MACHINING TITANIUM AND ITS ALLOYS

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This article was downloaded by: [Dalhousie University] On: 03 December 2012, At: 04:49 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office:
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This article was downloaded by: [Dalhousie University] On: 03 December 2012, At: 04:49 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Machining Science and Technology: An International Journal Publication details, including instructions for authors and subscription information: MACHINING TITANIUM AND ITS ALLOYS Xiaoping Yang a & C. Richard Liu a a School of Industrial Engineering, Purdue University, West Lafayette, IN, Version of record first published: 27 Apr To cite this article: Xiaoping Yang & C. Richard Liu (1999): MACHINING TITANIUM AND ITS ALLOYS, Machining Science and Technology: An International Journal, 3:1, To link to this article: PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material. MACHINING SCIENCE AND TECHNOLOGY, 3(1), (1999) REVIEW ARTICLE MACHINING TITANIUM AND ITS ALLOYS Xiaoping Yang and C. Richard Liu School of Industrial Engineering, Purdue University West Lafayette, IN Received March 15, 1998; Accepted November 10, 1998 ABSTRACT Titanium and its alloys are attractive materials due to their unique high strength-weight ratio that is maintained at elevated temperatures and their exceptional corrosion resistance. The major application of titanium has been in the aerospace industry. However, the focus shift of market trends from military to commercial and aerospace to industry has also been reported. On the other hand, titanium and its alloys are notorious for their poor thermal properties and are classified as difficult-to-machine materials. These properties limit the use of these materials especially in the commercial markets where cost is much more of a factor than in aerospace. Machining is an important manufacturing process because it is almost always involved if precision is required and is the most cost effective process for small volume production. This paper reviews the machining of titanium and its alloys and proposes potential research issues. Keywords: Titanium. Machining; Machinability; Residual Stress; Surface Integrity; INTRODUCTION Titanium alloys are attractive materials due to their unique high strengthweight ratio that is maintained at elevated temperatures, and their exceptional corrosion resistance. The major application of titanium is in the aerospace industry, where it is used both in airframes and engine components. Non-aerospace applications take advantage mainly of their excellent strength properties or corrosion resistance [1,2,3]. Typical application areas include automotive, chemical! 107 Copyright 1999 by Marcel Dekker, Inc. 108 YANG AND LIU energy, medical, and sporting goods [4]. It has been reported that the applications of titanium to non-aerospace industry have increased [5,6J. The Ti-6AI-4V comprises about 45% to 60% of the total titanium production [2,7,8,9J. Adams [IOJ also reports the focus shift of market trends from military to commercial and aerospace to industrial. Initial high cost, availability, and manufacturability have limited titanium's use. Improved fabrication methods could result in reduced scrap losses and fabrication time, and thus reduced cost and increased availability [IIJ. A literature search reveals that the machining of titanium and its alloys have not received much attention in recent years. This may result from the difficulties associated with machining titanium and its alloys. Siekmann pointed out in 1955 that 'machining of titanium and its alloys would always be a problem, no matter what techniques are employed to transform this metal into chips [12J. Komanduri and Reed have commented that 'this is still true in so far as cutting tool materials are concerned' (1983) [13J. Adams [IOJ emphasizes the lower cost in the R&D trends and requirements for success for titanium industry. Froes et al. [9J also assert that now the expansion of the titanium market will be even more dependent on reducing the cost. This can be achieved best if the machinability of titanium and its alloys can be improved because machining is almost always involved if precision is required and is the most cost effective process for small volume production [14J. This paper reviews the issues related to the machining of titanium and its alloys and proposes potential research issues. METALLURGY OF TITANIUM ALLOYS To better understand the machining of titanium and its alloys, basic knowledge on these materials is necessary. This section summarizes the information from references [2,8,9,12,15,16,17,18, 19,49J. Classification of Titanium and Its Alloys Titanium exists in two crystalline states: in a low-temperature a phase (hcp) and a high temperature p phase (bee). The hcp structure of titanium affords a limited number of slip or shear planes. On the other hand, the bee structure has more slip systems, thereby enabling more deformation locally wherever the structure has transformed from hcp into bee. The allotropic transformation of pure titanium takes place around 882 C (Figure I). By adding certain elements this temperature can be either raised or lowered. The 'a stabilizers' such as AI, 0, N, Ga, and C produce an increase in the temperature. The 'P stabilizers' such as Mo, V, Ta (isomorphous formers), Cu, Cr, Fe, Mn, Ni, Co, and H (eutectoid formers) produce a decrease in temperature of transformation. The 'neutral elements' such as Sn, Si, and Zr have little influence on the transformation temperature. The classification (Figure 2) is: MACHINING TITANIUM AND ITS ALLOYS 109 Body-centered cubic I Bela Transus Temperature 882 C a Hexagonal close packed Figure J. The two allotropic forms of titanium (after Froes et al. [9]). Commercially Pure (Unalloyed) Titanium: excellent corrosion resistance and low strength properties. a and near-a Alloys: containing a stabilizers and possessing excellent creep resistance. a- Alloys: containing both a stabilizers and stabilizers. They account for 70% of all titanium used, among which Ti-6AI-4V is the most common one comprising about 45% to 60% of the total titanium production. Alloys: containing significant quantities of stabilizers. High hardenability and a higher density. Titanium Aluminides, Ti 3AI (a2) and TiAI (y): can be used at as high as 900 C. Low room temperature ductility. Selected Titanium Properties Titanium has a density of 4.50 Mg/rn , and titanium alloys have densities in the range Mg/rn'. Table I is the physical and mechanical properties of elemental titanium. Table 2 is a simple comparison of selected properties between Ti-6AI-4V and AISI 1045 steel. Table 3 is a summary of the strength of commercial and semicommercial grades and alloys of titanium. Table 4 illustrates the technical advantages that resulted from the introduction of titanium in the JT-3 gas turbine. Figure 3 is a comparison of strength-density behavior of two titanium alloys vs that of three steels as a function of temperature. 110 YANG AND L1U t I Bin.ry Tit.nium Alloy, I I I (J-,tobilinct I I I o-st.bi!ized I I I I I Simpl. EutllCtoid!'eritl1Ctoid (J - 0 TraMform.tion TraMformetion Simple TrafJIformetion ((3.f,omorphous) Peritectic (p-eutllctoid) (poperitectoid) I I I Solutes Solutes Solutes Solutes V I Cr I Mn I Fe I Co I Ni I Cu N,O B, Sc, Go, La Zr Nb Mo I I I I I Pd I AlII Ce, Gd, Nd, Ge Hf To Re IW I I I I Pt I Aul AI,C 2+(J i Ti H, Bo, Si, Sn, Pb, Bi, U I 2/ I I 2+0/ 2 I??:;) (J 2:; 0 : (J 0+' T, / TI, !l.. Soluto Content -_! (J / p+.., 17 0+' TI Figure 2, Classification scheme for binary titanium alloys (after Froes et al. [9]). MACHINABILITY OF TITANIUM AND ITS ALLOYS Webster [68] defines machinability as the quality or state of being machinable ; machinable is defined as capable of or suitable for being machined ; and to machine is defined as to turn, shape, plane, mill, or otherwise reduce or finish by machine-operated tools, In general, there are three main aspects of machinability [20]: tool life, surface finish, and power required to cut. In addition, the chip form/chip breakability and part accuracy are also used in an assessment of machinability. Under normal circumstances the best criterion for rating machinability is machining cost per part. Under special conditions where machine capacity is lim- MACHINING TITANIUM AND ITS ALLOYS 111 Table 1. Physical and Mechanical Properties of Elemental Titanium (after Froes et al. [9]) Atomic number Atomic weight Atomic volume Covalent radius First ionization energy Thermal neutron absorption cross section Crystal structure Color Density Melting point Soliduslliquidus Boiling point Specific heat (at 298 K) Thermal conductivity Heat of fusion Heat of vaporization Specific gravity Hardness Tensile strength Modulus of elasticity Young's modulus of elasticity Poisson's ratio Coefficient of friction Specific resistance Coefficient of thermal expansion Electrical conductivity Electrical resistivity Electronegativity Temperature coefficient of electrical resistance Magnetic susceptibility Machinability rating weight/density nm MJ/(kj mol) 560 fm'latom a: close-packed, hexagonal :S 1156 K : body-centered, cubic I 156 K Dark gray 4510 kg/m! 1941 :t 285 K 1998 K 3533 K J/(kg K) 21 W/(m K) 440 kj/kg 9.83 MJ/kg 4.5 HRB 70 to GPa GPa GPa at 40 m/min 0.68 at 300 m/min J.lnm 8.64 x 10-6 K- 1 3% lacs (copper 100%) J.lnm 1.5 Pauling's K- l 1.25 X 10-6 ernulg 40 (equivalent to 'I, hardness stainless steel) ited and production output is of major concern, the proper machinability criterion is the number of parts per unit time [21]. Table 5 shows that power requirements of cutting titanium alloys are lower than those of cutting steels and Nickle and Cobalt Base Alloys [22]. This result is in agreement with the finding of Motonishi et al. [30] who state that no definite relation between cutting force and hardness and tensile strength of the titanium materials exists and conclude that it can hardly be said that the magnitude of cutting force causes difficulty in cutting. However, the machinability of titanium and its alloys is poor in terms of tool life. The tool wears fast and the cutting speed must be kept low, which give rise to high machining cost per part. Table 6 shows some machining time ratios for various types of titanium alloys compared to AISI 4340 steel at 300 BHN. Similarly, Catt and Milwain [33] observe that it ;:; Table 2. Properties of Ti-6AI-4V Compared to a Medium Carbon Steel (adapted from Machado and Wallbank [2]) MODULUS OF SPECIFIC VOLUMETRIC YIELD ELASTICITY STRENGTH- HEAT AT SPECIFIC THERMAL STRENGTH ELONGATION TENSION DENSITY WEIGHT C HEAT CONDUCTIVITY MATERIAL (Mpa) (%) (Gpa) (g/cm') RATIO J/kg. K (J/K. em') (W/m K) Ti-6AI-4V annealed bar Ti-6AI-4V solution treated aged bar AISI-I045 cold drawn : Z Z o r- ;: MACHINING TITANIUM AND ITS ALLOYS 113 Table 3. Strength of Commercial and Semicommercial Grades and Alloys of Titanium (after Donachie [8]) TENSILE 0.2% YIELD STRENGTH STRENGTH DESIGNATION (MPa) (min) (MPa) Unalloyed grades ASTM Grade I ASTM Grade ASTM Grade ASTM Grade ASTM Grade a and near-a alloys Ti Code Ti-5AI-2.5Sn Ti-5AI-2.5Sn-ELI Ti-8AI-1 Mo-I V Ti-6AI-2Sn-4Zr-2Mo Ti-6AI-2Nb-1 Ta-0.8Mo Ti-2.25AI-11 Sn-5Zr-1 Mo Ti-5AI-5Sn-2Zr-2Mo(a) a- alloys Ti-6AI-4V(b) Ti-6AI-4V-ELI(b) Ti-6AI-6V -2Sn(b) Ti-8Mn(b) Ti-7AI-4Mo(b) Ti-6AI-2Sn-4Zr-6Mo(c) Ti-5AI-2Sn-2Zr-2Mo- 2Cr(a)(c) Ti-6A 1-2Sn-2Zr-2Mo- 2Cr(a)(b) Ti-IOV-2Fe-3AI(a)(c) Ti-3AI-2.5V(d) alloys Ti-I3V-IICr-3AI(c) Ti-8Mo-8V-2Fe-3AI(a)(c) Ti-3AI-8V -6Cr-4Mo-4Zr(a)(b) Ti-11.5Mo-6Zr-4.5Sn(b) (a) Semicommercial alloy: mechanical properties and composition limits subject to negotiation with suppliers. (b) Mechanical properties given for annealed condition: may be solution treated and aged to increase strength. (c) Mechanical properties given for solution treated and aged condition; alloy not normally applied in annealed condition. Properties may be sensitive to section size and processing. (d) Primarily a tubing alloy; may be cold drawn to increase strength. 114 YANG AND LIU Table 4. Effect of Titanium in the JT-3 Gas Turbine (Titanium Made a Fan Configuration Possible) (after Donachie [8]) JT-3D (fan engine) vs JT-3C (no fan) Take-off thrust Climb thrust-slto Cruise thrust Specific fuel consumption Specific weight 42% more 23% more 16% more 13% less 18% less Figure ). Temperature:C n 8At-1Mo 1V E 2 I 2 co 0; C.. C.. 3! 3! III I III s AMS 5616 I- :) \AMS Temperature, OF Strength-density behavior of two titanium alloys vs that of three steels (after Donachie Table 5. Average Unit Power Requirements for Turning, Drilling, and Milling (Horsepower per Cubic inch/minute) (after Kahles et al. [22]) TURNING HSS MILLING HSS HARDNESS AND CARBIDE DRILLING AND CARBIDE MATERIAL Bhn (3000 kg) TOOLS HSS DRILLS TOOLS Steels R, Titanium Alloys High Temperature Nickel and Cobalt Alloys Aluminum Alloys (500 kg) Power requirements at spindle drive motor, corrected for 80% spindle drive efficiency. Dull tools may require 25% more power. MACHINING TITANIUM AND ITS ALLOYS lis Table 6. Machining Time Ratios for Various Types of Titanium Alloys Compared to AISI 4340 Steel at 300 BUN (after Zlatin and Field [24]) TITANIUM ALLOY TURNING (Carbide Tool) FACE MILLING (Carbide Tool) DRILLING (USS Tool) Commercially pure 175 BUN a Ti-8AI-IMo-IV 300 BUN a- Ti-6AI-4V 365 BHN Ti-I3V-IICr-3AI 400 BHN 0.7: I 1.4: I 2.5: I 5: I 1.4: I 2.5: I 3.3: I 10: I 0.7: I I: I 1.7: I 10: I takes over three times as long to manufacture parts from titanium as to manufacture them from aluminum alloys. It is seen from Table 6 that the hardness has a big impact on the machinability, which is also reported in Trucks [3] who claims that the machining characteristics of titanium alloys change significantly at hardness levels of 38 Rockwell (C scale). It is also seen in this Table that alloy type has an impact on the machinability. Truck [3] reports similar results. When ranked in a descending order in terms of machinability, the materials are commercially pure titanium, a alloys, a-p alloys, and p alloys. Possible reasons for making titanium and its alloys difficult-to-cut are listed below. In addition to fast tool wear, there are problems that could cause thermal damage (e.g. items I, 2, 3, and 7) and poor surface finish or part accuracy in cutting titanium and its alloys (e.g. items 1,5, and 6, built-up edge at low cutting speed). 1. The poor thermal properties of the materials ([ 1,2,3, 12,22,23,24], Table 2). This problem may be more pronounced in drilling operation using conventional twist drills because cutting speed diminishes towards the center resulting in considerable cutting forces and excessive heat [27]. 2. Titanium's chip is very thin with consequently an unusually small contact area with the tool (113 to 112 of that for turning steel) [2,12,24,28], which causes high stresses on the tip of the tool. The combination of a small contact area and the low thermal conductivity results in very high cutting temperatures. The cutting speed of the titanium must be low to avoid too short tool life. The high unit pressure resulting from the thin chip, high surface friction and high heat generated could give rise to pressure welding and galling [27,29]. 3. The high strength is maintained to elevated temperatures (Figure 3) that are generated in machining and this opposes the plastic deformation needed to form a chip [2]. 4. High chemical affinity [I]. There is strong chemical reactivity of titanium at the cutting temperature ( 500 C) with almost all tool materials available [2]. High affinity of titanium and its alloys for the interstitial oxygen and nitrogen gives rise to the pick up of interstitial of the heated outer surface layer of the workpiece during machining, which contrib- 116 YANG AND LlU utes partially to the hardening of titanium and its alloys in addition to the strain hardening [29]. The active properties of titanium alloys control tool wear rate (especially crater wear) [30]. 5. Low modulus of elasticity (Table 2) which can cause chatter, deflection, and rubbing problems [2,12]. The forces perpendicular to the workpiece may increase three to four times as a result of a build up of titanium on the wearland of the tool (the cutting forces will generally increase 25 to 50% as the tool dulls when cutting a steel) [24]. Because of this high thrust force and the low elastic modulus of titanium, the deflection of the workpiece can be a serious problem. 6. The long stringy swarf is difficult to handle and has stymied most titanium machining automation projects [31]. 7. Care' must be taken about titanium's tendency to ignite during machining because of the high temperatures involved [2]. Sparks have been observed during cutting experiments in the authors' laboratory. There are conflicting reports about machining titanium and its alloys: the coefficient of friction between the chip and the tool face is said to be high (coefficient is approximately 0.6 [29] and 1.0 [28]), but Zlatin [69] shows this to be in line with that obtained in machining many steels and Rabinowicz [32] reports that friction between titanium and the tool face is low owing to transfer of titanium to the tool face and formation of a thin oxide acting as a lubricant; even though the built-up edge is said not to occur, some authors have confirmed its presence at low cutting speeds, and this could lead to a poor surface finish in some operations [2]. However, the built-up edge could have positive impact on the machinability as well. Child and Dalton [29] claim that the absence of a built-up edge to dissipate heat leads to much higher surface heating under the tool and thereby increase the contamination of titanium and decrease tool life. Colwell and Truckenmiller [28J attempt to induce a built-up edge since the presence of it can reduce both temperature and pressure on the cutting tool; the rate of work hardening is said to be high [2], but Zlatin, Child and Dalton [29] and Trucks [3] have reported that in fact it work-hardens to a lesser extent than steel, also Shahan and Taheri [26] claim that low strain-hardening rate is one of the characteristics of a + P two-phase titanium alloys. There are several parameters that have impact on the machinability of titanium and its alloys. In addition to the hardness and alloy type discussed above, the tool materials, lubricant, and temperature also have impact on the machinability. Hong et al. [34] conclude that overbased sodium sulfonate and calcium sulfonate showed better tapping performance than overbased magnesium sulfonate in the tapping test of 304 stainless steel and T-6AI-4V alloy. The addition of sulfurized olefin to sodium and calcium sulfonates improved the tapping efficiencies as well as the surface finish of 304 stainless steel and Ti-6AI-4V alloy. Dillon et al. [50] report that high temperature (280 C) definitively posed problems with titanium. The machinability of titanium is improved at low workpiece temperature MACHINING TITANIUM AND ITS ALLOYS 117 (-190 C). The relationship between the tool materials and tool wear in machining titanium and its alloys will be discussed in the section on Tool Wear. Much research has been conducted to find machining conditions that give satisfactory to
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