Machining of titanium alloys used in aviation

High mechanical strength, low density and excellent corrosion resistance are the main properties that make titanium and its alloys attractive materials in various sectors: Aeronautics, aerospace, medicine... [1]

By their good behaviour at high temperatures and its high cost, which hinders the application of titanium alloys in many other sectors, its employment is widespread mainly with the development of aeronautical technology and aerospace, sector that consumes 50% of world production [1-3, 4]. Thus, in the new Airbus 380 and Boeing 787 in development, the use of titanium is located between 12-15% [5], which is a significant increase from earlier models that stood at 4-5% [6]. Figure 1 shows the evolution of the use of titanium in aeronautics from 2007 until 2015 in relation to the number of manufactured aircraft.

Figure 1:Evolution of the use of titanium alloys in the development of different models of aircraft [7].

Pure titanium suffers an allotropic transformation at 882 ° C, temperature called β transus, in which a change of phase α (hcp) to β (bcc). This transition temperature can soar, adding elements α-stabilizers; or decrease with elements β-stabilizers. Therefore, depending on their chemical composition, titanium alloys are classified mainly into three groups: α, α + β, and β or near β [1, 8]. The type α, is dominated by the percentage of elements α-stabilizers (A, C, O, N). Highlights its low mechanical properties in comparison to the rest of alloys of titanium and its excellent resistance to corrosion. TI-3Al-2.5V and Ti-5Al-2.5V are most used in this group.

Alloys α + β, so far the most studied, contain elements α - and β - stabilizers, which results in the formation of a mixed microstructure that gives the material a good combination of mechanical properties. Today's alloy

Ti6Al4V is most used, absorbing between 45 and 60% of the global consumption [1-3]. Timetal54M, hereinafter referred to as Ti54M, is a new alloy, α + β, very similar to the Ti6Al4V in terms of mechanical properties, but of machinability improved due to a finer grit [10.28] size.

Alloys near-β have been developed in recent decades years in order to increase the mechanical properties of the material, and improve life to fatigue, corrosion behaviour, ductility... [1-3, 10]. These alloys contain a high quantity of elements β stabilizing (Fe, Mo, V...), in such a way that, after the treatment of temple is retained the phase β by 100%, forming a matrix β−metaestable [1.22]. When you apply a treatment of aging, small particles of α phase precipitate evenly in the matrix of β, thus increasing the resistance of the material. Some of the most commonly used in recent years near β alloys are the Ti10.2.3, Ti555.3 and Ti17.

Figure 2 shows the location of the different alloys described according to percentage of stabilizing elements.

Figure 2:Phase diagram β-isomorhous indicating the position of some alloys of titanium depending on their chemical composition.

Several authors have examined the difficulty in machining of Ti6Al4V alloy, [12-20]. Armendia et to the. [21] conducted a comparative study of machinability, which concluded a better workability of the Ti54M. Comparing the maximum cutting speeds that can be used in the face to achieve a lifespan of tool, speed remachining Ti54M was one 10-15% higher than remachining Ti6Al4V. Because of the similarity in mechanical properties between both materials, the difference in machinability is attributed to differences in microstructure, and thus the results are understood

However, near β alloys publications focus more on aspects of the material itself (mechanical properties, microstructure...) [8, 11, 22-24], but just there are publications that address research on Machinability of the β titanium near. Arrazola et to the. [25, 26] carried out a comparative study between Ti555.3 and alloys Ti-6Al-4V, which noted a better Machinability of alloy α + β. C. Machai and D. Biermann [27,28] conducted an analysis of Machinability of alloy Ti10.2.3 with different microstructures, which found the high influence of conditions of court and microstructure on wear on the tool. The different microstructures are linked to different heat treatments, and affect the mechanical properties of the material, which are key in determining the life of the tool.

In general the difficulties of the machining of titanium alloys are associated with a:

• Bad thermal properties (low thermal conductivity and high specific heat) hinder the evacuation of heat generated in the manufacturing process, causing high temperatures in the cutting area that accelerate the wear of the tool [7].
• An effective area of contact between the chip and tiny tool makes that the Thermo-mechanical loads will focus on the area of the edge of cut, reducing the life of the tool [29]. The crater generated on the surface of detachment is located closer to the cutting edge, which weakens the edge and consequently decreases the life of the tool.
• A low modulus of elasticity (110 GPa) gives rise to harmful vibrations for the part, but mostly for the tool [6, 30].
• The high chemical reactivity at high temperatures with the majority of the materials used to manufacture the cutting tools [12-18], causes premature wear of the same by dissemination to other mechanisms [29, 31-34].
• Formation of segmented chip [35-38], which produces a cyclical, both thermal and mechanical loads on the tool, causing a more rapid deterioration of the same.

Commercial cutting tools used to machining titanium alloys are the plates of hard metal uncoated [6-7, 29, 32-33], primarily grades K. However, increasingly used more the plates of hard metal coating (TiN, TiCN and TiN - TiC...), mainly in milling. Another alternative, which results in roughing are comparable to the hard metal tools uncoated, are the tools of sintered steel [43]. In addition to increasing the productivity of conventional high-speed steel tools, their cost is much lower than the hard metal.

The CBN and PCD have very good performance when machining titanium alloys, reaching speeds of up to 150 m•min-1 [14] Court, and considerably improving the response of the hard metal plates. However, its applicability is limited by its high price [4-7, 15, 23].

Studied alloys have been the following: Ti6Al4V, Ti10.2.3, Ti17, Ti555.3 and Ti54M [10.26]. Chemical composition and mechanical properties of these are detailed in table 1.

Table 1:Chemical composition in percentages of weight, equivalent values of at the Mo and mechanical properties of titanium alloys studied.

The value of equivalent aluminum indicated in table 1, is an indicator of the quantity of elements α-stabilizers containing alloy, and gives an idea of the capacity of each alloy to obtain a certain hardness. At the same time, the value of equivalent molybdenum is an indicator of the amount of β - stabilizers in the alloy elements, therefore the amount of β phase which will retain the material at room temperature. A higher content of molybdenum equivalent translates into a greater ability to obtain high values of tensile strength limit. In this way, we can say that there is a direct relationship between the properties of each alloy and its chemical composition.

Figure 3 shows the microstructures of all alloys studied. They show significant differences in the number and morphology of primary phase α, β phase morphology and the grain size can be seen. In the case of alloys α + β is shown an already microstructure, formed mainly by grain equiaxicos of the primary phase residual α and α and β sheets zippered phases forming a Widmansttaten structure (Figure 3.a). As you can see, in addition to having a smaller grain size, Ti54M alloy also has largest number of phase β to the Ti6Al4V (Figure 3.b), as it also indicates the equivalent molybdenum content.

Near β alloys have a microstructure that is completely different from the one of the α + β, which mainly consists of phase α dispersed in a matrix of β phase precipitates. Alloy Ti10.2.3 has 50% of the globular primary phase α (fig. 3.c), while the Ti17 alloy is characterized by phase α on the boards of important phase β grain size and grain, obtaining a Widmanstätten structure in α β phase transformation (Figure 3.d). Finally, alloy Ti555.3 has 20% of the globular residual phase α, along with another transformed β α phase getting a Widmanstätten structure (Figure 3.e).

Figure 3:Microstructure of different alloys studied. The clear zones correspond to the α phase while to dark with the β. to) Ti6Al4V; (b) Ti54M; (c) Ti10.2.3; (d) Ti17; (e) Ti555.3.

Studying alloys have a wide field of applications, mainly in motor [2]. Ti10.2.3 alloy has a higher resistance to corrosion the Ti6Al4V, besides a good combination between ductility and toughness, so it is primarily used in landing gear, in order to absorb impacts of certain magnitude and cushioning the landing [1,11,22] or connection between wings and structure elements. Alloy Ti17 has better mechanical properties, such as the fatigue life, so its application is mostly aimed at wheels compressors and turbines for engines. Alloy Ti555.3 shows an exceptional combination of easy obtaining e major hardness and fracture toughness. Its main application is for critical items such as landing gear, competing in many cases with the Ti10.2.3. Alloy Ti54M is a mechanical properties similar to the Ti6Al4V alloy, where what is sought is an increase of the Machinability of the same.

Tests

In order to be able to understand in greater detail the phenomena that occur in the process of machining of titanium alloys, the research study is divided into two groups:

• (3D) tool life trials: in conditions close to the industrial.
• Trials of orthogonal cutting (2D): in order to understand the fundamentals of the cutting process.

In both cases, to perform rigorous analysis of machinability, the first aspect to consider is the use of a well defined methodology. Therefore discussed the input parameters of the trials (avant-process), in this case, (1) the State of the machine, (2) tools that are intended to be used, measuring the radii of tools and the edges of cutting, a lubricant (3), (4) material to study, analyzing the microstructure of the piece and the hardness of it, and (5) verified that the working conditions are those really established (through parameters of CNC).

Likewise, defined the cutting parameters to be used in the trials, which depend on the pattern of trials that are planned. In the course of the same (in-process), forces of cutting and power measurements are performed in both trials and measurements of temperatures in the face of detachment in orthogonal cutting trials. Finally, the output parameters will be analyzed post-process that, in this case, the plates (wear measurement and analysis of bowel movements) and chips in tool life trials are defined. The methodology followed in the life of tool tests shown in figures 4 and 5. Below explains each of the test types in more detail.

Figure 4:Diagram phases of the methodology applied to the trials of life of tool (3D)

Figure 5:Diagram phases of the methodology applied to trials of orthogonal cutting (2D)

• Trials of end-of-life tool (3D)

The end-of-life tests are performed in machining conditions close to industry (see table 2), using a sweep of cutting speeds for each material, maintaining constant progress and depth of past values. It consists of 15-minute testing and measuring the wear of the plate after each sweep.

Test execution methodology has been developed following the guidelines set by the standard ISO 3685 [44]. According to proposed criteria to define the end of tool life, tool reaches its end of life when there is a uniform flank (Vbmedia) of 0.3 mm wear or wear to maximum flank (Vbmax) of 0.6 mm (Figure 6). With this type of test determines the speed with which the tool reaches its end of life after 15 minutes of machining (Vcmax). To wholesale Vcmax, best machinability.

Figure 6:Criteria to define different levels of wear in the face of incidence.

To has been used them Danobat II CNC, coolant Quakercool 710 h. lathe The trials have been conducted with the inserts CNMG 10408-23 H13A (grade K 15). For the analysis of wear, Macroscopio Leica Z16 APO were used to measure the wear, and the electron microscope of scanning JEOL JSM-5600LV to detect bowel movements and identify the main wear mechanisms. Before each test is performed a control of the cutting edge, using Profiler Confocal Sensofar PLμ. For the acquisition of forces has been used dynamometer Kistler 9121 of three components located in porta-tools, measuring the dynamic variations in cutting forces during machining with an error of ±4%. The value of the forces has been measured for 10 seconds machining time, so the presence of wear will not interfere at the time of the measurement. Finally, an analysis of chip extracted from the trials, to study them in the optical microscope Leica Qwin takes place. Figure 7 the set-up of trials, and in table 2 shows a summary of the parameters of input used.

Figure 7: Experimental set-up for tool life trials.

• Trials of orthogonal cutting (2D)

Orthogonal cutting occurs when the relative motion between tool and workpiece is perpendicular (Figure 8). The orthogonal cutting trials have been conducted in the Lagun vertical machining center, without using any type of lubricant. The used plates are 16 TNMG 0408-23 H13A. Specimens for orthogonal cutting are tubes with an outer diameter of 48 mm and wall thickness of 2 mm. Trials at two different speeds (40 and 80 m/min) and with three different advances have been made by return (0,1, 0,15 and 0,25 mm), as shown in table 2. The experimental set-up is shown in Figure 9.

Figure 8:Orthogonal cutting test.

By means of dynamometer Kistler 9121, also used in trials of end of life, have been measured shear forces. For each of the tests has been used a new cutting edge, so wear phenomena do not interfere with the measurement of the forces and temperatures.

Camera FLIR Titanium 550 M. has been used to measure the temperature in the face of detachment, It has been measured the value of the temperature of 30 pixels (spatial resolution of 10 μm) along the face of detachment, by calculating the average value of them to represent the temperature in the tool. The measurements have an uncertainty of ±6% [45].

Figure 9: Experimental set-up for tool life trials.

Due to the limitation of material, trials of orthogonal cutting only performed in alloy Ti6Al4V, for being the reference alloy; in the Ti54M, to be able to compare the results between two alloys α + β; and in the alloy Ti10.2.3, for the comparative study between alloys α + β and the near β. He is assumed that the results obtained from the Ti10.2.3 alloy are qualitatively extrapolated the rest of near β alloys.

Table 2:Summary given input parameters used in the trials.

Results

This section shows the results of both types of test. The results of the trials of tool life, which are explained in the first place, is intended to understand and justify with the results of the tests carried out in orthogonal cutting, being able to establish correlations between machinability and these parameters, and giving more robustness to the conclusions drawn.

• Trials of end-of-life tool (3D)

Below are the results obtained in the different alloys tool life trials.

Tool life

Figure 10 shows the wear to 15 minutes of machining (according to standard tool life time) for different cutting speeds and different alloys studied. Maximum cutting speed for each alloy, which can be defined by the sudden increase in the curve of wear by a small increase in cutting speed can be observed.

In this way, the maximum speed of cut for these alloys can be removed. These values are aproximarían to 90, 80, 70, 60 and 45 m•min-1 for the Ti54M, Ti6Al4V, Ti10.2.3, Ti17 and Ti555.3 respectively, still alloy Ti54M the best machinability and the Ti555.3 the more difficult to machine.

Figure 10:Wear on the face of incidence (Vbmax) 15 minutes of machining (fv = 0.1 mm, p = 2 mm).

Specific cutting force (K_c), specific feed force (K_f) and specific penetration force (K_s)

The results of specific cutting forces (K_c), forward (K_f) and penetration (K_s) 11.a, 11.b and 11.c shown in figures respectively. In the three graphs can be observed near β alloys to generate efforts of court superior to the alloys of α + β during machining. This may be due to better mechanical properties of alloys near β (table 1). This trend can be seen more clearly in the graph of the specific force of progress (K_f), where the difference between forces that are specific to the types of alloy, α + β and near β is much greater.

According to these results higher shear forces are generated to be machined alloy Ti555.3 (2780 N/mm², 60 m/min), whereas the minor efforts have been reported during the machining of the alloy Ti54M (2248 N/mm², 60 m/min), which represents a difference of 20%. The sequence is repeated in the case of specific forces of progress, and also coincides with the results at end of life. A similar trend is observed in the penetration forces. Therefore it can be said that there is a direct relationship between machinability and specific cutting forces generated during machining.

You can see that the values of specific cutting force (K_c) decrease as it increases cutting speed. This can be attributed to a reduction of the thickness of the chip as it increases speed with constant progress.

On the other hand, the force specifies advance (Kf) is directly related to the effects of friction along the contact surface and the radius of the cutting edge effects, and therefore, the amount of heat generated in that area [25]. Therefore, it is expected that in the case of alloys near β generates more heat than in the case of Ti6Al4V, especially in the case of the alloy Ti555.3.

Figure 11:Specific forces obtained in the tests: to) specific cutting force (K_c); ((b) specific feed force (K_f), c) specific strength of penetration (K_s).

Analysis of the type of wear

After machining the two main types of wear on the tool (Figure 12) were analysed [41]: flank (in the face of incidence) wear and tear in the face of detachment. Even so, when analyzing the end of life, only took into account the flank wear since it is the type of wear that mostly prevails and that it is easier to be measured. In addition, it is the limiting effect of face to the machining industry.

Figure 12:Ways to wear

Figure 13 shows wear on the face of the impact of the tools after 15 minutes machining.

Figure 13:Wear of flank after being machined for 15 minutes at their respective V_cmax. a) Ti54M (V_c= 90 m/min); (b) Ti6Al4V (V_c = 80 m/min); (c) Ti10.2.3 (V_c = 70 m/min); (d) Ti17 (V_c = 60 m/min); (e) Ti555.3 (V_c = 45 m/min).

As you can see, wear occurs evenly along the edge of the Court, except for the alloy Ti10.2.3, which shows a not so homogeneous wear. Specifically in the alloy Ti54M uniform wear is completely uniform. In all cases it is observed that the surface area has been thermally affected during the cutting process. This area is also lower in the case of the alloy Ti54M. In addition, the 555.3 shows a slight depression of the cutting edge.

The analysis of the contact surface on the face of the tool release confirms the presence of materials in that area to mechanize any alloys, phenomenon which, until now, many researchers had observed to be machined alloy Ti6Al-4V [29, 32]. During the cutting operation, this bonded material can be started from the face of detachment, taking particles of the tool itself. This phenomenon, of the broadcast, joins by the Chemical reactivity of titanium and the high temperatures reached. This removal of material gives rise to a craterizacion of the face of detachment [29] (Figure 14).

Figure 15 shows an image of the face of release of Ti6Al4V alloy in electron microscope. Shown that there is material adhered on the cutting edge. Despite the high reactivity of titanium with the elements that constitute the own cutting tool, is not observed formation of titanium carbide coating.

Figure 14:Deposition of material in the tool a) Ti6Al4V: V_c= 50 m.min-1, T = 15 min b) Ti10.2.3: V_c= 70 m.min-1, T = 15 min.

Analysis of chip

Collected after each test chip has been prepared metalograficamente (setting, sanding, polishing and chemical attacks) for analysis by means of optical microscopy. The objective of this analysis is to observe the morphology of the chip and its microstructure. The images obtained are shown in Figure 16.

While Ti6Al4V and Ti54M alloys have a chip segmented areas of adiabatic bands from speeds of 90 m/min, Ti17 and Ti555.3 alloys have chips with narrow adiabatic bands from 40 m/min, even at lower speeds. The formation of these bands adiabatic frequency is much larger in the case of the α + β. For example, the cutting speed of 90 m/min in the Ti6Al4V adiabatic bands have a close to 30-40 kHz frequency, while close to 50-80 kHz frequencies are observed in the Ti555.3 and Ti17 to 50 m/min.

Figure 15: Microstructure of chips after 15 min of machining (f_v= 0,1 mm, p = 2 mm): to) Ti6Al4V; (b) Ti54M; (c) Ti10.2.3; (d) Ti17; (e) Ti555.3.

To a speed of 50 m/min, alloys α + β presents a semisegmentada chip called 'temporary chip'. On the other hand, the alloy Ti10.2.3 presents a continuous chip in all analyzed cutting speed ranges and the α-primary phase appears more deformed as it increases cutting speed. For this alloy, the short tool life is related to the phenomenon of craterizacion mentioned in the section 'Analysis of wear'.

The adiabatic bands are characteristics of titanium alloys [14.39, 40]. The high formation of such bands explains high wear and tear suffered by the tool. In terms of 'temporary chip' the creation of a layer of titanium carbide, titanium oxide, vanadium and other alloy elements protects the surface of the tool. On the other hand, in conditions of the adiabatic striping, tool is subjected to cyclic thermal variations and mechanical loads which avoid the possibility of creating protective coating. Lack of protective coating and cyclic loading causing a shorter life of the tool.

• Trials in court orthogonal (2D)

Once the study of end-of-life of different alloys, and have established the ratio of machinability for different alloys, they are the results obtained in the study of orthogonal cutting machining. The objective is to measure forces and temperatures and verify that there is a relationship with the results obtained in tests of end of life.

Force a specific court (K_C) and specific force of progress (K_K)

Figures 17.a 17.b, displays and graphs of forces specific to machining to 40 and 80 m/min, respectively. As you can see, these values decrease with increasing cutting speed.

Figure 16: Court and advance specific forces remachining Ti6Al4V, Ti54M and Ti10.2.3 a) Vc = 40 m/min, VF = 0.1, p = 2 mm b) Vc = 80 m/min, VF = 0.1, p = 2 mm

At low speeds (40 m/min) cutting, both of court and advance specific forces are greater to be machined alloy near β (Figure 17.a). However, just there are differences between alloys α + β. The highest values of specific cutting forces have been registered with the advance of 0.1 mm, which is the lowest. These values are 2,248 N/mm² in the case of the Ti10.2.3, 2047 N/mm² in the Ti6Al4V and 1998 N/mm², having a 12% difference between the maximum and the minimum. The trend is the same in advance specific forces: 1,769 N/mm² in the case of the Ti10.2.3, 1,455 N/mm² in the Ti6Al4V and 1426 N/mm² with a 20% difference between the extreme values. As noted at the end of tool life trials, friction forces play an important role in the wear of the tools.

At high speeds (80 m/min) cutting, differences in specific cutting forces both advance are minimal, although those registered for the Ti10.2.3 alloy remain slightly higher β (Figure 17.b). As well as low cutting speeds, maximum values are given with the lowest advance: 2.040 N/mm² in the case of the Ti6Al4V, 2,028 N/mm² in the Ti54M and 2,021 N/mm² in the Ti10.2.3. In this case, while alloys near α + β have higher specific cutting forces near β, this difference is 1%, which falls within the uncertainty of the measuring equipment. Values maximum in advance specific forces are 1,406 N/mm² in the Ti6Al4V, 1,509 N/mm² in the Ti54M and 1,469 N/mm² in the Ti10.2.3.

The graph of temperatures is shown in Figure 18. As it is expected the temperature increases with conditions of cutting, cutting speed both the forward. Alloy Ti10.2.3 always reaches higher temperatures than the Ti54M and the Ti6Al4V. The maximum temperatures to 40 m/min are 864 ° C (Ti10.2.3), 784 ° C (Ti54M) and 715 ° C (Ti6Al4V), having a difference of 12%. 80 M/min, this difference remains constant, and the maximum temperatures are 1.057 ° C (Ti10.2.3), 1,014 (Ti54M) and 946 ° C (Ti6Al4V).

( Figure 17: to) temperatures in the face of detachment to V_c= 40 and 80 m/min

• Summary of results

A correlation of the ratio of machinability compared to the rest of measured parameters, for a trial of reference can be seen in table 3. To compare the specific forces, in trials in 3D has been taken as terms of reference_cV = 60 m/min, f_v= 0.1 mm and p = 2 mm, whereas for those of orthogonal cutting V_c= 40 m/min, f_v= 0.15 mm and p = 2.

As you can see there is a degree of agreement, at least qualitatively, between the decrement of the machinability, and the increase of termo-mecanicas loads to which it subjected the material to the tool. For example, in the case of Ti-10V-2Fe-3Al alloy a decrement of the Machinability of a 12.5% due, inter alia, that can be directly related to the increase of the specific court (+ 4%), advance forces (+ 7%) and penetration there (+ 18%) in the 3D tests. Likewise, shows that trials of orthogonal cutting shows that there is an increase in the specific cutting forces (+ 6%), feed (14%) and temperature (14%).

This opens the possibility that, through orthogonal (2D) court trials of shorter duration than the trials of 3D, the degree of Machinability of titanium alloys can anticipate at least in a qualitative way.

In the field of specific forces is observed that there are differences between the results in 2D and 3D, which is owed, as well as the conditions of court, mainly, to the geometry of the tool.

Table 3: Correlation of the ratio of machinability with forces cut in end-of-life and cutting forces and temperature in orthogonal cutting.

The following conclusions are drawn from this study of machinability:

1. Alloys α + β have one machinability greater than β, depending on the value of V near the_cmax defined in tool life trials. In function that compare alloys I entered if machinability can be from 10 to 50% more.
2. Machinability of titanium alloys studied ratio decreases in the following order: Ti54M, Ti6Al4V, Ti10.2.3, Ti17 and Ti5.5.5.3, being their respective V_cmax of 90, 80, 70, 60 and 45 m/min.
3. As for the measurement of cutting forces, there is good correlation between the results obtained in end-of-life and orthogonal cutting, since the trends are the same: cutting and advance specific forces decrease with increasing cutting speed. This may be due to a local softening of the material before being machined. The specific forces also increase to reduce the advance due to a possible effect of cutting edge.
4. Both end-of-life testing in orthogonal cutting, superior cutting forces are obtained by machining near β alloys. This is due to higher mechanical properties. The differences are most noticeable in K_k. This translates into greater sensitivity to the effects of friction in the face of detachment to the machining of near β alloys.
5. There is a clear relationship between the temperature and the machinability. Higher temperatures are reached to be machined alloy near β studied (approximately 11% higher). Due to the low thermal properties of titanium alloys, by increasing the cutting conditions heat generated during machining can not be dissipated effectively, and is concentrated near the tip of the tool which leads to premature wear of the tool.
6. For cutting conditions studied, is observed the formation of chip segment in the alloy Ti6Al4V and Ti54M, whose chips are very similar. However, in the worst machinability alloys (Ti17 and Ti555.3) are adiabatic shear bands, that apparently accelerated wear of the tool.
7. There is an at least qualitative correlation between machinability ratio, and specific forces and temperature.

[1] Lutjerin, g., J.C. Williams, 2007, Titanium, Springer.

[2] Veiga, C., Davim, j., Loureiro, a., 2012, Properties and Applications of Titanium Alloys: A brief Review - to Review Advanced Materials Science, 32: 133-148.

[3] Boyer, R-R., 1995, Titanium for Aerospace: rationale and applications, Advanced Performance Materials, 2:349-368.

[4] Rahman, M., Wong, Y.S., Zareena, R.A., 2003, Machinability of titanium alloys, JSME International Journal, Series C: Mechanical Systems, Machine Elements and Manufacturing, 2003, 46, 107-115.

[5] Bruhus Y., Sebring W., Noland D., 2007, Meeting the winery of Milling Aerospace Materials, Aerospace Manufacturing and design.

[6] Cui, C. Hu, B., Zhao, l., Liu, S., 2011, Titanium alloy production technology, market prospects and industry development, Materials & Design, 32: 1684-1691.

[7] Rahman Rashid, r., Sun, S., Wang, g., Dargusch, M. S., 2011, Machinability of to near beta titanium alloy, Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture, 000: 1-12.

[8] Rahman, M., Wang, Z.G., Wong, Y.S., 2006, A Review on High-Speed Machining of Titanium Alloys, JSME International Journal, 49: 11-20.

[9] Ezugwu, e., Wang, z., 1997, Titanium alloys and their machinability - A review, Journal of Materials Processing Technology, 68, 262-274.

[10] Chen, w., Sun, q., Xiao, l., Sun, j., 2011, Deformation-Induced Grain Refinement and Amorphization in Ti-10V-2Fe-3Al Alloy, Metallurgical and Materials Transactions A, 43: 316-326.

[11] Armendia, M., 2011, Machining of Titanium Alloys: A machinability Study of Ti6Al4V and Timetal54M alloys, thesis Mondragon University.

[12] Yang, x. Liu, r.-C., 1999, Machining titanium and its alloys; Machining Science and Technology, 3 (1): 107-139.

[13] Machado, A. R., Wallbank, j., 1990, Machining of Titanium and Its alloys - A Review, Journal of Engineering Manufacture, 204: 53-60.

[14] Narutaki, N., Murakoshi, a., 1983, Study on Machining Titanium Alloys, Annals of the CIRP, 32/1:65-69.

[15] W. Konig, 1979, Applied Research on the Machinability of Titanium and Its Alloys, Proceedings of the Forty-seventh Meeting of AGARD, AGARD, CP256, 1.1-1.10, AGARD Structural and Material Panel.

[16] S. Motonishi, S. Isoda, H. Itoh, y. Tsumori, and y. Terada, 1987, Study on Machining of Titanium and Its Alloys, Kobelto Technology Review, 2 (1987) 28-31.

[17] Calamaz, M., 2006, Feasibility of the dry machining process on the aeronautic titanium alloys, Fifth International conference on High speed Machining.

[18] Ribeiro, M., Moreira, M., Ferreira, j., 2003, Optimization of titanium (6Al-4V) alloy machining, Journal of Materials Processing Technology, 2003, 143-144, 458-463.

[19] Boyer, R-R., 1992, New Titanium Applications on the Boeing 777 Airplane. Journal of Metals, May: 23-25.

[20] Fanning, j.-C., 2005, Properties of Timetal 555 (Ti-5.5Al-5mø-5V-3Cr), J. Materials Engineering and performance. 14:788-791.

[21] Armendia, M., Garay, a., Iriarte, L-M., Arrazola, P-j.., 2010, Comparison of the machinabilities of Ti6Al4V and Timetal54M using uncoated WC-Co tools, Journal of Materials Processing Technology, 210: 197-203.

[22] Aeby-Gautier, e., Settefrati, a., Bruneseaux, f., Appolaire, B., Denand, B., Dehmas, M., Geandier, g., Boulet, p., 2012, Isothermal phase formation in titanium alloys metastable beta alpha, Journal of Alloys and Compounds.

[23] Settefrati, a., Aeby-Gautier, e., Dehmas, M., Geandier, g., Appolaire, B., Audion, S., Delfosse, j., 2011, Precipitation in a near Beta Titanium Alloy on Ageing: Influence of Heating Rate and Chemical Composition of the Beta - Metastable Phase, Solid State Phenomena, 172-174: 760-765.

[24] Dehmas, M., Kovac, j., Appolaire, B., Aeby-Gautier, e., Denand, B., Da Costa Teixeira, j., 2011, β → α + β Phase Transformation in Ti17 Titanium Alloy: Chemical Composition and Crystallographic Aspect, Solid State Phenomena, 172-174: 396-401.

[25] Arrazola, P.J., Garay, a., Iriarte, L. M., Marya, S., Le Maître, f., 2007, Machinability of Near Beta Titanium Alloys Employed in Aeronautics, The 11th World Conference on Titanium (Japan), June 2007.

[26] Arrazola, P.J., Garay, a., Iriarte, L. M., Marya, S., Le Maître, f., 2007, Machinability of Titanium alloys (Ti6Al4V and Ti555.3), Journal of Materials Processing Technology (review pending).

[27] Machai, Biermann, C., D., 2011, Machining of b-titanium-alloy Ti-10V-2Fe-3Al under cryogenic conditions: Cooling with carbon dioxide snow, Journal of Materials Processing Technology, 211: 1175-1183.

[28] Machai, C., Abrahams, H., Biermann, D., 2012, Machinability of - titanium alloy Ti-10V-2Fe-3Al with different microstructures, TMS Annual Meeting, 1: 919 - 926.

[29] Dearnley, P.A., Grearson, a. N, 1986, Evaluation of main Wear Mechanisms of Cemented Carbides and Ceramics Used for Machining Titanium Alloy IMI 318, Materials Science and Technology, 2: 47-58.

[30] Yang, x., Richard Liu, C., 1999, Machining Titanium and its alloys, Machining Science and Technology, 3: 107-13.

[31] Hartung, P-D., Kramer, B-M., 1982, Tool wear in Titanium machining, Annals of the CIRP, 32/1:75-80.

[32] Corduan, N., Himbert, T., Pulachon, g., Dessoly, M., Lambertin, M., Vigneau, j., Payoux, B., 2003, Wear Mechanisms of New Tool Materials for Ti-6Al-4V High Performance Machining, Annals of the CIRP, 52/1:73-76.

[33] Farhad Nabhani, 2001, Wear mechanism of ultra-hard cutting tools materials, Journal of Materials processing Technology 115 (2001) 402-412.

[34] Jawaid, A.; Che-Haron, C., Abdullah, a. 1999, Tool wear characteristics in turning of titanium alloy Ti-6246 Journal of Materials Processing Technology, 1999, 92-93, 329-334.

[35] Bayoumi, A.E., Xie, J.Q., 1995, Some metallurgical aspects of chip formation in cutting Ti-6wt.%Al-4wt.%V alloy, "Materials Science & Engineering" A190 (1995) 173-180.

[36] Komanduri, r., 1982, Some Clarification on the Mechanics of Chip Formation When

Machining Titanium Alloys, Wear, 76 (1982) 15-34.

[37] Komanduri, r., von Turkovich, B.F., 1981, New observations on the mechanism of chip formation when machining titanium alloys, Wear, 1981, 69, 179-188.

[38] A. people, h.-w.. Hoffmeister, 2001, Chip Formation in Machining Ti6Al4V at extremely High Cutting Speeds, Annals of the CIRP, 50/1, p. 49-52.

[39] Shahan, Taheri, A.K., r. 1993, Adiabatic shear bands in titanium and titanium alloys: a critical review, Materials & Design, 1993, 14, 243-250.

[40] Shaw, M., Vyas, A, 1993, Chip formation in the machining of hardened steel CIRP Annals, 1993, 42, 29-33.

[41] JC, l. & WA, B., 1972, Adiabatic instability in the orthogonal cutting of steel, 3, 477 - 81.

[42] Elmagrabi, N., Shuaeib, f., Haron, C., 2007, An overview on the cutting tool factors in machinability assessment, Journal of Achievements in Materials and Manufacturing Engineering, 23: 87-90.

[43] IMTS 2012, Chicago. www.imts.com/education/detail. Last visit 12/01/2012. [44] ISO 3685:1977, Tool-Life testing with single-point turning tools.

[45] Arriola, i., Whitenton, e., Heigel, j., Arrazola, p., 2011, Relationship between machinability index and in-process parameters during orthogonal cutting of steels, CIRP Annals-Manufacturing Technology, Elsevier, 60: 93-96.