanium alloys. In the literature that is available, tests are detailed using cutting speeds up to 400 rn/min, with tool life decreasing severely as the cutting
Table 1. Coatings and selected physical property data Coating Hardness Oxidation Co-efficient
(HV) resistance (°C) of friction
TiN 1930-2200 600 0.5
TiC 2800-3000 400 -
TiCN 3000 400 0.4
TiAIN 3000-3500 540 0.4
TiAICrN 3500 920 0.4
TiAICrYN 2700 950 -
CrN 1650-2150 700 0.5
Al203 2100-3000 1200 -
ZrN 2800 600 0.6
MoS2 1500 - 0.02
WC/C 1500 300 0.2
Figure 1. Ball crater micrographs of selected coatings.
EFFECT OF ENVIRONMENT AND COATINGS WHEN HSM Ti-6Al-4V 183 speed is increased14-21
• When high speed milling Ti-6AI-4V, Brinksmeier et al14 found that, as with twning, uncoated carbide tooling outperformed coated tools. Similar results were also found by Derrien and Vigneau15 when comparing uncoated WC to TiCrN, CrC, TiCN and TiAlN coated tools. Tool fuilure has been reported to occur typically by flank wear and depth of cut notching with relatively little crater wear16• 17• In addition, high mechanical and thermal stresses due to the interrupted cutting process have been shown to lead to fatigue of the cutting edge and associated tool chipping18• 19• Derrien and Vigneau 15 found that the use of spray mist cutting fluid produced significantly longer tool life than obtained with flood application, due to a less severe heating and cooling cycle.
Although less beneficial than spray mist, they concluded that flood cutting fluid application was better than dry cutting as regards tool life due to the lowering of cutting tern peratures.
2. EXPERIMENT AL WORK
The following experimental work was carried out to determine the effects of cutting environment, cutting speed and tool coating on tool life, when high speed ball nose end milling titanium alloy Ti-6Al-4V. Forged rectangular blocks approximately 200xl30x95 mm were used which were annealed at 700°C for 4 hours and then air cooled to provide a bulk hardness of -35 HRC.
All the cutting tools used were 2 flute, 8 mm diameter solid carbide ball nose end mills with a fixed tool geometry and employed exactly the same carbide grade (from one supplier). The tools were collet mounted in a precision toolholder with a 40 mm overhang and run-out of :5:1 Oàm. A range of tool coatings were investigated, see Table 2 for selected mechanical and physical properties. These were all ostensibly hard, wear resistant commercial products, however, a 'soft' lubricant top coat was employed on one of the tools.
Tbl 2T I a e . oo coatmgs
Material TiCN TiAIN TiAIN+WC/C TiAIYNNN ZrN MoCrN Confi1uration
Mono layer TiCN
Multi laver TiAIN TiAlN ZrN MoCrN
Superlattice TiAIYNNN
Top coat WC/C
Hanlness at room temperature (HVo.osl
Base coat 3,000 3,000 3,000 3,750 2,800 2,500
Top coat 1,000
Thickness (um)
Base coat 3-4 3-5 1.3-4 2.8 3 -
Top coat 0.67-2
Max. operatin2 temperature 100
Base coat 400 800 800 750 600 1,200
Top coat 300
Friction coefl'. 0.4 0.4 0.2 0.2 0.6 0.3
184 H. NIEMANN ET AL
The machining trials were undertaken on a vertical high speed (20,000 rpm) prismatic machining centre retrofitted with a separate high speed spindle (45,000 rpm, 4.5kW), see Figure 2a. A high pressure manifold ring provided the necessary coupling between the various hardware units and enabled fluid application at 70 and 150 bar pressure via two external nozzles, see Figure 2b which also details the cutting motion.
The cutting fluid employed was a 5% semi-synthetic soluble oil with extreme pressure (EP) additives. Tool wear was measured with a toolmaker's microscope fitted with a x-y digital micrometer measuring platform. Selected worn tools were further examined using an optical microscope attached to a digital camera.
a. High speed machine with retrofit 45,000rpm spindle.
b. Workpiece fixturing, cutting tool and fluid supply arrangment.
Figure 2. High speed machining set up.
Machining parameters fixed throughout the investigation were; radial depth of cut (0.2 mm), axial depth of cut (0.5 mm), feed rate (0.2 mm feed/tooth), workpiece tilt angle (0°) and down milling configuration. Table 3 details the variable operating parameters employed in the Phase I experimental work, which was carried out to establish the most suitable cutting speed based on a tool life criterion. The parameters employed relate to rotational speeds of 20,500 to 28,800 rpm and feedrates of 8,200 to 11,500 mm/min.
Table 4 details the variable operating parameters employed in Phase 2 which investigated the effect of tool coating and cutting environment on tool life. The cutting speed used in Phase 2 was that identified as giving the longest tool life in Phase 1. All trials were replicated to improve the reliability of the results (indicated by the double crosses in the relevant table). During the trials, ISO 8688-222 was followed as closely as possible.
Wear measurements were made after a set number of passes depending on the level of wear experienced, maximum flank wear and maximum notch wear were recorded against length cut. The tests were stopped when the maximum flank wear reached 0.3mm or the maximum notch wear reached 0.5mm.
Workpiece cross-sections were hot mounted, ground using SiC paper and polished with Si02 solution. After polishing, they were etched in two stages, firstly a solution of 2% hydrogen fluoride and 98% water was used for 2 minutes. The second stage involved a 5 second etch in a mixture of 2ml hydrogen fluoride, 50ml nitric acid and 48ml water.
Subsurface microstructural analysis was conducted using an optical microscope connected to a digital camera. Knoop microhardness measurements using a load of 50g for 15 seconds were taken to a depth of I OOàm at 1 Oàm intervals.
Cuttin s d Tool coating Cuttin environment
70 bar, 17 I/min
EFFECT OF ENVIRONMENT AND COATINGS WHEN HSM Ti-6Al-4V
Cuttin s d
Table 3. Phase 1 ex erimental work Cutting speed
m/min 250 300 350
Cuttin environment
Tool coating
None TiCN
70 bar, 17 I/min
Table 4. Phase 2 ex erimental work 250 m/min
185
Tool coating None TiCN TiAIN TiAIN + TiAIYN ZrN MoCrN
WC/C /VN
Cuttin environment
70 bar, 17 I/min * *
150 bar, 20 I/min t
• these trials were conducted in Phase I.
3. RESULTS AND DISCUSSION
Tool failure under all machining conditions irrespective of the cutting speed, cutting fluid pressure/flow rate and type of tool coating, occurred predominantly by notching as shown in Figure 3. The wear occurred predominantly on the flank face of the tool at the depth of cut position, where the highest cutting speeds and thermal/mechanical loads are situated. Crater wear and flank wear were marginal. Notch wear is frequently found when machining high temperature nickel-based superalloys and titanium alloys which show a strong tendency to strain harden.
Machining parameters
Cutting speed : 250 m/min Spindle speed : 20,546 rpm Radial depth of cut : 0.2 mm Feed/ tooth : 0.2 mm
Feedrate : 8,218 mm/min
Axial depth of cut : 0.5 mm Cutting environment : 70 bar,
171/min Tool diameter : 8 mm Tool material : Micrograin
carbide Tool coating : TiAIN
Figure 3. Typical tool flank/notch wear on one tooth ofa ball nose end mill.
Figure 4 gives the results of Phase 1 and shows the effect of tool coating and cutting speed on length cut. For a given cutting speed, little or no difference was found in the length cut when using either uncoated or TiCN coated tools. However, the lowest cutting speed (250 m/min) produced the longest length cut values and this speed was therefore used in Phase 2. The reduction in tool life/length cut followed convention, in that higher cutting speeds would be expected to produce higher temperatures causing rapid tool dissolution.
186 H. NIEMANN ET AL.
Cutting speed (m/min)
Machining parameters 120 ~--25~o----t---3...,oo _ _ ___,c--__ 3_,50 _ _ ----1 Radial depth of cut : 0.2 mm
Feed/ tooth : 0.2 mm Axial depth of cut : 0.5 mm 100 f-:::±:::--l--l~r-1r----1----1r----1----1 Cutting environment : 70 bar,
~ 17 Vmin
.§. 80 .t--t---=i=-t--~-+----t---; Workpiece tilt angle : 0°
'ii Milling operation : Point
~ 60 H----i:rnr--t!-f- ,r--t---...,.,,rl Millin~ direction : Down
Oil Tool diameter : 8 mm
j 40 Tool overhang : 40 mm
Note:
20 • Length cut values correspond to
a max flank wear of0.3 mm or a
o '---'-'-1--'""""~1---.1.-~l---.l."-"'-"~-'"---''--l___...,'--l max notch wear of0.5 mm
• Range bars indicate maximum and minimum values Uncoated TiCN Uncoated TiCN Uncoated TiCN
Coatings
Figure 4. Effect of cutting speed and tool coating on length cut.
Figure 5 shows the Phase 2 results and gives the effect of cutting environment and tool coating on mean length cut. Cutting environment effects were less pronounced than anticipated, the maximum difference in tool performance being of the order of-20 %. A possible explanation for this result was given by Smith et al23, who found that above a certain cutting fluid pressure/flow rate and hence cooling regime, no additional reduction in cutting temperature was seen. Comparable results were also found by Kaminski and Alvelid24• The use of high pressure cutting fluid supply would act to reduce the cutting temperatures generated. Ezugwu and Wang4 stated that titanium alloys react chemically with almost all tool materials available at temperatures above 500°C. Although cutting temperatures were not measured in the current trials, similar work on the high speed ball nose end milling of Inconel 718 nickel based superalloy has shown that when using comparable operating parameters the cutting temperatures generated were a maximum of -300°C25. It is likely that the low temperatures that would be encountered in these trials prevented significant diffusion wear of the cutting tools from occurring.
The majority of coatings gave similar results despite the wide range of mechanical and physical properties, coating thickness, coating methods and chemistry. The particular TiAIN product used, however, gave a -50% improvement in performance over other coatings and the uncoated tools. The reasons for this are not obvious, other than the combination of relatively high hardness, oxidation resistance and low coefficient of friction of the coating. Unfortunately, the performance of apparently similar coatings from different manufacturers can vary widely. Results presented by Dewes and Aspinwall26 when high speed machining hardened steel showed an 800% difference in tool life for TiAIN coated tools. Furthermore scratch test data presented by Ng et al25 has shown no direct correlation with actual cutting performance. For the most part, the results confirm previously published data, which suggest that tool coatings do not provide any significant advantage.
Figures 6 and 7 detail sample workpiece microstructure and microhardness (new and worn tools) results for the test using TiCN coated tools, which were measured parallel to the feed direction. The deformation shown is minimal, i.e. within lOàm of the machined workpiece surface. No deformation was visible with new tools. The corresponding microhardness profiles show no discernible strain hardening27•
EFFECT OF ENVIRONMENT AND COATINGS WHEN HSM Ti-6Al-4V 187 180
160 140
~ D r , -• -,__ -- - --,-Cutting speed --r ., 250m/min' - ~
70 bar 150 bar Radial depth of cut 0.2mm
I - - 1 - - - Feed/ tooth 0.2mm ~
g 120 Axial depth of cut O.Smm
~ I - -
:; " 100 -
t c
-3 80
~I- ã-~ ~- ~
- -- -- ~I- ~~ ~~ ~- _,_ I - - - ~c.. - C-'-- ~
60 - - - -- ~- ~I- ~- ~- -'-- '-'-- - '-'-- - '-'-- C-C- ' -
40 - -- e-- C-f- 1-1- -- ~- - ' - - ~I- ~I- ~I- -- - + - + - - ~
20 - -- ~-- C-f- 1-1- 1-1- - - -e- c...+- ~I- ~I- - - - + - I -I - ~
0 _....___...._ ~
Coating Uncoated TiCN TiAIN MoCrN TiAIYN/VN TiAIN+WC/C ZrN
Figure 5. Effect of cutting environment and tool coating on mean length cut (Phase 2).
a. low magnification. b. high magnification.
Figure 6. Workpiece microstructure using worn TiCN coated tool.
~ 450
0 400
ci
~ 350 :!:.
~ . . . -
,...
.. 300 .. ., 250
"E c .. 200
.c .. I!? 150
~ 100
.. 50
c
Cutting speed : 250m/min Radial depth of cut :0.2mm Feed/ tooth :0.2mm Axial depth of cut :O.Smm
I I I
~ 0
,_
10 20 30 40 50 60 70 80 90 100
Depth beneath machined surface (um)
Figure 7. Workpiece microhardness using new and worn TiCN coated tools.
188 H. NIEMANN ET AI-
CONCLUSIONS
• Tool failure under all machining conditions irrespective of the cutting speed, cutting fluid pressure /flow rate and type of tool coating, occurred by notching at the depth of cut position.
• The use of the 150 bar cutting fluid environment did not produce a consistent trend in tool life when compared to 70 bar.
• The majority of coatings used gave similar results to the uncoated WC tool, however, the particular multilayer TiAIN product used gave a -50%
improvement in performance. Unfortunately, previous work by the authors suggests that this may not be the case for all TiAlN coated products.
• The microstructure and microhardness of the workpiece remained largely unaffected by the high speed ball nose end milling process even when using worn tools.
ACKNOWLEDGEMENTS
The authors would like to thank Prof. A.A. Ball, Head of the School of Manufacturing and Mechanical Engineering and Pro£ M.H. Loretto, Director of the IRC in Materials for High Performance Applications, for provision of facilities and funding.
Thanks also go to Mr M. Kirby at Technicut Ltd, Miss N. Renevier at Teer Coatings Ltd and Dr. S. Davey at Timet UK Ltd for technical support. Additional thanks go to Mr L.S.
Goi, undergraduate student, for surface integrity evaluation.
REFERENCES
I 2
3 4 5
6 7
8
Martin J, Materials for Engineering, The Institute of Materials, ISBN 1-86125-0 I 2-6, (1996).
Machado A.Rand Wallbank J, "Machining of titanium and its alloys - a review", Proceedings of the Jnstition of Mechanical Engineers, Vol.204, pp 53-59, (1990).
Wang M and Zhang Y, "Diffusion wear in milling titanium alloys", Materials Science and Technology, Jun., Vol. 4, pp 548-553, (1988).
Ezugwu E.O and Wang Z.M, "Titanium alloys and their machinability - a review", Journal of Materials Processing Technology, Vol. 68, pp 262-274, (1997).
Deamley P.A and Grearson A.N, "Evaluation of principal wear mechanisms of cemented carbides and ceramics used for machining titanium alloy !MI 318", Materials Science and Technology, Jan., Vol. 2, pp 47-58, (1986).
Komanduri R and Reed W.R, "Evaluation of carbide grades and a new cutting geometry for machining titanium alloys", Wear, Vol.92, pp 113-123, (1983).
Jawaid A, Che-Haron C.H and Fallah A, "Tool wear in machining of titanium alloy Ti 6242", Proceedings of the 3nJ International Coriference on Progress of Cutting and Grinding, Osaka, Japan, Nov., pp 126-131, (1996).
Narutaki N and Murakoshi A, "Study on machining of titanium alloys", Annals of the CJRP, Vol. 32, No. 1, pp 65-69, (1983).
9 Kitagawa T, Kubo A and Maekawa K, "Temperature and wear of cutting tools in high speed machining oflnconel 718 and Ti-6AJ-6V-2Sn", Wear, Vol. 202, pp 142-148, (1997).
10 Trent E.M, Metal Cutting, 3"1 Edition, Butterworth-Heinemann Ltd, ISBN 0-7506-1068-9, (1991).
11 AB Sandvik Coromant, Modern Metal Cutting, ISBN 91-972299-0-3.
12 Wang Z.M and Ezugwu E.O, '~Performance of PVD coated carbide tools when machining Ti-6Al- 4V", ASME!STLE Tribology Conference, San Francisco, California, Oct 13-17, pp 81-86, (1996).
EFFECT OF ENVIRONME1'1 13 Donachie M.J, "Titaniun 14 Brinksmeier E, Berger l Proceedings of the I' Jun, pp 295-306, (1911 15 Derrien S, and Vigneau J
French and German (1997).
I 6 Eckstein M, Lebkuc schnittgeschwindi speeds -Part I : R 17 Eckstein M, Lebk
18 19
20 21 22 23 24 25 26
27
EFFECT OF ENVIRONMENT AND COATINGS WHEN USM Ti-6Al-4V 189 13 Donachie M.J, "Titanium a technical guide", ASM International, OH, ISBN 0871703092, (1988).
14 Brinksmeier E, Berger U and Janssen R, "High speed milling of TiAl4V4 for aircraft applications'', Proceedings of the I" French and German Conference on High Speed Machining, Metz, France, Jun, pp 295-306, ( 1997).
15 Derrien S, and Vigneau J, "High speed milling of difficult to machine alloys", Proceedings of the I"
French and German Conference on High Speed Machining, Metz, France, Jun., pp 284-294, (1997).
16 Eckstein M, Lebkilchner G and Blum D, "Schattfiiisen von titanlegierungen mit hohen schnittgeschwindigkeiten - Teil I : Schruppen (End milling of titanium alloys using high cutting speeds-Part I : Rough machining)", VDJ-Z. 133(12), Dec., pp 28-34, (1991).
17 Eckstein M, Lebkilchner G and Blum D, "Schattfiiisen von titanlegierungen mit bohen schnittgeschwindigkeiten - Teil 2 : Schlichten (End milling of titanium alloys using high cutting speeds- Part 2: Finish machining)", VDJ-Z, 134(6), Jun., pp 61-67, (1992).
18 Schulz H, Hochgeschwindigkeitsfrasen schwer zerspanbarer Legierungen, Dissertation TH Darmstadt, Carl Hanser Verlag Munchen Wien, (I991).
19 Maekawa K., Ohshima I and Nakano Y, "High speed end milling ofTi-6Al-6V-2Sn titanium alloy", Advancement of Intelligent Production, Japan Society for Precision Engineering, Edited by E.
Usui, pp 431-436, ( 1994 ).
20 Sage C, "Cost effective milling of titanium alloy : High speed or conventional technique?", Dusseldorff, Germany, VD/ Berichte Number 1399, Nov., pp 451-464, (1998).
21 Toller D, "Developments in compressor blade machining", Proceedings of the 3"' International Conference on Progress of Cutting and Grinding, Osaka, Japan, Nov., pp 1-7, (1996).
22 ISO 8688-2 "Tool life testing in machining - Part 2", International Standards Institution, ( 1989).
23 Smith A, Lagerberg S, Dahlman P and Kaminski J, "High pressure jet assisted turning", Industrial Tooling 99, Southampton, 7-8 Sept., pp 62-71, (1999).
24 Kaminski J and Alvelid B, "Temperature reduction in the cutting zone in water-jet assisted turning", Journal of Materials Processing Technology, Vol. 106, pp 68-73, (2000).
25 Ng E-G, Lee D.W, Sharman AR.C, Dewes R.C and Aspinwall D.K., "High speed ball nosed end milling oflnconel 718",Annals of the CIRP, Vol. 49, pp 41-46, (2000).
26 Dewes R.C and Aspinwall D.K, "High speed machining of hardened steels using coated tungsten carbide ball nose end mills", Proceedings of the r' French and German Conference on High Speed Machining, Darmstadt, Germany, Mar 10-11, pp 165-174, (1999).
27 Goi, L.S, Final year report, School of Manufilcturing and Mechanical Engineering, University of Birmingham, Confidential, (200 I).