The technology supporting the machining of titanium alloys is basically very similar to that for other alloy systems. Efficient metal ing requires access to data relating the machin-ing parameters of a cuttmachin-ing tool to the work ma-terial for the given operation. The important parameters include:
•
Tool life•
Forces•
Power requirements•
Cutting tools and fluidsGuidelines. The following guidelines, based in large part on the inherent factors affecting the machinability of titanium described above, contribute to the efficient machining of tita-nium:
•
Use low cutting speeds: A low cutting speed helps to minimize tool edge temperature and maximize tool life. Tool tip temperature is strongly affected by cutting speed. Lower speeds are required for alloys such as Ti-6Al-4V than are necessary for unalloyed titanium.•
Maintain high feed rates: The highest rate of feed consistent with good practice should be used. Tool temperature is affected less by feed rate than by speed. The depth of each succeeding cut should be greater than the work-hardened layer resulting from the pre-vious cut.•
Use a generous quantity of cutting fluid: A coolant provides more effective heat trans-fer. It also washes away chips and reduces cutting forces, thus improving tool life.•
Maintain sharp tools: Tool wear results in a buildup of metal on the cutting edges and causes poor surface finish, tearing, and de-flection of the workpiece.•
Never stop feeding while tool and titanium are in moving contact: Allowing a tool todwell when it is in moving contact with tita-nium causes work hardening and promotes smearing, galling, and seizing. This can lead to a total tool breakdown.
•
Use rigid setups: Rigidity of the machine tool and workpiece ensures a controlled depth of cut.Although the basic machining properties of titanium metal cannot be altered significantly, their effects can be greatly minimized by de-creasing temperatures generated at the tool face and cutting edge. Economical production tech-niques have been developed through applica-tion of the previously menapplica-tioned basic rules in machining titanium.
Tool Life. Tool life data have been devel-oped experimentally for a wide variety of tita-nium alloys. A common way of representing such data is shown in Fig. 10.2, where tool life (as time) is plotted against cutting speed for a given cutting tool material at a constant feed and depth in relation to Ti-6Al-4V. It can be seen that tools for machining titanium alloys are very sensitive to changes in feed. At a high cutting speed, tool life is extremely short; as the cutting speed decreases, tool life dramatically increases. Industry generally operates at cutting speeds promoting long tool life.
Forces and Power Requirements. Cutting force is important because, when multiplied by the cutting velocity, it determines the power re-quirements in machining. For general approxi-mations, the power requirements in turning and milling can be obtained by measuring the power input to the drive motor of the machine tool during a cutting operation and by subtract-ing from it the tare, or idle power. A good ap-proximation of the horsepower required in most machining operations can be predicted from unit power requirements. Table 10.2 shows the power requirements for titanium in comparison with other alloys.
Tool Materials. Cutting tools used to ma-chine titanium require abrasion resistance and adequate hot hardness. Despite the use of new tool materials—such as special ceramics, coated carbides, polycrystalline diamonds, and cubic boron nitride—in metal removal of steels, cast irons, and heat-resistant alloys, none of these newer developments have found appli-cation in increasing the productivity of titanium machined parts.
Generally, only straight carbide and gen-eral-purpose high-speed or highly alloyed tool 80 / Titanium: A Technical Guide
Fig. 10.2 Effect of cutting speed and feed on tool life during the turning of Ti-6Al-4V alpha-beta alloy
Table 10.2 Average unit power requirements for turning, drilling, or milling of titanium alloys compared with other alloys systems
Unit power for sharp tools (a), hp/in.3per min
Material Hardness, HB (3000 kg) Turning HSS and carbide tools Drilling HSS drills Milling HSS and carbide tools
Steels 35–40 HRC 1.4 1.4 1.5
Titanium alloys 250–375 1.2 1.1 1.1
High-temperature nickel and cobalt-base alloys
200–360 2.5 2.0 2.0
Aluminum alloys 30–150 (500 kg) 0.25 0.16 0.32
(a) Power requirements at spindle drive motor, corrected to 80% spindle drive efficiency. Dull tools may require 25% more power. HSS, high-speed steel
steels can be used. Carbide tools (such as grades C-2 and C-3), if feasible, optimize pro-duction rates. General-purpose high-speed tool steels (such as grades Ml, M2, M7, and M10) also are used. However, better results are gen-erally obtained with more highly alloyed tool steel grades, such as T5, T15, M33, and the M40 series. Cutting tool performance is influ-enced by many factors. Setup, processing meth-ods, grinding techniques, material quality, and the condition of the machine tool and fixturing all influence cutter performance.
In early studies, the straight tungsten carbide cutting tools, typically C-2 grades, performed best in operations such as turning and face mill-ing, while the high-cobalt, high-speed steels were most applicable in drilling, tapping, and end milling. The situation remains much the same today. C-2 carbides are used extensively in engine and airframe manufacturing for turn-ing and face millturn-ing operations. Solid C-2 end mills and end mills with replaceable C-2 car-bides find application, particularly in aerospace plants. M7 and the M42 and M33 high-speed steels are recommended for end milling, drill-ing, and tapping of titanium alloys.
Cutting Fluids. The correct use of coolants during machining operations greatly extends cutting tool life, and this is particularly true for titanium alloys. Chemically active cutting flu-ids transfer heat efficiently and reduce cutting forces between tool and workpiece. Of course, cutting fluids should not cause any degradation of the properties of the workpiece. Chlorine at one time was considered a suspect element in cutting fluids, regardless of the concentration and specific conditions used in titanium alloy manufacturing operations. The aversion to cut-ting fluids containing chlorine was based on the early discovery of hot-salt stress-corrosion damage in titanium alloys through mechanical property studies (see Chapter 13) and on the unexpected cracking of titanium alloys in cleaning and heat-treatment operations.
Although the presence of chlorine ions (e.g., those found in fingerprints on a part) can cause stress corrosion in some alloys during process-ing, it is not thought to always damage titanium alloys during machining. Nevertheless, cutting fluids used in machining titanium alloys require special consideration. If chlorinated cutting flu-ids are used on alloys that may be subject to stress-corrosion cracking, carefully controlled postmachining cleaning operations must be fol-lowed. The general prohibition against the use of cutting fluids containing chlorine is not uni-versally observed.
When specifying cutting fluids for machin-ing titanium, some companies have practically no restrictions other than the use of con-trolled-washing procedures on parts after ma-chining. Other manufacturers do likewise, ex-cept that they do not use cutting fluids containing chlorine on parts that are subjected to higher temperatures in welding processes or in service. Also, when assemblies are ma-chined, the same restrictions apply due to the difficulty of doing a good cleaning job after
machining. Still other organizations in aero-space manufacturing permit no active chlorine in any cutting fluid used for machining titanium alloys.
Mechanical property evaluations to define the effect of experimental chlorinated and sulfurized cutting fluids on Ti-6Al-4V alloy in-dicated that no degradation of mechanical prop-erties relative to those obtained from neutral cutting fluids occurred. Similar results were ob-tained by using chlorinated and sulfurized flu-ids in machining, or by having those cutting fluids present as an environment during testing.
These results and others suggest that under cer-tain conditions, chlorine-concer-taining cutting flu-ids are not detrimental to titanium alloys.
Usually the heavy chlorine-bearing fluids ex-cel in operations such as drilling, tapping, and broaching. The use of chlorine-containing (or halogen-containing) cutting fluids generally is not a recommended practice, however. There are excellent cutting fluids available that do not contain any halogen compounds. Actually, for certain alloys and operations, dry machining is preferred. Figure 10.3 shows the effect of vari-ous cutting fluids on tool life in drilling Ti-6Al-4V.
Machining Speeds and Feeds. Cutting speed and feed are two of the most important parameters for all types of machining opera-tions. Table 10.3 gives some speed and feed data on turning of selected titanium alloys. Be-cause speed and feed rates have a direct influ-ence on tool life, it is desirable to have charts or graphs for all possible tool and titanium alloy combinations, as well as machining techniques.
Considering the range of alloys, tool composi-tions, and machining techniques possible, such charts are not likely to be available for all situa-tions. However, charts such as Table 10.3 have been compiled for some other machining tech-niques. (See Appendix G for more detailed in-formation on machining of titanium and its al-loys.)
Machining recommendations, such as noted above in Table 10.3 and similar sources, can require modification to fit particular circum-stances in a given shop. For example, cost, storage, or other requirements can make it im-practical to accommodate a very large number of different cutting fluids. Savings achieved by making a change in cutting fluid can be offset by the cost of changing fluids. Likewise, it might not be economical to inventory cutting tools that have only infrequent use. Further-more, the design of parts can limit the rate of metal removal in order to minimize distortion (e.g., of thin flanges) and to corner without ex-cessive inertia effects.
An illustration of typical machining parame-ters used to machine Ti-6Al-4V bulkheads con-taining deep pockets, thin flanges, and floors at an airframe manufacturer is given in Table 10.4. A bulkhead frequently contains numer-ous pockets and some flanges as thin as 0.76 mm (0.030 in.). Typical bulkhead rough forgings can weigh in excess of 450 kg (1000 lb), but the finished part is less than 67.5 kg
(150 lb) after machining. Extensive machining is done on gas turbine engine components, just as is done on the larger airframe components.
Table 10.5 lists typical parameters for machin-ing Ti-6Al-4V jet engine components, such as fan disks, spacers, shafts, and rotating seals.
Increased Productivity with Special Tech-niques. The inability to improve cutting tool performance for titanium alloys by developing new cutting tool materials—coatings in particu-lar—has been very frustrating. Likewise, very little improvement in productivity has been ex-perienced by exploring new combinations of speeds, feeds, and depths. Some developments of interest include specially designed turning tools and milling cutters, along with the use of a special end-mill pocketing technique.
One of the practical techniques for increas-ing productivity is to determine the optimum cost in machining a given titanium part for a specific machining operation. If specific data are available relating tool life to speed, feed, and depth for a given operation and cutter, it is possible to calculate the overall cost and time of machining as a function of the cutting pa-rameters. Some companies are using computers to perform such cost analyses and to arrive at minimum costs and optimum production rates for specific machining operations.
Fire Prevention. Fine particles of titanium can ignite and burn. Use of water-base coolants or large volumes of oil-base coolants generally eliminates the danger of ignition during ma-chining operations. However, an accumulation of titanium fines can pose a fire hazard. Chips, turnings, and other titanium fines should be collected regularly to prevent undue accumula-tion and should always be removed from ma-chines at the end of the day.
Machining / 81
Fig. 10.3 Effect of various cutting fluids and speeds on tool life when drilling Ti-6Al-4V (375 HB).
HSS, high-speed steel
Salvageable material should be placed in covered, labeled, clean, dry, steel containers and stored, preferably in an outside yard area.
Unsalvageable fines should be properly dis-posed. Titanium sludge should not be permitted
to dry out before being removed to an isolated, outside location.
Dry powders developed for extinguishing combustible metal fines are recommended for the control of titanium fires. For maximum
safety, such extinguishers should be readily available to each machinist working with tita-nium. Dry sand retards, but does not extin-guish, titanium fires. Carbon dioxide and chlo-rinated hydrocarbons are not recommended.
82 / Titanium: A Technical Guide
Table 10.3 Nominal speeds and feeds for turning titanium and titanium alloys with high-speed tool steel and carbide tools
Carbide tool, uncoated
High-speed tool steel Tool
Hardness, Depth of cut(a), Speed, Feed, mm/rev Tool material Speed, m/min (sfm) Feed, material
Material HB Condition mm (in.) m/min (sfm) (in./rev) grade(b), AISI Brazed Indexable mm/rev (in./rev) grade
Commercially pure: Ti (99.0) 110–170 Annealed 1.0 (0.040) 76 (250) 0.13 (0.005)(b) T15, M42 160 (525) 172 (565) 0.13 (0.005) C-3 4.0 (0.150) 67 (220) 0.25 (0.010) T15, M42 137 (450) 148 (485) 0.25 (0.010) C-2 7.5 (0.300) 53 (175) 0.38 (0.015) T15, M42 104 (340) 110 (360) 0.38 (0.015) C-2
16.0 (0.625) … … … 52 (170) 55 (180) 0.50 (0.020) C-2
140–200 Annealed 1.0 (0.040) 58 (190) 0.13 (0.005) T15, M42 137 (450) 152 (500) 0.13 (0.005) C-3 4.0 (0.150) 52 (170) 0.25 (0.010) T15, M42 119 (390) 130 (425) 0.25 (0.010) C-2 7.5 (0.300) 46 (150) 0.38 (0.015) T15, M42 88 (290) 98 (320) 0.38 (0.015) C-2
16.0 (0.625) … … … 44 (145) 49 (160) 0.50 (0.020) C-2
200–275 Annealed 1.0 (0.040) 35 (115) 0.13 (0.005) T15, M42 88 (290) 113 (370) 0.13 (0.005) C-3 4.0 (0.150) 32 (105) 0.25 (0.010) T15, M42 76 (250) 98 (320) 0.20 (0.008) C-2 7.5 (0.300) 29 (95) 0.38 (0.015) T15, M42 58 (190) 73 (240) 0.38 (0.015) C-2
16.0 (0.625) … … … 29 (95) 37 (120) 0.50 (0.020) C-2
Alpha alloys: Ti-5Al-2.5Sn, 300–340 Annealed 1.0 (0.040) 24 (80) 0.13 (0.005) T15, M42 66 (215) 76 (250) 0.13 (0.005) C-3
Ti-5Al-2.5Sn-ELI, 4.0 (0.150) 21 (70) 0.25 (0.010) T15, M42 56 (185) 66 (215) 0.20 (0.008) C-2
Ti-6Al-2Nb-1Ta-0.80Mo 7.5 (0.300) 18 (60) 0.38 (0.015) T15, M42 43 (140) 49 (160) 0.25 (0.010) C-2
16.0 (0.625) … … … 21 (70) 24 (80) 0.38 (0.015) C-2
Alpha-beta alloys: Ti-6Al-4V, 310–350 Annealed 1.0 (0.040) 21 (70) 0.13 (0.005) T15, M42 52 (170) 69 (225) 0.13 (0.005) C-3
Ti-6Al-4V-ELI, Ti-6Al-2Sn-4Zr-2Mo, 4.0 (0.150) 18 (60) 0.25 (0.010) T15, M42 44 (145) 59 (195) 0.20 (0.008) C-2
Ti-6Al-2Sn-4Zr-2Mo-0.25Si, 7.5 (0.300) 15 (50) 0.38 (0.015) T15, M42 34 (110) 44 (145) 0.25 (0.010) C-2
Ti-6Al-2Sn-4Zr-6Mo 16.0 (0.625) … … … 17 (55) 21 (70) 0.38 (0.015) C-2
320–380 Solution treated and 1.0 (0.040) 20 (65) 0.13 (0.005) T15, M42 49 (160) 58 (190) 0.13 (0.005) C-3
aged 4.0 (0.150) 17 (55) 0.25 (0.010) T15, M42 41 (135) 50 (165) 0.20 (0.008) C-2
7.5 (0.300) 14 (45) 0.38 (0.015) T15, M42 26 (85) 37 (120) 0.25 (0.010) C-2
16.0 (0.625) … … … 15 (50) 18 (60) 0.38 (0.015) C-2
Beta alloys: Ti-3Al-8V-6Cr-4Mo-4Zr, 275–350 Annealed or solution 1.0 (0.040) 12 (40) 0.13 (0.005) T15, M42 38 (125) 49 (160) 0.13 (0.005) C-3
Ti-8Mo-8V-2Fe-3Al, Ti-11.5 treated 4.0 (0.150) 9 (30) 0.25 (0.010) T15,M42 32 (105) 41 (135) 0.20 (0.008) C-2
Mo-6Zr-4.5Sn, Ti-10V-2Fe-3Al, 7.5 (0.300) 7 (25) 0.38 (0.015) T15, M42 24 (80) 26 (85) 0.25 (0.010) C-2
Ti-13V-11Cr-3Al 16.0 (0.625) … … … 12 (40) 15 (50) 0.38 (0.015) C-2
350–440 Solution treated and 1.0 (0.040) 11 (35) 0.13 (0.005) T15, M42 36 (110) 38 (125) 0.13 (0.005) C-3
aged 4.0 (0.150) 7 (25) 0.25 (0.010) T15, M42 27 (90) 32 (105) 0.20 (0.008) C-2
7.5 (0.300) … … … 21 (70) 24 (80) 0.25 (0.010) C-2
16.0 (0.625) … … … 11 (35) 12 (40) 0.38 (0.015) C-2
ELI, extra-low interstitial. (a) Caution: check power requirements on heavier depths of cut. (b) Any premium high-speed tool steel can be used. Source: Metcut Research Associates Inc.
Table 10.4 Some typical machining parameters used to machine airframe bulkheads from an alpha-beta (Ti-6Al-4V) alloy
Operation Part surface Cutter description and material Speed, m/min (ft/min) Feed, mm/tooth (in./tooth)
Milling rough/finish Peripheral ML flanges 50.8 mm (2 in.) diam × 152.4 mm (6 in.) flute length, 6 flute, 35° helix, M42 15 (50) 0.2/0.0096 (0.0066/0.0096) Milling rough Thin flanges, walls 31.8 mm (1.25 in.) diam × 50.8 mm (2 in.) flute length, 4 flute, 35° helix, M42 15 (50) 0.2/0.009 (0.0062/0.009) Milling finish Thin flanges 19.1 mm (0.75 in.) diam × 63.5 mm (2.5 in.) flute length, 4 flute, 35° helix, M42 15 (50) 0.1/0.0034 (0.0024/0.0034) Milling finish Pocket floor 31.8 mm (1.25 in.) diam × 50.8 mm (2 in.) flute length, 4 flute, 35° helix, M42 15 (50) 0.2/0.009 (0.0062/0.009)
Table 10.5 Example of typical parameters for machining gas turbine components from an alpha-beta (Ti-6Al-4V) alloy
Cutting speed, Depth of cut,
Operation Tool material m/min (ft/min) Feed mm (in.)
Turn (rough) C-2 45 (150) 0.254 mm (0.010 in.)/rev 5.207 (0.250)
Turn (finish) C-2 60 (200) 0.152–0.203 mm (0.006–0.008 in.)/rev 0.254–0.762 (0.010–0.030)
Turn (finish) C-2 90 (300) 0.152–0.203 mm (0.006–0.008 in.)/rev 0.254–0.762 (0.010–0.030)
End mill (19.05–25.4 mm, or34–1 in. diam) M42 HSS (a) 18 (60) 0.076 mm (0.003 in.)/tooth Axial depth 3.175 (0.125) Radial depth: up to two-thirds cutter diameter End mill (19.05–25.4 mm, or34–1 in. diam) C-10 60 (200) 0.127 mm (0.005 in.)/tooth Axial depth: 3.810–5.080 (0.150–0.200) Radial depth: up to two-thirds
cutter diameter Drill (6.35–12.70 mm, or14–12in. diam) M42 HSS(a) 9 (30) 0.127 mm (0.005 in.)/rev
Drill (6.35–12.70 mm, or14–12in. diam) C-2 12 (40) 0.102 mm (0.004 in.)/rev
Ream M42 HSS (a) 6 (20) 0.254 mm (0.010 in.)/rev
C-2 10.5 (35) 0.254 mm (0.010 in.)/rev
Tap M7 HSS 4.5 (15) …
Broach M3 HSS 3.6 (12) 0.076 mm (0.003 in.)/tooth max
Spline shape M42 HSS 3.6 (12) 0.305 mm (0.012 in.)/stroke
(a)Designates tool material most widely used. HSS, high-speed steel
Water should never be applied directly to a tita-nium fire.