Titanium alloys and their machinability.pdf

ELSEVIER Journal of Materials Processing Technology 68 (1997) 262-274

]awa~(ff Materials Processing Technology

Titanium alloys and their machinability a review

E.O. Ezugwu *, Z.M. Wang School of Engineering Systems and Design, South Bank University, London SEI OAA, UK

Received 20 October 1995

Abstract

Although there have been great advances in the development of cutting tool materials which have significantly improved the machinability of a large number of metallic materials, including cast irons, steels and some high temperature alloys such as nickel-based alloys, no equivalent development has been made for cutting titanium alloys due primarily to their peculiar characteristics. This paper reviews the main problems associated with the machining of titanium as well as tool wear and the mechanisms responsible for tool failure. It was found that the straight tungsten carbide (WC/Co) cutting tools continue to maintain their superiority in almost all machining processes of titanium alloys, whilst CVD coated carbides and ceramics have not replaced cemented carbides due to their reactivity with titanium and their relatively low fracture toughness as well as the poor thermal conductivity of most ceramics. This paper also discusses special machining methods, such as rotary cutting and the use of ledge tools, which have shown some success in the machining of titanium alloys. 1997 Elsevier Science S.A.

Keywords: Titanium alloys; Machinability; Notching; Flank wear; Cratering; Chipping; AttrRion wear; Dissolution-diffusiom Plastic deformation; Rotary cutting; Ledge tools

1. Introduction

Titanium and its alloys are used extensively in aerospace because of their excellent combination of h i~ specific strength (strength-to-weight ratio) which is maintained at elevated temperature, their fracture resis- tant characteristics , and their exceptional resistance to corrosion. They are also being used increasingly (or being considered for use) in other industrial and commercial applications, such as petroleum refining, chemical pro~ssing, surgical implantation, pulp and paper, pollution control, nuclear waste storage, food processing, electrochemical (including cathodic protection and extractive metallurgy) and marine applications [1]. They have become established engineering materials available in a range of alloys and in all the wrought forms, such as billet, bar, plate, sheet, strip, hollows, extrusions, wire, etc.

Despite the increased usage and production of titanium and its alloys, they are expensive when compared to many other metals because of the complexity of the extraction process, difficulty of melting, and problems

* Corresponding author. Fax: +44 171 8157699.

0924-0136/97/$1Z00 1997 Elsevier Science S.A. All rights reserved. Pll S0924-0136(96)00030-1

during fabrication and machining [2,3]. Near net-shape methods such as castings, isothermal forging, and pow- der metallurgy have been introduced to reduce the cost of titanium components [4-9]. However, most titanium parts are still manufactured by conventional machining methods. Virtually all types of machining operations, such as turning, milling, drilling, reaming, tapping, sawing, and grinding, are employed in producing aerospace components [10]. For the manufacture of gas turbine engines, turning and drilling are the major machining operations, whilst in airframe production, end milling and drilling are amongst the most important machining operations.

The machinability of titanium and its alloys is generally considered to be poor owing to several inherent properties of the materials. Titanium is very chemically reactive and, therefore, has a tendency to weld to the cutting tool during machining, thus leading to chipping and premature tool failure. Its low thermal conductivity increases the temperature at the tool/workpiece interface, which affects the tool life adversely. Additionally, its high strength maintained at elevated temperature and its low modulus of elasticity further impairs its machinability [11]. In 1955, Siekmann [12] pointed out

E.O. Ezugwu, Z.M. Wang/Journal of Materials Processing Technology 68 (1997) 262-274 263

that '"machining of titanium and its alloys would always be a problem, no matter what techniques are employed to transform this metal into chips". The poor machinability of titanium and its alloys have led many large companies (for example Rolls-Royce and General Electrics) to invest large sums of money in developing techniques to minimise machining cost. Reasonable production rates and excellent surface quality can be achieved with conventional machining methods if the unique characteristics of the metal and its alloys are taken into account [13|.

2. Metallurgy of titanium alloys

2.1. Alloying additions of titanium alloys

Pure titanium undergoes an allotropic transformation at 882C, changing from the low-temperature close-packed hexagonal phase to the higher-temperature body-centred cubic fl-phase. Alloying elements in titanium alloying tend to stabilise either the phase, or the allotrope fl phase that alters the transformation temperature and changes the shape and ex- tent of the e-/3 field [14,15]. Elements that raise the transformation temperature are e-stabilisers, these being aluminium (AI), oxygen (O), nitrogen (N) and carbon (C), of which AI is a very effective ~-strengthening element at ambient and elevated temperatures up to 550C. The low density of AI is an important additional advantage. O, N and C are regarded as impurities in commercial alloys. However, O is used as a strengthening agent to provide several grades of commercially-pure titanium offering various combina- tions of strength and fabricability [16].

Although the addition of tin (Sn) or zirconium (Zr) also strengthen the 0~ phase, these elements have little influence on the transformation temperature because they exhibit extensive solubility in e- and fl-titanium and are known as 'neutral elements'.

Elements that produce a decrease in the transformation temperature are fl-stabilisers, involving twc types, fl-isomorphous and fl-eutectoid [16]. The most important fl-isomorphous alloying additions are molybdenum (Mo), vanadium (V), niobium (Nb). These elements are mutually soluble with fl-titanium, increasing addition of the solute element progressively depressing the fl to e transformation up to ambient temperature, fl-eutectoid elements have restricted solubility in fl-titanium and form intermetaUic compounds by eutectoid decomposition of the fl-phase. The two most important examples of such elements used in commercial alloys are copper (Cu) and silicon (Si).

2.2. Ch~.ss(fication o/' titanium alloys

Titanium alloys may be divided into four main groups, according to their basic metallurgical characteristics: e alloys, near e alloys, e-fl alloys and alloys [10,14,17-19].

e alloys: These contain ~-stabilisers, sometimes in combination with neutral elements, and hence have an e- phase microstructure. One such single phase a-alloy, Ti 5-2 (Ti-SAI-2Sn), is still available commercially and is the only one of its type to survive besides commercially-pure titanium. The alloy has excellent tensile properties and creep stability at room and elevated temperatures up to 300C. a-alloys are used chiefly for corrosion resistance and cryogenic applications. Near ~ alloys: These alloys are highly e-stabilised and contain only limited quantities of fl-stabilising elements. They are characterised by a microstructure consisting of c~ phase containing only small quantities of fl phase. Ti 8-1-1 (T i -8A I - IMo- IV ) and IM! 685 (Ti-6AI-5Zr-0.5Mo-0.25Si) are examples of near e alloys. They behave more like ~-alloys and are capable of operating at greater temperatures of between 400 and 520C. e-fl alloys: This group of alloys contains addition of .e- and fl-stabilisers and they possess microstruetures consisting of mixtures of e- and /3-phases. Ti 6-4 (Ti- 6AI-4V, designated IMI 318) and IMI 550 (Ti-4AI-2Sn-4Mo-0.5Si) are its most common alloys. They can be heat-treated to high strength lev- els and hence are used chiefly for high-strength applications at elevated temperatures of between 350 and 400C. fl alloys: These alloys contain significant quantities of/3-stabilisers and are characterised by high hardenability, improved forgeability and cold formability, as well as high density. Basically, these alloys offer an ambient temperature strength equivalent to that of e-B alloys, but their elevated temperature properties are inferior to those of the e-fl alloys. As far as the gas turbine engine is concerned,

the most important alloys are those in the near and e-/3 groups, the ct-/3 alloy Ti--6AI-4V being the most commonly used titanium alloy, accounting for over 45% of the total titanium production [20]. Table 1 gives important properties of Ti-6AI-4V, and of AISI 1045 steel, as a basis for comparison

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264 E.O. Ezugwu, Z.M. Wang/Journal of Materials Processing Technology 68 (1997) 262-274

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3. Machining of titanium alloys

Progress in the machining of titanium alloys has not kept pace with advances in the machining of other materials due to their high temperature strength, very low thermal conductivity, relatively low modulus of elasticity and high chemical reactivity. Therefore, success in the machining of titanium alloys depends largely on the overcoming of the principal problems associated with the inherent properties of these materials, as discussed below:

High cutting temperature: It is well known that high cutting temperatures are generated when machining titanium alloys and the fact that the high temperatures act close to the cutting edge of the tool are the principal reasons for the rapid tool wear commonly observed. As illus- trated in Fig. l, a large proportion (about 80%) of the heat generated when machining titanium alloy Ti-6AI-4V is conducted into the tool because it cannot be removed with the fast flowing chip or bed into the workpiece due to the low thermal conductivity of titanium alloys, which is about 1/6 that of steels [21,22]. About 50% of the" heat generated is absorbed into the tool when machining steel. Investi- gation of the distribution of the cutting temperature has shown that the temperature gradients are much steeper and the heat-affected zone much smaller and much closer to the cutting edge when machining titanium alloys because of the thinner chips produced (hence short chip-tool contact length) and the presence of a very thin flow zone between the chip and the tool (approximately 8 ~tm compared with 50 lam when cutting iron under the same cutting conditions) which causes high tool-tip temperatures of up to about 1100C [23-27]. High cutting pressures: The cutting forces recorded when machining titanium alloys are reported to be similar to those obtained when machining steels [28], thus the power consump- tion during machining is approximately the same or lower (Table 2) [10]. Much higher mechanical stresses do, however, occur in the immediate vicinity of the cutting edge when machining titanium alloy. Konig [21] has reported higher stresses on the tool when machining Ti-6AI-4V (titanium alloy) than when machining Nimonic 105 (nickel-based alloy) and three to four times those observed when machining steel Ck 53N (Fig. 2). This may be attributed to the unusually small chip-tool contact area on the rake face, (which is about one-third that of the contact area for steel at the same feed rate and depth of cut) [29] and partly to the high resistance of Ti-aUoy to deformation at elevated temperatures, which only reduces considerably at temperatures in excess of 800C [21,30].

E O. Ezugwu, Z.M. H"atlg Journal o/Materials Process'ing Technology 68 (1997)262 274 265

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Thermal Conductivity~,. n ( J lmm s C )

Fig. I. Distr ibut ion of thermal load when machin ing t i tan ium and steel (after Konig [2t]).

Chatter: Chatter is another main problem to be overcome when machining titanium alloys, especially for finish machining, the low modulus of elasticity of titanium alloys being a principal cause of the chatter during machining. When subjected to cutting pressure, titanium deflects nearly twice as much as carbon steel the greater spring-back behind the cutting edge resulting in premature flank wear. vibration and higher cutting temperature [30]. In effect, there is a bounc- ing action as the cutting edge enters the cut. The appearance of chatter may also be partly ascribed to the high dynamic cutting forces in the machining of titanium. This can be up to 30% of the value of the static forces [21] due to the 'adiabatic or catastrophic thermoplastic shear' process by which titanium chips are formed [31-36]. Additional criteria for tool materials: Besides high cutting temperatures~ high mechanical pressure and high dynamic loads in the machining of titanium alloys, which result in plastic deformation and/or rapid tool wear, cutting tools also suffer from strong the chemical reactivity of titanium. Titanium and its alloys react chemically with almost all tool materials available at cutting temperature in excess of 500C due to their strong chemical reactivity. The tendency for chips to pressure weld to cutting tools, severe dissolution-diffusion wear, which rises with increasing temperature, and other peculiar characteristics already mentioned, demand additional criteria in the choice of the cutting tool materials.

These problems may be minimised by employing very rigid machines, using proper cutting tools and set-ups, minimising cutting pressures, providing copious coolant flow and designing special tools or non-conventional cutting methods.

3.1. Tool materials for machin#lg tit,~ziunl alloys

Major improvements in the rate at which workpieces are machined usually result from the development and application of new tool materials. Over the last few decades, there have been great advancements in the development of cutting tools, including coated carbides, ceramics, cubic boron nitride and polycrystalline diamond. These have found useful applications in the machining of cast irons, steels and high temperature alloys such as nickel-based alloys. However, none of these newer developments in cutting tool materials have had successful application in improving the machinability of titanium alloys because of the paramount quali- ties required of tool materials, which are: (i) high hot hardness to resist the high stresses involved; (ii) good thermal conductivity to minimise thermal gradients and thermal shock; (iii) good chemical inertness to depress the tendency to react with titanium; (iv) toughness and fatigue resistance to withstand the chip segmentaron process; and (v) high compressive, tensile and shear strength.

Straight tungsten carbide (WC/Co) cutting tools have proven their superiority in almost all machining processes of titanium alloys and interrupted cutting (end

266 E.O. E'.,ugwu, Z.M. Wang/Journal of Materials Process#tg Technology 68 (1997) 262-274

Table 2 Average unit power requirements for turning, drilling and milling (horsepower per cubic inch/minute) (after Kahles et al. [10])

Material Hardness Rc or Bhn Turning with HSS and carbide Drilling with HSS (3000 kg) tools d rills

Milling with HSS and carbide tools

Steels 35-40 Rc 1.4 1.4 1.5 Titanium alloys 250- 375 1.2 1. I I. 1 Nickel based al- 200-360 2.5 2.0 2.0

loys

milling, tapping, broaching and planing), drilling and reaming being performed best by high-speed steel tools. Freeman [25] established the better performance of the WC/Co grades, no matter which wear mechanism is taking place. It has been found that the best grades in cutting applications are the C-2, represented by ISO K20 [37-39]. Dearnley, Grearson and Aucote [23,40], carrying out many trials involving various tool materials in the continuous turning of Ti-6AI-4V, also confi- rmed the K grade carbides as the best choice. They suggested that those WC/Co alloys with Co contents of 6 wt% and a medium WC grain size (about 0.8 and 1.4 ~tm) gave the optimum performance. A recent study [41] advises that straight cobalt-base tungsten carbide cutting tools implanted with either chlorine or indium are very effective in the machining of titanium and their alloys.

It has been proven that steel cutting grades (P grades of ISO codes) of cemented carbides are not suitable for machining titanium alloys because of the greater wear rate of the mixed carbide grains than that of the WC grains and because of their thermal properties [21,23,25,42]. All coated carbide tools tested (cemented carbides coated by TiC, TiCN, TiN-TiC, AI20~-TiC, TiN-Ti(C,N)-TiC, A1203, HfN, and TiB_,) also show greater wear rates than those of straight grade cemented carbides [15,42,43]. Ezugwu and Pashby have, however, reported that a very fine grain TiN/steel com- pound coated with a layer of TiN (using the PVD technique) shows outstanding performance when end milling titanium alloy (IMI 318) at high cutting conditions beyond those possible with carbide end mills [44].

General-purpose high-speed steel tools (such as M 1, M2, M7, and M10) are often suitable in the machining of titanium. However, the best results have been achieved with highly alloyed grades, such as M33, M40, and M42 [10,13].

Even though ceramics have improved in quality and found: increased application in the machining of difficult-to-cut materials, especially high-temperature alloys (such as nickel-based alloys), they have not replaced cemented carbides and high-speed steels due to the poor thermal conductivity of most ceramics, their relatively low fracture toughness and their reactivity with titanium [45].

The superhard cutting tool materials (cubic boron nitride and polycrystalline diamond) have also shown a good performance in terms of wear rate in the machining of titanium [23,46]. However, their application are limited due to their high price.

3.2. Tool failure modes and wear mechanisms

Some specific studies on tool failure modes and wear mechanisms when machining titanium alloys have been conducted [23,25,40,42,43,47-49]. Cutting tool materials encounter severe thermal and mechanical shocks when machining titanium alloys, the high cutting stresses and high temperatures generated at and/or close to the cutting edge greatly influencing the wear rate and hence the tool life. Notching, flank wear, crater wear, chipping and catastrophic failure are the prominent failure modes when machining titanium alloys, these being caused by a combination of high temperature, high cutting stresses, the strong chemical reactivity of titanium, the formation process of catastrophic shear (lamellar) chips, etc.

Different tool materials tend to have different re- sponses to different wear mechanisms when machining titanium alloys. Because of the rapid loss of their hardness at elevated temperatures above 600C, high- speed steel tools suffer severe plastic deformation which accelerates the rate of wear [21]. Plastic deformation can also be a major contributor to wear mechanisms of other tool materials when machining titanium alloys, especially in the case of high-speed machining, due to the presence of high compressive stresses and the development of high temperatures close to the cutting edge [18,23,46].

Freeman [25] carried out a tool life study of steel cutting grades (containing carbides other than tungsten) and straight WC/Co grades of cemented carbides when machining two commercially-available titanium alloys (an alpha-beta alloy and a beta alloy) and reported that plastic deformation occurred, especially at higher cutting speeds, and that a crater can also be formed by shearing on the rake face, both of these effects acceler- ating other wear mechanisms considerably. The tools tested also suffered diffusion during machining. The steel-cutting grades of cemented carbide are inferior to

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6 1810190 185 o 15~18Ol0Ol70 o 0ol8ob4 r = 0.5 ram, Chip cross-section: a s = 1.5 0.25 mm 2

Fig. 2. Normal and tangential stresses

straight grades because of the presence of the mixed carbide grains (such as TiC and TaC). The mechanism of attrition acts preferentially on the mixed carbide grains, and tools containing mixed carbides also wear by diffusion quicker than WC/Co tools because these mixed carbides dissolve preferentially in titanium.

According to Dearnley et al. [23,40], the rake and flank wear of all of the tool materials tested resulted from dissolution-diffusion and attrition when turning titanium alloys. Dissolution-diffusion wear predomi- nated on the 'rake face' of all the uncoated cemented carbides and ceramics, except for sialon, where attrition is the competitive wear mechanism. On the 'flank face', attrition wear controls the wear rates of ceramics and steel-cutting grades of cemented carbides, whilst it is less predominant on the flank faces of straight grades of cemented carbides, which can probably be attributed to the increased toughness of WC/Co alloys compared to that of other grades. For these materials, dissolution- diffusion wear controls the wear rates of flank wear. Coatings of TiN, TiC, A1203 and HfN on both the rake and flank faces are worn more rapidly than uncoated WC/Co by either dissolution-diffusion or attrition wear mechanisms. Coatings of TiB2 are relatively more resis- tant than others, as are CBN tools [23]. Notch wear, which severely affects ceramic tools, is caused mainly by a fracture process, which agrees with the fracture

in machining (alter Konig [21]).

mechanism proposed by Katayama and lmai [43]. However, a smoother notch wear surface (perhaps caused by reaction with the atmosphere) has also been reported with Sialon tools [23].

Hartung and Kramer [42] have suggested that the presence of a 'flow zone" at the chip-tool interface will eliminate the sliding between them, thus maximising the wear resistance. If a flow zone is formed the wear will be limited by the diffusion rate of the tool constituents through this layer. This process of weal" is believed to occur at a lower rate compared to that caused by physical motion of the chip under sliding conditions (i.e. attrition); however, attrition has been found in other machining operations when a flow zone is present, It was found that WC/Co grades of cemented carbide and polycrystailine diamond are the best tool materials to machine titanium because a stable reaction layer is formed between the tool and the chip. The carbon from either WC/Co-based composites or polycrystalline diamond reacts with the workpiece to form TiC. This reaction layer has high deformation resistance at the cutting temperature and adheres strongly to both the tool and the chip. This layer quickly becomes saturated, limiting the mass transport of tool constituents from the tool surface and reducing the wear rate. This, however, seems to conflict with the fact that titanium carbide formed by chemical vapour depo-

268 E.O. Ezugwu, Z.M. Wang/Journal of" Materials Processing Tectmology 68 (1997) 262-274

Table 3 Typical parameters for machining Ti-6AI-4V jet engine components (after Kahles et al. [10])

Operation Tool materials Cutting speed (in./min) Feed rate Depth of cut (in3

Turning (rough) C-2 150 001,~ in./rev. Turning (finish) C-2 200 0.006-0.008 in./rev. Turning (finish) C-2 300 0.006-0.008 in./rev. End mill (~-1' dia.) M42 HSS 60 0.003 in./tooth End mill (~-1' dia.) C-!0 200 0.005 in./tooth Drill i 1, ~-~ dta.) M42 HSS 30 0.005 in./rev. Drill i i, (~-~ dia.) C-2 40 0.004 in./rev. Ream M42 HSS 20 0.010 in./rev. Ream C-2 35 0.010 in./rev. Tap M7 HSS 15 - - Broach M3 HSS 12 0.003 in.,'tooth max. Spline shape M42 HSS 12 0.012 in.-',,stroke

0.250 0.010 0.030 0.0t0 0.030 0.125" 0.150-0.200"

~Axial depth. Radial depth is up to two-thirds the cutter diameter.

sition on commercial tool tips is not effective in sup- pressing wear.

It has been reported that plastic deformation and the development of cracks by a thermal shock process will dominate the wear mechanisms when machining titanium at high cutting speeds with cemented carbide and ceramic tools and that the crater wear is closely related to the chemical composition of the tools [43,50].

In the milling of titanium with WC/Co grades and coated tools, chipping is a major failure mode of the tools [39]. This type of wear is a result of the combination of high temperature, and high thermal, mechanical and cyclical stresses, as well as adhesion of the work material onto the tool faces [50,51]. Ezugwu and Machado [39] found that prior to chipping, an initial normal flank wear takes place and that this contributes to enhancing the critical conditions for the first appearance of chipping. Min and Youzhen [47,52] suggested that a carbide-rich layer in the tool surface region and a carbide deficient layer in the tool subsurface region are formed by diffusion between the too! and the workpiece. The carbon redistribution results in surface weakening and embrittlement of the tool, which en- courages chipping and increased tool wear rate. Bhat- tacharyya et al. [51] have found that at high cutting speeds the high temperature developed enables chemical interactions between the work material and the coating layers to take place and the layers are thus rapidly removed resulting in the substrate acting as the cutting edge over most of the tool life.

3.3. Cutting parameters and tool geometry

Data on cutting parameters have been developed experimentally on a wide variety of titanium alloys. An example of typical machining parameters currently used for machining Ti-6AI-4V jet engine components (such as fan disks, spacers, shafts, and rotating seals) are shown in Table 3 [10].

Cutting speed has the most considerable influence on tool life. The latter can be plotted against cutting speed for a given cutting tool material at a constant feed rate and depth of cut [10], Fig. 3 being a typical example of a large number of tool-life charts available elsewhere [53-58]. It can be seen that tool life is extremely short at high cutting speeds but improves dramatically as the speed is reduced.

Another important variable affecting the tool life is the feed rate. Often the tool life is not changed dramatically with a change in feed, but titanium alloys, however, are very sensitive to changes in feed (Fig. 3). Chandler [l 3] has suggested that operation at high feeds is more desirable to increase productivity.

When machining titanium, the effect of the depth of cut must be considered also. As indicated in Fig. 4 [13], increasing the depth of cut from 0.75 to 3 mm decreases the tool life from 46 to 14 min at a cutting speed of 60 m/min.

When machining titanium alloys, the tool geometry has a considerable influence on the tool life. Koman- duri and Reed [59] suggested that a new tool geometry, consisting of a high clearance angle (from l0 to 15 ) together with a high negative rake angle (from - l0 to -150), increases the tool life of straight~ cemented tungsten carbide (WC/Co) significantly compared with the standard tool geometry ( - 5 rake angle and 5 clearance angle).

3.4. Cutting fluid

The high temperature and the high stresses developed at the cutting edge of the tool are the principal problems when machining titanium alloys. To minimise the problem, a cutting fluid must be applied, as a basic rule. The cutting fluid not only acts as a coolant but also functions as a lubricant, reducing the tool temperatures and lessening the cutting forces and chip welding that are commonly experienced with titanium alloys,

E.O. Ezugwu, Z,M. ~ 'ang Jmtrmd ~! Materiat~ Pr~ces.~in~ Techm,h,y 68 (1997J 262 274 260

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35

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20

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i \ .005in/rev

, i L I I . . . . . .

50 150 250

Cutting speed - feet/minute

Ti-6AI-4V (Solution treated and aged 388 BHN) C-2 (883) Carbide

350

Fig. 3. Effect of cutting speed and feed on tool

thus improving the tool life. The correct choice of cutting fluid has a significant effect on tool life. Copi- ous, uninterrupted flow of coolant will also provide a good flushing action to remove chips, minimise thermal shock of milling tools and prevent chips from igniting, especially when grinding titanium [10,13,18,30,60,61]. Additionally, a high pressure coolant supply can result in small, discontinuous and easily disposable chips, unlike the long continuous chips produced when machining with a conventional coolant supply [62].

Catt and Milwain r601 fbund that an extreme-pres- L J

sure emulsion oil gives reasonable results, whilst those containing phosphates give the best results due to their good cooling properties and great anti-welding properties with a suitable lubricant. Difficulties were, however, experienced due to the activity of the fluid, which caused corrosion of the machine tool. Chlorine compounds are used partly because of their undoubted superiority for particular operations, such as grinding, broaching, and tapping. It was found that sulphur compounds led to sulphur attack on turbine blades made in titanium alloys, which led to an embargo on their use. Many ~f the early chlorinated cutting fluids containing chlorinated hydrocarbons also were effective, but these were banned because of their toxicity, it have been found that chlorokerosenes are equally effective without the attendant risks [60].

Konig and Schroder [61] suggested that the application of coolants could suppress the built-up edge that

life in t,:,ning T i -6AI -4V (after Kahles et al. [10]L

was observed generally during the face milling of titanium with HSS- and carbide-tools. Tests did however show, that the application of coolants as concentrates, emulsions, or solutions on a mineral oil-, mineral oil free-, or synthetic, basis in liquid jet or in spray cooling causes more wear than does dry cutting. Work carried out at the Air Force Materials Laboratory [53-58], concluded that chlorine-containing cutting fluids do not always provide a better tool life. For particular alloys and operatious, dry machining is preferred, which agrees with the observations of Konig and Schroder. Usually the heavy chlorine-bearing fluids excel in operations such as drilling, tapping, and broaching.

According to Chandler [13], water-base fluids are more efficient than oils. He found that a weak solution of rust inhibitor and/or water-oil (5-10%) solution is the most practical fluid for high-speed cutting operations. Slow speed and complex operations may require chlorinated or sulfurized oils to minimise frictional forces and the galling and seizing tendency of titanium. Chandler [13] pointed our that chlorinated cutting fluids should be used with great caution because of their potential to cause stress corrosion cracking.

A series of tests on the chlorine problem was run in Germany [63]. It was found that the machining of titanium with a lubricant containing a chlorine additive developed surface films of a thickness equal to or less than 150 lam (1500 ~,) and a chlorine content of at most 3 at.%. Similar films with i.5 at.% and 100--150 ~tm

270 E.O. Ezugwu. Z.M. Wang/Journal of Materials ProcessOJg Technology 68 (1997) 262-274

5O

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0 ~5o ~Ts 2oo 225 2so 275

Cutting speed, feet/rain

Work Material: Ti-5AI-2Sn (Annealed 321 BHN) Tool material: C-2 (883) Carbide

Fig. 4. Effect of cutting speed and depth of cut on tool life in turning Ti-5AI-2Sn (after Chandler [13]).

(1000-1500 ,~,) thickness were obtained by machining titanium with demineralized water. The work concluded that the prohibition of machining titanium with lubri- cants containing chlorine additives can no longer be maintained.

3.4.1. Special machining techniques The inability to improve cutting-tool performance by

developing new cutting-tool materials has been very frustrating, Likewise, very little improvement in productivity has been experienced by exploring new combi- nations of speeds, feeds, and depths of cut. However, increased productivity and long tool life have been achieved by special machining techniques, including specially designed ledge tools and rotary tools.

3.4.2. Ledge tools Ledge tools are characterised by a thin cutting edge

that overhangs a small distance equal to the desired depth of cut (Fig. 5) [64,65]. The advantage of these tools, developed by the General Electric Company, lies in the limited maximum flank wear of the tools during machining. As cutting proceeds, they first achieve maximum flank wear and then the length of the overhang wears back without further development in flank wear due to a restricted clearance face. Thus the tools can perform for a long time, as the tool life is not limited by the amount of flank wear but by the size of the edge [65]. Because of its restricted geometry these tools are applicable only to straight cuts in turning, facing, bor-

ing, face milling, and some peripheral milling operations.

3.4.3. Rotary tools Rotary cutting tools are in the form of circular discs

that rotate about their central axis in addition to the main cutting and feed motion (Fig. 6). It has been shown conclusively that rotary tools give rise to several hundred degrees centigrade lower cutting temperatures when machining titanium alloys (Fig. 7) [66]. The tool- life improvements are considerable when machining difficult-to-cut materials with rotary tools due to their superior wear-resistivity (Fig. 8), which may be attributed to their peculiar characteristics such as continuous shifting of the cutting edge during machining and lower cutting temperature [67,68]. Komanduri et al. concluded that the tool lives are approximately seven times those of conventional tools when machining Ti- 6A1-4V using rotary tools at very high feed rates (up to 1 mm/rev), achieved without sacrificing the surface finish or the stability of the cutting process [64]. Due to the cutting edge being circular the rotary tool can also lead to a very fine machined surface, provided that the tool spindle assembly is adequately rigid.

Although the improvement in tool life achieved by rotary cutting is very significant, very few industrial applications, have been reported. The reasons for this may be their reduced effectiveness for machining complex surfaces and the requirement for either rigid machine-work systems or light cuts.

E.O. Eztgwu, Z.AI. Wang Jottrna! qf Materials Proce.ssin.~' TeJm~logy 68 11997) 262 274 271

Ledg~

~Too l holder (o) Ledge toot r~oun~ec~ on o conven't,onol oo~ holder

q ~-Feec~ rote

- GO) T~rnin() operot lon

w;'th o Ledge "too~

~or~

l Lea~ -tool ed I Ledge [

Fig. 5. Ledge tool (after Komanduri and Lee [64.05]).

3.5. Surface hltegrity

Titanium is generally used for a material for parts requiring the greatest reliability, and therefore the surface integrity must be maintained. However, the surFace of titanium alloys is easily damaged during machining and grinding operations due to their poor machinability, damage appearing in the form of microcracks, built-up edge, plastic deformation, heat-affccted zones, and tensile residual stresses. Specific studies on surface integrity parameters (microstructures hardness, surface roughness and residual stress) have been carried out [26,69-76]. When machining titanium in an abusive manner (such as using a dull tool) an overheated white layer can be produced which may be harder or softer than the base materials [70]. Under both gentle and abusive machining conditions, however, the surface residual stresses appear compressive and their values differ according to the cutting conditions (such as the cutting speed). In grinding, abusive grinding practices

(o) ~ (u)

Fig. 6. Rotary cutting tools (after Ping Chen [66]).

produce high-residual tensile surface stresses, whilst gentle grinding produces beneficial shallow compressive stresses [10]. The surfaces produced under abusive conditions are also damaged by deformation and microcracks, which contribute to the loss of fatigue strength and stress corrosion resistance in combination with the resi,:lual stress pattern discussed above.

4. Conclusions

I. Titanium and its alloys are considered as difficult- to-cut materials due to the high cutting temperature and the high stresses at and/or close to the cutting edge during machining. The high cutting temperature is due to the heat generated during machining (catastrophic thermoplastic shear process), the thin chips, a thin seco~.dary zone, a short chip-tool contact length and the poor heat-conductivity of the metal, whilst the high stresses are due to the small contact area and the strength of titanium even at elevated temperature.

2. Straight grade (WC/Co) cemented carbides are regarded as the most suitable tool material available commercially for the machining of titanium alloys as a continuous operation. The C-2, identical to ISO K20, is the best carbide grade. High-speed steel tools are also very useful for some interrupted cuts, but the development of new tool materials is still required.

3. Cutting tool materials undergo severe thermal and mechanical loads when machining titanium alloys due

272 E.O. Ezugwu, Z.M. Wang/Journal of Materials Processing Technology 68 (1997)262-274

1500

g~ r~

1300

1100

900

Fixed round tool

~ ~ ~ ~ gutting tool

700 ' ' ' 0.5 1 1.5 2

Cutting speed (m/s)

Feed rate: 0.4 mm/rev, Depth of cut: 0.25 mm

2.5

Fig. 7. Measured temperature in turning Ti-6AI-4V (after Ping Chen [66]).

to the high cutting stresses and temperatures near the cutting edge, which greatly influence the wear rate and hence the tool life. Flank wear, crater wear, notch wear, chipping and catastrophic failure are the prominent failure modes when machining titanium alloys. Flank and crater wear may be attributed to dissolution-diffusion, attrition and plastic deformation, depending on the cutting conditions and the Iool material, whilst

notch wear is caused mainly by a fracture process and/or chemical reaction.

4. As a basic rule, a cutting fluid must be applied when machining titanium alloys. The correct use of coolants during machining operations greatly extends the life of the cutting tool. Chemically active cutting fluids transfer heat efficiently and reduce the cutting forces between the tool and the workpiece.

0.45

0.4

0,35

0.3

o.2s

~ 0.2

-~ 0.IS

0.1

0.05

I I I I

5 10 15 20 25

Cutting time (min)

o. . Cutting speed 60 m/min, . . Cutting speed 120 m/min

Fig. 8. Tool wear curves in machining Ti-6AI-4V (after Ping Chen [67,68]).

E.O. Ezugwu, Z.M. Wang/Journal o/ Materkds Processklg Tectmology 6g (19971 262 274 273

5. The machining methods used for titanium are essentially those that have been used since titanium became used widely in the early 1960s. However, some special machining techniques (such as the use of [edge tools and rotary tools and other non-conventional machining methods~ may be thought of as alternative methods to increase the metal removal rate in the production of titanium components, provided that the component geometry integrity permits this.

6. Great care must be exercised to avoid loss of surface integrity in the machining of titanium, especially grinding, or a dramatic loss in mechanical behaviour such as fatigue can result. Generally, the crack-free, compressive residual stress produced during machining gives excellent fatigue properties, whilst surface damage and a tensile residual-stress pattern will result in a dramatic loss in performance.

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Titanium alloys and their machinability.pdf