Machining Titanium and It's Alloys

Introduction to Machining
Influencing Characteristics
Traditional Machining
Machining Data
Productivity Increase
Non-Traditional Machining
Surface Integrity

INTRODUCTION                                                 

Many of titanium’s material and component design characteristics make it expensive to machine.  A considerable amount of stock must be removed from primary forms such as forgings, plates, bars, etc.  In some instance, as much as 50 to 90% of the primary form’s weight ends up as chips.  (The complexity of some finished parts, such as bulkhead, makes difficult the use of near-net-shape methods that would minimize chip forming.)  Maximum machining efficiency for titanium alloys is required to minimize the costs of stock removal. 

Historically, titanium has been perceived as a material that is difficult to machine.  Due to titanium’s growing acceptance in many industries, along with the experience gained by progressive fabricators, a broad base of titanium machining knowledge now exists.  Manufacturers now know that, with proper procedures, titanium can be fabricated using techniques no more difficult than those used for machining 316 stainless steel. 

Stories about problems encountered when machining titanium have usually originated in shops working with aircraft alloys.  The fact is that commercially pure grades of titanium (ASTM B, Grades 1, 2, 3, and 4) with tensile strengths of 241 to 552 MPa (35 to 80 ksi) machine much easier than aircraft alloys (i.e. ASTM B, Grade 5:  Ti-6AL-4V).          

With higher alloy content and hardness, the machinability of titanium alloys by traditional chip-making methods generally decreases.  (This is true of most other metals.) At a hardness level over 38 R_C  (350 BHN) increased difficulty in operations such as drilling tapping, milling, and broaching can be expected.  In general, however, if the particular characteristics of titanium are taken into account, the machining of titanium and its alloys should not present undue problems.            

Machining of titanium alloys requires cutting forces only slightly higher than those needed to machine steels, but these alloys have metallurgical characteristics that make them somewhat more difficult to machine than steels of equivalent hardness.  The beta alloys are the most difficult titanium alloys to machine.  When machining conditions are selected properly for a specific alloy composition and processing sequence, reasonable production rates of machining can be achieved at acceptable cost levels.   

Care must be exercised to avoid loss of surface integrity, especially during grinding; otherwise a dramatic loss in mechanical behavior such as fatigue can result.  To date, techniques such as high-speed machining have not improved the machinability of titanium.  A breakthrough appears to require the development of new tool materials.

CHARACTERISTICS INFLUENCING MACHINABILITY                       

The fact that titanium sometimes is classified as difficult to machine by traditional methods in part can be explained by the physical, chemical, and mechanical properties of the metal.  For example:

Titanium is a poor conductor of heat.  Heat, generated by the cutting action, does not dissipate quickly.  Therefore, most of the heat is concentrated on the cutting edge and the tool face.

Titanium has a strong alloying tendency or chemical reactivity with materials in the cutting tools at tool operating temperatures.  This causes galling, welding, and smearing along with rapid destruction of the cutting tool. 

Titanium has a relatively low modulus of elasticity, thereby having more “springiness” than steel.  Work has a tendency to move away from the cutting tool unless heavy cuts are maintained or proper backup is employed.  Slender parts tend to deflect under tool pressures, causing chatter, tool rubbing, and tolerance problems.  Rigidity of the entire system is consequently very important, as is the use of sharp, properly shaped cutting tools.

Titanium’s fatigue properties are strongly influenced by a tendency to surface damage if certain machining techniques are used.  Care must be exercised to avoid the loss of surface integrity, especially during grinding.  (This characteristic is described in greater detail below.)

Titanium’s work-hardening characteristics are such that titanium alloys demonstrate a complete absence of “built-up edge.”  Because of the lack of a stationary mass of metal (built-up edge) ahead of the cutting tool, a high shearing angle is formed.  This causes a thin chip to contact a relatively small area on the cutting tool face and results in high bearing loads per unit area.  The high bearing force, combined with the friction developed by the chip as it rushes over the bearing area, results in a great increase in heat on a very localized portion of the cutting tool.  Furthermore, the combination of high bearing forces and heat produces cratering action close to the cutting edge, resulting in rapid tool breakdown.

 With respect to titanium’s fatigue properties, briefly noted in the above list, the following details are of interest.

As stated, loss of surface integrity must be avoided.  If this precaution is not observed, a dramatic loss of mechanical behavior (such as fatigue) can result.  Even proper grinding practices using conventional parameters (wheel speed, downfeed, etc.) may result in appreciably lower fatigue strength due to surface damage.  The basic fatigue properties of many titanium alloys rely on a favorable compressive surface stress induced by tool action during machining.  Electromechanical removal of material, producing a stress-free surface, can cause a debit from the customary design fatigue strength properties.  (These results are similar when mechanical processes such as grinding are involved, although the reasons are different.)   

TRADITIONAL MACHINING OF TITANIUM                                        

 General    

The term “machining” has broad application and refers to all types of metal removal and cutting processes.  These include turning, boring, milling, drilling, reaming, tapping, both sawing and gas cutting, broaching, planing, gear hobbing, shaping, shaving, and grinding. 

The technology supporting the machining of titanium alloys basically is very similar to that for other alloy systems.  Efficient metal machining requires access to data relating the machining parameters of a cutting tool to the work material for the given operation.  The important parameters include:

• Tool life
• Forces
• Power requirements
• Cutting tools and fluids

Subsequent paragraphs discuss these parameters in general terms.   

Tool Life

Tool-life data have been developed experimentally for a wide variety of titanium alloys.  A common way of representing such data is shown in Figure 6.1 where tool life (as time) is plotted against cutting speed (fpm) for a given cutting tool material at a constant feed and depth in relation to Ti-6Al-4V.  It can be seen that at a high cutting speed, tool life is extremely short.  As the cutting speed decreases, tool life dramatically increases.

Titanium alloys are very sensitive to changes in feed, as in Figure 6.1.  Industry generally operates at cutting speeds providing long tool life.  Curve fitting of tool life to feed, speed, and other machining parameters is commonly being done by means of computer techniques.  However, in cases where no data base exists, certain rules of thumb should be recognized.  For example, when cutting titanium, a high shear angle is produced between the workpiece and chip, resulting in a thin chip flowing at high velocity over the tool face.  High temperatures develop, and, since titanium has low thermal conductivity, the chips have a tendency to gall and weld themselves to the tool cutting edges.  This speeds up tool wear and failure.  When dealing with high-fixed-cost machine tools production output may be much more important than a cutting tool’s life!  It thus may be wise to work a tool at its maximum capacity, and then replace it as soon as its cutting efficiency starts to drop off noticeably, thereby maintaining uptime as much as possible.

 When machining titanium in circumstances in which production costs are not of paramount concern, it is still unsound practice to allow tools to run to destruction.  The other extreme, premature tool changing, may result in a low number of pieces per tool grind, but the lower the tool wear, the less expensive the regrinding. 

Ideally, a tool should be permitted to continue cutting as long as possible without risking damage to the tool or the work but with the retention of surface integrity.  The only way to find a safe stopping point is to check a few runs by counting the pieces produced and inspecting the surface finish, dimensions, and surface integrity.  In this manner it can be established how many acceptable pieces can be produced before the tool fails. 

Forces and Power Requirements

 The forces in machining can be determined with a tool dynamometer.  In turning, the tool dynamometer usually measures three components:

• Tangential, or cutting force
• Thrust, or separating force
• Feed, or axial force

 The cutting force is important since, when multiplied by the cutting velocity, it determines the power requirements in machining.  The thrust, or separating force, determines the accuracy produced on a part. 

 For general approximations, the power requirements in turning and milling can be obtained by measuring the power input to the machine tool’s drive motor during a cutting operation and by subtracting from it the tare, or idle power.  A good approximation of the horsepower required in most machining operations can be predicted from unit power requirements.  Table 6.1 shows the power requirements for titanium in comparison to other alloys. 

 Table 6.1:  Average unit power requirements for turning, drilling, or milling of titanium compared with other competitive alloy systems

Cutting Tools

Major improvements in the rate at which workpieces are machined usually result from the development and application of new tool materials.  In the past several years, there have been major advancements in the development of cutting tools including coated carbides, ceramics, cermets, cubic boron nitride, and polycrystalline diamond.  These have found useful applications in the machining of cast irons, steels, and high-temperature and aluminum alloys.

Unfortunately, none of these or other new materials has improved the removal rate of titanium alloys.  In studies conducted as early as 1950, the straight tungsten carbide (WC) cutting tools, typically C-2 grades, performed best in operations such as turning and face milling, while the high-cobalt, high-speed steels were most applicable in drilling, tapping, and end milling. 

Today, the situation is much the same.  C-2 carbides are used extensively in engine and airframe manufacturing for turning and face milling operations.  In recent years, in the United States as well as in Europe, solid C-2 end mills and end mills with replaceable C-2 carbides are finding applications, particularly in aerospace plants.  Today, the M7 and, more frequently, the M42 and M33 high-speed steels are recommended for end milling, drilling, and tapping of titanium alloys. 

 Cutting Fluids

 Cutting fluids used in machining titanium alloys require special consideration because chlorine ions have, under certain circumstances, caused stress-corrosion cracking in laboratory testing of these alloys for mechanical properties.  Consequently, chlorine at one time was considered a suspect element regardless of the concentration and specific conditions used in manufacturing operations, such as machining. 

When specifying cutting fluids for machining titanium, some companies have practically no restrictions other than using controlled-washing procedures on parts after machining.  Other manufacturers do likewise, except that they do not use cutting fluids containing chlorine on parts which are subjected to higher temperatures in welding processes or in service.  Also when assemblies are machined, the same restrictions apply because of the difficulty in doing a good cleaning job after machining.  Still other organizations in aerospace manufacturing permit no active chlorine in any cutting fluid used for machining titanium alloys.

 A program to define the effect of experimental chlorinated and sulfurized cutting fluids on the mechanical properties of the Ti-6AL-4V alloy (annealed, 34 R_c) was performed.  Mechanical property evaluations included:        

• High-cycle fatigue at both room and elevated temperatures
• crack propagation at two cyclic frequencies
• Fracture
• Stress-corrosion/surface-embrittlement exposures

Within the scope of the program, and within the range of variables investigated, the results indicated generally that no degradation of mechanical properties relative to those obtained from neutral cutting fluids occurred.  Similar results were obtained by using chlorinated and sulfurized fluids in machining, or by having those cutting fluids present as an environment during testing.  The use of chlorine-containing (or halogen-containing) cutting fluids generally is not a recommended practice, despite the above-noted results which pertain to only a single titanium alloy.

 There are excellent cutting fluids available which do not contain any halogen compounds.  In fact, from extensive test data collected by the Air Force Materials Laboratory, it can be concluded that chlorine-containing cutting fluids do not always provide better tool life.  For certain alloys and operations, dry machining is preferred.  Usually the heavy chlorine-bearing fluids excel in operations such as drilling, tapping, and broaching.  Figure 6.2 shows the effect of various cutting fluids on tool life in drilling Ti-6Al-4V.   

MACHINING DATA:  SPEEDS AND FEEDS                                         

 Cutting speed and feed are two of the most important parameters for all types of machining operations.  Extensive testing has developed the tool-life data, as illustrated in Figure 6.1, for turning Ti-6Al-4V.  Tool life charts are available, as noted in Appendix C, "Machining Practices."  One Manufacturer offers the following general guidelines for typical machining operations. 

Although the basic machining properties of titanium metal cannot be altered significantly, their effects can be greatly minimized by decreasing temperatures generated at the tool face and cutting edge.  Economical production techniques have been developed through application of these basic rules in machining titanium:

Use low cutting speeds.  Tool tip temperatures are affected more by cutting speed than by any other single variable.   A change from 6 to 46 meters per min (20 to 150 sfm) with carbide tools results in a temperature change from 427ºC to 927ºC (800ºF to 1700ºF). 

Maintain high feed rates.  Temperature is not affected by feed rate so much as by speed, and the highest feed rates consistent with good machining practice should be used.  A change from 0.05 to 0.51 mm (0.002 in. to 0.020 in.) per revolution results in a temperature increase of only 149ºC (300ºF). 

Use generous amounts of cutting fluid.  Coolant carries away heat, washes away chips, and reduces cutting forces. 

Use sharp tools and replace them at the first sign of wear, or as determined by production/cost considerations.  Tool wear is not linear when cutting titanium.  Complete tool failure occurs rather quickly after small initial amount of wear takes place. 

Never stop feeding while a tool and a workpiece are in moving contact.  Permitting a tool to dwell in moving contact causes work hardening and promotes smearing, galling, seizing, and total tool breakdown. 

Machining recommendations, such as noted above, may require modification to fit particular circumstances in a given shop.  For example, cost, storage, or requirements may make it impractical to accommodate a very large number of different cutting fluids.  Savings achieved by making a change in cutting fluid may be offset by the cost of changing fluids.  Likewise, it may be uneconomical to inventory cutting tools which may have only infrequent use.  Also, the design of parts may limit the rate of metal removal in order to minimize distortion (of thin flanges, for example) and to corner without excessive inertia effects. 

An example of typical machining parameters currently used to machine Ti-6Al-4V bulkheads containing deep pockets, thin flanges, and floors at an important United States airframe manufacturer are shown in Table 6.2. A bulkhead frequently contains numerous pockets and some flanges as thin as 0.76 mm (0.030 in.).  Typical example bulkhead rough forgings weigh in excess of 450 kg (1000 lb), but the finished part is less than 67.5 kg (150 lb) after machining. Extensive machining is done on gas turbine engine components, just as is done on the larger airframe components.  Table 6.3 lists typical parameters for machining Ti-6Al-4V jet engine components such as fan disks, spacers, shafts, and rotating seals.

Table 6.2:  Example of typical machining parameters currently used

to machine Ti-6Al-4V airframe bulkheads

INCREASED PRODUCTIVITY BY SPECIAL TECHNIQUES             

 The inability to improve cutting-tool performance by developing new cutting-tool materials—coatings in particular—has been very frustrating.  Likewise, very little improvement in productivity has been experienced by exploring new combinations of speeds, feeds, and depths.  However, developments of interest include specially designed turning tools and milling cuttings along with the use of a special end mill pocketing technique.

 In recent years, ceramic tools have been used successfully in machining high-temperature alloy jet-engine components at speeds much higher than those conventionally used.  At speeds of 183 to 213 m/min (600 to 700 ft/min), tool life is short (3 to 5 min), but it is possible to finish a cut at these speeds and then index the cutting tip for making the next pass.  This same technique has potential in machining of titanium with C-2 carbides.  Data are needed to determine the speeds at which reproducible and reliable tool life of the order of 3 to 5 min can be obtained, and to determine whether these conditions improve the economics of titanium machining.

Table 6.3:  Example of typical parameters for machining Ti-6Al-4V gas turbine components

 One of the practical techniques for increasing productivity is to determine the optimum cost in machining a given titanium part for a specific machining operation.  If specific data are available relating tool life to speed, feed, and depth for a given operation and cutter, it is possible to calculate the overall cost and time of machining as a function of the cutting parameters.  Some companies are now using computers to perform such cost analyses and to arrive at  minimum costs and optimum production rates for specific machining operations. 

NON-TRADITIONAL MACHINING                                                     

 The design of titanium alloy components often requires the use of the so-called nontraditional machining methods.  Among these electrochemical machining (ECM), chemical milling (CHM), and laser beam torch (LBT) are probably the most widely used.   Technical information on procedures and techniques is generally proprietary, however. 

 Chemical and electrochemical methods of metal removal are expected to be used increasingly in years to come, because of their many favorable features.  They are particularly useful for rapid removal of metal from the surface of formed or complex-shaped parts, from thin sections, and from large areas down to shallow depths.  These processes have no damaging effect on the mechanical properties of the metal.  (See the earlier comments about fatigue properties of stress-free surfaces.)  There is no hydrogen entry into the metal to cause embrittlement or loss of ductility.

ECM is the removal of electrically conductive material by anodic dissolution in a rapidly flowing electrolyte which separates the workpiece from a shaped electrode.  ECM can generate difficult contours and provide distortion-free, high quality surfaces.  For ECM of titanium alloys, a very common electrolyte is sodium chloride used at concentrations of about 1 lb/gal.

CHM is the controlled dissolution of a workpiece material by contact with a strong chemical reagent.  The part being processed is cleaned thoroughly and covered with a strippable, chemically-resistant mask.  Areas where chemical action is desired are stripped off the mask, and then the part is submerged in the chemical reagent to dissolve the exposed material. 

Another operation usable in the processing of titanium alloys is the LBT method. In this process, material is removed by focusing a laser beam and a gas stream on a workpiece. The laser energy causes localized melting, and an oxygen gas stream promotes an exothermic reaction and purges the molten material from the cut. Titanium alloys are cut at very rapid rates using a continuous wave CO laser with oxygen assist.

SURFACE INTEGRITY                                                                         

The surface of titanium alloys is thought to be easily damaged during some traditional machining operations.  Damage appears in the form of microcracks; built-up edge; plastic deformation; heat-affected zones; and tensile residual stresses.  In service, this damage can lead to degraded fatigue strength and stress corrosion resistance.  In a study of grinding effects on Ti-6Al-4V alloy, gentle or low-stress grinding parameters displayed no readily identifiable changes at the surface, while conventional and abusive practices altered the surface layer noticeably.  There was an appreciable drop in hardness in the gently ground specimen, but very good high-cycle-fatigue values were noted.

 Figure 6.3 indicates an endurance limit of 372 MPa (54 ksi) for the gentle grinding and values of 83 and 97 MPa (12 and 14 ksi) for conventional and abusive conditions, respectively.  Figure 6.3 also presents values for other machining operations including electrical discharge machining (EDM) and chemical milling (CHM).  As can be seen, in operations like end mill cutting or turning, the same sensitivity to abusive conditions was not observed, possibly due to residual surface compressive stresses.

Machinists and companies specializing in the machining of aerospace materials generally will have developed techniques to maximize surface integrity of titanium alloys.  Thus optimum properties usually are achieved during the production machining of titanium.  In those areas of application where maximum fatigue strength is required, not only are appropriated machining parameters used, but also selected surface areas of components may be glass bead blasted to restore, or to retain, a high level of favorable compressive surface stress.

Thanks To:

Titanium: A Technical Guide (1988), ASM International, Materials Park, OH, 44073-0002, page 75-85